Spring后置器postProcess的源码解析（二）

上篇我们解析了Spring后置器的一种BeanFactoryPostProcessor的接口实现bean，这次我们解析另一种BeanPostProcessor接口的实现bean在Spring初始化的执行步骤。
来看refresh()方法：

public void refresh() throws BeansException, IllegalStateException {
        synchronized (this.startupShutdownMonitor) {
            // Prepare this context for refreshing.
            //设置环境变量和容器的开关标志
            prepareRefresh();

            // Tell the subclass to refresh the internal bean factory.
            //刷新beanFactory，删除旧的beanFactory，创建新的beanFactory
            ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();

            // Prepare the bean factory for use in this context.
            // 准备beanfactory来使用这个上下文.做一些准备工作，例如classloader，beanPostProcessor等 
            prepareBeanFactory(beanFactory);

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

                // Invoke factory processors registered as beans in the context.
                //执行注册到该上下文的BeanFactoryPostProcessors  
                invokeBeanFactoryPostProcessors(beanFactory);

                // Register bean processors that intercept bean creation.
//注册BeanPostProcessors，注意只是注册，和上一行不同，没有invoke
                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();
            }
        }
    }

可以看到我们registerBeanPostProcessors就是我们要解析的方法，含义是注册BeanPostProcessors，并没有和BeanFactoryPostProcessor的注册过程一样，注册完就执行，这里只是注册了BeanPostProcessors并没有invoke。
我们跟进registerBeanPostProcessors(beanFactory);

/**
     * Instantiate and invoke all registered BeanFactoryPostProcessor beans,
     * respecting explicit order if given.
     * <p>Must be called before singleton instantiation.
     */
    protected void invokeBeanFactoryPostProcessors(ConfigurableListableBeanFactory beanFactory) {
        PostProcessorRegistrationDelegate.invokeBeanFactoryPostProcessors(beanFactory, getBeanFactoryPostProcessors());
    }

    /**
     * Instantiate and invoke all registered BeanPostProcessor beans,
     * respecting explicit order if given.
     * <p>Must be called before any instantiation of application beans.
     */
    protected void registerBeanPostProcessors(ConfigurableListableBeanFactory beanFactory) {
        PostProcessorRegistrationDelegate.registerBeanPostProcessors(beanFactory, this);
    }

这里我留了两个方法，第一个方法是上篇的BeanFactoryPostProcessor的执行方法，第二个是BeanPostProcessor的执行过程，这两个后置器都是通过PostProcessorRegistrationDelegate这个委托类来实现各自的过程的，我们打开这个委托类就会看到我们上篇的核心方法，当然也有本篇的核心方法，分别是

public static void invokeBeanFactoryPostProcessors(ConfigurableListableBeanFactory beanFactory, List<BeanFactoryPostProcessor> beanFactoryPostProcessors)
public static void registerBeanPostProcessors(ConfigurableListableBeanFactory beanFactory, AbstractApplicationContext applicationContext) 

我们今天分析registerBeanPostProcessors，这个方法的入参没有后置器集合，而是把AbstractApplicationContext 实例作为参数传了进去

public static void registerBeanPostProcessors(
            ConfigurableListableBeanFactory beanFactory, AbstractApplicationContext applicationContext) {
第一步
        String[] postProcessorNames = beanFactory.getBeanNamesForType(BeanPostProcessor.class, true, false);

        // Register BeanPostProcessorChecker that logs an info message when
        // a bean is created during BeanPostProcessor instantiation, i.e. when
        // a bean is not eligible for getting processed by all BeanPostProcessors.
        int beanProcessorTargetCount = beanFactory.getBeanPostProcessorCount() + 1 + postProcessorNames.length;
        beanFactory.addBeanPostProcessor(new BeanPostProcessorChecker(beanFactory, beanProcessorTargetCount));

        // Separate between BeanPostProcessors that implement PriorityOrdered,
        // Ordered, and the rest.
        List<BeanPostProcessor> priorityOrderedPostProcessors = new ArrayList<BeanPostProcessor>();
        List<BeanPostProcessor> internalPostProcessors = new ArrayList<BeanPostProcessor>();
        List<String> orderedPostProcessorNames = new ArrayList<String>();
        List<String> nonOrderedPostProcessorNames = new ArrayList<String>();
第二步
        for (String ppName : postProcessorNames) {
            if (beanFactory.isTypeMatch(ppName, PriorityOrdered.class)) {
                BeanPostProcessor pp = beanFactory.getBean(ppName, BeanPostProcessor.class);
                priorityOrderedPostProcessors.add(pp);
                if (pp instanceof MergedBeanDefinitionPostProcessor) {
                    internalPostProcessors.add(pp);
                }
            }
            else if (beanFactory.isTypeMatch(ppName, Ordered.class)) {
                orderedPostProcessorNames.add(ppName);
            }
            else {
                nonOrderedPostProcessorNames.add(ppName);
            }
        }
第三步
        // First, register the BeanPostProcessors that implement PriorityOrdered.
        sortPostProcessors(beanFactory, priorityOrderedPostProcessors);
        registerBeanPostProcessors(beanFactory, priorityOrderedPostProcessors);

        // Next, register the BeanPostProcessors that implement Ordered.
        List<BeanPostProcessor> orderedPostProcessors = new ArrayList<BeanPostProcessor>();
        for (String ppName : orderedPostProcessorNames) {
            BeanPostProcessor pp = beanFactory.getBean(ppName, BeanPostProcessor.class);
            orderedPostProcessors.add(pp);
            if (pp instanceof MergedBeanDefinitionPostProcessor) {
                internalPostProcessors.add(pp);
            }
        }
        sortPostProcessors(beanFactory, orderedPostProcessors);
        registerBeanPostProcessors(beanFactory, orderedPostProcessors);

        // Now, register all regular BeanPostProcessors.
        List<BeanPostProcessor> nonOrderedPostProcessors = new ArrayList<BeanPostProcessor>();
        for (String ppName : nonOrderedPostProcessorNames) {
            BeanPostProcessor pp = beanFactory.getBean(ppName, BeanPostProcessor.class);
            nonOrderedPostProcessors.add(pp);
            if (pp instanceof MergedBeanDefinitionPostProcessor) {
                internalPostProcessors.add(pp);
            }
        }
        registerBeanPostProcessors(beanFactory, nonOrderedPostProcessors);

        // Finally, re-register all internal BeanPostProcessors.
        sortPostProcessors(beanFactory, internalPostProcessors);
        registerBeanPostProcessors(beanFactory, internalPostProcessors);
第四步
        beanFactory.addBeanPostProcessor(new ApplicationListenerDetector(applicationContext));
    }

我在源码里标注了四个步骤，接下里分别解释：

1.通过beanFactory.getBeanNamesForType(BeanPostProcessor.class, true, false);方法获取beanFactory里继承了BeanPostProcessor接口的name的集合；
2.把后置器beans分为PriorityOrdered、Ordered、nonOrdered三大类，前两类是增加了排序条件的后置器；
3.前两类后置器执行sortPostProcessors和registerBeanPostProcessors方法，也就是先执行排序方法，后执行注册方法。
4.最后一步用到了上面提到的BeanPostProcessor和BeanFactoryPostProcessor的入参不同的AbstractApplicationContext，这一步执行了什么呢？

public void addBeanPostProcessor(BeanPostProcessor beanPostProcessor) {
        Assert.notNull(beanPostProcessor, "BeanPostProcessor must not be null");
        this.beanPostProcessors.remove(beanPostProcessor);
        this.beanPostProcessors.add(beanPostProcessor);
        if (beanPostProcessor instanceof InstantiationAwareBeanPostProcessor) {
            this.hasInstantiationAwareBeanPostProcessors = true;
        }
        if (beanPostProcessor instanceof DestructionAwareBeanPostProcessor) {
            this.hasDestructionAwareBeanPostProcessors = true;
        }
    }

可以看到，在addBeanPostProcessor方法里把BeanPostProcessor注册进了AbstractBeanFactory，这也就是为什么BeanFactoryPostProcessor执行了后置接口实现类，而BeanPostProcessor仅仅执行了注册，而没有执行的原因。