SpringBoot自动配置原理

1.什么是自动配置

个人理解SpringBoot的自动配置就是在系统启动的过程中自动扫描加载starter和自定义的配置类和配置文件中的bean，并且能根据当前环境和条件动态加载bean，达到开箱即用的目的。

2.从注解反向看自动配置

说到自动配置，很多帖子会直接从启动类的main函数说起，从@SpringBootApplication这个入手，进而找到加载Bean的入口，一般情况下是可以这样看的。

@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 {

SpringBootApplication 这个注解是个复合注解，

@SpringBootConfiguration 这个注解的作用和@Configuration的作用是一致的，声明当前类为一个配置类。

@ComponentScan 这个注解的作用是声明Bean扫描的相关信息，扫描路径和排除，包含关系

@EnableAutoConfiguration 字面意思是开启自动配置。它本身也是一个复合注解

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

@AutoConfigurationPackage注解其实是向容器导入了一个Bean，

@Import(AutoConfigurationImportSelector.class)向容器导入了AutoConfigurationImportSelector 这个Bean,这个Bean的作用是加载自动配置类

public String[] selectImports(AnnotationMetadata annotationMetadata) {
        if (!isEnabled(annotationMetadata)) {
            return NO_IMPORTS;
        }
        AutoConfigurationMetadata autoConfigurationMetadata = AutoConfigurationMetadataLoader
                .loadMetadata(this.beanClassLoader);
        //加载自动配置类
        AutoConfigurationEntry autoConfigurationEntry = getAutoConfigurationEntry(autoConfigurationMetadata,
                annotationMetadata);
        return StringUtils.toStringArray(autoConfigurationEntry.getConfigurations());
    }

protected AutoConfigurationEntry getAutoConfigurationEntry(AutoConfigurationMetadata autoConfigurationMetadata,
            AnnotationMetadata annotationMetadata) {
        if (!isEnabled(annotationMetadata)) {
            return EMPTY_ENTRY;
        }
        AnnotationAttributes attributes = getAttributes(annotationMetadata);
        //获取候选的配置类名称，其实是加载包路径下的META-INF/spring.factories 中的类路径
        List<String> configurations = getCandidateConfigurations(annotationMetadata, attributes);
        //排重
        configurations = removeDuplicates(configurations);
        Set<String> exclusions = getExclusions(annotationMetadata, attributes);
        checkExcludedClasses(configurations, exclusions);
        //移除要排除的配置类
        configurations.removeAll(exclusions);
        //筛选满足条件的配置类，这个地方其实是根据Condition过滤到满足条件的配置类
        configurations = filter(configurations, autoConfigurationMetadata);
        fireAutoConfigurationImportEvents(configurations, exclusions);
        return new AutoConfigurationEntry(configurations, exclusions);
    }

到此为止，我们仅仅能看到加载了哪些类，但是每个类中又有很多Bean，每个Bean上又加了很多Condition注解，那这些Bean是如何加载的？这个AutoConfigurationImportSelector是什么时候被触发的？如果单从注解反向看的话是很难回答的？

3.从启动流程正向看自动配置

要想真正了解自动配置原理，还是要从启动流程中下手，启动过程中有很重要的一步是

refreshContext(context);
它调用的是AbstractApplicationContext的 refresh方法， 启动有一步是执行BeanFactory的后置处理器

invokeBeanFactoryPostProcessors(beanFactory);
这里面有一段执行 BeanDefinitionRegistryPostProcessor 的代码，这个是和bean注册相关的

    // Do not initialize FactoryBeans here: We need to leave all regular beans
            // uninitialized to let the bean factory post-processors apply to them!
            // Separate between BeanDefinitionRegistryPostProcessors that implement
            // PriorityOrdered, Ordered, and the rest.
            List<BeanDefinitionRegistryPostProcessor> currentRegistryProcessors = new ArrayList<>();

            // First, invoke the BeanDefinitionRegistryPostProcessors that implement PriorityOrdered.
            
            // 这个地方拿到的是org.springframework.context.annotation.internalConfigurationAnnotationProcessor 这个类，
            String[] postProcessorNames =
                    beanFactory.getBeanNamesForType(BeanDefinitionRegistryPostProcessor.class, true, false);
            for (String ppName : postProcessorNames) {
                if (beanFactory.isTypeMatch(ppName, PriorityOrdered.class)) {
                    currentRegistryProcessors.add(beanFactory.getBean(ppName, BeanDefinitionRegistryPostProcessor.class));
                    processedBeans.add(ppName);
                }
            }
            sortPostProcessors(currentRegistryProcessors, beanFactory);
            registryProcessors.addAll(currentRegistryProcessors);
　　　　　　　　//currentRegistryProcessors中存放的是ConfigurationClassPostProcessor，这个类是自动配置的核心
            invokeBeanDefinitionRegistryPostProcessors(currentRegistryProcessors, registry);
            currentRegistryProcessors.clear();

invoke 方法调用的org.springframework.context.annotation.ConfigurationClassPostProcessor的processConfigBeanDefinitions方法

String[] candidateNames = registry.getBeanDefinitionNames();

        for (String beanName : candidateNames) {
            BeanDefinition beanDef = registry.getBeanDefinition(beanName);
            if (beanDef.getAttribute(ConfigurationClassUtils.CONFIGURATION_CLASS_ATTRIBUTE) != null) {
                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));
            }
        }

从候选bean中选取有@Configuration的类，候选的有下面几个，其中只有我们自己的主启动类包含了这个注解

org.springframework.context.annotation.internalConfigurationAnnotationProcessor
org.springframework.context.annotation.internalAutowiredAnnotationProcessor
org.springframework.context.annotation.internalCommonAnnotationProcessor
org.springframework.context.event.internalEventListenerProcessor
org.springframework.context.event.internalEventListenerFactory
conditionTestApplication（主启动类）
org.springframework.boot.autoconfigure.internalCachingMetadataReaderFactory

接下来执行parser，这里面循环调用，将所有@Configuration的配置类都处理了一遍，将使用@Bean注解的方法都提取出来了

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 {
            //记载配置类
            parser.parse(candidates);
            parser.validate();
　　　　　　　　//存放配置类及Bean信息
            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());
            }
            //根据条件注解，加载Bean
            this.reader.loadBeanDefinitions(configClasses);
            alreadyParsed.addAll(configClasses);

parser.parse(candidates); 方法中调用了 this.deferredImportSelectorHandler.process()，而 AutoConfigurationImportSelector 是 DeferredImportSelector的实现类

public void process() {
            List<DeferredImportSelectorHolder> deferredImports = this.deferredImportSelectors;
            this.deferredImportSelectors = null;
            try {
                if (deferredImports != null) {
                    DeferredImportSelectorGroupingHandler handler = new DeferredImportSelectorGroupingHandler();
                    deferredImports.sort(DEFERRED_IMPORT_COMPARATOR);
                    deferredImports.forEach(handler::register);
                    //进行导入
                    handler.processGroupImports();
                }
            }
            finally {
                this.deferredImportSelectors = new ArrayList<>();
            }
        }

public void processGroupImports() {
            for (DeferredImportSelectorGrouping grouping : this.groupings.values()) {
                grouping.getImports().forEach(entry -> {
                    ConfigurationClass configurationClass = this.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);
                    }
                });
            }
        }

grouping.getImports()的实现是

public Iterable<Group.Entry> getImports() {
            for (DeferredImportSelectorHolder deferredImport : this.deferredImports) {
                this.group.process(deferredImport.getConfigurationClass().getMetadata(),
                        deferredImport.getImportSelector());
            }
            return this.group.selectImports();
        }

group.process的调用了AutoConfigurationImportSelector.process 方法，这样就和前面的从注解看实现的步骤对上了，加载配置类的入口就找到了，并且经过一层过滤

public void process(AnnotationMetadata annotationMetadata, DeferredImportSelector deferredImportSelector) {
            Assert.state(deferredImportSelector instanceof AutoConfigurationImportSelector,
                    () -> String.format("Only %s implementations are supported, got %s",
                            AutoConfigurationImportSelector.class.getSimpleName(),
                            deferredImportSelector.getClass().getName()));
            // 这个地方就是执行的导入配置类
            AutoConfigurationEntry autoConfigurationEntry = ((AutoConfigurationImportSelector) deferredImportSelector)
                    .getAutoConfigurationEntry(getAutoConfigurationMetadata(), annotationMetadata);
            this.autoConfigurationEntries.add(autoConfigurationEntry);
            for (String importClassName : autoConfigurationEntry.getConfigurations()) {
                this.entries.putIfAbsent(importClassName, annotationMetadata);
            }
        }

再回到processGroupImports() 这个方法很复杂，作用是循环调用所有配置类，将自动配置类中的有 @ComponentScan @Import  @ImportResource 这些注解的类都找到，如果没有注解作为配置类使用，并且将结果存在在configClass中，这个类中存放的是所有配置类及配置类中产生Bean方法的信息，后面将通过这些信息确定要最终加载哪些Bean。

this.reader.loadBeanDefinitions(configClasses);

private void loadBeanDefinitionsForConfigurationClass(
            ConfigurationClass configClass, TrackedConditionEvaluator trackedConditionEvaluator) {
　　　　　　// 判断类是不是要跳过
        if (trackedConditionEvaluator.shouldSkip(configClass)) {
            String beanName = configClass.getBeanName();
            if (StringUtils.hasLength(beanName) && this.registry.containsBeanDefinition(beanName)) {
                this.registry.removeBeanDefinition(beanName);
            }
            this.importRegistry.removeImportingClass(configClass.getMetadata().getClassName());
            return;
        }

        if (configClass.isImported()) {
            registerBeanDefinitionForImportedConfigurationClass(configClass);
        }
        // 将configerClass 转成BeanDefinition并注册到BeanFactory
        for (BeanMethod beanMethod : configClass.getBeanMethods()) {
            loadBeanDefinitionsForBeanMethod(beanMethod);
        }

        loadBeanDefinitionsFromImportedResources(configClass.getImportedResources());
        loadBeanDefinitionsFromRegistrars(configClass.getImportBeanDefinitionRegistrars());
    }

 ConditionEvaluator.shouldSkip 的作用就是根据判断Condition注解能否生效

public boolean shouldSkip(@Nullable AnnotatedTypeMetadata metadata, @Nullable ConfigurationPhase phase) {
        if (metadata == null || !metadata.isAnnotated(Conditional.class.getName())) {
            return false;
        }

        if (phase == null) {
            if (metadata instanceof AnnotationMetadata &&
                    ConfigurationClassUtils.isConfigurationCandidate((AnnotationMetadata) metadata)) {
                return shouldSkip(metadata, ConfigurationPhase.PARSE_CONFIGURATION);
            }
            return shouldSkip(metadata, ConfigurationPhase.REGISTER_BEAN);
        }

        List<Condition> conditions = new ArrayList<>();
        for (String[] conditionClasses : getConditionClasses(metadata)) {
            for (String conditionClass : conditionClasses) {
                Condition condition = getCondition(conditionClass, this.context.getClassLoader());
                conditions.add(condition);
            }
        }

        AnnotationAwareOrderComparator.sort(conditions);

        for (Condition condition : conditions) {
            ConfigurationPhase requiredPhase = null;
            if (condition instanceof ConfigurationCondition) {
                requiredPhase = ((ConfigurationCondition) condition).getConfigurationPhase();
            }
            if ((requiredPhase == null || requiredPhase == phase) && !condition.matches(this.context, metadata)) {
                return true;
            }
        }

        return false;
    }

这样整个自动配置的链路就串起来了，加载自动配类，使用条件注解判断哪些Bean需要加载到容器中都可以找到实现。