Spring原始碼之BeanFactoryPostProcessor(後置處理器)。
有點水平的Spring開發人員想必都知道BeanFactoryPostProcessor也就是常說的後置管理器，這是Spirng生命週期中的一個介面，實現這個介面可以在beanFactory初始化前做一些事。
我們熟知的Spring和Mybatis的結合，正是因為Mybatis實現了BeanFactoryPostProcessor，它的重要性不言而喻，深入理解他對於切入Mybatis原始碼有著深刻的意義。
如下圖是簡單的應用：
還是先貼上refresh()的原始碼
@Override
public void refresh() throws BeansException, IllegalStateException {
synchronized (this.startupShutdownMonitor) {
// Prepare this context for refreshing.
//1、重新整理前的準備
prepareRefresh();
// Tell the subclass to refresh the internal bean factory.
//2、將會初始化 BeanFactory、載入 Bean、註冊 Bean
ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();
// Prepare the bean factory for use in this context.
//3、設定 BeanFactory 的類載入器，新增幾個 BeanPostProcessor，手動註冊幾個特殊的 bean
prepareBeanFactory(beanFactory);
try {
//4、模板方法
// Allows post-processing of the bean factory in context subclasses.
postProcessBeanFactory(beanFactory);
// Invoke factory processors registered as beans in the context.
//執行BeanFactory後置處理器
invokeBeanFactoryPostProcessors(beanFactory);
// 5、Register bean processors that intercept bean creation.
//註冊bean後置處理器
registerBeanPostProcessors(beanFactory);
// Initialize message source for this context.
//國際化
initMessageSource();
// Initialize event multicaster for this context.
initApplicationEventMulticaster();
// Initialize other special beans in specific context subclasses.
//6、模板方法--springboot實現了這個方法
onRefresh();
// Check for listener beans and register them.
//7、註冊監聽器
registerListeners();
// Instantiate all remaining (non-lazy-init) singletons.
//8、完成bean工廠的初始化**方法**********************************************
finishBeanFactoryInitialization(beanFactory);
//9、 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();
}
}
}
benafactory原始碼的處理位於第三個位置。
主要涉及到兩個方法postProcessBeanFactory(beanFactory);
和invokeBeanFactoryPostProcessors(beanFactory);
postProcessBeanFactory(beanFactory)點進去發現是一個空方法，具體的執行在invokeBeanFactoryPostProcessors(beanFactory);中
我們在invokeBeanFactoryPostProcessors方法上打斷點一探究竟。第一次看的時候覺得這是什麼玩意，這麼長，耐住性子一步步的往下看。
方法雖長大概總結一下就是，判斷beanFactory型別，然後將註冊的BeanPostFactory放入、排好順序、執行。
invokeBeanFactoryPostProcessors 方法的內容其實比較少，大部分過程在註釋都已經寫清楚，這邊在稍微總結一下。
整個 invokeBeanFactoryPostProcessors 方法圍繞兩個介面，BeanDefinitionRegistryPostProcessor 和 BeanFactoryPostProcessor，其中 BeanDefinitionRegistryPostProcessor 繼承了 BeanFactoryPostProcessor 。
BeanDefinitionRegistryPostProcessor 主要用來在常規 BeanFactoryPostProcessor 檢測開始之前註冊其他 Bean 定義，說的簡單點，就是 BeanDefinitionRegistryPostProcessor 具有更高的優先順序，執行順序在 BeanFactoryPostProcessor 之前。
具體的過程看註釋吧
public static void invokeBeanFactoryPostProcessors(
ConfigurableListableBeanFactory beanFactory, List<BeanFactoryPostProcessor> beanFactoryPostProcessors) {
// Invoke BeanDefinitionRegistryPostProcessors first, if any.
Set<String> processedBeans = new HashSet<>();
// 1.判斷beanFactory是否為BeanDefinitionRegistry，beanFactory為DefaultListableBeanFactory,
// 而DefaultListableBeanFactory實現了BeanDefinitionRegistry介面，因此這邊為true
if (beanFactory instanceof BeanDefinitionRegistry) {
BeanDefinitionRegistry registry = (BeanDefinitionRegistry) beanFactory;
// 用於存放普通的BeanFactoryPostProcessor
List<BeanFactoryPostProcessor> regularPostProcessors = new ArrayList<>();
// 用於存放BeanDefinitionRegistryPostProcessor
List<BeanDefinitionRegistryPostProcessor> registryProcessors = new ArrayList<>();
// 2.首先處理入參中的beanFactoryPostProcessors
// 遍歷所有的beanFactoryPostProcessors, 將BeanDefinitionRegistryPostProcessor和普通BeanFactoryPostProcessor區分開
for (BeanFactoryPostProcessor postProcessor : beanFactoryPostProcessors) {
if (postProcessor instanceof BeanDefinitionRegistryPostProcessor) {
// 2.1 如果是BeanDefinitionRegistryPostProcessor
BeanDefinitionRegistryPostProcessor registryProcessor =
(BeanDefinitionRegistryPostProcessor) postProcessor;
// 2.1.1 直接執行BeanDefinitionRegistryPostProcessor介面的postProcessBeanDefinitionRegistry方法
registryProcessor.postProcessBeanDefinitionRegistry(registry);
// 2.1.2 新增到registryProcessors(用於最後執行postProcessBeanFactory方法)
registryProcessors.add(registryProcessor);
}
else {
// 2.2 否則，只是普通的BeanFactoryPostProcessor
// 2.2.1 新增到regularPostProcessors(用於最後執行postProcessBeanFactory方法)
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.
// 用於儲存本次要執行的BeanDefinitionRegistryPostProcessor
List<BeanDefinitionRegistryPostProcessor> currentRegistryProcessors = new ArrayList<>();
// First, invoke the BeanDefinitionRegistryPostProcessors that implement PriorityOrdered.
// 3.呼叫所有實現PriorityOrdered介面的BeanDefinitionRegistryPostProcessor實現類
// 3.1 找出所有實現BeanDefinitionRegistryPostProcessor介面的Bean的beanName
String[] postProcessorNames =
beanFactory.getBeanNamesForType(BeanDefinitionRegistryPostProcessor.class, true, false);
// 3.2 遍歷postProcessorNames
for (String ppName : postProcessorNames) {
// 3.3 校驗是否實現了PriorityOrdered介面
if (beanFactory.isTypeMatch(ppName, PriorityOrdered.class)) {
// 3.4 獲取ppName對應的bean例項, 新增到currentRegistryProcessors中,
currentRegistryProcessors.add(beanFactory.getBean(ppName, BeanDefinitionRegistryPostProcessor.class));
// 3.5 將要被執行的加入processedBeans，避免後續重複執行
processedBeans.add(ppName);
}
}
// 3.6 進行排序(根據是否實現PriorityOrdered、Ordered介面和order值來排序)
sortPostProcessors(currentRegistryProcessors, beanFactory);
// 3.7 新增到registryProcessors(用於最後執行postProcessBeanFactory方法)
registryProcessors.addAll(currentRegistryProcessors);
// 3.8 遍歷currentRegistryProcessors, 執行postProcessBeanDefinitionRegistry方法
invokeBeanDefinitionRegistryPostProcessors(currentRegistryProcessors, registry);
// 3.9 執行完畢後, 清空currentRegistryProcessors
currentRegistryProcessors.clear();
// Next, invoke the BeanDefinitionRegistryPostProcessors that implement Ordered.
// 4.呼叫所有實現了Ordered介面的BeanDefinitionRegistryPostProcessor實現類（過程跟上面的步驟3基本一樣）
// 4.1 找出所有實現BeanDefinitionRegistryPostProcessor介面的類, 這邊重複查詢是因為執行完上面的BeanDefinitionRegistryPostProcessor,
// 可能會新增了其他的BeanDefinitionRegistryPostProcessor, 因此需要重新查詢
postProcessorNames = beanFactory.getBeanNamesForType(BeanDefinitionRegistryPostProcessor.class, true, false);
for (String ppName : postProcessorNames) {
// 校驗是否實現了Ordered介面，並且還未執行過
if (!processedBeans.contains(ppName) && beanFactory.isTypeMatch(ppName, Ordered.class)) {
currentRegistryProcessors.add(beanFactory.getBean(ppName, BeanDefinitionRegistryPostProcessor.class));
processedBeans.add(ppName);
}
}
sortPostProcessors(currentRegistryProcessors, beanFactory);
registryProcessors.addAll(currentRegistryProcessors);
// 4.2 遍歷currentRegistryProcessors, 執行postProcessBeanDefinitionRegistry方法
invokeBeanDefinitionRegistryPostProcessors(currentRegistryProcessors, registry);
currentRegistryProcessors.clear();
// Finally, invoke all other BeanDefinitionRegistryPostProcessors until no further ones appear.
// 5.最後, 呼叫所有剩下的BeanDefinitionRegistryPostProcessors
boolean reiterate = true;
while (reiterate) {
reiterate = false;
// 5.1 找出所有實現BeanDefinitionRegistryPostProcessor介面的類
postProcessorNames = beanFactory.getBeanNamesForType(BeanDefinitionRegistryPostProcessor.class, true, false);
for (String ppName : postProcessorNames) {
// 5.2 跳過已經執行過的
if (!processedBeans.contains(ppName)) {
currentRegistryProcessors.add(beanFactory.getBean(ppName, BeanDefinitionRegistryPostProcessor.class));
processedBeans.add(ppName);
// 5.3 如果有BeanDefinitionRegistryPostProcessor被執行, 則有可能會產生新的BeanDefinitionRegistryPostProcessor,
// 因此這邊將reiterate賦值為true, 代表需要再迴圈查詢一次
reiterate = true;
}
}
sortPostProcessors(currentRegistryProcessors, beanFactory);
registryProcessors.addAll(currentRegistryProcessors);
// 5.4 遍歷currentRegistryProcessors, 執行postProcessBeanDefinitionRegistry方法
invokeBeanDefinitionRegistryPostProcessors(currentRegistryProcessors, registry);
currentRegistryProcessors.clear();
}
// Now, invoke the postProcessBeanFactory callback of all processors handled so far.
// 6.呼叫所有BeanDefinitionRegistryPostProcessor的postProcessBeanFactory方法(BeanDefinitionRegistryPostProcessor繼承自BeanFactoryPostProcessor)
invokeBeanFactoryPostProcessors(registryProcessors, beanFactory);
// 7.最後, 呼叫入參beanFactoryPostProcessors中的普通BeanFactoryPostProcessor的postProcessBeanFactory方法
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!
// 到這裡 , 入參beanFactoryPostProcessors和容器中的所有BeanDefinitionRegistryPostProcessor已經全部處理完畢,
// 下面開始處理容器中的所有BeanFactoryPostProcessor
// Do not initialize FactoryBeans here: We need to leave all regular beans
// uninitialized to let the bean factory post-processors apply to them!
// 8.找出所有實現BeanFactoryPostProcessor介面的類
String[] postProcessorNames =
beanFactory.getBeanNamesForType(BeanFactoryPostProcessor.class, true, false);
// Separate between BeanFactoryPostProcessors that implement PriorityOrdered,
// Ordered, and the rest.
// 用於存放實現了PriorityOrdered介面的BeanFactoryPostProcessor
List<BeanFactoryPostProcessor> priorityOrderedPostProcessors = new ArrayList<>();
// 用於存放實現了Ordered介面的BeanFactoryPostProcessor的beanName
List<String> orderedPostProcessorNames = new ArrayList<>();
// 用於存放普通BeanFactoryPostProcessor的beanName
List<String> nonOrderedPostProcessorNames = new ArrayList<>();
// 8.1 遍歷postProcessorNames, 將BeanFactoryPostProcessor按實現PriorityOrdered、實現Ordered介面、普通三種區分開
for (String ppName : postProcessorNames) {
// 8.2 跳過已經執行過的
if (processedBeans.contains(ppName)) {
// skip - already processed in first phase above
}
else if (beanFactory.isTypeMatch(ppName, PriorityOrdered.class)) {
// 8.3 新增實現了PriorityOrdered介面的BeanFactoryPostProcessor
priorityOrderedPostProcessors.add(beanFactory.getBean(ppName, BeanFactoryPostProcessor.class));
}
else if (beanFactory.isTypeMatch(ppName, Ordered.class)) {
// 8.4 新增實現了Ordered介面的BeanFactoryPostProcessor的beanName
orderedPostProcessorNames.add(ppName);
}
else {
// 8.5 新增剩下的普通BeanFactoryPostProcessor的beanName
nonOrderedPostProcessorNames.add(ppName);
}
}
// First, invoke the BeanFactoryPostProcessors that implement PriorityOrdered.
// 9.呼叫所有實現PriorityOrdered介面的BeanFactoryPostProcessor
// 9.1 對priorityOrderedPostProcessors排序
sortPostProcessors(priorityOrderedPostProcessors, beanFactory);
// 9.2 遍歷priorityOrderedPostProcessors, 執行postProcessBeanFactory方法
invokeBeanFactoryPostProcessors(priorityOrderedPostProcessors, beanFactory);
// Next, invoke the BeanFactoryPostProcessors that implement Ordered.
// 10.呼叫所有實現Ordered介面的BeanFactoryPostProcessor
List<BeanFactoryPostProcessor> orderedPostProcessors = new ArrayList<>();
for (String postProcessorName : orderedPostProcessorNames) {
// 10.1 獲取postProcessorName對應的bean例項, 新增到orderedPostProcessors, 準備執行
orderedPostProcessors.add(beanFactory.getBean(postProcessorName, BeanFactoryPostProcessor.class));
}
// 10.2 對orderedPostProcessors排序
sortPostProcessors(orderedPostProcessors, beanFactory);
// 10.3 遍歷orderedPostProcessors, 執行postProcessBeanFactory方法
invokeBeanFactoryPostProcessors(orderedPostProcessors, beanFactory);
// Finally, invoke all other BeanFactoryPostProcessors.
// 11.呼叫所有剩下的BeanFactoryPostProcessor
List<BeanFactoryPostProcessor> nonOrderedPostProcessors = new ArrayList<>();
for (String postProcessorName : nonOrderedPostProcessorNames) {
// 11.1 獲取postProcessorName對應的bean例項, 新增到nonOrderedPostProcessors, 準備執行
nonOrderedPostProcessors.add(beanFactory.getBean(postProcessorName, BeanFactoryPostProcessor.class));
}
// 11.2 遍歷nonOrderedPostProcessors, 執行postProcessBeanFactory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...
// 12.清除後設資料快取（mergedBeanDefinitions、allBeanNamesByType、singletonBeanNamesByType），
// 因為後處理器可能已經修改了原始後設資料，例如， 替換值中的佔位符...
beanFactory.clearMetadataCache();
}