AutoConfigurationImportSelector 到底怎么初始化

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发布于: 2020 年 05 月 27 日
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。
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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。
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 Note: 本文基于SpringBoot：2.3.0.RELEASE。一般解析写在代码的注释上
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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）
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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。
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1. 从这SpringApplication#refresh当入口
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	protected void refresh(ConfigurableApplicationContext applicationContext) {
    // 调用父类的refresh方法
		applicationContext.refresh();
	}
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2. 进入 org.springframework.context.support.AbstractApplicationContext#refresh方法
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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);
	}
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3. 进入 org.springframework.context.support.PostProcessorRegistrationDelegate#invokeBeanFactoryPostProcessors，这有关键点，就是生成的postProcessorNames
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			// 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);
		
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Debug图片
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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。
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4. 接着进入org.springframework.context.annotation.ConfigurationClassPostProcessor#processConfigBeanDefinitions方法
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	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);
		
		...
	}
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Debug图片
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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）.
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5. 进入org.springframework.context.annotation.ConfigurationClassParser#parse()方法，开始解析启动类。
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	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();
	}
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6. 接下来进入org.springframework.context.annotation.ConfigurationClassParser#processConfigurationClass方法。
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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);
	}
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7. 进入org.springframework.context.annotation.ConfigurationClassParser#doProcessConfigurationClass方法，直接分析代码.
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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);
	}
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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，
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/**
 * <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"};
    }
}
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8. 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
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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);
						}
					}
        }
        ...
			}
		}
	}
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9. org.springframework.context.annotation.ConfigurationClassParser.DeferredImportSelectorHandler#handle方法
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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);
			}
		}
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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();
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10. org.springframework.context.annotation.ConfigurationClassParser.DeferredImportSelectorHandler#process代码
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	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<>();
			}
		}
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11. org.springframework.context.annotation.ConfigurationClassParser.DeferredImportSelectorGroupingHandler#register代码
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		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());
		}
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12. org.springframework.context.annotation.ConfigurationClassParser.DeferredImportSelectorGroupingHandler#processGroupImports方法，SpringSPI的调用点
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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);
					}
				});
			}
		}
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13.  org.springframework.context.annotation.ConfigurationClassParser.DeferredImportSelectorGrouping#getImports
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		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();
		}
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14. org.springframework.boot.autoconfigure.AutoConfigurationImportSelector.AutoConfigurationGroup#process方法
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		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);
			}
		}
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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
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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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感谢支持。推荐！！！ 课外文件，解忧星球
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