概述：
在spring的啓動過程中，bean工廠創建成功並加載所有的XML資源文件生成BeanDefinitions集合後，實例化所有singlelonBean對象之前，需要對beanDefinition做額外的操作，bean工廠後置處理器就發揮重要作用了。
一個經常用到的場景：業務代碼中或者XML文件中配置bean屬性的值經常採用${...}佔位符，把參數寫入properties文件中，xml中只需配置如下bean，spring就會完成佔位符的替換。
<bean id="propertyConfigurer" class="org.springframework.beans.factory.config.PropertyPlaceholderConfigurer">
<property name="locations">
<list>
<value>classpath:${profile.properties}/*.properties</value>
</list>
</property>
</bean>首先看下PropertyResourceConfigurer類靜態結構圖
可以發現該類實現了BeanFactoryPostProcessor、PriorityOrdered接口，下面我們看下實現這兩個接口的bean在spring容器啓動過程中何時被觸發
org.springframework.context.support.AbstractApplicationContext啓動Spring容器核心類
public void refresh() throws BeansException, IllegalStateException {
...
// Invoke factory processors registered as beans in the context.
invokeBeanFactoryPostProcessors(beanFactory);
...
} /**
* Instantiate and invoke all registered BeanFactoryPostProcessor beans,
* 實例化並調用所有註冊的BeanFactoryPostProcessor實例
* respecting explicit order if given.
* <p>Must be called before singleton instantiation.
*/
protected void invokeBeanFactoryPostProcessors(ConfigurableListableBeanFactory beanFactory) {
PostProcessorRegistrationDelegate.invokeBeanFactoryPostProcessors(beanFactory, getBeanFactoryPostProcessors());
} public static void invokeBeanFactoryPostProcessors(
ConfigurableListableBeanFactory beanFactory, List<BeanFactoryPostProcessor> beanFactoryPostProcessors) {
// Invoke BeanDefinitionRegistryPostProcessors first, if any.
Set<String> processedBeans = new HashSet<String>();
if (beanFactory instanceof BeanDefinitionRegistry) {
BeanDefinitionRegistry registry = (BeanDefinitionRegistry) beanFactory;
List<BeanFactoryPostProcessor> regularPostProcessors = new LinkedList<BeanFactoryPostProcessor>();
List<BeanDefinitionRegistryPostProcessor> registryPostProcessors =
new LinkedList<BeanDefinitionRegistryPostProcessor>();
for (BeanFactoryPostProcessor postProcessor : beanFactoryPostProcessors) {
if (postProcessor instanceof BeanDefinitionRegistryPostProcessor) {
BeanDefinitionRegistryPostProcessor registryPostProcessor =
(BeanDefinitionRegistryPostProcessor) postProcessor;
registryPostProcessor.postProcessBeanDefinitionRegistry(registry);
registryPostProcessors.add(registryPostProcessor);
}
else {
regularPostProcessors.add(postProcessor);
}
}
// Do not initialize FactoryBeans here: We need to leave all regular beans
// uninitialized to let the bean factory post-processors apply to them!
// Separate between BeanDefinitionRegistryPostProcessors that implement
// PriorityOrdered, Ordered, and the rest.
// BeanDefinitionRegistryPostProcessor核心接口，從bean工廠獲取實現了BeanDefinitionRegistryPostProcessor接口的beanDefinition集合
String[] postProcessorNames =
beanFactory.getBeanNamesForType(BeanDefinitionRegistryPostProcessor.class, true, false);
// First, invoke the BeanDefinitionRegistryPostProcessors that implement PriorityOrdered.
List<BeanDefinitionRegistryPostProcessor> priorityOrderedPostProcessors = new ArrayList<BeanDefinitionRegistryPostProcessor>();
for (String ppName : postProcessorNames) {
if (beanFactory.isTypeMatch(ppName, PriorityOrdered.class)) {
priorityOrderedPostProcessors.add(beanFactory.getBean(ppName, BeanDefinitionRegistryPostProcessor.class));
processedBeans.add(ppName);
}
}
sortPostProcessors(beanFactory, priorityOrderedPostProcessors);
registryPostProcessors.addAll(priorityOrderedPostProcessors);
// 觸發接口方法
invokeBeanDefinitionRegistryPostProcessors(priorityOrderedPostProcessors, registry);
// Next, invoke the BeanDefinitionRegistryPostProcessors that implement Ordered.
postProcessorNames = beanFactory.getBeanNamesForType(BeanDefinitionRegistryPostProcessor.class, true, false);
List<BeanDefinitionRegistryPostProcessor> orderedPostProcessors = new ArrayList<BeanDefinitionRegistryPostProcessor>();
for (String ppName : postProcessorNames) {
if (!processedBeans.contains(ppName) && beanFactory.isTypeMatch(ppName, Ordered.class)) {
orderedPostProcessors.add(beanFactory.getBean(ppName, BeanDefinitionRegistryPostProcessor.class));
processedBeans.add(ppName);
}
}
sortPostProcessors(beanFactory, orderedPostProcessors);
registryPostProcessors.addAll(orderedPostProcessors);
// 觸發接口方法
invokeBeanDefinitionRegistryPostProcessors(orderedPostProcessors, registry);
// Finally, invoke all other BeanDefinitionRegistryPostProcessors until no further ones appear.
boolean reiterate = true;
while (reiterate) {
reiterate = false;
postProcessorNames = beanFactory.getBeanNamesForType(BeanDefinitionRegistryPostProcessor.class, true, false);
for (String ppName : postProcessorNames) {
if (!processedBeans.contains(ppName)) {
BeanDefinitionRegistryPostProcessor pp = beanFactory.getBean(ppName, BeanDefinitionRegistryPostProcessor.class);
registryPostProcessors.add(pp);
processedBeans.add(ppName);
// 觸發接口方法
pp.postProcessBeanDefinitionRegistry(registry);
reiterate = true;
}
}
}
// Now, invoke the postProcessBeanFactory callback of all processors handled so far.
invokeBeanFactoryPostProcessors(registryPostProcessors, beanFactory);
invokeBeanFactoryPostProcessors(regularPostProcessors, beanFactory);
}
else {
// Invoke factory processors registered with the context instance.
invokeBeanFactoryPostProcessors(beanFactoryPostProcessors, beanFactory);
}
// Do not initialize FactoryBeans here: We need to leave all regular beans
// uninitialized to let the bean factory post-processors apply to them!
// BeanFactoryPostProcessor核心接口，從bean工廠獲取實現了BeanFactoryPostProcessor接口的beanDefinition集合
String[] postProcessorNames =
beanFactory.getBeanNamesForType(BeanFactoryPostProcessor.class, true, false);
// Separate between BeanFactoryPostProcessors that implement PriorityOrdered,
// Ordered, and the rest.
List<BeanFactoryPostProcessor> priorityOrderedPostProcessors = new ArrayList<BeanFactoryPostProcessor>();
List<String> orderedPostProcessorNames = new ArrayList<String>();
List<String> nonOrderedPostProcessorNames = new ArrayList<String>();
for (String ppName : postProcessorNames) {
if (processedBeans.contains(ppName)) {
// skip - already processed in first phase above
}
else if (beanFactory.isTypeMatch(ppName, PriorityOrdered.class)) {
priorityOrderedPostProcessors.add(beanFactory.getBean(ppName, BeanFactoryPostProcessor.class));
}
else if (beanFactory.isTypeMatch(ppName, Ordered.class)) {
orderedPostProcessorNames.add(ppName);
}
else {
nonOrderedPostProcessorNames.add(ppName);
}
}
// First, invoke the BeanFactoryPostProcessors that implement PriorityOrdered.
sortPostProcessors(beanFactory, priorityOrderedPostProcessors);
// 觸發接口方法
invokeBeanFactoryPostProcessors(priorityOrderedPostProcessors, beanFactory);
// Next, invoke the BeanFactoryPostProcessors that implement Ordered.
List<BeanFactoryPostProcessor> orderedPostProcessors = new ArrayList<BeanFactoryPostProcessor>();
for (String postProcessorName : orderedPostProcessorNames) {
orderedPostProcessors.add(beanFactory.getBean(postProcessorName, BeanFactoryPostProcessor.class));
}
sortPostProcessors(beanFactory, orderedPostProcessors);
// 觸發接口方法
invokeBeanFactoryPostProcessors(orderedPostProcessors, beanFactory);
// Finally, invoke all other BeanFactoryPostProcessors.
List<BeanFactoryPostProcessor> nonOrderedPostProcessors = new ArrayList<BeanFactoryPostProcessor>();
for (String postProcessorName : nonOrderedPostProcessorNames) {
nonOrderedPostProcessors.add(beanFactory.getBean(postProcessorName, BeanFactoryPostProcessor.class));
}
// 觸發接口方法
invokeBeanFactoryPostProcessors(nonOrderedPostProcessors, beanFactory);
// Clear cached merged bean definitions since the post-processors might have
// modified the original metadata, e.g. replacing placeholders in values...
beanFactory.clearMetadataCache();
}public interface BeanFactoryPostProcessor {
/**
* Modify the application context's internal bean factory after its standard
* initialization. All bean definitions will have been loaded, but no beans
* will have been instantiated yet. This allows for overriding or adding
* properties even to eager-initializing beans.
* @param beanFactory the bean factory used by the application context
* @throws org.springframework.beans.BeansException in case of errors
*/
void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) throws BeansException;
}public interface BeanDefinitionRegistryPostProcessor extends BeanFactoryPostProcessor {
/**
* Modify the application context's internal bean definition registry after its
* standard initialization. All regular bean definitions will have been loaded,
* but no beans will have been instantiated yet. This allows for adding further
* bean definitions before the next post-processing phase kicks in.
* @param registry the bean definition registry used by the application context
* @throws org.springframework.beans.BeansException in case of errors
*/
void postProcessBeanDefinitionRegistry(BeanDefinitionRegistry registry) throws BeansException;
}官方的解釋是：在實例化所有bean之前，可以添加額外的bean definition。
下面看下spring啓動過程中常用的後置處理器有哪些：
1. org.springframework.context.annotation.ConfigurationClassPostProcessor實現了BeanDefinitionRegistryPostProcessor接口
（該類主要處理通過註解的bean實例，後續會有通過註解定義bean的原理文章具體講解）
2. org.mybatis.spring.mapper.MapperScannerConfigurer實現了BeanDefinitionRegistryPostProcessor接口
（該類主要實現了mybatis掃描DAO接口轉換爲beanDefinition的過程，後續會有mybatis與spring集成原理文章具體講解）
3.org.springframework.beans.factory.config.PropertyPlaceholderConfigurer實現了BeanFactoryPostProcessor接口
（該類主要加載指定的properties文件並替換${...}佔位符，後續會有替換佔位符原理文章具體講解）
總結：
這篇文件主要說明了bean工廠後置處理器的作用及實現方式，spring容器利用這個功能做了大量的個性化集成如與mybatis的集成，後續文章會把常用的工廠處理器拿出來具體分析。剛開發了分佈式配置中心繫統，參數的動態植入就是利用了spring的bean工廠後置處理器原理。spring源碼解析的文章網上比比皆是，每個作者的出發點不同故而講解的重點也會不同，打算從個人的出發點並吸取別人的優點寫一系列spring源碼解析文章。歡迎各位拍磚，有問題請留言一起探討一起成長。