Spring源码分析之—— Bean的实例化和依赖注入细节

本文探讨注解方式的Bean实例化和属性注入，像构造器和setter方式，大体差不多，先做个大概介绍：

• 构造器方式，解析XML，用BeanDefinition存储构造信息，包含哪些属性作为构造器的参数，到时候一并构造；
• setter方式，解析XML，用BeanDefinition存储PropertyValues，在Bean实例化后通过set方式进行注入；
• 注解方式，首先要开启注解功能，注入是通过注解后置处理器来完成的；

OK，进入主题，共分两步走，虽然叫两步，但代码里其实是一起的，实例化后马上就进行初始化的属性注入

第一步：Bean实例化————

org.springframework.context.support.AbstractApplicationContext  

@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.
        // 重点：这是个非常重要的方法，它负责获取新的beanFactory，销毁原有beanFactory、为每个bean生成BeanDefinition等
        // 不过本篇看注入细节，这一段先过
        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.
            // 这个类是空的，由子类实现
            // 到这里的时候，所有的 Bean 都加载、注册完成了，但是都还没有初始化
            postProcessBeanFactory(beanFactory);

            // Invoke factory processors registered as beans in the context.
            // 如果有自定义的beanFactory后置处理器，继承BeanFactoryPostProcessor的
            // 使用getBeanNamesForType(BeanFactoryPostProcessor.class, true, false)获取所有的bean factory后置处理器
            // 在这一步就会把这些处理器实例化并且调用其postProcessBeanFactory方法，对所有的bean definition做相应操作
            invokeBeanFactoryPostProcessors(beanFactory);

            // Register bean processors that intercept bean creation.
            // 同上，根据BeanPostProcessor.class类型查找Bean后置处理器
            // 把这些处理器注册到AbstractBeanFactory类的集合中去：List beanPostProcessors
            // 注意：这里只是注册上去，并不调用，调用要等到倒数第二步
            registerBeanPostProcessors(beanFactory);

            // Initialize message source for this context.
            // 国际化处理，略过
            initMessageSource();

            // Initialize event multicaster for this context.
            // 初始化当前 ApplicationContext 的事件广播器，监听onApplicationEvent事件，也不做重点
            initApplicationEventMulticaster();

            // Initialize other special beans in specific context subclasses.
            // 一个hook（钩子）方法，空实现，子类可以重写加入自己的逻辑
            onRefresh();

            // Check for listener beans and register them.
            // 注册事件监听器，监听默认或自定义的事件，比如：监听下订单事件
            // 最终注册到ListenerRetriever的集合：Set applicationListenerBeans中去
            // ListenerRetriever是个内部类，在AbstractApplicationEventMulticaster中
            registerListeners();

            // Instantiate all remaining (non-lazy-init) singletons.
            // ========重点来了======
            // 初始化所有的 singleton beans（非懒加载）
            // 比如invokeBeanFactoryPostProcessors方法中根据各种注解解析出来的类，在这个时候都会被初始化。
            // 实例化的过程各种BeanPostProcessor开始起作用。
            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();
        }
    }
}


/**
 * Finish the initialization of this context's bean factory,
 * initializing all remaining singleton beans.
 * ======完成上下文bean factory初始化，并且初始化所有的单例bean
 */
protected void finishBeanFactoryInitialization(ConfigurableListableBeanFactory beanFactory) {
    // Initialize conversion service for this context.
    if (beanFactory.containsBean(CONVERSION_SERVICE_BEAN_NAME) &&
            beanFactory.isTypeMatch(CONVERSION_SERVICE_BEAN_NAME, ConversionService.class)) {
        beanFactory.setConversionService(
                beanFactory.getBean(CONVERSION_SERVICE_BEAN_NAME, ConversionService.class));
    }

    // Register a default embedded value resolver if no bean post-processor
    // (such as a PropertyPlaceholderConfigurer bean) registered any before:
    // at this point, primarily for resolution in annotation attribute values.
    if (!beanFactory.hasEmbeddedValueResolver()) {
        beanFactory.addEmbeddedValueResolver(new StringValueResolver() {
            @Override
            public String resolveStringValue(String strVal) {
                return getEnvironment().resolvePlaceholders(strVal);
            }
        });
    }

    // Initialize LoadTimeWeaverAware beans early to allow for registering their transformers early.
    String[] weaverAwareNames = beanFactory.getBeanNamesForType(LoadTimeWeaverAware.class, false, false);
    for (String weaverAwareName : weaverAwareNames) {
        getBean(weaverAwareName);
    }

    // Stop using the temporary ClassLoader for type matching.
    beanFactory.setTempClassLoader(null);

    // Allow for caching all bean definition metadata, not expecting further changes.
    beanFactory.freezeConfiguration();

    // Instantiate all remaining (non-lazy-init) singletons.
    // 初始化bean的重点在这里
    beanFactory.preInstantiateSingletons();
}

 

——BeanFactory核心类：

org.springframework.beans.factory.support.DefaultListableBeanFactory

// 确保所有非懒加载的单例bean都实例化
@Override
public void preInstantiateSingletons() throws BeansException {
    if (this.logger.isDebugEnabled()) {
        this.logger.debug("Pre-instantiating singletons in " + this);
    }

    // Iterate over a copy to allow for init methods which in turn register new bean definitions.
    // While this may not be part of the regular factory bootstrap, it does otherwise work fine.
    List beanNames = new ArrayList(this.beanDefinitionNames);

    // Trigger initialization of all non-lazy singleton beans...
    for (String beanName : beanNames) {
        // 对BeanDefinition先做个处理，不管原来是什么BeanDefinition，统统转换成RootBeanDefinition
        // 如果原来不是RootBeanDefinition，会用深拷贝方式转换成RootBeanDefinition
        // 深拷贝放在父类AbstractBeanDefinition中完成
        RootBeanDefinition bd = getMergedLocalBeanDefinition(beanName);
        if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) {
            if (isFactoryBean(beanName)) {
                final FactoryBean factory = (FactoryBean) getBean(FACTORY_BEAN_PREFIX + beanName);
                boolean isEagerInit;
                if (System.getSecurityManager() != null && factory instanceof SmartFactoryBean) {
                    isEagerInit = AccessController.doPrivileged(new PrivilegedAction() {
                        @Override
                        public Boolean run() {
                            return ((SmartFactoryBean) factory).isEagerInit();
                        }
                    }, getAccessControlContext());
                }
                else {
                    isEagerInit = (factory instanceof SmartFactoryBean &&
                            ((SmartFactoryBean) factory).isEagerInit());
                }
                if (isEagerInit) {
                    getBean(beanName);
                }
            }
            else {
                // ====== 核心方法在这。。。。
                // 咦！怎么是getBean呢，不是还没实例化吗？进去就知道了
                getBean(beanName);
            }
        }
    }

    // Trigger post-initialization callback for all applicable beans...
    for (String beanName : beanNames) {
        Object singletonInstance = getSingleton(beanName);
        if (singletonInstance instanceof SmartInitializingSingleton) {
            final SmartInitializingSingleton smartSingleton = (SmartInitializingSingleton) singletonInstance;
            if (System.getSecurityManager() != null) {
                AccessController.doPrivileged(new PrivilegedAction