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。
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