谈谈Spring的IoC之注解扫描

Posted lijianming180

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问题

??IoC是Inversion of Control的缩写，翻译过来即“控制反转”。IoC可以说是Spring的灵魂，想要读懂Spring，必先读懂IoC。不过有时候硬着头皮直接看源码，就像雾里看花，并不能一窥真谛。想要理解Spring的IoC，不如反过来思考一下，如果我们自己去实现IoC，需要考虑什么？遵循依赖大于配置的原则，我们这里只讨论注解式的控制反转容器，XML配置的方式暂不考虑。由此我想到下面几个问题：

  1. 注解扫描
  2. 实例化
  3. 循环依赖
  4. 泛型注入

本文的重点，就是围绕这几个问题，从spring的源码入手，解答心中的疑惑。

注解扫描

??我们从一个ApplicationContext示例开始启动Spring：

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 * 这是IoC的测试类
 */
public class  {
    public static void main(String[] args) {
        AnnotationConfigApplicationContext context = new AnnotationConfigApplicationContext(IoCTest.class);
        MyService service = context.getBean(MyService.class);
        System.out.println(service.getService());
    }
}

 * 测试注入Bean的接口
 */
public interface MyService {
    String getService();
}


 * 测试的Bean
 */

public class ServiceA implements MyService {
    public String getService() {
        return "serviceA";
    }
}

运行IoCTest的main，发现报错NoSuchBeanDefinitionException。因为IoCTest少了一个注解扫描的环节，我们加上一个@ComponentScan就可以运行并打印了：

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 * 这是IoC的测试类
 */
@ComponentScan
public class  {
    public static void main(String[] args) {
        AnnotationConfigApplicationContext context = new AnnotationConfigApplicationContext(IoCTest.class);
        MyService service = context.getBean(MyService.class);
        System.out.println(service.getService());
    }
}

我们直接由AnnotationConfigApplicationContext入手：

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public AnnotationConfigApplicationContext(Class<?>... annotatedClasses) {
   this();
   register(annotatedClasses);
   refresh();
}

register()这里是注册IoCTest，我们主要看一下refresh()，这里refresh()方法由AbstractApplicationContext实现：

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public void refresh() throws BeansException, IllegalStateException {
synchronized (this.startupShutdownMonitor) {
	// Prepare this context for refreshing.
	prepareRefresh();

	// Tell the subclass to refresh the internal bean factory.
	ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();

	// Prepare the bean factory for use in this context.
	prepareBeanFactory(beanFactory);

	try {
		// Allows post-processing of the bean factory in context subclasses.
		postProcessBeanFactory(beanFactory);

		// Invoke factory processors registered as beans in the context.
		invokeBeanFactoryPostProcessors(beanFactory);

		// Register bean processors that intercept bean creation.
		registerBeanPostProcessors(beanFactory);

		// Initialize message source for this context.
		initMessageSource();

		// Initialize event multicaster for this context.
		initApplicationEventMulticaster();

		// Initialize other special beans in specific context subclasses.
		onRefresh();

		// Check for listener beans and register them.
		registerListeners();

		// Instantiate all remaining (non-lazy-init) singletons.
		finishBeanFactoryInitialization(beanFactory);

		// 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();
	}
}
}

refresh()有一堆方法，不过还好Spring每一个方法的注释都很清晰。由于我们的重点在于分析注解扫描，所以我们主要看一下invokeBeanFactoryPostProcessors(beanFactory)：

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protected void invokeBeanFactoryPostProcessors(ConfigurableListableBeanFactory beanFactory) {
	PostProcessorRegistrationDelegate.invokeBeanFactoryPostProcessors(beanFactory, getBeanFactoryPostProcessors());

	// Detect a LoadTimeWeaver and prepare for weaving, if found in the meantime
	// (e.g. through an @Bean method registered by ConfigurationClassPostProcessor)
	if (beanFactory.getTempClassLoader() == null && beanFactory.containsBean(LOAD_TIME_WEAVER_BEAN_NAME)) {
		beanFactory.addBeanPostProcessor(new LoadTimeWeaverAwareProcessor(beanFactory));
		beanFactory.setTempClassLoader(new ContextTypeMatchClassLoader(beanFactory.getBeanClassLoader()));
	}
}

调用PostProcessorRegistrationDelegate执行BeanFactory的PostProcessors：

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public static void invokeBeanFactoryPostProcessors(
	ConfigurableListableBeanFactory beanFactory, List<BeanFactoryPostProcessor> beanFactoryPostProcessors) {
	Set<String> processedBeans = new HashSet<>();
    if (beanFactory instanceof BeanDefinitionRegistry) {
    	// ...上面省略了一部分代码...
    	List<BeanDefinitionRegistryPostProcessor> currentRegistryProcessors = new ArrayList<>();
    	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);
    		}
    	}
    	sortPostProcessors(currentRegistryProcessors, beanFactory);
    	registryProcessors.addAll(currentRegistryProcessors);
    	invokeBeanDefinitionRegistryPostProcessors(currentRegistryProcessors, registry);
    	// ...省略下面一部分代码...
   }
   // ...限于篇幅省略下面的代码...
}

从BeanFactory中获取BeanDefinitionRegistryPostProcessor的实例，而BeanDefinitionRegistryPostProcessor是一个接口，他只有一个实现ConfigurationClassPostProcessor，即这里currentRegistryProcessors中存放了一个ConfigurationClassPostProcessor的实例对象，看一下invokeBeanDefinitionRegistryPostProcessors方法：

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* Invoke the given BeanDefinitionRegistryPostProcessor beans.
*/
private static void invokeBeanDefinitionRegistryPostProcessors(
Collection<? extends BeanDefinitionRegistryPostProcessor> postProcessors, BeanDefinitionRegistry registry) {
  for (BeanDefinitionRegistryPostProcessor postProcessor : postProcessors) {
  postProcessor.postProcessBeanDefinitionRegistry(registry);
  }
}

只做了一件事，调用BeanDefinitionRegistryPostProcessor的postProcessBeanDefinitionRegistry(BeanDefinitionRegistry registry)方法，现在我们来看一下ConfigurationClassPostProcessor的实现：

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 * Derive further bean definitions from the configuration classes in the registry.
 */
@Override
public void postProcessBeanDefinitionRegistry(BeanDefinitionRegistry registry) {
	int registryId = System.identityHashCode(registry);
	if (this.registriesPostProcessed.contains(registryId)) {
		throw new IllegalStateException(
				"postProcessBeanDefinitionRegistry already called on this post-processor against " + registry);
	}
	if (this.factoriesPostProcessed.contains(registryId)) {
		throw new IllegalStateException(
				"postProcessBeanFactory already called on this post-processor against " + registry);
	}
	this.registriesPostProcessed.add(registryId);

	processConfigBeanDefinitions(registry);
}

继续看这里的processConfigBeanDefinitions(BeanDefinitionRegistry registry)：

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public void processConfigBeanDefinitions(BeanDefinitionRegistry registry) {
	// ...省略上面一部分代码...
	// Parse each @Configuration class
	ConfigurationClassParser parser = new ConfigurationClassParser(
			this.metadataReaderFactory, this.problemReporter, this.environment,
			this.resourceLoader, this.componentScanBeanNameGenerator, registry);
	// 这里的configCandidates是BeanDefinitionHolder集合
	Set<BeanDefinitionHolder> candidates = new LinkedHashSet<>(configCandidates);
	Set<ConfigurationClass> alreadyParsed = new HashSet<>(configCandidates.size());
	do {
		parser.parse(candidates);
		parser.validate();

		Set<ConfigurationClass> configClasses = new LinkedHashSet<>(parser.getConfigurationClasses());
		configClasses.removeAll(alreadyParsed);

		// Read the model and create bean definitions based on its content
		if (this.reader == null) {
			this.reader = new ConfigurationClassBeanDefinitionReader(
					registry, this.sourceExtractor, this.resourceLoader, this.environment,
					this.importBeanNameGenerator, parser.getImportRegistry());
		}
		this.reader.loadBeanDefinitions(configClasses);
		alreadyParsed.addAll(configClasses);

		candidates.clear();
		if (registry.getBeanDefinitionCount() > candidateNames.length) {
			String[] newCandidateNames = registry.getBeanDefinitionNames();
			Set<String> oldCandidateNames = new HashSet<>(Arrays.asList(candidateNames));
			Set<String> alreadyParsedClasses = new HashSet<>();
			for (ConfigurationClass configurationClass : alreadyParsed) {
				alreadyParsedClasses.add(configurationClass.getMetadata().getClassName());
			}
			for (String candidateName : newCandidateNames) {
				if (!oldCandidateNames.contains(candidateName)) {
					BeanDefinition bd = registry.getBeanDefinition(candidateName);
					if (ConfigurationClassUtils.checkConfigurationClassCandidate(bd, this.metadataReaderFactory) 大专栏  谈谈Spring的IoC之注解扫描 &&
							!alreadyParsedClasses.contains(bd.getBeanClassName())) {
						candidates.add(new BeanDefinitionHolder(bd, candidateName));
					}
				}
			}
			candidateNames = newCandidateNames;
		}
	}
	while (!candidates.isEmpty());
	// ...省略下面一部分代码...
}

candidates在这里其实是IoCTest的BeanDefinitionHolder集合。继续看ConfigurationClassParser的parse()方法：

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public void parse(Set<BeanDefinitionHolder> configCandidates) {
	for (BeanDefinitionHolder holder : configCandidates) {
		BeanDefinition bd = holder.getBeanDefinition();
		try {
			if (bd instanceof AnnotatedBeanDefinition) {
				parse(((AnnotatedBeanDefinition) bd).getMetadata(), holder.getBeanName());
			}
			else if (bd instanceof AbstractBeanDefinition && ((AbstractBeanDefinition) bd).hasBeanClass()) {
				parse(((AbstractBeanDefinition) bd).getBeanClass(), holder.getBeanName());
			}
			else {
				parse(bd.getBeanClassName(), holder.getBeanName());
			}
		}
		catch (BeanDefinitionStoreException ex) {
			throw ex;
		}
		catch (Throwable ex) {
			throw new BeanDefinitionStoreException(
					"Failed to parse configuration class [" + bd.getBeanClassName() + "]", ex);
		}
	}

	this.deferredImportSelectorHandler.process();
}

最终parse会调用：

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protected void processConfigurationClass(ConfigurationClass configClass) throws IOException {
	// ...省略上面一部分代码...
	// Recursively process the configuration class and its superclass hierarchy.
	SourceClass sourceClass = asSourceClass(configClass);
	do {
		sourceClass = doProcessConfigurationClass(configClass, sourceClass);
	}
	while (sourceClass != null);

	this.configurationClasses.put(configClass, configClass);
}

继续看doProcessConfigurationClass()：

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protected final SourceClass doProcessConfigurationClass(ConfigurationClass configClass, SourceClass sourceClass)
		throws IOException {
	// ...省略上部分代码...
	// Process any @ComponentScan annotations
	Set<AnnotationAttributes> componentScans = AnnotationConfigUtils.attributesForRepeatable(
			sourceClass.getMetadata(), ComponentScans.class, ComponentScan.class);
	if (!componentScans.isEmpty() &&
			!this.conditionEvaluator.shouldSkip(sourceClass.getMetadata(), ConfigurationPhase.REGISTER_BEAN)) {
		for (AnnotationAttributes componentScan : componentScans) {
			// The config class is annotated with @ComponentScan -> perform the scan immediately
			Set<BeanDefinitionHolder> scannedBeanDefinitions =
					this.componentScanParser.parse(componentScan, sourceClass.getMetadata().getClassName());
			// Check the set of scanned definitions for any further config classes and parse recursively if needed
			for (BeanDefinitionHolder holder : scannedBeanDefinitions) {
				BeanDefinition bdCand = holder.getBeanDefinition().getOriginatingBeanDefinition();
				if (bdCand == null) {
					bdCand = holder.getBeanDefinition();
				}
				if (ConfigurationClassUtils.checkConfigurationClassCandidate(bdCand, this.metadataReaderFactory)) {
					parse(bdCand.getBeanClassName(), holder.getBeanName());
				}
			}
		}
	}
	// ...省略下部分代码...
}

至此终于进入我们的正题，注解扫描的主角this.componentScanParser即ComponentScanAnnotationParser登场。继续看ComponentScanAnnotationParser的parse(AnnotationAttributes componentScan, final String declaringClass)：

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public Set<BeanDefinitionHolder> parse(AnnotationAttributes componentScan, final String declaringClass) {
	ClassPathBeanDefinitionScanner scanner = new ClassPathBeanDefinitionScanner(this.registry,
			componentScan.getBoolean("useDefaultFilters"), this.environment, this.resourceLoader);

	Class<? extends BeanNameGenerator> generatorClass = componentScan.getClass("nameGenerator");
	boolean useInheritedGenerator = (BeanNameGenerator.class == generatorClass);
	scanner.setBeanNameGenerator(useInheritedGenerator ? this.beanNameGenerator :
			BeanUtils.instantiateClass(generatorClass));

	ScopedProxyMode scopedProxyMode = componentScan.getEnum("scopedProxy");
	if (scopedProxyMode != ScopedProxyMode.DEFAULT) {
		scanner.setScopedProxyMode(scopedProxyMode);
	}
	else {
		Class<? extends ScopeMetadataResolver> resolverClass = componentScan.getClass("scopeResolver");
		scanner.setScopeMetadataResolver(BeanUtils.instantiateClass(resolverClass));
	}

	scanner.setResourcePattern(componentScan.getString("resourcePattern"));

	for (AnnotationAttributes filter : componentScan.getAnnotationArray("includeFilters")) {
		for (TypeFilter typeFilter : typeFiltersFor(filter)) {
			scanner.addIncludeFilter(typeFilter);
		}
	}
	for (AnnotationAttributes filter : componentScan.getAnnotationArray("excludeFilters")) {
		for (TypeFilter typeFilter : typeFiltersFor(filter)) {
			scanner.addExcludeFilter(typeFilter);
		}
	}

	boolean lazyInit = componentScan.getBoolean("lazyInit");
	if (lazyInit) {
		scanner.getBeanDefinitionDefaults().setLazyInit(true);
	}

	Set<String> basePackages = new LinkedHashSet<>();
	String[] basePackagesArray = componentScan.getStringArray("basePackages");
	for (String pkg : basePackagesArray) {
		String[] tokenized = StringUtils.tokenizeToStringArray(this.environment.resolvePlaceholders(pkg),
				ConfigurableApplicationContext.CONFIG_LOCATION_DELIMITERS);
		Collections.addAll(basePackages, tokenized);
	}
	for (Class<?> clazz : componentScan.getClassArray("basePackageClasses")) {
		basePackages.add(ClassUtils.getPackageName(clazz));
	}
	if (basePackages.isEmpty()) {
		basePackages.add(ClassUtils.getPackageName(declaringClass));
	}

	scanner.addExcludeFilter(new AbstractTypeHierarchyTraversingFilter(false, false) {
		@Override
		protected boolean matchClassName(String className) {
			return declaringClass.equals(className);
		}
	});
	return scanner.doScan(StringUtils.toStringArray(basePackages));
}

declaringClass在这个例子中的入参即spring.ioc.test.IoCTest，装配basePackages后使用ClassPathBeanDefinitionScanner的doScan(String… basePackages)进行扫描：

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protected Set<BeanDefinitionHolder> doScan(String... basePackages) {
		Assert.notEmpty(basePackages, "At least one base package must be specified");
		Set<BeanDefinitionHolder> beanDefinitions = new LinkedHashSet<>();
		for (String basePackage : basePackages) {
			Set<BeanDefinition> candidates = findCandidateComponents(basePackage);
			for (BeanDefinition candidate : candidates) {
				ScopeMetadata scopeMetadata = this.scopeMetadataResolver.resolveScopeMetadata(candidate);
				candidate.setScope(scopeMetadata.getScopeName());
				String beanName = this.beanNameGenerator.generateBeanName(candidate, this.registry);
				if (candidate instanceof AbstractBeanDefinition) {
					postProcessBeanDefinition((AbstractBeanDefinition) candidate, beanName);
				}
				if (candidate instanceof AnnotatedBeanDefinition) {
					AnnotationConfigUtils.processCommonDefinitionAnnotations((AnnotatedBeanDefinition) candidate);
				}
				if (checkCandidate(beanName, candidate)) {
					BeanDefinitionHolder definitionHolder = new BeanDefinitionHolder(candidate, beanName);
					definitionHolder =
							AnnotationConfigUtils.applyScopedProxyMode(scopeMetadata, definitionHolder, this.registry);
					beanDefinitions.add(definitionHolder);
					registerBeanDefinition(definitionHolder, this.registry);
				}
			}
		}
		return beanDefinitions;
	}

终于到了最后一步，findCandidateComponents(String basePackage)，来看下Spring如何根据basePackages来扫描组装BeanDefinition：

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public Set<BeanDefinition> findCandidateComponents(String basePackage) {
	if (this.componentsIndex != null && indexSupportsIncludeFilters()) {
		return addCandidateComponentsFromIndex(this.componentsIndex, basePackage);
	}
	else {
		return scanCandidateComponents(basePackage);
	}
}

在这个示例中，this.componentsIndex为null，进入scanCandidateComponents(String basePackage)：

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private Set<BeanDefinition> scanCandidateComponents(String basePackage) {
	Set<BeanDefinition> candidates = new LinkedHashSet<>();
	try {
		String packageSearchPath = ResourcePatternResolver.CLASSPATH_ALL_URL_PREFIX +
				resolveBasePackage(basePackage) + '/' + this.resourcePattern;
		Resource[] resources = getResourcePatternResolver().getResources(packageSearchPath);
		boolean traceEnabled = logger.isTraceEnabled();
		boolean debugEnabled = logger.isDebugEnabled();
		for (Resource resource : resources) {
			if (traceEnabled) {
				logger.trace("Scanning " + resource);
			}
			if (resource.isReadable()) {
				try {
					MetadataReader metadataReader = getMetadataReaderFactory().getMetadataReader(resource);
					if (isCandidateComponent(metadataReader)) {
						ScannedGenericBeanDefinition sbd = new ScannedGenericBeanDefinition(metadataReader);
						sbd.setResource(resource);
						sbd.setSource(resource);
						if (isCandidateComponent(sbd)) {
							if (debugEnabled) {
								logger.debug("Identified candidate component class: " + resource);
							}
							candidates.add(sbd);
						}
						else {
							if (debugEnabled) {
								logger.debug("Ignored because not a concrete top-level class: " + resource);
							}
						}
					}
					else {
						if (traceEnabled) {
							logger.trace("Ignored because not matching any filter: " + resource);
						}
					}
				}
				catch (Throwable ex) {
					throw new BeanDefinitionStoreException(
							"Failed to read candidate component class: " + resource, ex);
				}
			}
			else {
				if (traceEnabled) {
					logger.trace("Ignored because not readable: " + resource);
				}
			}
		}
	}
	catch (IOException ex) {
		throw new BeanDefinitionStoreException("I/O failure during classpath scanning", ex);
	}
	return candidates;
}

加载packageSearchPath（**classpath:spring/ioc/test/**/.class**）下所有的class文件，并通过isCandidateComponent(MetadataReader metadataReader)进行过滤，上面的测试例子中只有MyService符合条件加入candidates集合。

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