SpringBoot学习（三）探究Springboot自动装配

目录

　　什么是自动装配

　　何时自动装配

　　原理分析

　　注：以下展示的代码springboot的版本为2.0.3版。因源码过长，大家选择展开代码 ㄟ( ▔, ▔ )ㄏ 

什么是自动装配

　　自动装配还是利用了SpringFactoriesLoader来加载META-INF/spring.factoires文件里所有配置的EnableAutoConfgruation，它会经过exclude和filter等操作，最终确定要装配的类

　　流程：@Configuration 配置的Bean -> BeanFactory -> ImportSelector -> AutoConfigurationSelectImporter ->
　　　　　SpringFactoriesLoader -> META-INF/spring.factories->
　　　　　所有配置的@EnableAutoConfiguration -> @Configuration -> 收集好后注册到IOC容器里

何时自动装配，过程是什么

　　处理@Configuration的核心还是ConfigurationClassPostProcessor，这个类实现了BeanFactoryPostProcessor, 因此当AbstractApplicationContext执行refresh方法里的invokeBeanFactoryPostProcessors(beanFactory)方法时会执行自动装配

　　通过：ConfigurationClassPostProcessor这个类中的 ConfigurationClassParser  parser()方法

　　过程：refresh() -> 注册BeanFatoryPostProcessor  -> ConfigurationClassParser-> 内部类DeferredImportSelectorHolder
　　　　　　-> DeferredImportSeletor -> AutoConfigurationImportSelector -> selectImports方法

原理分析

什么是自动装配？

　　@EnableAutoConfiguration源码里import了EnableAutoConfigurationImportSelector这个类，

EnableAutoConfiguration接口

/*
 * Copyright 2012-2017 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.springframework.boot.autoconfigure;

import java.lang.annotation.Documented;
import java.lang.annotation.ElementType;
import java.lang.annotation.Inherited;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;

import org.springframework.boot.autoconfigure.condition.ConditionalOnBean;
import org.springframework.boot.autoconfigure.condition.ConditionalOnClass;
import org.springframework.boot.autoconfigure.condition.ConditionalOnMissingBean;
import org.springframework.boot.web.embedded.tomcat.TomcatServletWebServerFactory;
import org.springframework.boot.web.servlet.server.ServletWebServerFactory;
import org.springframework.context.annotation.Conditional;
import org.springframework.context.annotation.Configuration;
import org.springframework.context.annotation.Import;
import org.springframework.core.io.support.SpringFactoriesLoader;

/**
 * Enable auto-configuration of the Spring Application Context, attempting to guess and
 * configure beans that you are likely to need. Auto-configuration classes are usually
 * applied based on your classpath and what beans you have defined. For example, if you
 * have {@code tomcat-embedded.jar} on your classpath you are likely to want a
 * {@link TomcatServletWebServerFactory} (unless you have defined your own
 * {@link ServletWebServerFactory} bean).
 * <p>
 * When using {@link SpringBootApplication}, the auto-configuration of the context is
 * automatically enabled and adding this annotation has therefore no additional effect.
 * <p>
 * Auto-configuration tries to be as intelligent as possible and will back-away as you
 * define more of your own configuration. You can always manually {@link #exclude()} any
 * configuration that you never want to apply (use {@link #excludeName()} if you don't
 * have access to them). You can also exclude them via the
 * {@code spring.autoconfigure.exclude} property. Auto-configuration is always applied
 * after user-defined beans have been registered.
 * <p>
 * The package of the class that is annotated with {@code @EnableAutoConfiguration},
 * usually via {@code @SpringBootApplication}, has specific significance and is often used
 * as a 'default'. For example, it will be used when scanning for {@code @Entity} classes.
 * It is generally recommended that you place {@code @EnableAutoConfiguration} (if you're
 * not using {@code @SpringBootApplication}) in a root package so that all sub-packages
 * and classes can be searched.
 * <p>
 * Auto-configuration classes are regular Spring {@link Configuration} beans. They are
 * located using the {@link SpringFactoriesLoader} mechanism (keyed against this class).
 * Generally auto-configuration beans are {@link Conditional @Conditional} beans (most
 * often using {@link ConditionalOnClass @ConditionalOnClass} and
 * {@link ConditionalOnMissingBean @ConditionalOnMissingBean} annotations).
 *
 * @author Phillip Webb
 * @author Stephane Nicoll
 * @see ConditionalOnBean
 * @see ConditionalOnMissingBean
 * @see ConditionalOnClass
 * @see AutoConfigureAfter
 * @see SpringBootApplication
 */
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@AutoConfigurationPackage
@Import(AutoConfigurationImportSelector.class)
public @interface EnableAutoConfiguration {

    String ENABLED_OVERRIDE_PROPERTY = "spring.boot.enableautoconfiguration";

    /**
     * Exclude specific auto-configuration classes such that they will never be applied.
     * @return the classes to exclude
     */
    Class<?>[] exclude() default {};

    /**
     * Exclude specific auto-configuration class names such that they will never be
     * applied.
     * @return the class names to exclude
     * @since 1.3.0
     */
    String[] excludeName() default {};

}

但是该类在SpringBoot1.5.X版本已经过时了，因此我们看一下它的父类AutoConfigurationImportSelector。（2.0.3版直接引入了父类AutoConfigurationImportSelector）

AutoConfigurationImportSelector类

/*
 * Copyright 2012-2018 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.springframework.boot.autoconfigure;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.LinkedHashMap;
import java.util.LinkedHashSet;
import java.util.List;
import java.util.Map;
import java.util.Set;
import java.util.concurrent.TimeUnit;
import java.util.stream.Collectors;

import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;

import org.springframework.beans.BeansException;
import org.springframework.beans.factory.Aware;
import org.springframework.beans.factory.BeanClassLoaderAware;
import org.springframework.beans.factory.BeanFactory;
import org.springframework.beans.factory.BeanFactoryAware;
import org.springframework.beans.factory.NoSuchBeanDefinitionException;
import org.springframework.beans.factory.config.ConfigurableListableBeanFactory;
import org.springframework.boot.context.properties.bind.Binder;
import org.springframework.context.EnvironmentAware;
import org.springframework.context.ResourceLoaderAware;
import org.springframework.context.annotation.DeferredImportSelector;
import org.springframework.core.Ordered;
import org.springframework.core.annotation.AnnotationAttributes;
import org.springframework.core.env.ConfigurableEnvironment;
import org.springframework.core.env.Environment;
import org.springframework.core.io.ResourceLoader;
import org.springframework.core.io.support.SpringFactoriesLoader;
import org.springframework.core.type.AnnotationMetadata;
import org.springframework.core.type.classreading.CachingMetadataReaderFactory;
import org.springframework.core.type.classreading.MetadataReaderFactory;
import org.springframework.util.Assert;
import org.springframework.util.ClassUtils;
import org.springframework.util.StringUtils;

/**
 * {@link DeferredImportSelector} to handle {@link EnableAutoConfiguration
 * auto-configuration}. This class can also be subclassed if a custom variant of
 * {@link EnableAutoConfiguration @EnableAutoConfiguration}. is needed.
 *
 * @author Phillip Webb
 * @author Andy Wilkinson
 * @author Stephane Nicoll
 * @author Madhura Bhave
 * @since 1.3.0
 * @see EnableAutoConfiguration
 */
public class AutoConfigurationImportSelector
        implements DeferredImportSelector, BeanClassLoaderAware, ResourceLoaderAware,
        BeanFactoryAware, EnvironmentAware, Ordered {

    private static final String[] NO_IMPORTS = {};

    private static final Log logger = LogFactory
            .getLog(AutoConfigurationImportSelector.class);

    private static final String PROPERTY_NAME_AUTOCONFIGURE_EXCLUDE = "spring.autoconfigure.exclude";

    private ConfigurableListableBeanFactory beanFactory;

    private Environment environment;

    private ClassLoader beanClassLoader;

    private ResourceLoader resourceLoader;

    @Override
    public String[] selectImports(AnnotationMetadata annotationMetadata) {
        if (!isEnabled(annotationMetadata)) {
            return NO_IMPORTS;
        }
        AutoConfigurationMetadata autoConfigurationMetadata = AutoConfigurationMetadataLoader
                .loadMetadata(this.beanClassLoader);
        AnnotationAttributes attributes = getAttributes(annotationMetadata);
        List<String> configurations = getCandidateConfigurations(annotationMetadata,
                attributes);
        configurations = removeDuplicates(configurations);
        Set<String> exclusions = getExclusions(annotationMetadata, attributes);
        checkExcludedClasses(configurations, exclusions);
        configurations.removeAll(exclusions);
        configurations = filter(configurations, autoConfigurationMetadata);
        fireAutoConfigurationImportEvents(configurations, exclusions);
        return StringUtils.toStringArray(configurations);
    }

    @Override
    public Class<? extends Group> getImportGroup() {
        return AutoConfigurationGroup.class;
    }

    protected boolean isEnabled(AnnotationMetadata metadata) {
        if (getClass() == AutoConfigurationImportSelector.class) {
            return getEnvironment().getProperty(
                    EnableAutoConfiguration.ENABLED_OVERRIDE_PROPERTY, Boolean.class,
                    true);
        }
        return true;
    }

    /**
     * Return the appropriate {@link AnnotationAttributes} from the
     * {@link AnnotationMetadata}. By default this method will return attributes for
     * {@link #getAnnotationClass()}.
     * @param metadata the annotation metadata
     * @return annotation attributes
     */
    protected AnnotationAttributes getAttributes(AnnotationMetadata metadata) {
        String name = getAnnotationClass().getName();
        AnnotationAttributes attributes = AnnotationAttributes
                .fromMap(metadata.getAnnotationAttributes(name, true));
        Assert.notNull(attributes,
                () -> "No auto-configuration attributes found. Is "
                        + metadata.getClassName() + " annotated with "
                        + ClassUtils.getShortName(name) + "?");
        return attributes;
    }

    /**
     * Return the source annotation class used by the selector.
     * @return the annotation class
     */
    protected Class<?> getAnnotationClass() {
        return EnableAutoConfiguration.class;
    }

    /**
     * Return the auto-configuration class names that should be considered. By default
     * this method will load candidates using {@link SpringFactoriesLoader} with
     * {@link #getSpringFactoriesLoaderFactoryClass()}.
     * @param metadata the source metadata
     * @param attributes the {@link #getAttributes(AnnotationMetadata) annotation
     * attributes}
     * @return a list of candidate configurations
     */
    protected List<String> getCandidateConfigurations(AnnotationMetadata metadata,
            AnnotationAttributes attributes) {
        List<String> configurations = SpringFactoriesLoader.loadFactoryNames(
                getSpringFactoriesLoaderFactoryClass(), getBeanClassLoader());
        Assert.notEmpty(configurations,
                "No auto configuration classes found in META-INF/spring.factories. If you "
                        + "are using a custom packaging, make sure that file is correct.");
        return configurations;
    }

    /**
     * Return the class used by {@link SpringFactoriesLoader} to load configuration
     * candidates.
     * @return the factory class
     */
    protected Class<?> getSpringFactoriesLoaderFactoryClass() {
        return EnableAutoConfiguration.class;
    }

    private void checkExcludedClasses(List<String> configurations,
            Set<String> exclusions) {
        List<String> invalidExcludes = new ArrayList<>(exclusions.size());
        for (String exclusion : exclusions) {
            if (ClassUtils.isPresent(exclusion, getClass().getClassLoader())
                    && !configurations.contains(exclusion)) {
                invalidExcludes.add(exclusion);
            }
        }
        if (!invalidExcludes.isEmpty()) {
            handleInvalidExcludes(invalidExcludes);
        }
    }

    /**
     * Handle any invalid excludes that have been specified.
     * @param invalidExcludes the list of invalid excludes (will always have at least one
     * element)
     */
    protected void handleInvalidExcludes(List<String> invalidExcludes) {
        StringBuilder message = new StringBuilder();
        for (String exclude : invalidExcludes) {
            message.append("	- ").append(exclude).append(String.format("%n"));
        }
        throw new IllegalStateException(String
                .format("The following classes could not be excluded because they are"
                        + " not auto-configuration classes:%n%s", message));
    }

    /**
     * Return any exclusions that limit the candidate configurations.
     * @param metadata the source metadata
     * @param attributes the {@link #getAttributes(AnnotationMetadata) annotation
     * attributes}
     * @return exclusions or an empty set
     */
    protected Set<String> getExclusions(AnnotationMetadata metadata,
            AnnotationAttributes attributes) {
        Set<String> excluded = new LinkedHashSet<>();
        excluded.addAll(asList(attributes, "exclude"));
        excluded.addAll(Arrays.asList(attributes.getStringArray("excludeName")));
        excluded.addAll(getExcludeAutoConfigurationsProperty());
        return excluded;
    }

    private List<String> getExcludeAutoConfigurationsProperty() {
        if (getEnvironment() instanceof ConfigurableEnvironment) {
            Binder binder = Binder.get(getEnvironment());
            return binder.bind(PROPERTY_NAME_AUTOCONFIGURE_EXCLUDE, String[].class)
                    .map(Arrays::asList).orElse(Collections.emptyList());
        }
        String[] excludes = getEnvironment()
                .getProperty(PROPERTY_NAME_AUTOCONFIGURE_EXCLUDE, String[].class);
        return (excludes != null ? Arrays.asList(excludes) : Collections.emptyList());
    }

    private List<String> filter(List<String> configurations,
            AutoConfigurationMetadata autoConfigurationMetadata) {
        long startTime = System.nanoTime();
        String[] candidates = StringUtils.toStringArray(configurations);
        boolean[] skip = new boolean[candidates.length];
        boolean skipped = false;
        for (AutoConfigurationImportFilter filter : getAutoConfigurationImportFilters()) {
            invokeAwareMethods(filter);
            boolean[] match = filter.match(candidates, autoConfigurationMetadata);
            for (int i = 0; i < match.length; i++) {
                if (!match[i]) {
                    skip[i] = true;
                    skipped = true;
                }
            }
        }
        if (!skipped) {
            return configurations;
        }
        List<String> result = new ArrayList<>(candidates.length);
        for (int i = 0; i < candidates.length; i++) {
            if (!skip[i]) {
                result.add(candidates[i]);
            }
        }
        if (logger.isTraceEnabled()) {
            int numberFiltered = configurations.size() - result.size();
            logger.trace("Filtered " + numberFiltered + " auto configuration class in "
                    + TimeUnit.NANOSECONDS.toMillis(System.nanoTime() - startTime)
                    + " ms");
        }
        return new ArrayList<>(result);
    }

    protected List<AutoConfigurationImportFilter> getAutoConfigurationImportFilters() {
        return SpringFactoriesLoader.loadFactories(AutoConfigurationImportFilter.class,
                this.beanClassLoader);
    }

    protected final <T> List<T> removeDuplicates(List<T> list) {
        return new ArrayList<>(new LinkedHashSet<>(list));
    }

    protected final List<String> asList(AnnotationAttributes attributes, String name) {
        String[] value = attributes.getStringArray(name);
        return Arrays.asList(value != null ? value : new String[0]);
    }

    private void fireAutoConfigurationImportEvents(List<String> configurations,
            Set<String> exclusions) {
        List<AutoConfigurationImportListener> listeners = getAutoConfigurationImportListeners();
        if (!listeners.isEmpty()) {
            AutoConfigurationImportEvent event = new AutoConfigurationImportEvent(this,
                    configurations, exclusions);
            for (AutoConfigurationImportListener listener : listeners) {
                invokeAwareMethods(listener);
                listener.onAutoConfigurationImportEvent(event);
            }
        }
    }

    protected List<AutoConfigurationImportListener> getAutoConfigurationImportListeners() {
        return SpringFactoriesLoader.loadFactories(AutoConfigurationImportListener.class,
                this.beanClassLoader);
    }

    private void invokeAwareMethods(Object instance) {
        if (instance instanceof Aware) {
            if (instance instanceof BeanClassLoaderAware) {
                ((BeanClassLoaderAware) instance)
                        .setBeanClassLoader(this.beanClassLoader);
            }
            if (instance instanceof BeanFactoryAware) {
                ((BeanFactoryAware) instance).setBeanFactory(this.beanFactory);
            }
            if (instance instanceof EnvironmentAware) {
                ((EnvironmentAware) instance).setEnvironment(this.environment);
            }
            if (instance instanceof ResourceLoaderAware) {
                ((ResourceLoaderAware) instance).setResourceLoader(this.resourceLoader);
            }
        }
    }

    @Override
    public void setBeanFactory(BeanFactory beanFactory) throws BeansException {
        Assert.isInstanceOf(ConfigurableListableBeanFactory.class, beanFactory);
        this.beanFactory = (ConfigurableListableBeanFactory) beanFactory;
    }

    protected final ConfigurableListableBeanFactory getBeanFactory() {
        return this.beanFactory;
    }

    @Override
    public void setBeanClassLoader(ClassLoader classLoader) {
        this.beanClassLoader = classLoader;
    }

    protected ClassLoader getBeanClassLoader() {
        return this.beanClassLoader;
    }

    @Override
    public void setEnvironment(Environment environment) {
        this.environment = environment;
    }

    protected final Environment getEnvironment() {
        return this.environment;
    }

    @Override
    public void setResourceLoader(ResourceLoader resourceLoader) {
        this.resourceLoader = resourceLoader;
    }

    protected final ResourceLoader getResourceLoader() {
        return this.resourceLoader;
    }

    @Override
    public int getOrder() {
        return Ordered.LOWEST_PRECEDENCE - 1;
    }

    private static class AutoConfigurationGroup implements DeferredImportSelector.Group,
            BeanClassLoaderAware, BeanFactoryAware, ResourceLoaderAware {

        private ClassLoader beanClassLoader;

        private BeanFactory beanFactory;

        private ResourceLoader resourceLoader;

        private final Map<String, AnnotationMetadata> entries = new LinkedHashMap<>();

        @Override
        public void setBeanClassLoader(ClassLoader classLoader) {
            this.beanClassLoader = classLoader;
        }

        @Override
        public void setBeanFactory(BeanFactory beanFactory) {
            this.beanFactory = beanFactory;
        }

        @Override
        public void setResourceLoader(ResourceLoader resourceLoader) {
            this.resourceLoader = resourceLoader;
        }

        @Override
        public void process(AnnotationMetadata annotationMetadata,
                DeferredImportSelector deferredImportSelector) {
            String[] imports = deferredImportSelector.selectImports(annotationMetadata);
            for (String importClassName : imports) {
                this.entries.put(importClassName, annotationMetadata);
            }
        }

        @Override
        public Iterable<Entry> selectImports() {
            return sortAutoConfigurations().stream()
                    .map((importClassName) -> new Entry(this.entries.get(importClassName),
                            importClassName))
                    .collect(Collectors.toList());
        }

        private List<String> sortAutoConfigurations() {
            List<String> autoConfigurations = new ArrayList<>(this.entries.keySet());
            if (this.entries.size() <= 1) {
                return autoConfigurations;
            }
            AutoConfigurationMetadata autoConfigurationMetadata = AutoConfigurationMetadataLoader
                    .loadMetadata(this.beanClassLoader);
            return new AutoConfigurationSorter(getMetadataReaderFactory(),
                    autoConfigurationMetadata).getInPriorityOrder(autoConfigurations);
        }

        private MetadataReaderFactory getMetadataReaderFactory() {
            try {
                return this.beanFactory.getBean(
                        SharedMetadataReaderFactoryContextInitializer.BEAN_NAME,
                        MetadataReaderFactory.class);
            }
            catch (NoSuchBeanDefinitionException ex) {
                return new CachingMetadataReaderFactory(this.resourceLoader);
            }
        }

    }

}

该类实现了DeferredImportSelector接口，这个接口继承了ImportSelector。该接口主要是为了导入@Configuration的配置项。

DeferredImportSelector接口

/*
 * Copyright 2002-2018 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.springframework.context.annotation;

import java.util.Objects;

import org.springframework.core.type.AnnotationMetadata;
import org.springframework.lang.Nullable;

/**
 * A variation of {@link ImportSelector} that runs after all {@code @Configuration} beans
 * have been processed. This type of selector can be particularly useful when the selected
 * imports are {@code @Conditional}.
 *
 * <p>Implementations can also extend the {@link org.springframework.core.Ordered}
 * interface or use the {@link org.springframework.core.annotation.Order} annotation to
 * indicate a precedence against other {@link DeferredImportSelector}s.
 *
 * <p>Implementations may also provide an {@link #getImportGroup() import group} which
 * can provide additional sorting and filtering logic across different selectors.
 *
 * @author Phillip Webb
 * @author Stephane Nicoll
 * @since 4.0
 */
public interface DeferredImportSelector extends ImportSelector {

    /**
     * Return a specific import group or {@code null} if no grouping is required.
     * @return the import group class or {@code null}
     */
    @Nullable
    default Class<? extends Group> getImportGroup() {
        return null;
    }


    /**
     * Interface used to group results from different import selectors.
     */
    interface Group {

        /**
         * Process the {@link AnnotationMetadata} of the importing @{@link Configuration}
         * class using the specified {@link DeferredImportSelector}.
         */
        void process(AnnotationMetadata metadata, DeferredImportSelector selector);

        /**
         * Return the {@link Entry entries} of which class(es) should be imported for this
         * group.
         */
        Iterable<Entry> selectImports();

        /**
         * An entry that holds the {@link AnnotationMetadata} of the importing
         * {@link Configuration} class and the class name to import.
         */
        class Entry {

            private final AnnotationMetadata metadata;

            private final String importClassName;

            public Entry(AnnotationMetadata metadata, String importClassName) {
                this.metadata = metadata;
                this.importClassName = importClassName;
            }

            /**
             * Return the {@link AnnotationMetadata} of the importing
             * {@link Configuration} class.
             */
            public AnnotationMetadata getMetadata() {
                return this.metadata;
            }

            /**
             * Return the fully qualified name of the class to import.
             */
            public String getImportClassName() {
                return this.importClassName;
            }

            @Override
            public boolean equals(Object o) {
                if (this == o) {
                    return true;
                }
                if (o == null || getClass() != o.getClass()) {
                    return false;
                }
                Entry entry = (Entry) o;
                return Objects.equals(this.metadata, entry.metadata) &&
                        Objects.equals(this.importClassName, entry.importClassName);
            }

            @Override
            public int hashCode() {
                return Objects.hash(this.metadata, this.importClassName);
            }
        }
    }

}

ImportSelector接口

/*
 * Copyright 2002-2013 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.springframework.context.annotation;

import org.springframework.core.type.AnnotationMetadata;

/**
 * Interface to be implemented by types that determine which @{@link Configuration}
 * class(es) should be imported based on a given selection criteria, usually one or more
 * annotation attributes.
 *
 * <p>An {@link ImportSelector} may implement any of the following
 * {@link org.springframework.beans.factory.Aware Aware} interfaces, and their respective
 * methods will be called prior to {@link #selectImports}:
 * <ul>
 * <li>{@link org.springframework.context.EnvironmentAware EnvironmentAware}</li>
 * <li>{@link org.springframework.beans.factory.BeanFactoryAware BeanFactoryAware}</li>
 * <li>{@link org.springframework.beans.factory.BeanClassLoaderAware BeanClassLoaderAware}</li>
 * <li>{@link org.springframework.context.ResourceLoaderAware ResourceLoaderAware}</li>
 * </ul>
 *
 * <p>ImportSelectors are usually processed in the same way as regular {@code @Import}
 * annotations, however, it is also possible to defer selection of imports until all
 * {@code @Configuration} classes have been processed (see {@link DeferredImportSelector}
 * for details).
 *
 * @author Chris Beams
 * @since 3.1
 * @see DeferredImportSelector
 * @see Import
 * @see ImportBeanDefinitionRegistrar
 * @see Configuration
 */
public interface ImportSelector {

    /**
     * Select and return the names of which class(es) should be imported based on
     * the {@link AnnotationMetadata} of the importing @{@link Configuration} class.
     */
    String[] selectImports(AnnotationMetadata importingClassMetadata);

}

　　然后我们回头看AutoConfigurationImportSelector重写的selectImport方法，该方法刚开始会先判断是否进行自动装配，而后会从META-INF/spring-autoconfigure-metadata.properties读取元数据与元数据的相关属性

    @Override
    public String[] selectImports(AnnotationMetadata annotationMetadata) {
        if (!isEnabled(annotationMetadata)) {
            return NO_IMPORTS;
        }
        AutoConfigurationMetadata autoConfigurationMetadata = AutoConfigurationMetadataLoader
                .loadMetadata(this.beanClassLoader);
        AnnotationAttributes attributes = getAttributes(annotationMetadata);
        List<String> configurations = getCandidateConfigurations(annotationMetadata,
                attributes);
        configurations = removeDuplicates(configurations);
        Set<String> exclusions = getExclusions(annotationMetadata, attributes);
        checkExcludedClasses(configurations, exclusions);
        configurations.removeAll(exclusions);
        configurations = filter(configurations, autoConfigurationMetadata);
        fireAutoConfigurationImportEvents(configurations, exclusions);
        return StringUtils.toStringArray(configurations);
    }

　　紧接着会调用getCandidateConfigurations方法。这个方法里有个 SpringFactoryiesLoader, 它会读取META-INF/spring.factories下的EnableAutoConfiguration的配置

    /**
     * Return the auto-configuration class names that should be considered. By default
     * this method will load candidates using {@link SpringFactoriesLoader} with
     * {@link #getSpringFactoriesLoaderFactoryClass()}.
     * @param metadata the source metadata
     * @param attributes the {@link #getAttributes(AnnotationMetadata) annotation
     * attributes}
     * @return a list of candidate configurations
     */
    protected List<String> getCandidateConfigurations(AnnotationMetadata metadata,
            AnnotationAttributes attributes) {
        List<String> configurations = SpringFactoriesLoader.loadFactoryNames(
                getSpringFactoriesLoaderFactoryClass(), getBeanClassLoader());
        Assert.notEmpty(configurations,
                "No auto configuration classes found in META-INF/spring.factories. If you "
                        + "are using a custom packaging, make sure that file is correct.");
        return configurations;
    }

    /**
     * Return the class used by {@link SpringFactoriesLoader} to load configuration
     * candidates.
     * @return the factory class
     */
    protected Class<?> getSpringFactoriesLoaderFactoryClass() {
        return EnableAutoConfiguration.class;
    }

　　紧接着在进行排除与过滤，进而得到需要装配的类。最后让所有配置在META-INF/spring.factories下的AutoConfigurationImportListener执行AutoConfigurationImportEvent事件
所有配置的@EnableAutoConfiguration 收集好后注册到IOC容器里

    private void fireAutoConfigurationImportEvents(List<String> configurations,
            Set<String> exclusions) {
        List<AutoConfigurationImportListener> listeners = getAutoConfigurationImportListeners();
        if (!listeners.isEmpty()) {
            AutoConfigurationImportEvent event = new AutoConfigurationImportEvent(this,
                    configurations, exclusions);
            for (AutoConfigurationImportListener listener : listeners) {
                invokeAwareMethods(listener);
                listener.onAutoConfigurationImportEvent(event);
            }
        }
    }

    protected List<AutoConfigurationImportListener> getAutoConfigurationImportListeners() {
        return SpringFactoriesLoader.loadFactories(AutoConfigurationImportListener.class,
                this.beanClassLoader);
    }

何时自动装配？

　上面只是确定最终要将哪些类进行自动装配，那么SpringBoot何时处理这些自动装配的类呢？下面我们简要的分析一下：

AbstractApplicationContext的refresh方法：

　　当AbstractApplicationContext执行refresh方法里的invokeBeanFactoryPostProcessors(beanFactory)方法时会执行自动装配

AbstractApplicationContext类的refresh()方法

    @Override
    public void refresh() throws BeansException, IllegalStateException {
        synchronized (this.startupShutdownMonitor) {
            // Prepare this context for refreshing.
            prepareRefresh();

            // Tell the subclass to refresh the internal bean factory.
            ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();

            // Prepare the bean factory for use in this context.
            prepareBeanFactory(beanFactory);

            try {
                // Allows post-processing of the bean factory in context subclasses.
                postProcessBeanFactory(beanFactory);

                // Invoke factory processors registered as beans in the context.
                invokeBeanFactoryPostProcessors(beanFactory);

                // Register bean processors that intercept bean creation.
                registerBeanPostProcessors(beanFactory);

                // Initialize message source for this context.
                initMessageSource();

                // Initialize event multicaster for this context.
                initApplicationEventMulticaster();

                // Initialize other special beans in specific context subclasses.
                onRefresh();

                // Check for listener beans and register them.
                registerListeners();

                // Instantiate all remaining (non-lazy-init) singletons.
                finishBeanFactoryInitialization(beanFactory);

                // Last step: publish corresponding event.
                finishRefresh();
            }

            catch (BeansException ex) {
                if (logger.isWarnEnabled()) {
                    logger.warn("Exception encountered during context initialization - " +
                            "cancelling refresh attempt: " + ex);
                }

                // Destroy already created singletons to avoid dangling resources.
                destroyBeans();

                // Reset 'active' flag.
                cancelRefresh(ex);

                // Propagate exception to caller.
                throw ex;
            }

            finally {
                // Reset common introspection caches in Spring's core, since we
                // might not ever need metadata for singleton beans anymore...
                resetCommonCaches();
            }
        }
    }

　　在这里是处理BeanFactoryPostProcessor的，那么我们在来看一下这个接口BeanDefinitionRegistryPostProcessor

BeanDefinitionRegistryPostProcessor接口

/*
 * Copyright 2002-2010 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.springframework.beans.factory.support;

import org.springframework.beans.BeansException;
import org.springframework.beans.factory.config.BeanFactoryPostProcessor;

/**
 * Extension to the standard {@link BeanFactoryPostProcessor} SPI, allowing for
 * the registration of further bean definitions <i>before</i> regular
 * BeanFactoryPostProcessor detection kicks in. In particular,
 * BeanDefinitionRegistryPostProcessor may register further bean definitions
 * which in turn define BeanFactoryPostProcessor instances.
 *
 * @author Juergen Hoeller
 * @since 3.0.1
 * @see org.springframework.context.annotation.ConfigurationClassPostProcessor
 */
public interface BeanDefinitionRegistryPostProcessor extends BeanFactoryPostProcessor {

    /**
     * Modify the application context's internal bean definition registry after its
     * standard initialization. All regular bean definitions will have been loaded,
     * but no beans will have been instantiated yet. This allows for adding further
     * bean definitions before the next post-processing phase kicks in.
     * @param registry the bean definition registry used by the application context
     * @throws org.springframework.beans.BeansException in case of errors
     */
    void postProcessBeanDefinitionRegistry(BeanDefinitionRegistry registry) throws BeansException;

}

　　该类又实现 BeanFactoryPostProcessor

BeanFactoryPostProcessor接口

/*
 * Copyright 2002-2016 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.springframework.beans.factory.config;

import org.springframework.beans.BeansException;

/**
 * Allows for custom modification of an application context's bean definitions,
 * adapting the bean property values of the context's underlying bean factory.
 *
 * <p>Application contexts can auto-detect BeanFactoryPostProcessor beans in
 * their bean definitions and apply them before any other beans get created.
 *
 * <p>Useful for custom config files targeted at system administrators that
 * override bean properties configured in the application context.
 *
 * <p>See PropertyResourceConfigurer and its concrete implementations
 * for out-of-the-box solutions that address such configuration needs.
 *
 * <p>A BeanFactoryPostProcessor may interact with and modify bean
 * definitions, but never bean instances. Doing so may cause premature bean
 * instantiation, violating the container and causing unintended side-effects.
 * If bean instance interaction is required, consider implementing
 * {@link BeanPostProcessor} instead.
 *
 * @author Juergen Hoeller
 * @since 06.07.2003
 * @see BeanPostProcessor
 * @see PropertyResourceConfigurer
 */
@FunctionalInterface
public interface BeanFactoryPostProcessor {

    /**
     * Modify the application context's internal bean factory after its standard
     * initialization. All bean definitions will have been loaded, but no beans
     * will have been instantiated yet. This allows for overriding or adding
     * properties even to eager-initializing beans.
     * @param beanFactory the bean factory used by the application context
     * @throws org.springframework.beans.BeansException in case of errors
     */
    void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) throws BeansException;

}

ConfigurationClassPostProcessor 类

　　处理@Configuration的核心还是ConfigurationClassPostProcessor。该类主要处理@Configuration注解的，它实现了BeanDefinitionRegistryPostProcessor,  那么也间接实现了BeanFactoryPostProcessor，代码如下：

ConfigurationClassPostProcessor类

/*
 * Copyright 2002-2018 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.springframework.context.annotation;

import java.beans.PropertyDescriptor;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.HashSet;
import java.util.LinkedHashMap;
import java.util.LinkedHashSet;
import java.util.List;
import java.util.Map;
import java.util.Set;

import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;

import org.springframework.aop.framework.autoproxy.AutoProxyUtils;
import org.springframework.beans.PropertyValues;
import org.springframework.beans.factory.BeanClassLoaderAware;
import org.springframework.beans.factory.BeanDefinitionStoreException;
import org.springframework.beans.factory.BeanFactory;
import org.springframework.beans.factory.config.BeanDefinition;
import org.springframework.beans.factory.config.BeanDefinitionHolder;
import org.springframework.beans.factory.config.BeanFactoryPostProcessor;
import org.springframework.beans.factory.config.ConfigurableListableBeanFactory;
import org.springframework.beans.factory.config.InstantiationAwareBeanPostProcessorAdapter;
import org.springframework.beans.factory.config.SingletonBeanRegistry;
import org.springframework.beans.factory.parsing.FailFastProblemReporter;
import org.springframework.beans.factory.parsing.PassThroughSourceExtractor;
import org.springframework.beans.factory.parsing.ProblemReporter;
import org.springframework.beans.factory.parsing.SourceExtractor;
import org.springframework.beans.factory.support.AbstractBeanDefinition;
import org.springframework.beans.factory.support.BeanDefinitionRegistry;
import org.springframework.beans.factory.support.BeanDefinitionRegistryPostProcessor;
import org.springframework.beans.factory.support.BeanNameGenerator;
import org.springframework.context.EnvironmentAware;
import org.springframework.context.ResourceLoaderAware;
import org.springframework.context.annotation.ConfigurationClassEnhancer.EnhancedConfiguration;
import org.springframework.core.Ordered;
import org.springframework.core.PriorityOrdered;
import org.springframework.core.env.Environment;
import org.springframework.core.env.StandardEnvironment;
import org.springframework.core.io.DefaultResourceLoader;
import org.springframework.core.io.ResourceLoader;
import org.springframework.core.type.AnnotationMetadata;
import org.springframework.core.type.classreading.CachingMetadataReaderFactory;
import org.springframework.core.type.classreading.MetadataReaderFactory;
import org.springframework.lang.Nullable;
import org.springframework.util.Assert;
import org.springframework.util.ClassUtils;

import static org.springframework.context.annotation.AnnotationConfigUtils.*;

/**
 * {@link BeanFactoryPostProcessor} used for bootstrapping processing of
 * {@link Configuration @Configuration} classes.
 *
 * <p>Registered by default when using {@code <context:annotation-config/>} or
 * {@code <context:component-scan/>}. Otherwise, may be declared manually as
 * with any other BeanFactoryPostProcessor.
 *
 * <p>This post processor is {@link Ordered#HIGHEST_PRECEDENCE} as it is important
 * that any {@link Bean} methods declared in Configuration classes have their
 * respective bean definitions registered before any other BeanFactoryPostProcessor
 * executes.
 *
 * @author Chris Beams
 * @author Juergen Hoeller
 * @author Phillip Webb
 * @since 3.0
 */
public class ConfigurationClassPostProcessor implements BeanDefinitionRegistryPostProcessor,
        PriorityOrdered, ResourceLoaderAware, BeanClassLoaderAware, EnvironmentAware {

    private static final String IMPORT_REGISTRY_BEAN_NAME =
            ConfigurationClassPostProcessor.class.getName() + ".importRegistry";


    private final Log logger = LogFactory.getLog(getClass());

    private SourceExtractor sourceExtractor = new PassThroughSourceExtractor();

    private ProblemReporter problemReporter = new FailFastProblemReporter();

    @Nullable
    private Environment environment;

    private ResourceLoader resourceLoader = new DefaultResourceLoader();

    @Nullable
    private ClassLoader beanClassLoader = ClassUtils.getDefaultClassLoader();

    private MetadataReaderFactory metadataReaderFactory = new CachingMetadataReaderFactory();

    private boolean setMetadataReaderFactoryCalled = false;

    private final Set<Integer> registriesPostProcessed = new HashSet<>();

    private final Set<Integer> factoriesPostProcessed = new HashSet<>();

    @Nullable
    private ConfigurationClassBeanDefinitionReader reader;

    private boolean localBeanNameGeneratorSet = false;

    /* Using short class names as default bean names */
    private BeanNameGenerator componentScanBeanNameGenerator = new AnnotationBeanNameGenerator();

    /* Using fully qualified class names as default bean names */
    private BeanNameGenerator importBeanNameGenerator = new AnnotationBeanNameGenerator() {
        @Override
        protected String buildDefaultBeanName(BeanDefinition definition) {
            String beanClassName = definition.getBeanClassName();
            Assert.state(beanClassName != null, "No bean class name set");
            return beanClassName;
        }
    };


    @Override
    public int getOrder() {
        return Ordered.LOWEST_PRECEDENCE;  // within PriorityOrdered
    }

    /**
     * Set the {@link SourceExtractor} to use for generated bean definitions
     * that correspond to {@link Bean} factory methods.
     */
    public void setSourceExtractor(@Nullable SourceExtractor sourceExtractor) {
        this.sourceExtractor = (sourceExtractor != null ? sourceExtractor : new PassThroughSourceExtractor());
    }

    /**
     * Set the {@link ProblemReporter} to use.
     * <p>Used to register any problems detected with {@link Configuration} or {@link Bean}
     * declarations. For instance, an @Bean method marked as {@code final} is illegal
     * and would be reported as a problem. Defaults to {@link FailFastProblemReporter}.
     */
    public void setProblemReporter(@Nullable ProblemReporter problemReporter) {
        this.problemReporter = (problemReporter != null ? problemReporter : new FailFastProblemReporter());
    }

    /**
     * Set the {@link MetadataReaderFactory} to use.
     * <p>Default is a {@link CachingMetadataReaderFactory} for the specified
     * {@linkplain #setBeanClassLoader bean class loader}.
     */
    public void setMetadataReaderFactory(MetadataReaderFactory metadataReaderFactory) {
        Assert.notNull(metadataReaderFactory, "MetadataReaderFactory must not be null");
        this.metadataReaderFactory = metadataReaderFactory;
        this.setMetadataReaderFactoryCalled = true;
    }

    /**
     * Set the {@link BeanNameGenerator} to be used when triggering component scanning
     * from {@link Configuration} classes and when registering {@link Import}'ed
     * configuration classes. The default is a standard {@link AnnotationBeanNameGenerator}
     * for scanned components (compatible with the default in {@link ClassPathBeanDefinitionScanner})
     * and a variant thereof for imported configuration classes (using unique fully-qualified
     * class names instead of standard component overriding).
     * <p>Note that this strategy does <em>not</em> apply to {@link Bean} methods.
     * <p>This setter is typically only appropriate when configuring the post-processor as
     * a standalone bean definition in XML, e.g. not using the dedicated
     * {@code AnnotationConfig*} application contexts or the {@code
     * <context:annotation-config>} element. Any bean name generator specified against
     * the application context will take precedence over any value set here.
     * @since 3.1.1
     * @see AnnotationConfigApplicationContext#setBeanNameGenerator(BeanNameGenerator)
     * @see AnnotationConfigUtils#CONFIGURATION_BEAN_NAME_GENERATOR
     */
    public void setBeanNameGenerator(BeanNameGenerator beanNameGenerator) {
        Assert.notNull(beanNameGenerator, "BeanNameGenerator must not be null");
        this.localBeanNameGeneratorSet = true;
        this.componentScanBeanNameGenerator = beanNameGenerator;
        this.importBeanNameGenerator = beanNameGenerator;
    }

    @Override
    public void setEnvironment(Environment environment) {
        Assert.notNull(environment, "Environment must not be null");
        this.environment = environment;
    }

    @Override
    public void setResourceLoader(ResourceLoader resourceLoader) {
        Assert.notNull(resourceLoader, "ResourceLoader must not be null");
        this.resourceLoader = resourceLoader;
        if (!this.setMetadataReaderFactoryCalled) {
            this.metadataReaderFactory = new CachingMetadataReaderFactory(resourceLoader);
        }
    }

    @Override
    public void setBeanClassLoader(ClassLoader beanClassLoader) {
        this.beanClassLoader = beanClassLoader;
        if (!this.setMetadataReaderFactoryCalled) {
            this.metadataReaderFactory = new CachingMetadataReaderFactory(beanClassLoader);
        }
    }


    /**
     * Derive further bean definitions from the configuration classes in the registry.
     */
    @Override
    public void postProcessBeanDefinitionRegistry(BeanDefinitionRegistry registry) {
        int registryId = System.identityHashCode(registry);
        if (this.registriesPostProcessed.contains(registryId)) {
            throw new IllegalStateException(
                    "postProcessBeanDefinitionRegistry already called on this post-processor against " + registry);
        }
        if (this.factoriesPostProcessed.contains(registryId)) {
            throw new IllegalStateException(
                    "postProcessBeanFactory already called on this post-processor against " + registry);
        }
        this.registriesPostProcessed.add(registryId);

        processConfigBeanDefinitions(registry);
    }

    /**
     * Prepare the Configuration classes for servicing bean requests at runtime
     * by replacing them with CGLIB-enhanced subclasses.
     */
    @Override
    public void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) {
        int factoryId = System.identityHashCode(beanFactory);
        if (this.factoriesPostProcessed.contains(factoryId)) {
            throw new IllegalStateException(
                    "postProcessBeanFactory already called on this post-processor against " + beanFactory);
        }
        this.factoriesPostProcessed.add(factoryId);
        if (!this.registriesPostProcessed.contains(factoryId)) {
            // BeanDefinitionRegistryPostProcessor hook apparently not supported...
            // Simply call processConfigurationClasses lazily at this point then.
            processConfigBeanDefinitions((BeanDefinitionRegistry) beanFactory);
        }

        enhanceConfigurationClasses(beanFactory);
        beanFactory.addBeanPostProcessor(new ImportAwareBeanPostProcessor(beanFactory));
    }

    /**
     * Build and validate a configuration model based on the registry of
     * {@link Configuration} classes.
     */
    public void processConfigBeanDefinitions(BeanDefinitionRegistry registry) {
        List<BeanDefinitionHolder> configCandidates = new ArrayList<>();
        String[] candidateNames = registry.getBeanDefinitionNames();

        for (String beanName : candidateNames) {
            BeanDefinition beanDef = registry.getBeanDefinition(beanName);
            if (ConfigurationClassUtils.isFullConfigurationClass(beanDef) ||
                    ConfigurationClassUtils.isLiteConfigurationClass(beanDef)) {
                if (logger.isDebugEnabled()) {
                    logger.debug("Bean definition has already been processed as a configuration class: " + beanDef);
                }
            }
            else if (ConfigurationClassUtils.checkConfigurationClassCandidate(beanDef, this.metadataReaderFactory)) {
                configCandidates.add(new BeanDefinitionHolder(beanDef, beanName));
            }
        }

        // Return immediately if no @Configuration classes were found
        if (configCandidates.isEmpty()) {
            return;
        }

        // Sort by previously determined @Order value, if applicable
        configCandidates.sort((bd1, bd2) -> {
            int i1 = ConfigurationClassUtils.getOrder(bd1.getBeanDefinition());
            int i2 = ConfigurationClassUtils.getOrder(bd2.getBeanDefinition());
            return Integer.compare(i1, i2);
        });

        // Detect any custom bean name generation strategy supplied through the enclosing application context
        SingletonBeanRegistry sbr = null;
        if (registry instanceof SingletonBeanRegistry) {
            sbr = (SingletonBeanRegistry) registry;
            if (!this.localBeanNameGeneratorSet) {
                BeanNameGenerator generator = (BeanNameGenerator) sbr.getSingleton(CONFIGURATION_BEAN_NAME_GENERATOR);
                if (generator != null) {
                    this.componentScanBeanNameGenerator = generator;
                    this.importBeanNameGenerator = generator;
                }
            }
        }

        if (this.environment == null) {
            this.environment = new StandardEnvironment();
        }

        // Parse each @Configuration class
        ConfigurationClassParser parser = new ConfigurationClassParser(
                this.metadataReaderFactory, this.problemReporter, this.environment,
                this.resourceLoader, this.componentScanBeanNameGenerator, registry);

        Set<BeanDefinitionHolder> candidates = new LinkedHashSet<>(configCandidates);
        Set<ConfigurationClass> alreadyParsed = new HashSet<>(configCandidates.size());
        do {
            parser.parse(candidates);
            parser.validate();

            Set<ConfigurationClass> configClasses = new LinkedHashSet<>(parser.getConfigurationClasses());
            configClasses.removeAll(alreadyParsed);

            // Read the model and create bean definitions based on its content
            if (this.reader == null) {
                this.reader = new ConfigurationClassBeanDefinitionReader(
                        registry, this.sourceExtractor, this.resourceLoader, this.environment,
                        this.importBeanNameGenerator, parser.getImportRegistry());
            }
            this.reader.loadBeanDefinitions(configClasses);
            alreadyParsed.addAll(configClasses);

            candidates.clear();
            if (registry.getBeanDefinitionCount() > candidateNames.length) {
                String[] newCandidateNames = registry.getBeanDefinitionNames();
                Set<String> oldCandidateNames = new HashSet<>(Arrays.asList(candidateNames));
                Set<String> alreadyParsedClasses = new HashSet<>();
                for (ConfigurationClass configurationClass : alreadyParsed) {
                    alreadyParsedClasses.add(configurationClass.getMetadata().getClassName());
                }
                for (String candidateName : newCandidateNames) {
                    if (!oldCandidateNames.contains(candidateName)) {
                        BeanDefinition bd = registry.getBeanDefinition(candidateName);
                        if (ConfigurationClassUtils.checkConfigurationClassCandidate(bd, this.metadataReaderFactory) &&
                                !alreadyParsedClasses.contains(bd.getBeanClassName())) {
                            candidates.add(new BeanDefinitionHolder(bd, candidateName));
                        }
                    }
                }
                candidateNames = newCandidateNames;
            }
        }
        while (!candidates.isEmpty());

        // Register the ImportRegistry as a bean in order to support ImportAware @Configuration classes
        if (sbr != null && !sbr.containsSingleton(IMPORT_REGISTRY_BEAN_NAME)) {
            sbr.registerSingleton(IMPORT_REGISTRY_BEAN_NAME, parser.getImportRegistry());
        }

        if (this.metadataReaderFactory instanceof CachingMetadataReaderFactory) {
            // Clear cache in externally provided MetadataReaderFactory; this is a no-op
            // for a shared cache since it'll be cleared by the ApplicationContext.
            ((CachingMetadataReaderFactory) this.metadataReaderFactory).clearCache();
        }
    }

    /**
     * Post-processes a BeanFactory in search of Configuration class BeanDefinitions;
     * any candidates are then enhanced by a {@link ConfigurationClassEnhancer}.
     * Candidate status is determined by BeanDefinition attribute metadata.
     * @see ConfigurationClassEnhancer
     */
    public void enhanceConfigurationClasses(ConfigurableListableBeanFactory beanFactory) {
        Map<String, AbstractBeanDefinition> configBeanDefs = new LinkedHashMap<>();
        for (String beanName : beanFactory.getBeanDefinitionNames()) {
            BeanDefinition beanDef = beanFactory.getBeanDefinition(beanName);
            if (ConfigurationClassUtils.isFullConfigurationClass(beanDef)) {
                if (!(beanDef instanceof AbstractBeanDefinition)) {
                    throw new BeanDefinitionStoreException("Cannot enhance @Configuration bean definition '" +
                            beanName + "' since it is not stored in an AbstractBeanDefinition subclass");
                }
                else if (logger.isWarnEnabled() && beanFactory.containsSingleton(beanName)) {
                    logger.warn("Cannot enhance @Configuration bean definition '" + beanName +
                            "' since its singleton instance has been created too early. The typical cause " +
                            "is a non-static @Bean method with a BeanDefinitionRegistryPostProcessor " +
                            "return type: Consider declaring such methods as 'static'.");
                }
                configBeanDefs.put(beanName, (AbstractBeanDefinition) beanDef);
            }
        }
        if (configBeanDefs.isEmpty()) {
            // nothing to enhance -> return immediately
            return;
        }

        ConfigurationClassEnhancer enhancer = new ConfigurationClassEnhancer();
        for (Map.Entry<String, AbstractBeanDefinition> entry : configBeanDefs.entrySet()) {
            AbstractBeanDefinition beanDef = entry.getValue();
            // If a @Configuration class gets proxied, always proxy the target class
            beanDef.setAttribute(AutoProxyUtils.PRESERVE_TARGET_CLASS_ATTRIBUTE, Boolean.TRUE);
            try {
                // Set enhanced subclass of the user-specified bean class
                Class<?> configClass = beanDef.resolveBeanClass(this.beanClassLoader);
                if (configClass != null) {
                    Class<?> enhancedClass = enhancer.enhance(configClass, this.beanClassLoader);
                    if (configClass != enhancedClass) {
                        if (logger.isDebugEnabled()) {
                            logger.debug(String.format("Replacing bean definition '%s' existing class '%s' with " +
                                    "enhanced class '%s'", entry.getKey(), configClass.getName(), enhancedClass.getName()));
                        }
                        beanDef.setBeanClass(enhancedClass);
                    }
                }
            }
            catch (Throwable ex) {
                throw new IllegalStateException("Cannot load configuration class: " + beanDef.getBeanClassName(), ex);
            }
        }
    }


    private static class ImportAwareBeanPostProcessor extends InstantiationAwareBeanPostProcessorAdapter {

        private final BeanFactory beanFactory;

        public ImportAwareBeanPostProcessor(BeanFactory beanFactory) {
            this.beanFactory = beanFactory;
        }

        @Override
        public PropertyValues postProcessPropertyValues(
                PropertyValues pvs, PropertyDescriptor[] pds, Object bean, String beanName) {

            // Inject the BeanFactory before AutowiredAnnotationBeanPostProcessor's
            // postProcessPropertyValues method attempts to autowire other configuration beans.
            if (bean instanceof EnhancedConfiguration) {
                ((EnhancedConfiguration) bean).setBeanFactory(this.beanFactory);
            }
            return pvs;
        }

        @Override
        public Object postProcessBeforeInitialization(Object bean, String beanName)  {
            if (bean instanceof ImportAware) {
                ImportRegistry ir = this.beanFactory.getBean(IMPORT_REGISTRY_BEAN_NAME, ImportRegistry.class);
                AnnotationMetadata importingClass = ir.getImportingClassFor(bean.getClass().getSuperclass().getName());
                if (importingClass != null) {
                    ((ImportAware) bean).setImportMetadata(importingClass);
                }
            }
            return bean;
        }
    }

}

　　其关键代码为：postProcessBeanFactory和processConfigBeanDefinitions方法

　　这里可以看到解析每一个@ConfigurationClass的关键类是：ConfigurationClassParser 会调用该类的parse方法

ConfigurationClassParser类

/*
 * Copyright 2002-2018 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.springframework.context.annotation;

import java.io.FileNotFoundException;
import java.io.IOException;
import java.lang.annotation.Annotation;
import java.net.UnknownHostException;
import java.util.ArrayDeque;
import java.util.ArrayList;
import java.util.Collection;
import java.util.Collections;
import java.util.Comparator;
import java.util.Deque;
import java.util.HashMap;
import java.util.Iterator;
import java.util.LinkedHashMap;
import java.util.LinkedHashSet;
import java.util.LinkedList;
import java.util.List;
import java.util.Map;
import java.util.Set;

import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;

import org.springframework.beans.BeanUtils;
import org.springframework.beans.factory.BeanDefinitionStoreException;
import org.springframework.beans.factory.annotation.AnnotatedBeanDefinition;
import org.springframework.beans.factory.config.BeanDefinition;
import org.springframework.beans.factory.config.BeanDefinitionHolder;
import org.springframework.beans.factory.parsing.Location;
import org.springframework.beans.factory.parsing.Problem;
import org.springframework.beans.factory.parsing.ProblemReporter;
import org.springframework.beans.factory.support.AbstractBeanDefinition;
import org.springframework.beans.factory.support.BeanDefinitionReader;
import org.springframework.beans.factory.support.BeanDefinitionRegistry;
import org.springframework.beans.factory.support.BeanNameGenerator;
import org.springframework.context.annotation.ConfigurationCondition.ConfigurationPhase;
import org.springframework.context.annotation.DeferredImportSelector.Group;
import org.springframework.core.NestedIOException;
import org.springframework.core.OrderComparator;
import org.springframework.core.Ordered;
import org.springframework.core.annotation.AnnotationAttributes;
import org.springframework.core.annotation.AnnotationAwareOrderComparator;
import org.springframework.core.annotation.AnnotationUtils;
import org.springframework.core.env.CompositePropertySource;
import org.springframework.core.env.ConfigurableEnvironment;
import org.springframework.core.env.Environment;
import org.springframework.core.env.MutablePropertySources;
import org.springframework.core.env.PropertySource;
import org.springframework.core.io.Resource;
import org.springframework.core.io.ResourceLoader;
import org.springframework.core.io.support.DefaultPropertySourceFactory;
import org.springframework.core.io.support.EncodedResource;
import org.springframework.core.io.support.PropertySourceFactory;
import org.springframework.core.io.support.ResourcePropertySource;
import org.springframework.core.type.AnnotationMetadata;
import org.springframework.core.type.MethodMetadata;
import org.springframework.core.type.StandardAnnotationMetadata;
import org.springframework.core.type.classreading.MetadataReader;
import org.springframework.core.type.classreading.MetadataReaderFactory;
import org.springframework.core.type.filter.AssignableTypeFilter;
import org.springframework.lang.Nullable;
import org.springframework.util.Assert;
import org.springframework.util.ClassUtils;
import org.springframework.util.CollectionUtils;
import org.springframework.util.LinkedMultiValueMap;
import org.springframework.util.MultiValueMap;
import org.springframework.util.StringUtils;

/**
 * Parses a {@link Configuration} class definition, populating a collection of
 * {@link ConfigurationClass} objects (parsing a single Configuration class may result in
 * any number of ConfigurationClass objects because one Configuration class may import
 * another using the {@link Import} annotation).
 *
 * <p>This class helps separate the concern of parsing the structure of a Configuration
 * class from the concern of registering BeanDefinition objects based on the
 * content of that model (with the exception of {@code @ComponentScan} annotations which
 * need to be registered immediately).
 *
 * <p>This ASM-based implementation avoids reflection and eager class loading in order to
 * interoperate effectively with lazy class loading in a Spring ApplicationContext.
 *
 * @author Chris Beams
 * @author Juergen Hoeller
 * @author Phillip Webb
 * @author Sam Brannen
 * @author Stephane Nicoll
 * @since 3.0
 * @see ConfigurationClassBeanDefinitionReader
 */
class ConfigurationClassParser {

    private static final PropertySourceFactory DEFAULT_PROPERTY_SOURCE_FACTORY = new DefaultPropertySourceFactory();

    private static final Comparator<DeferredImportSelectorHolder> DEFERRED_IMPORT_COMPARATOR =
            (o1, o2) -> AnnotationAwareOrderComparator.INSTANCE.compare(o1.getImportSelector(), o2.getImportSelector());


    private final Log logger = LogFactory.getLog(getClass());

    private final MetadataReaderFactory metadataReaderFactory;

    private final ProblemReporter problemReporter;

    private final Environment environment;

    private final ResourceLoader resourceLoader;

    private final BeanDefinitionRegistry registry;

    private final ComponentScanAnnotationParser componentScanParser;

    private final ConditionEvaluator conditionEvaluator;

    private final Map<ConfigurationClass, ConfigurationClass> configurationClasses = new LinkedHashMap<>();

    private final Map<String, ConfigurationClass> knownSuperclasses = new HashMap<>();

    private final List<String> propertySourceNames = new ArrayList<>();

    private final ImportStack importStack = new ImportStack();

    @Nullable
    private List<DeferredImportSelectorHolder> deferredImportSelectors;


    /**
     * Create a new {@link ConfigurationClassParser} instance that will be used
     * to populate the set of configuration classes.
     */
    public ConfigurationClassParser(MetadataReaderFactory metadataReaderFactory,
            ProblemReporter problemReporter, Environment environment, ResourceLoader resourceLoader,
            BeanNameGenerator componentScanBeanNameGenerator, BeanDefinitionRegistry registry) {

        this.metadataReaderFactory = metadataReaderFactory;
        this.problemReporter = problemReporter;
        this.environment = environment;
        this.resourceLoader = resourceLoader;
        this.registry = registry;
        this.componentScanParser = new ComponentScanAnnotationParser(
                environment, resourceLoader, componentScanBeanNameGenerator, registry);
        this.conditionEvaluator = new ConditionEvaluator(registry, environment, resourceLoader);
    }


    public void parse(Set<BeanDefinitionHolder> configCandidates) {
        this.deferredImportSelectors = new LinkedList<>();

        for (BeanDefinitionHolder holder : configCandidates) {
            BeanDefinition bd = holder.getBeanDefinition();
            try {
                if (bd instanceof AnnotatedBeanDefinition) {
                    parse(((AnnotatedBeanDefinition) bd).getMetadata(), holder.getBeanName());
                }
                else if (bd instanceof AbstractBeanDefinition && ((AbstractBeanDefinition) bd).hasBeanClass()) {
                    parse(((AbstractBeanDefinition) bd).getBeanClass(), holder.getBeanName());
                }
                else {
                    parse(bd.getBeanClassName(), holder.getBeanName());
                }
            }
            catch (BeanDefinitionStoreException ex) {
                throw ex;
            }
            catch (Throwable ex) {
                throw new BeanDefinitionStoreException(
                        "Failed to parse configuration class [" + bd.getBeanClassName() + "]", ex);
            }
        }

        processDeferredImportSelectors();
    }

    protected final void parse(@Nullable String className, String beanName) throws IOException {
        Assert.notNull(className, "No bean class name for configuration class bean definition");
        MetadataReader reader = this.metadataReaderFactory.getMetadataReader(className);
        processConfigurationClass(new ConfigurationClass(reader, beanName));
    }

    protected final void parse(Class<?> clazz, String beanName) throws IOException {
        processConfigurationClass(new ConfigurationClass(clazz, beanName));
    }

    protected final void parse(AnnotationMetadata metadata, String beanName) throws IOException {
        processConfigurationClass(new ConfigurationClass(metadata, beanName));
    }

    /**
     * Validate each {@link ConfigurationClass} object.
     * @see ConfigurationClass#validate
     */
    public void validate() {
        for (ConfigurationClass configClass : this.configurationClasses.keySet()) {
            configClass.validate(this.problemReporter);
        }
    }

    public Set<ConfigurationClass> getConfigurationClasses() {
        return this.configurationClasses.keySet();
    }


    protected void processConfigurationClass(ConfigurationClass configClass) throws IOException {
        if (this.conditionEvaluator.shouldSkip(configClass.getMetadata(), ConfigurationPhase.PARSE_CONFIGURATION)) {
            return;
        }

        ConfigurationClass existingClass = this.configurationClasses.get(configClass);
        if (existingClass != null) {
            if (configClass.isImported()) {
                if (existingClass.isImported()) {
                    existingClass.mergeImportedBy(configClass);
                }
                // Otherwise ignore new imported config class; existing non-imported class overrides it.
                return;
            }
            else {
                // Explicit bean definition found, probably replacing an import.
                // Let's remove the old one and go with the new one.
                this.configurationClasses.remove(configClass);
                this.knownSuperclasses.values().removeIf(configClass::equals);
            }
        }

        // Recursively process the configuration class and its superclass hierarchy.
        SourceClass sourceClass = asSourceClass(configClass);
        do {
            sourceClass = doProcessConfigurationClass(configClass, sourceClass);
        }
        while (sourceClass != null);

        this.configurationClasses.put(configClass, configClass);
    }

    /**
     * Apply processing and build a complete {@link ConfigurationClass} by reading the
     * annotations, members and methods from the source class. This method can be called
     * multiple times as relevant sources are discovered.
     * @param configClass the configuration class being build
     * @param sourceClass a source class
     * @return the superclass, or {@code null} if none found or previously processed
     */
    @Nullable
    protected final SourceClass doProcessConfigurationClass(ConfigurationClass configClass, SourceClass sourceClass)
            throws IOException {

        // Recursively process any member (nested) classes first
        processMemberClasses(configClass, sourceClass);

        // Process any @PropertySource annotations
        for (AnnotationAttributes propertySource : AnnotationConfigUtils.attributesForRepeatable(
                sourceClass.getMetadata(), PropertySources.class,
                org.springframework.context.annotation.PropertySource.class)) {
            if (this.environment instanceof ConfigurableEnvironment) {
                processPropertySource(propertySource);
            }
            else {
                logger.warn("Ignoring @PropertySource annotation on [" + sourceClass.getMetadata().getClassName() +
                        "]. Reason: Environment must implement ConfigurableEnvironment");
            }
        }

        // Process any @ComponentScan annotations
        Set<AnnotationAttributes> componentScans = AnnotationConfigUtils.attributesForRepeatable(
                sourceClass.getMetadata(), ComponentScans.class, ComponentScan.class);
        if (!componentScans.isEmpty() &&
                !this.conditionEvaluator.shouldSkip(sourceClass.getMetadata(), ConfigurationPhase.REGISTER_BEAN)) {
            for (AnnotationAttributes componentScan : componentScans) {
                // The config class is annotated with @ComponentScan -> perform the scan immediately
                Set<BeanDefinitionHolder> scannedBeanDefinitions =
                        this.componentScanParser.parse(componentScan, sourceClass.getMetadata().getClassName());
                // Check the set of scanned definitions for any further config classes and parse recursively if needed
                for (BeanDefinitionHolder holder : scannedBeanDefinitions) {
                    BeanDefinition bdCand = holder.getBeanDefinition().getOriginatingBeanDefinition();
                    if (bdCand == null) {
                        bdCand = holder.getBeanDefinition();
                    }
                    if (ConfigurationClassUtils.checkConfigurationClassCandidate(bdCand, this.metadataReaderFactory)) {
                        parse(bdCand.getBeanClassName(), holder.getBeanName());
                    }
                }
            }
        }

        // Process any @Import annotations
        processImports(configClass, sourceClass, getImports(sourceClass), true);

        // Process any @ImportResource annotations
        AnnotationAttributes importResource =
                AnnotationConfigUtils.attributesFor(sourceClass.getMetadata(), ImportResource.class);
        if (importResource != null) {
            String[] resources = importResource.getStringArray("locations");
            Class<? extends BeanDefinitionReader> readerClass = importResource.getClass("reader");
            for (String resource : resources) {
                String resolvedResource = this.environment.resolveRequiredPlaceholders(resource);
                configClass.addImportedResource(resolvedResource, readerClass);
            }
        }

        // Process individual @Bean methods
        Set<MethodMetadata> beanMethods = retrieveBeanMethodMetadata(sourceClass);
        for (MethodMetadata methodMetadata : beanMethods) {
            configClass.addBeanMethod(new BeanMethod(methodMetadata, configClass));
        }

        // Process default methods on interfaces
        processInterfaces(configClass, sourceClass);

        // Process superclass, if any
        if (sourceClass.getMetadata().hasSuperClass()) {
            String superclass = sourceClass.getMetadata().getSuperClassName();
            if (superclass != null && !superclass.startsWith("java") &&
                    !this.knownSuperclasses.containsKey(superclass)) {
                this.knownSuperclasses.put(superclass, configClass);
                // Superclass found, return its annotation metadata and recurse
                return sourceClass.getSuperClass();
            }
        }

        // No superclass -> processing is complete
        return null;
    }

    /**
     * Register member (nested) classes that happen to be configuration classes themselves.
     */
    private void processMemberClasses(ConfigurationClass configClass, SourceClass sourceClass) throws IOException {
        Collection<SourceClass> memberClasses = sourceClass.getMemberClasses();
        if (!memberClasses.isEmpty()) {
            List<SourceClass> candidates = new ArrayList<>(memberClasses.size());
            for (SourceClass memberClass : memberClasses) {
                if (ConfigurationClassUtils.isConfigurationCandidate(memberClass.getMetadata()) &&
                        !memberClass.getMetadata().getClassName().equals(configClass.getMetadata().getClassName())) {
                    candidates.add(memberClass);
                }
            }
            OrderComparator.sort(candidates);
            for (SourceClass candidate : candidates) {
                if (this.importStack.contains(configClass)) {
                    this.problemReporter.error(new CircularImportProblem(configClass, this.importStack));
                }
                else {
                    this.importStack.push(configClass);
                    try {
                        processConfigurationClass(candidate.asConfigClass(configClass));
                    }
                    finally {
                        this.importStack.pop();
                    }
                }
            }
        }
    }

    /**
     * Register default methods on interfaces implemented by the configuration class.
     */
    private void processInterfaces(ConfigurationClass configClass, SourceClass sourceClass) throws IOException {
        for (SourceClass ifc : sourceClass.getInterfaces()) {
            Set<MethodMetadata> beanMethods = retrieveBeanMethodMetadata(ifc);
            for (MethodMetadata methodMetadata : beanMethods) {
                if (!methodMetadata.isAbstract()) {
                    // A default method or other concrete method on a Java 8+ interface...
                    configClass.addBeanMethod(new BeanMethod(methodMetadata, configClass));
                }
            }
            processInterfaces(configClass, ifc);
        }
    }

    /**
     * Retrieve the metadata for all <code>@Bean</code> methods.
     */
    private Set<MethodMetadata> retrieveBeanMethodMetadata(SourceClass sourceClass) {
        AnnotationMetadata original = sourceClass.getMetadata();
        Set<MethodMetadata> beanMethods = original.getAnnotatedMethods(Bean.class.getName());
        if (beanMethods.size() > 1 && original instanceof StandardAnnotationMetadata) {
            // Try reading the class file via ASM for deterministic declaration order...
            // Unfortunately, the JVM's standard reflection returns methods in arbitrary
            // order, even between different runs of the same application on the same JVM.
            try {
                AnnotationMetadata asm =
                        this.metadataReaderFactory.getMetadataReader(original.getClassName()).getAnnotationMetadata();
                Set<MethodMetadata> asmMethods = asm.getAnnotatedMethods(Bean.class.getName());
                if (asmMethods.size() >= beanMethods.size()) {
                    Set<MethodMetadata> selectedMethods = new LinkedHashSet<>(asmMethods.size());
                    for (MethodMetadata asmMethod : asmMethods) {
                        for (MethodMetadata beanMethod : beanMethods) {
                            if (beanMethod.getMethodName().equals(asmMethod.getMethodName())) {
                                selectedMethods.add(beanMethod);
                                break;
                            }
                        }
                    }
                    if (selectedMethods.size() == beanMethods.size()) {
                        // All reflection-detected methods found in ASM method set -> proceed
                        beanMethods = selectedMethods;
                    }
                }
            }
            catch (IOException ex) {
                logger.debug("Failed to read class file via ASM for determining @Bean method order", ex);
                // No worries, let's continue with the reflection metadata we started with...
            }
        }
        return beanMethods;
    }


    /**
     * Process the given <code>@PropertySource</code> annotation metadata.
     * @param propertySource metadata for the <code>@PropertySource</code> annotation found
     * @throws IOException if loading a property source failed
     */
    private void processPropertySource(AnnotationAttributes propertySource) throws IOException {
        String name = propertySource.getString("name");
        if (!StringUtils.hasLength(name)) {
            name = null;
        }
        String encoding = propertySource.getString("encoding");
        if (!StringUtils.hasLength(encoding)) {
            encoding = null;
        }
        String[] locations = propertySource.getStringArray("value");
        Assert.isTrue(locations.length > 0, "At least one @PropertySource(value) location is required");
        boolean ignoreResourceNotFound = propertySource.getBoolean("ignoreResourceNotFound");

        Class<? extends PropertySourceFactory> factoryClass = propertySource.getClass("factory");
        PropertySourceFactory factory = (factoryClass == PropertySourceFactory.class ?
                DEFAULT_PROPERTY_SOURCE_FACTORY : BeanUtils.instantiateClass(factoryClass));

        for (String location : locations) {
            try {
                String resolvedLocation = this.environment.resolveRequiredPlaceholders(location);
                Resource resource = this.resourceLoader.getResource(resolvedLocation);
                addPropertySource(factory.createPropertySource(name, new EncodedResource(resource, encoding)));
            }
            catch (IllegalArgumentException | FileNotFoundException | UnknownHostException ex) {
                // Placeholders not resolvable or resource not found when trying to open it
                if (ignoreResourceNotFound) {
                    if (logger.isInfoEnabled()) {
                        logger.info("Properties location [" + location + "] not resolvable: " + ex.getMessage());
                    }
                }
                else {
                    throw ex;
                }
            }
        }
    }

    private void addPropertySource(PropertySource<?> propertySource) {
        String name = propertySource.getName();
        MutablePropertySources propertySources = ((ConfigurableEnvironment) this.environment).getPropertySources();

        if (this.propertySourceNames.contains(name)) {
            // We've already added a version, we need to extend it
            PropertySource<?> existing = propertySources.get(name);
            if (existing != null) {
                PropertySource<?> newSource = (propertySource instanceof ResourcePropertySource ?
                        ((ResourcePropertySource) propertySource).withResourceName() : propertySource);
                if (existing instanceof CompositePropertySource) {
                    ((CompositePropertySource) existing).addFirstPropertySource(newSource);
                }
                else {
                    if (existing instanceof ResourcePropertySource) {
                        existing = ((ResourcePropertySource) existing).withResourceName();
                    }
                    CompositePropertySource composite = new CompositePropertySource(name);
                    composite.addPropertySource(newSource);
                    composite.addPropertySource(existing);
                    propertySources.replace(name, composite);
                }
                return;
            }
        }

        if (this.propertySourceNames.isEmpty()) {
            propertySources.addLast(propertySource);
        }
        else {
            String firstProcessed = this.propertySourceNames.get(this.propertySourceNames.size() - 1);
            propertySources.addBefore(firstProcessed, propertySource);
        }
        this.propertySourceNames.add(name);
    }


    /**
     * Returns {@code @Import} class, considering all meta-annotations.
     */
    private Set<SourceClass> getImports(SourceClass sourceClass) throws IOException {
        Set<SourceClass> imports = new LinkedHashSet<>();
        Set<SourceClass> visited = new LinkedHashSet<>();
        collectImports(sourceClass, imports, visited);
        return imports;
    }

    /**
     * Recursively collect all declared {@code @Import} values. Unlike most
     * meta-annotations it is valid to have several {@code @Import}s declared with
     * different values; the usual process of returning values from the first
     * meta-annotation on a class is not sufficient.
     * <p>For example, it is common for a {@code @Configuration} class to declare direct
     * {@code @Import}s in addition to meta-imports originating from an {@code @Enable}
     * annotation.
     * @param sourceClass the class to search
     * @param imports the imports collected so far
     * @param visited used to track visited classes to prevent infinite recursion
     * @throws IOException if there is any problem reading metadata from the named class
     */
    private void collectImports(SourceClass sourceClass, Set<SourceClass> imports, Set<SourceClass> visited)
            throws IOException {

        if (visited.add(sourceClass)) {
            for (SourceClass annotation : sourceClass.getAnnotations()) {
                String annName = annotation.getMetadata().getClassName();
                if (!annName.startsWith("java") && !annName.equals(Import.class.getName())) {
                    collectImports(annotation, imports, visited);
                }
            }
            imports.addAll(sourceClass.getAnnotationAttributes(Import.class.getName(), "value"));
        }
    }

    private void processDeferredImportSelectors() {
        List<DeferredImportSelectorHolder> deferredImports = this.deferredImportSelectors;
        this.deferredImportSelectors = null;
        if (deferredImports == null) {
            return;
        }

        deferredImports.sort(DEFERRED_IMPORT_COMPARATOR);
        Map<Object, DeferredImportSelectorGrouping> groupings = new LinkedHashMap<>();
        Map<AnnotationMetadata, ConfigurationClass> configurationClasses = new HashMap<>();
        for (DeferredImportSelectorHolder deferredImport : deferredImports) {
            Class<? extends Group> group = deferredImport.getImportSelector().getImportGroup();
            DeferredImportSelectorGrouping grouping = groupings.computeIfAbsent(
                    (group == null ? deferredImport : group),
                    (key) -> new DeferredImportSelectorGrouping(createGroup(group)));
            grouping.add(deferredImport);
            configurationClasses.put(deferredImport.getConfigurationClass().getMetadata(),
                    deferredImport.getConfigurationClass());
        }
        for (DeferredImportSelectorGrouping grouping : groupings.values()) {
            grouping.getImports().forEach((entry) -> {
                ConfigurationClass configurationClass = configurationClasses.get(
                        entry.getMetadata());
                try {
                    processImports(configurationClass, asSourceClass(configurationClass),
                            asSourceClasses(entry.getImportClassName()), false);
                }
                catch (BeanDefinitionStoreException ex) {
                    throw ex;
                }
                catch (Throwable ex) {
                    throw new BeanDefinitionStoreException(
                            "Failed to process import candidates for configuration class [" +
                                    configurationClass.getMetadata().getClassName() + "]", ex);
                }
            });
        }
    }

    private Group createGroup(@Nullable Class<? extends Group> type) {
        Class<? extends Group> effectiveType = (type != null ? type
                : DefaultDeferredImportSelectorGroup.class);
        Group group = BeanUtils.instantiateClass(effectiveType);
        ParserStrategyUtils.invokeAwareMethods(group,
                ConfigurationClassParser.this.environment,
                ConfigurationClassParser.this.resourceLoader,
                ConfigurationClassParser.this.registry);
        return group;
    }

    private void processImports(ConfigurationClass configClass, SourceClass currentSourceClass,
            Collection<SourceClass> importCandidates, boolean checkForCircularImports) {

        if (importCandidates.isEmpty()) {
            return;
        }

        if (checkForCircularImports && isChainedImportOnStack(configClass)) {
            this.problemReporter.error(new CircularImportProblem(configClass, this.importStack));
        }
        else {
            this.importStack.push(configClass);
            try {
                for (SourceClass candidate : importCandidates) {
                    if (candidate.isAssignable(ImportSelector.class)) {
                        // Candidate class is an ImportSelector -> delegate to it to determine imports
                        Class<?> candidateClass = candidate.loadClass();
                        ImportSelector selector = BeanUtils.instantiateClass(candidateClass, ImportSelector.class);
                        ParserStrategyUtils.invokeAwareMethods(
                                selector, this.environment, this.resourceLoader, this.registry);
                        if (this.deferredImportSelectors != null && selector instanceof DeferredImportSelector) {
                            this.deferredImportSelectors.add(
                                    new DeferredImportSelectorHolder(configClass, (DeferredImportSelector) selector));
                        }
                        else {
                            String[] importClassNames = selector.selectImports(currentSourceClass.getMetadata());
                            Collection<SourceClass> importSourceClasses = asSourceClasses(importClassNames);
                            processImports(configClass, currentSourceClass, importSourceClasses, false);
                        }
                    }
                    else if (candidate.isAssignable(ImportBeanDefinitionRegistrar.class)) {
                        // Candidate class is an ImportBeanDefinitionRegistrar ->
                        // delegate to it to register additional bean definitions
                        Class<?> candidateClass = candidate.loadClass();
                        ImportBeanDefinitionRegistrar registrar =
                                BeanUtils.instantiateClass(candidateClass, ImportBeanDefinitionRegistrar.class);
                        ParserStrategyUtils.invokeAwareMethods(
                                registrar, this.environment, this.resourceLoader, this.registry);
                        configClass.addImportBeanDefinitionRegistrar(registrar, currentSourceClass.getMetadata());
                    }
                    else {
                        // Candidate class not an ImportSelector or ImportBeanDefinitionRegistrar ->
                        // process it as an @Configuration class
                        this.importStack.registerImport(
                                currentSourceClass.getMetadata(), candidate.getMetadata().getClassName());
                        processConfigurationClass(candidate.asConfigClass(configClass));
                    }
                }
            }
            catch (BeanDefinitionStoreException ex) {
                throw ex;
            }
            catch (Throwable ex) {
                throw new BeanDefinitionStoreException(
                        "Failed to process import candidates for configuration class [" +
                        configClass.getMetadata().getClassName() + "]", ex);
            }
            finally {
                this.importStack.pop();
            }
        }
    }

    private boolean isChainedImportOnStack(ConfigurationClass configClass) {
        if (this.importStack.contains(configClass)) {
            String configClassName = configClass.getMetadata().getClassName();
            AnnotationMetadata importingClass = this.importStack.getImportingClassFor(configClassName);
            while (importingClass != null) {
                if (configClassName.equals(importingClass.getClassName())) {
                    return true;
                }
                importingClass = this.importStack.getImportingClassFor(importingClass.getClassName());
            }
        }
        return false;
    }

    ImportRegistry getImportRegistry() {
        return this.importStack;
    }


    /**
     * Factory method to obtain a {@link SourceClass} from a {@link ConfigurationClass}.
     */
    private SourceClass asSourceClass(ConfigurationClass configurationClass) throws IOException {
        AnnotationMetadata metadata = configurationClass.getMetadata();
        if (metadata instanceof StandardAnnotationMetadata) {
            return asSourceClass(((StandardAnnotationMetadata) metadata).getIntrospectedClass());
        }
        return asSourceClass(metadata.getClassName());
    }

    /**
     * Factory method to obtain a {@link SourceClass} from a {@link Class}.
     */
    SourceClass asSourceClass(@Nullable Class<?> classType) throws IOException {
        if (classType == null) {
            return new SourceClass(Object.class);
        }
        try {
            // Sanity test that we can reflectively read annotations,
            // including Class attributes; if not -> fall back to ASM
            for (Annotation ann : classType.getAnnotations()) {
                AnnotationUtils.validateAnnotation(ann);
            }
            return new SourceClass(classType);
        }
        catch (Throwable ex) {
            // Enforce ASM via class name resolution
            return asSourceClass(classType.getName());
        }
    }

    /**
     * Factory method to obtain {@link SourceClass}s from class names.
     */
    private Collection<SourceClass> asSourceClasses(String... classNames) throws IOException {
        List<SourceClass> annotatedClasses = new ArrayList<>(classNames.length);
        for (String className : classNames) {
            annotatedClasses.add(asSourceClass(className));
        }
        return annotatedClasses;
    }

    /**
     * Factory method to obtain a {@link SourceClass} from a class name.
     */
    SourceClass asSourceClass(@Nullable String className) throws IOException {
        if (className == null) {
            return new SourceClass(Object.class);
        }
        if (className.startsWith("java")) {
            // Never use ASM for core java types
            try {
                return new SourceClass(ClassUtils.forName(className, this.resourceLoader.getClassLoader()));
            }
            catch (ClassNotFoundException ex) {
                throw new NestedIOException("Failed to load class [" + className + "]", ex);
            }
        }
        return new SourceClass(this.metadataReaderFactory.getMetadataReader(className));
    }


    @SuppressWarnings("serial")
    private static class ImportStack extends ArrayDeque<ConfigurationClass> implements ImportRegistry {

        private final MultiValueMap<String, AnnotationMetadata> imports = new LinkedMultiValueMap<>();

        public void registerImport(AnnotationMetadata importingClass, String importedClass) {
            this.imports.add(importedClass, importingClass);
        }

        @Override
        @Nullable
        public AnnotationMetadata getImportingClassFor(String importedClass) {
            return CollectionUtils.lastElement(this.imports.get(importedClass));
        }

        @Override
        public void removeImportingClass(String importingClass) {
            for (List<AnnotationMetadata> list : this.imports.values()) {
                for (Iterator<AnnotationMetadata> iterator = list.iterator(); iterator.hasNext();) {
                    if (iterator.next().getClassName().equals(importingClass)) {
                        iterator.remove();
                        break;
                    }
                }
            }
        }

        /**
         * Given a stack containing (in order)
         * <ul>
         * <li>com.acme.Foo</li>
         * <li>com.acme.Bar</li>
         * <li>com.acme.Baz</li>
         * </ul>
         * return "[Foo->Bar->Baz]".
         */
        @Override
        public String toString() {
            StringBuilder builder = new StringBuilder("[");
            Iterator<ConfigurationClass> iterator = iterator();
            while (iterator.hasNext()) {
                builder.append(iterator.next().getSimpleName());
                if (iterator.hasNext()) {
                    builder.append("->");
                }
            }
            return builder.append(']').toString();
        }
    }


    private static class DeferredImportSelectorHolder {

        private final ConfigurationClass configurationClass;

        private final DeferredImportSelector importSelector;

        public DeferredImportSelectorHolder(ConfigurationClass configClass, DeferredImportSelector selector) {
            this.configurationClass = configClass;
            this.importSelector = selector;
        }

        public ConfigurationClass getConfigurationClass() {
            return this.configurationClass;
        }

        public DeferredImportSelector getImportSelector() {
            return this.importSelector;
        }
    }


    private static class DeferredImportSelectorGrouping {

        private final DeferredImportSelector.Group group;

        private final List<DeferredImportSelectorHolder> deferredImports = new ArrayList<>();

        DeferredImportSelectorGrouping(Group group) {
            this.group = group;
        }

        public void add(DeferredImportSelectorHolder deferredImport) {
            this.deferredImports.add(deferredImport);
        }

        /**
         * Return the imports defined by the group.
         * @return each import with its associated configuration class
         */
        public Iterable<Group.Entry> getImports() {
            for (DeferredImportSelectorHolder deferredImport : this.deferredImports) {
                this.group.process(deferredImport.getConfigurationClass().getMetadata(),
                        deferredImport.getImportSelector());
            }
            return this.group.selectImports();
        }
    }


    private static class DefaultDeferredImportSelectorGroup implements Group {

        private final List<Entry> imports = new ArrayList<>();

        @Override
        public void process(AnnotationMetadata metadata, DeferredImportSelector selector) {
            for (String importClassName : selector.selectImports(metadata)) {
                this.imports.add(new Entry(metadata, importClassName));
            }
        }

        @Override
        public Iterable<Entry> selectImports() {
            return this.imports;
        }
    }


    /**
     * Simple wrapper that allows annotated source classes to be dealt with
     * in a uniform manner, regardless of how they are loaded.
     */
    private class SourceClass implements Ordered {

        private final Object source;  // Class or MetadataReader

        private final AnnotationMetadata metadata;

        public SourceClass(Object source) {
            this.source = source;
            if (source instanceof Class) {
                this.metadata = new StandardAnnotationMetadata((Class<?>) source, true);
            }
            else {
                this.metadata = ((MetadataReader) source).getAnnotationMetadata();
            }
        }

        public final AnnotationMetadata getMetadata() {
            return this.metadata;
        }

        @Override
        public int getOrder() {
            Integer order = ConfigurationClassUtils.getOrder(this.metadata);
            return (order != null ? order : Ordered.LOWEST_PRECEDENCE);
        }

        public Class<?> loadClass() throws ClassNotFoundException {
            if (this.source instanceof Class) {
                return (Class<?>) this.source;
            }
            String className = ((MetadataReader) this.source).getClassMetadata().getClassName();
            return ClassUtils.forName(className, resourceLoader.getClassLoader());
        }

        public boolean isAssignable(Class<?> clazz) throws IOException {
            if (this.source instanceof Class) {
                return clazz.isAssignableFrom((Class<?>) this.source);
            }
            return new AssignableTypeFilter(clazz).match((MetadataReader) this.source, metadataReaderFactory);
        }

        public ConfigurationClass asConfigClass(ConfigurationClass importedBy) throws IOException {
            if (this.source instanceof Class) {
                return new ConfigurationClass((Class<?>) this.source, importedBy);
            }
            return new ConfigurationClass((MetadataReader) this.source, importedBy);
        }

        public Collection<SourceClass> getMemberClasses() throws IOException {
            Object sourceToProcess = this.source;
            if (sourceToProcess instanceof Class) {
                Class<?> sourceClass = (Class<?>) sourceToProcess;
                try {
                    Class<?>[] declaredClasses = sourceClass.getDeclaredClasses();
                    List<SourceClass> members = new ArrayList<>(declaredClasses.length);
                    for (Class<?> declaredClass : declaredClasses) {
                        members.add(asSourceClass(declaredClass));
                    }
                    return members;
                }
                catch (NoClassDefFoundError err) {
                    // getDeclaredClasses() failed because of non-resolvable dependencies
                    // -> fall back to ASM below
                    sourceToProcess = metadataReaderFactory.getMetadataReader(sourceClass.getName());
                }
            }

            // ASM-based resolution - safe for non-resolvable classes as well
            MetadataReader sourceReader = (MetadataReader) sourceToProcess;
            String[] memberClassNames = sourceReader.getClassMetadata().getMemberClassNames();
            List<SourceClass> members = new ArrayList<>(memberClassNames.length);
            for (String memberClassName : memberClassNames) {
                try {
                    members.add(asSourceClass(memberClassName));
                }
                catch (IOException ex) {
                    // Let's skip it if it's not resolvable - we're just looking for candidates
                    if (logger.isDebugEnabled()) {
                        logger.debug("Failed to resolve member class [" + memberClassName +
                                "] - not considering it as a configuration class candidate");
                    }
                }
            }
            return members;
        }

        public SourceClass getSuperClass() throws IOException {
            if (this.source instanceof Class) {
                return asSourceClass(((Class<?>) this.source).getSuperclass());
            }
            return asSourceClass(((MetadataReader) this.source).getClassMetadata().getSuperClassName());
        }

        public Set<SourceClass> getInterfaces() throws IOException {
            Set<SourceClass> result = new LinkedHashSet<>();
            if (this.source instanceof Class) {
                Class<?> sourceClass = (Class<?>) this.source;
                for (Class<?> ifcClass : sourceClass.getInterfaces()) {
                    result.add(asSourceClass(ifcClass));
                }
            }
            else {
                for (String className : this.metadata.getInterfaceNames()) {
                    result.add(asSourceClass(className));
                }
            }
            return result;
        }

        public Set<SourceClass> getAnnotations() throws IOException {
            Set<SourceClass> result = new LinkedHashSet<>();
            for (String className : this.metadata.getAnnotationTypes()) {
                try {
                    result.add(getRelated(className));
                }
                catch (Throwable ex) {
                    // An annotation not present on the classpath is being ignored
                    // by the JVM's class loading -> ignore here as well.
                }
            }
            return result;
        }

        public Collection<SourceClass> getAnnotationAttributes(String annType, String attribute) throws IOException {
            Map<String, Object> annotationAttributes = this.metadata.getAnnotationAttributes(annType, true);
            if (annotationAttributes == null || !annotationAttributes.containsKey(attribute)) {
                return Collections.emptySet();
            }
            String[] classNames = (String[]) annotationAttributes.get(attribute);
            Set<SourceClass> result = new LinkedHashSet<>();
            for (String className : classNames) {
                result.add(getRelated(className));
            }
            return result;
        }

        private SourceClass getRelated(String className) throws IOException {
            if (this.source instanceof Class) {
                try {
                    Class<?> clazz = ((Class<?>) this.source).getClassLoader().loadClass(className);
                    return asSourceClass(clazz);
                }
                catch (ClassNotFoundException ex) {
                    // Ignore -> fall back to ASM next, except for core java types.
                    if (className.startsWith("java")) {
                        throw new NestedIOException("Failed to load class [" + className + "]", ex);
                    }
                    return new SourceClass(metadataReaderFactory.getMetadataReader(className));
                }
            }
            return asSourceClass(className);
        }

        @Override
        public boolean equals(Object other) {
            return (this == other || (other instanceof SourceClass &&
                    this.metadata.getClassName().equals(((SourceClass) other).metadata.getClassName())));
        }

        @Override
        public int hashCode() {
            return this.metadata.getClassName().hashCode();
        }

        @Override
        public String toString() {
            return this.metadata.getClassName();
        }
    }


    /**
     * {@link Problem} registered upon detection of a circular {@link Import}.
     */
    private static class CircularImportProblem extends Problem {

        public CircularImportProblem(ConfigurationClass attemptedImport, Deque<ConfigurationClass> importStack) {
            super(String.format("A circular @Import has been detected: " +
                    "Illegal attempt by @Configuration class '%s' to import class '%s' as '%s' is " +
                    "already present in the current import stack %s", importStack.element().getSimpleName(),
                    attemptedImport.getSimpleName(), attemptedImport.getSimpleName(), importStack),
                    new Location(importStack.element().getResource(), attemptedImport.getMetadata()));
        }
    }

}

　　注意parse方法的最后一句processDeferredImportSelectors方法，在这里将会对DeferredImportSelector进行处理，这样我们就和AutoConfigurationSelectImporter结合到一起了。

　　注意processDeferredImportSelectors中的

　　　　 Class<? extends Group> group = deferredImport.getImportSelector().getImportGroup();这句代码

这里deferredImport的类型为DeferredImportSelectorHolder：

    private static class DeferredImportSelectorHolder {

        private final ConfigurationClass configurationClass;

        private final DeferredImportSelector importSelector;

        public DeferredImportSelectorHolder(ConfigurationClass configClass, DeferredImportSelector selector) {
            this.configurationClass = configClass;
            this.importSelector = selector;
        }

        public ConfigurationClass getConfigurationClass() {
            return this.configurationClass;
        }

        public DeferredImportSelector getImportSelector() {
            return this.importSelector;
        }
    }

　　在这个内部类里持有了一个DeferredImportSelector的引用，至此将会执行自动装配的所有操作

ㄟ( ▔, ▔ )ㄏ