AutoConfigurationImportSelector到底怎么初始化
1. 前言
我们知道，在spring中，一般的实现ImportSelector接口，然后重写selectImports方法，就可以使用到spring的SPI技术，加载spring.factories中配置的org.springframework.boot.autoconfigure.EnableAutoConfiguration.EnableAutoConfiguration的类。
抱着测试的心态，给@SpringBootApplication的注解上实现的SelectorAutoConfigurationImportSelector#selectImports打上断点测试，这一测试，不得了，心态崩了，debug没进去。Demo有问题？换上开发的项目，还是没有进debug，难道是大家说的有问题？不行，我这暴脾气忍不了，要一探究竟。
Note: 本文基于SpringBoot：2.3.0.RELEASE。一般解析写在代码的注释上
2. 序列图
先摆上debug的时序图，方法返回没画（太丑，me嫌弃，有个大概了解一下就行）
3. 代码分析
代码太多，跳过了部分简单代码，然后挑关键点说。
- 从这
SpringApplication#refresh当入口
protected void refresh(ConfigurableApplicationContext applicationContext) {
// 调用父类的refresh方法
applicationContext.refresh();
}
- 进入
org.springframework.context.support.AbstractApplicationContext#refresh方法
public void refresh() throws BeansException, IllegalStateException {
synchronized (this.startupShutdownMonitor) {
// Invoke factory processors registered as beans in the context.
// 调用BeanFactory前置处理器
invokeBeanFactoryPostProcessors(beanFactory);
}
- 进入
org.springframework.context.support.PostProcessorRegistrationDelegate#invokeBeanFactoryPostProcessors，这有关键点，就是生成的postProcessorNames
// First, invoke the BeanDefinitionRegistryPostProcessors that implement PriorityOrdered.
//
String[] postProcessorNames =
beanFactory.getBeanNamesForType(BeanDefinitionRegistryPostProcessor.class, true, false);
for (String ppName : postProcessorNames) {
if (beanFactory.isTypeMatch(ppName, PriorityOrdered.class)) {
currentRegistryProcessors.add(beanFactory.getBean(ppName, BeanDefinitionRegistryPostProcessor.class));
processedBeans.add(ppName);
}
}
...
// 调用BeanDefinitionRegistry前置处理器
invokeBeanDefinitionRegistryPostProcessors(currentRegistryProcessors, registry);
Debug图片
我们通过Debug可以看出，currentRegistryProcessors中放的是ConfigurationClassPostProcessor的Bean对象，接着就调用了invokeBeanDefinitionRegistryPostProcessors方法并传入ConfigurationClassPostProcessor。
- 接着进入
org.springframework.context.annotation.ConfigurationClassPostProcessor#processConfigBeanDefinitions方法
public void processConfigBeanDefinitions(BeanDefinitionRegistry registry) {
...
// 从BeanDefinition中找出带有Configuration.class的，自己Debug可以进入if的两个方法中查看
for (String beanName : candidateNames) {
BeanDefinition beanDef = registry.getBeanDefinition(beanName);
if (beanDef.getAttribute(ConfigurationClassUtils.CONFIGURATION_CLASS_ATTRIBUTE) != null) {
if (logger.isDebugEnabled()) {
logger.debug("Bean definition has already been processed as a configuration class: " + beanDef);
}
}
else if (ConfigurationClassUtils.checkConfigurationClassCandidate(beanDef, this.metadataReaderFactory)) {
configCandidates.add(new BeanDefinitionHolder(beanDef, beanName));
}
}
...
// 传入候选人
parser.parse(candidates);
...
}
Debug图片
通过图片，可以看出，candidates只有一个，那就是启动类SpringDemoApplication（测试项目的启动类）.
- 进入
org.springframework.context.annotation.ConfigurationClassParser#parse()方法，开始解析启动类。
public void parse(Set<BeanDefinitionHolder> configCandidates) {
for (BeanDefinitionHolder holder : configCandidates) {
BeanDefinition bd = holder.getBeanDefinition();
try {
if (bd instanceof AnnotatedBeanDefinition) {
// 进入此方法了。 这我是Debug进去的，没有探究启动类在被解析成BeanDefinition的时候，被解析成
// AnnotatedBeanDefinition， 有兴趣的同学自己Debug追究一下
parse(((AnnotatedBeanDefinition) bd).getMetadata(), holder.getBeanName());
}
}
...
// 这个有用，所以我留在这了，关键点。
this.deferredImportSelectorHandler.process();
}
- 接下来进入
org.springframework.context.annotation.ConfigurationClassParser#processConfigurationClass方法。
protected void processConfigurationClass(ConfigurationClass configClass, Predicate<String> filter) throws IOException {
...
// Recursively process the configuration class and its superclass hierarchy.
SourceClass sourceClass = asSourceClass(configClass, filter);
do {
// 我们可以通过注解，看出这个是循环调用,找到configClass 自己的configuration注解或继承的注解中包含configuration的
// 不用多纠结，我们直接找到
sourceClass = doProcessConfigurationClass(configClass, sourceClass, filter);
}
while (sourceClass != null);
}
- 进入
org.springframework.context.annotation.ConfigurationClassParser#doProcessConfigurationClass方法，直接分析代码.
protected final SourceClass doProcessConfigurationClass(
ConfigurationClass configClass, SourceClass sourceClass, Predicate<String> filter)
throws IOException {
if (configClass.getMetadata().isAnnotated(Component.class.getName())) {
// Recursively process any member (nested) classes first
// 在这递归的，会回到上一步代码中
processMemberClasses(configClass, sourceClass, filter);
}
// Process any @Import annotations
// 别的代码不看，就这个名字，我们也知道这个类是干嘛的了吧！
processImports(configClass, sourceClass, getImports(sourceClass), filter, true);
}
getImports(sourceClass)这个方法是递归调用，找到注解Import中的值。 放个Debug图给大家瞅一下。CustomizedImportSelector是我自己测试的，
/**
* <br>自定义importSelector</br>
*
* @author fattyca1
*/
public class CustomizedImportSelector implements ImportSelector {
@Override
public String[] selectImports(AnnotationMetadata importingClassMetadata) {
return new String[]{"com.spring.demo.config.MyConfig"};
}
}
- 进入
org.springframework.context.annotation.ConfigurationClassParser#processImports方法，核心来了，就是问题的关键，到底是怎么使用SpringSPI的
private void processImports(ConfigurationClass configClass, SourceClass currentSourceClass,
Collection<SourceClass> importCandidates, Predicate<String> exclusionFilter,
boolean checkForCircularImports) {
...
if (checkForCircularImports && isChainedImportOnStack(configClass)) {
this.problemReporter.error(new CircularImportProblem(configClass, this.importStack));
}
else {
this.importStack.push(configClass);
try {
for (SourceClass candidate : importCandidates) {
if (candidate.isAssignable(ImportSelector.class)) {
// Candidate class is an ImportSelector -> delegate to it to determine imports
Class<?> candidateClass = candidate.loadClass();
ImportSelector selector = ParserStrategyUtils.instantiateClass(candidateClass, ImportSelector.class,
this.environment, this.resourceLoader, this.registry);
Predicate<String> selectorFilter = selector.getExclusionFilter();
if (selector instanceof DeferredImportSelector) {
// 因为AutoConfigurationImportSelector继承了DeferredImportSelector，所以会进入这个方法，放到
// 列表里处理，直接放到一个List中。
this.deferredImportSelectorHandler.handle(configClass, (DeferredImportSelector) selector);
}
}
}
...
}
}
}
org.springframework.context.annotation.ConfigurationClassParser.DeferredImportSelectorHandler#handle方法
public void handle(ConfigurationClass configClass, DeferredImportSelector importSelector) {
DeferredImportSelectorHolder holder = new DeferredImportSelectorHolder(configClass, importSelector);
if (this.deferredImportSelectors == null) {
DeferredImportSelectorGroupingHandler handler = new DeferredImportSelectorGroupingHandler();
handler.register(holder);
handler.processGroupImports();
}
else {
// deferredImportSelectors 是一个ArrayList，在类部类中被初始化，所以走的此方法
this.deferredImportSelectors.add(holder);
}
}
自此，我们分析完AutoConfigurationImportSelector在第一遍解析完后，被放在哪，那接下来就是如何解析了。激动人心的时刻来了。那就是在ConfigurationClassParser#parse()中执行的代码了this.deferredImportSelectorHandler.process();
org.springframework.context.annotation.ConfigurationClassParser.DeferredImportSelectorHandler#process代码
public void process() {
List<DeferredImportSelectorHolder> deferredImports = this.deferredImportSelectors;
this.deferredImportSelectors = null;
try {
if (deferredImports != null) {
DeferredImportSelectorGroupingHandler handler = new DeferredImportSelectorGroupingHandler();
deferredImports.sort(DEFERRED_IMPORT_COMPARATOR);
// 把list中的DeferredImportSelectorHolder注册到DeferredImportSelectorGroupingHandler
// 这个register方法会对DeferredImportSelectorHolder进行封装
deferredImports.forEach(handler::register);
handler.processGroupImports();
}
}
finally {
this.deferredImportSelectors = new ArrayList<>();
}
}
org.springframework.context.annotation.ConfigurationClassParser.DeferredImportSelectorGroupingHandler#register代码
public void register(DeferredImportSelectorHolder deferredImport) {
// AutoConfigurationImportSelector返回的是AutoConfigurationGroup.class，代码中已写死
Class<? extends Group> group = deferredImport.getImportSelector().getImportGroup();
// 封装成 DeferredImportSelector.Group 对象，并放到了groupings中，groupings是LinkedHashMap
// Group对象是用AutoConfigurationGroup.class生成
DeferredImportSelectorGrouping grouping = this.groupings.computeIfAbsent(
(group != null ? group : deferredImport),
key -> new DeferredImportSelectorGrouping(createGroup(group)));
grouping.add(deferredImport);
this.configurationClasses.put(deferredImport.getConfigurationClass().getMetadata(),
deferredImport.getConfigurationClass());
}
org.springframework.context.annotation.ConfigurationClassParser.DeferredImportSelectorGroupingHandler#processGroupImports方法，SpringSPI的调用点
public void processGroupImports() {
for (DeferredImportSelectorGrouping grouping : this.groupings.values()) {
Predicate<String> exclusionFilter = grouping.getCandidateFilter();
//遍历放入到grouping中的group，并执行getImports()方法，此方法就是SPI调用点！！！！
grouping.getImports().forEach(entry -> {
ConfigurationClass configurationClass = this.configurationClasses.get(entry.getMetadata());
try {
processImports(configurationClass, asSourceClass(configurationClass, exclusionFilter),
Collections.singleton(asSourceClass(entry.getImportClassName(), exclusionFilter)),
exclusionFilter, false);
}
catch (BeanDefinitionStoreException ex) {
throw ex;
}
catch (Throwable ex) {
throw new BeanDefinitionStoreException(
"Failed to process import candidates for configuration class [" +
configurationClass.getMetadata().getClassName() + "]", ex);
}
});
}
}
org.springframework.context.annotation.ConfigurationClassParser.DeferredImportSelectorGrouping#getImports
public Iterable<Group.Entry> getImports() {
for (DeferredImportSelectorHolder deferredImport : this.deferredImports) {
// 调用group的process方法, 也就是上面分析，AutoConfigurationGroup.class类的process方法
this.group.process(deferredImport.getConfigurationClass().getMetadata(),
deferredImport.getImportSelector());
}
return this.group.selectImports();
}
org.springframework.boot.autoconfigure.AutoConfigurationImportSelector.AutoConfigurationGroup#process方法
public void process(AnnotationMetadata annotationMetadata, DeferredImportSelector deferredImportSelector) {
Assert.state(deferredImportSelector instanceof AutoConfigurationImportSelector,
() -> String.format("Only %s implementations are supported, got %s",
AutoConfigurationImportSelector.class.getSimpleName(),
deferredImportSelector.getClass().getName()));
// getAutoConfigurationEntry 熟悉的方法，SPI的具体执行逻辑
AutoConfigurationEntry autoConfigurationEntry = ((AutoConfigurationImportSelector) deferredImportSelector)
.getAutoConfigurationEntry(annotationMetadata);
this.autoConfigurationEntries.add(autoConfigurationEntry);
for (String importClassName : autoConfigurationEntry.getConfigurations()) {
this.entries.putIfAbsent(importClassName, annotationMetadata);
}
}
自此，我们的代码分析结束，发现AutoConfigurationImportSelector.class在SpringBoot启动中，并不是调用的selectImports方法，而是直接调用的getAutoConfigurationEntry方法
4. 总结
SpringBoot在启动中，AutoConfigurationImportSelector在被加载中，调用的不是selectImports方法， 而是直接被调用了getAutoConfigurationEntry方法。 骚年，你可长点心吧！
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