Spring实例化源码解析之registerBeanPostProcessors(六)
BeanPostProcessors是Spring框架中的一个扩展机制，它允许开发人员在Spring容器实例化、配置和初始化Bean的过程中干预和定制化。BeanPostProcessor接口定义了两个方法：postProcessBeforeInitialization和postProcessAfterInitialization，分别在Bean初始化之前和之后被调用。
BeanPostProcessors的作用是在Bean的初始化过程中提供额外的处理逻辑。通过实现BeanPostProcessor接口并注册到Spring容器中，开发人员可以在Bean实例化后的早期和后期阶段对Bean进行修改、增强或执行其他自定义逻辑。这样可以实现很多功能，如属性注入、AOP代理、资源初始化等。
猜测
入口是在AbstractApplicationContext的refresh方法中，在方法中调用了registerBeanPostProcessors(beanFactory)，从字面意思来说就是注册BeanPostProcessors，BeanPostProcessor具体做什么用的上面也有介绍，本章就分析一下这个注册做了哪些事情。
分析
老规矩，从入口的注释开始，这个方法就是实例化和注册所有的BeanPostProcessor，尊重期望的排序，如果有给出的话，这个方法必须在我们应用bean实力化之前被调用。
/**
* Instantiate and register all BeanPostProcessor beans,
* respecting explicit order if given.
* Must be called before any instantiation of application beans.
*/
protected void registerBeanPostProcessors(ConfigurableListableBeanFactory beanFactory) {
PostProcessorRegistrationDelegate.registerBeanPostProcessors(beanFactory, this);
}
方法实际调用的是PostProcessorRegistrationDelegate类中的registerBeanPostProcessors方法，如果调用参数的详细入下截图所示：
PostProcessorRegistrationDelegate
这个类中的方法其实就是整个代码的核心，接下来我们继续用拆解的方式来进行分析，下面是其所有的代码块。
public static void registerBeanPostProcessors(
ConfigurableListableBeanFactory beanFactory, AbstractApplicationContext applicationContext) {
// WARNING: Although it may appear that the body of this method can be easily
// refactored to avoid the use of multiple loops and multiple lists, the use
// of multiple lists and multiple passes over the names of processors is
// intentional. We must ensure that we honor the contracts for PriorityOrdered
// and Ordered processors. Specifically, we must NOT cause processors to be
// instantiated (via getBean() invocations) or registered in the ApplicationContext
// in the wrong order.
//
// Before submitting a pull request (PR) to change this method, please review the
// list of all declined PRs involving changes to PostProcessorRegistrationDelegate
// to ensure that your proposal does not result in a breaking change:
// https://github.com/spring-projects/spring-framework/issues?q=PostProcessorRegistrationDelegate+is%3Aclosed+label%3A%22status%3A+declined%22
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<>();
List<BeanPostProcessor> internalPostProcessors = new ArrayList<>();
List<String> orderedPostProcessorNames = new ArrayList<>();
List<String> nonOrderedPostProcessorNames = new ArrayList<>();
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);
}
}
// First, register the BeanPostProcessors that implement PriorityOrdered.
sortPostProcessors(priorityOrderedPostProcessors, beanFactory);
registerBeanPostProcessors(beanFactory, priorityOrderedPostProcessors);
// Next, register the BeanPostProcessors that implement Ordered.
List<BeanPostProcessor> orderedPostProcessors = new ArrayList<>(orderedPostProcessorNames.size());
for (String ppName : orderedPostProcessorNames) {
BeanPostProcessor pp = beanFactory.getBean(ppName, BeanPostProcessor.class);
orderedPostProcessors.add(pp);
if (pp instanceof MergedBeanDefinitionPostProcessor) {
internalPostProcessors.add(pp);
}
}
sortPostProcessors(orderedPostProcessors, beanFactory);
registerBeanPostProcessors(beanFactory, orderedPostProcessors);
// Now, register all regular BeanPostProcessors.
List<BeanPostProcessor> nonOrderedPostProcessors = new ArrayList<>(nonOrderedPostProcessorNames.size());
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(internalPostProcessors, beanFactory);
registerBeanPostProcessors(beanFactory, internalPostProcessors);
// Re-register post-processor for detecting inner beans as ApplicationListeners,
// moving it to the end of the processor chain (for picking up proxies etc).
beanFactory.addBeanPostProcessor(new ApplicationListenerDetector(applicationContext));
}
步骤一：
直接通过类型去获取所有的BeanPostProcessor的BeanNames，存在postProcessorNames数组中。
String[] postProcessorNames = beanFactory.getBeanNamesForType(BeanPostProcessor.class, true, false);
步骤二：
注册一个名为BeanPostProcessorChecker的BeanPostProcessor，并将其添加到beanFactory中。
具体分析如下：
-
获取已注册的BeanPostProcessor数量：
int beanProcessorTargetCount = beanFactory.getBeanPostProcessorCount(); ``` 这行代码获取已经注册到beanFactory的BeanPostProcessor的数量。 -
计算新的BeanPostProcessor的目标数量：
beanProcessorTargetCount = beanProcessorTargetCount + 1 + postProcessorNames.length; ``` 在已有的BeanPostProcessor数量基础上，加上1（表示新注册的BeanPostProcessorChecker）以及postProcessorNames的长度（表示后续要注册的BeanPostProcessor的数量），计算出新的目标数量。 -
创建并注册BeanPostProcessorChecker：
beanFactory.addBeanPostProcessor(new BeanPostProcessorChecker(beanFactory, beanProcessorTargetCount)); ``` 创建一个名为BeanPostProcessorChecker的BeanPostProcessor实例，并将其添加到beanFactory中。 BeanPostProcessorChecker是一个自定义的BeanPostProcessor，它在BeanPostProcessor实例化期间检查并记录信息。具体来说，当创建BeanPostProcessor期间创建了一个Bean（即一个Bean不符合所有BeanPostProcessor的处理条件）时，BeanPostProcessorChecker会记录一个信息日志。
步骤三：
对已注册的BeanPostProcessor进行分类，并按照优先级顺序注册到beanFactory中。
-
创建用于存储实现PriorityOrdered接口的BeanPostProcessor的列表：
List<BeanPostProcessor> priorityOrderedPostProcessors = new ArrayList<>(); ``` 创建一个名为priorityOrderedPostProcessors的列表，用于存储实现了PriorityOrdered接口的BeanPostProcessor。 -
创建用于存储内部BeanPostProcessor的列表：
List<BeanPostProcessor> internalPostProcessors = new ArrayList<>(); ``` 创建一个名为internalPostProcessors的列表，用于存储内部的BeanPostProcessor。 -
创建用于存储实现Ordered接口的BeanPostProcessor名称的列表：
List<String> orderedPostProcessorNames = new ArrayList<>(); ``` 创建一个名为orderedPostProcessorNames的列表，用于存储实现了Ordered接口的BeanPostProcessor的名称。 -
创建用于存储未实现Ordered接口的BeanPostProcessor名称的列表：
List<String> nonOrderedPostProcessorNames = new ArrayList<>(); ``` 创建一个名为nonOrderedPostProcessorNames的列表，用于存储未实现Ordered接口的BeanPostProcessor的名称。 -
遍历已注册的BeanPostProcessor名称列表，根据其类型进行分类：
for (String ppName : postProcessorNames) { if (beanFactory.isTypeMatch(ppName, PriorityOrdered.class)) { // BeanPostProcessor实现了PriorityOrdered接口 BeanPostProcessor pp = beanFactory.getBean(ppName, BeanPostProcessor.class); priorityOrderedPostProcessors.add(pp); if (pp instanceof MergedBeanDefinitionPostProcessor) { // 内部的BeanPostProcessor internalPostProcessors.add(pp); } } else if (beanFactory.isTypeMatch(ppName, Ordered.class)) { // BeanPostProcessor实现了Ordered接口 orderedPostProcessorNames.add(ppName); } else { // 未实现Ordered接口的BeanPostProcessor nonOrderedPostProcessorNames.add(ppName); } } ``` 遍历已注册的BeanPostProcessor名称列表postProcessorNames，根据名称对应的BeanPostProcessor的类型进行分类。如果BeanPostProcessor实现了PriorityOrdered接口，则将其添加到priorityOrderedPostProcessors列表中；如果是MergedBeanDefinitionPostProcessor的实例，则也添加到internalPostProcessors列表中；如果实现了Ordered接口，则将其名称添加到orderedPostProcessorNames列表中；否则，将其名称添加到nonOrderedPostProcessorNames列表中。 -
对实现PriorityOrdered接口的BeanPostProcessor进行排序：
sortPostProcessors(priorityOrderedPostProcessors, beanFactory); ``` 调用sortPostProcessors方法对priorityOrderedPostProcessors列表中的BeanPostProcessor进行排序，按照优先级顺序进行排序。 -
注册实现PriorityOrdered接口的BeanPostProcessor到beanFactory中：
registerBeanPostProcessors(beanFactory, priorityOrderedPostProcessors); ``` 调用registerBeanPostProcessors方法，将priorityOrderedPostProcessors列表中的BeanPostProcessor注册到beanFactory中。
步骤四：
实现了Ordered接口的BeanPostProcessor到beanFactory中。
-
创建用于存储实现Ordered接口的BeanPostProcessor的列表：
List<BeanPostProcessor> orderedPostProcessors = new ArrayList<>(orderedPostProcessorNames.size()); ``` 创建一个名为orderedPostProcessors的列表，用于存储实现了Ordered接口的BeanPostProcessor。 -
遍历实现了Ordered接口的BeanPostProcessor的名称列表，并将它们实例化并添加到orderedPostProcessors列表中：
for (String ppName : orderedPostProcessorNames) { BeanPostProcessor pp = beanFactory.getBean(ppName, BeanPostProcessor.class); orderedPostProcessors.add(pp); if (pp instanceof MergedBeanDefinitionPostProcessor) { // 内部的BeanPostProcessor internalPostProcessors.add(pp); } } ``` 遍历实现了Ordered接口的BeanPostProcessor的名称列表orderedPostProcessorNames，通过beanFactory.getBean方法实例化每个BeanPostProcessor，并将其添加到orderedPostProcessors列表中。如果BeanPostProcessor是MergedBeanDefinitionPostProcessor的实例，则也添加到internalPostProcessors列表中。 -
对实现Ordered接口的BeanPostProcessor进行排序：
sortPostProcessors(orderedPostProcessors, beanFactory); ``` 调用sortPostProcessors方法对orderedPostProcessors列表中的BeanPostProcessor进行排序，按照Ordered接口的顺序进行排序。 -
注册实现Ordered接口的BeanPostProcessor到beanFactory中：
registerBeanPostProcessors(beanFactory, orderedPostProcessors); ``` 调用registerBeanPostProcessors方法，将orderedPostProcessors列表中的BeanPostProcessor注册到beanFactory中。
步骤五：
注册剩余的未实现Ordered接口的BeanPostProcessor，并重新注册内部的BeanPostProcessor。
-
创建用于存储未实现Ordered接口的BeanPostProcessor的列表：
List<BeanPostProcessor> nonOrderedPostProcessors = new ArrayList<>(nonOrderedPostProcessorNames.size()); ``` 创建一个名为nonOrderedPostProcessors的列表，用于存储未实现Ordered接口的BeanPostProcessor。 -
遍历未实现Ordered接口的BeanPostProcessor的名称列表，并将它们实例化并添加到nonOrderedPostProcessors列表中：
for (String ppName : nonOrderedPostProcessorNames) { BeanPostProcessor pp = beanFactory.getBean(ppName, BeanPostProcessor.class); nonOrderedPostProcessors.add(pp); if (pp instanceof MergedBeanDefinitionPostProcessor) { // 内部的BeanPostProcessor internalPostProcessors.add(pp); } } ``` 遍历未实现Ordered接口的BeanPostProcessor的名称列表nonOrderedPostProcessorNames，通过beanFactory.getBean方法实例化每个BeanPostProcessor，并将其添加到nonOrderedPostProcessors列表中。如果BeanPostProcessor是MergedBeanDefinitionPostProcessor的实例，则也添加到internalPostProcessors列表中。 -
注册未实现Ordered接口的BeanPostProcessor到beanFactory中：
registerBeanPostProcessors(beanFactory, nonOrderedPostProcessors); ``` 调用registerBeanPostProcessors方法，将nonOrderedPostProcessors列表中的BeanPostProcessor注册到beanFactory中。 -
对内部的BeanPostProcessor进行排序：
sortPostProcessors(internalPostProcessors, beanFactory); ``` 调用sortPostProcessors方法对internalPostProcessors列表中的BeanPostProcessor进行排序。 -
重新注册内部的BeanPostProcessor到beanFactory中：
registerBeanPostProcessors(beanFactory, internalPostProcessors); ``` 调用registerBeanPostProcessors方法，将internalPostProcessors列表中的BeanPostProcessor重新注册到beanFactory中。 -
重新注册用于检测内部Bean作为ApplicationListeners的后处理器：
beanFactory.addBeanPostProcessor(new ApplicationListenerDetector(applicationContext)); ``` 创建一个ApplicationListenerDetector的实例，并将其作为BeanPostProcessor添加到beanFactory中。这个后处理器用于检测内部Bean是否为ApplicationListeners，并将其移动到处理器链的末尾，以便在处理代理等情况时能够正确拾取内部Bean。