SpringBoot自动装配源码解析

序：众所周知spring-boot入门容易精通难，说到底spring-boot是对spring已有的各种技术的整合封装，因为封装了所以使用简单，也因为封装了所以越来越多的"拿来主义"者们不愿意去关注其具体实现！为了更好的使用spring-boot所以必要的源码探索是非常有必要的！今天开始探索的第一步：自动装配原理-----------------（此处默认各位看官熟悉spring的各种基础注解）

1.要谈自动装配我们需要从项目的初始注解入手:@SpringBootApplication

@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@SpringBootConfiguration
@EnableAutoConfiguration
@ComponentScan(
    excludeFilters = {@Filter(
    type = FilterType.CUSTOM,
    classes = {TypeExcludeFilter.class}
), @Filter(
    type = FilterType.CUSTOM,
    classes = {AutoConfigurationExcludeFilter.class}
)}
)
public @interface SpringBootApplication {
    ...38 }

2.这个下面实现自动装配的注解为：@EnableAutoConfiguration

@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@AutoConfigurationPackage
@Import({AutoConfigurationImportSelector.class})
public @interface EnableAutoConfiguration {
    String ENABLED_OVERRIDE_PROPERTY = "spring.boot.enableautoconfiguration";

    Class<?>[] exclude() default {};

    String[] excludeName() default {};
}

3.在这个注解下我们需要关注两个注解：@AutoConfigurationPackage、@Import({AutoConfigurationImportSelector.class})

a.我们先来看看@AutoConfigurationPackage

@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@Import({Registrar.class})
public @interface AutoConfigurationPackage {
}

这个下面引入了:Registrar

static class Registrar implements ImportBeanDefinitionRegistrar, DeterminableImports {
        Registrar() {
        }

        public void registerBeanDefinitions(AnnotationMetadata metadata, BeanDefinitionRegistry registry) {
            AutoConfigurationPackages.register(registry, (new AutoConfigurationPackages.PackageImport(metadata)).getPackageName());
        }

        public Set<Object> determineImports(AnnotationMetadata metadata) {
            return Collections.singleton(new AutoConfigurationPackages.PackageImport(metadata));
        }
    }

这是AutoConfigurationPackage的一个静态类，标蓝处的动作为为指定@ComponentScan的扫描路径,此处打断点启动项目我们可以发现这个为项目的顶级包名。（这个下面不做深入介绍，此内容不属于本文重点，有兴趣自行深入了解）如图：

这儿确保了将项目目录下所有的bean注入到容器 

 b.接下来我们看看引入的AutoConfigurationImportSelector(个人觉得这个是自动配置的灵魂)

public class AutoConfigurationImportSelector implements DeferredImportSelector, BeanClassLoaderAware, ResourceLoaderAware, BeanFactoryAware, EnvironmentAware, Ordered {
    private static final AutoConfigurationImportSelector.AutoConfigurationEntry EMPTY_ENTRY = new AutoConfigurationImportSelector.AutoConfigurationEntry();
    private static final String[] NO_IMPORTS = new String[0];
    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;
　　....
    protected AutoConfigurationImportSelector.AutoConfigurationEntry getAutoConfigurationEntry(AutoConfigurationMetadata autoConfigurationMetadata, AnnotationMetadata annotationMetadata) {
        if (!this.isEnabled(annotationMetadata)) {
            return EMPTY_ENTRY;
        } else {
            AnnotationAttributes attributes = this.getAttributes(annotationMetadata);
            List<String> configurations = this.getCandidateConfigurations(annotationMetadata, attributes);
            configurations = this.removeDuplicates(configurations);
            Set<String> exclusions = this.getExclusions(annotationMetadata, attributes);
            this.checkExcludedClasses(configurations, exclusions);
            configurations.removeAll(exclusions);
            configurations = this.filter(configurations, autoConfigurationMetadata);
            this.fireAutoConfigurationImportEvents(configurations, exclusions);
            return new AutoConfigurationImportSelector.AutoConfigurationEntry(configurations, exclusions);
        }
    }
　　....
}

此处是去获取真正自动配置类的集合,我们需要关注标蓝的方法：

protected List<String> getCandidateConfigurations(AnnotationMetadata metadata, AnnotationAttributes attributes) {
        List<String> configurations = SpringFactoriesLoader.loadFactoryNames(this.getSpringFactoriesLoaderFactoryClass(), this.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;
    }

还看不出来什么，还需要往下走一层：

public static List<String> loadFactoryNames(Class<?> factoryClass, @Nullable ClassLoader classLoader) {
        String factoryClassName = factoryClass.getName();
        return (List)loadSpringFactories(classLoader).getOrDefault(factoryClassName, Collections.emptyList());
    }

    private static Map<String, List<String>> loadSpringFactories(@Nullable ClassLoader classLoader) {
        MultiValueMap<String, String> result = (MultiValueMap)cache.get(classLoader);
        if (result != null) {
            return result;
        } else {
            try {
                Enumeration<URL> urls = classLoader != null ? classLoader.getResources("META-INF/spring.factories") : ClassLoader.getSystemResources("META-INF/spring.factories");
                LinkedMultiValueMap result = new LinkedMultiValueMap();

                while(urls.hasMoreElements()) {
                    URL url = (URL)urls.nextElement();
                    UrlResource resource = new UrlResource(url);
                    Properties properties = PropertiesLoaderUtils.loadProperties(resource);
                    Iterator var6 = properties.entrySet().iterator();

                    while(var6.hasNext()) {
                        Entry<?, ?> entry = (Entry)var6.next();
                        String factoryClassName = ((String)entry.getKey()).trim();
                        String[] var9 = StringUtils.commaDelimitedListToStringArray((String)entry.getValue());
                        int var10 = var9.length;

                        for(int var11 = 0; var11 < var10; ++var11) {
                            String factoryName = var9[var11];
                            result.add(factoryClassName, factoryName.trim());
                        }
                    }
                }

                cache.put(classLoader, result);
                return result;
            } catch (IOException var13) {
                throw new IllegalArgumentException("Unable to load factories from location [META-INF/spring.factories]", var13);
            }
        }
    }

我们可以发现spring-boot会去META-INF/spring.factories找org.springframework.boot.autoconfigure.EnableAutoConfiguration的value.这个的具体位置如图:

这个下面配置了所有自动配置，以其中一个为例（其余类似），分析其可配参数。（AopAutoConfiguration）

@Configuration
@ConditionalOnClass({EnableAspectJAutoProxy.class, Aspect.class, Advice.class, AnnotatedElement.class})
@ConditionalOnProperty(
    prefix = "spring.aop",
    name = {"auto"},
    havingValue = "true",
    matchIfMissing = true
)
public class AopAutoConfiguration {
    public AopAutoConfiguration() {
    }

    @Configuration
    @EnableAspectJAutoProxy(
        proxyTargetClass = true
    )
    @ConditionalOnProperty(
        prefix = "spring.aop",
        name = {"proxy-target-class"},
        havingValue = "true",
        matchIfMissing = true
    )
    public static class CglibAutoProxyConfiguration {
        public CglibAutoProxyConfiguration() {
        }
    }

    @Configuration
    @EnableAspectJAutoProxy(
        proxyTargetClass = false
    )
    @ConditionalOnProperty(
        prefix = "spring.aop",
        name = {"proxy-target-class"},
        havingValue = "false",
        matchIfMissing = false
    )
    public static class JdkDynamicAutoProxyConfiguration {
        public JdkDynamicAutoProxyConfiguration() {
        }
    }
}

我们可以在spring-configuration-metadata.json中找到其对应的配置如下：

{
      "name": "spring.aop.proxy-target-class",
      "type": "java.lang.Boolean",
      "description": "Whether subclass-based (CGLIB) proxies are to be created (true), as opposed to standard Java interface-based proxies (false).",
      "defaultValue": true
    },

介绍到这里spring-boot的自动装配过程我们就基本看完了～