SpringBoot自动配置分析

一、注解

@SpringBootApplication 注解

@SpringBootApplication 注解实际上是一个组合注解，它由三个注解组合而成，分别是 @SpringBootConfiguration、@EnableAutoConfiguration 和 @ComponentScan

@ComponentScan 注解

@ComponentScan 注解不是 Spring Boot 引入的新注解，而是属于 Spring 容器管理的内容。@ComponentScan 注解就是扫描基于 @Component 等注解所标注的类所在包下的所有需要注入的类，并把相关 Bean 定义批量加载到容器中。

@ComponentScan注解的处理逻辑在Configuration解析时的ConfigurationClassParser中进行处理

#调用栈
doProcessConfigurationClass:281, ConfigurationClassParser (org.springframework.context.annotation)
processConfigurationClass:245, ConfigurationClassParser (org.springframework.context.annotation)
parse:202, ConfigurationClassParser (org.springframework.context.annotation)
parse:170, ConfigurationClassParser (org.springframework.context.annotation)
processConfigBeanDefinitions:331, ConfigurationClassPostProcessor (org.springframework.context.annotation)
postProcessBeanDefinitionRegistry:233, ConfigurationClassPostProcessor (org.springframework.context.annotation)
invokeBeanDefinitionRegistryPostProcessors:303, PostProcessorRegistrationDelegate (org.springframework.context.support)
invokeBeanFactoryPostProcessors:106, PostProcessorRegistrationDelegate (org.springframework.context.support)
invokeBeanFactoryPostProcessors:739, AbstractApplicationContext (org.springframework.context.support)
refresh:536, AbstractApplicationContext (org.springframework.context.support)

// @ComponentScan注解处理
protected final SourceClass doProcessConfigurationClass(ConfigurationClass configClass, SourceClass sourceClass)
            throws IOException {
    ......
    Set 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 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());
                }
            }
        }
    }
    ......
}

使用ClassPathBeanDefinitionScanner扫描basePackage下的@Component注解类，递归解析成BeanDefinition并通过 BeanDefinitionReaderUtils.registerBeanDefinition(definitionHolder, registry) 添加到 DefaultListableBeanFactory - beanDefinitionMap 中(这个时候bean并没有进行实例化，而是进行了注册。具体的实例化在finishBeanFactoryInitialization方法中执行)。

@SpringBootConfiguration 注解

@SpringBootConfiguration 注解比较简单，只是使用了 Spring 中的 @Configuration 注解。@Configuration 注解比较常见，提供了 JavaConfig 配置类实现。具体实现在 ConfigurationClassParser#parse 中，涉及对 @PropertySources、@ComponentScan、@Import、@ImportResource、@Bean、@DeferredImportSelector 注解的处理。

@EnableAutoConfiguration 注解

@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 {};
}

这里我们关注两个新注解，@AutoConfigurationPackage 和 @Import(AutoConfigurationImportSelector.class)

@AutoConfigurationPackage 注解

@AutoConfigurationPackage 注解主要就是引用@Import(AutoConfigurationPackages.Registrar.class)

@Import 注解

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface Import {
    Class[] value();
}

在 @Import 注解的属性中可以设置需要引入的类名，例如 @AutoConfigurationPackage 注解上的 @Import(AutoConfigurationPackages.Registrar.class)。

private void processImports(ConfigurationClass configClass, SourceClass currentSourceClass,
        Collection 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 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();
        }
    }
}

根据该类的不同类型，Spring 容器针对 @Import 注解有以下四种处理方式：

• 如果该类实现了 ImportSelector 接口，调用其 selectImports 方法，返回spring.factories 中配置的类，然后递归调用 processImports 进行处理，最终会被保存在 configurationClasses 中；
• 如果该类实现了 DeferredImportSelector 接口，则 Spring 容器也会实例化该类并调用其 selectImports方法。DeferredImportSelector 继承了 ImportSelector，区别在于DeferredImportSelector 实例先保存在 deferredImportSelectors 中，要等到 @Configuration 注解中相关的业务全部都处理完了才会通过 processDeferredImportSelectors 调用 selectImports 方法；
• 如果该类实现了 ImportBeanDefinitionRegistrar 接口，会把Metadata 保存importBeanDefinitionRegistrars 中，在解析完Configuration 后通过 this.reader.loadBeanDefinitions(configClasses)，最后在loadBeanDefinitionsFromRegistrars(configClass.getImportBeanDefinitionRegistrars()) 中调用其 registerBeanDefinitions 方法；
• 如果该类没有实现上述三种接口中的任何一个，把这个类当成是@Configuration注解修饰的类递归重头开始解析这个类；

关于注解就到这里，下面重点分析下 @Import 的两个类 AutoConfigurationPackages.Registrar 和 AutoConfigurationImportSelector

二、AutoConfigurationPackages.Registrar

static class Registrar implements ImportBeanDefinitionRegistrar, DeterminableImports {

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