深入理解Spring IOC源码分析

Spring容器初始化

本文使用的是Spring 5.1.7版本

写在前面：我们看源码一般有3种方式。

• 第一种直接用class文件，IDEA会帮我们反编译成看得懂的java代码
• 第二种是用maven的download Sources
• 第三种是直接下载源码编译

如果随便看看，那么第二种就行了。如果想仔细研究，可以选第三种，这样可以给代码加注释，或者修改一些东西等等。

Spring源码构建

• 下载源码（github）
• 安装gradle
• 导入项目
• 编译工程（顺序：core->xom->context->beans->aspects->aop）
• 编译方法：工程->tasks->compileTestJava

• 编译完成后，新建一个测试module，用gradle的方式，build.gradle为：

group 'org.springframework'
version '5.1.21.BUILD-SNAPSHOT'

apply plugin: 'java'

sourceCompatibility = 1.8

repositories {
    mavenCentral()
}

dependencies {
    compile(project(":spring-context"))
    testCompile group: 'junit', name: 'junit', version: '4.12'
}

如果运行报错，需要设置IDEA。
需要将Preferences > Build, Execution, Deployment > Build Tools > Gradle > Runner里的 Delegate IDE build/run actions to gradle勾选上。

---

Spring IOC容器初始化的关键环节就在AbstractApplicationContext的refresh方法中。

public void refresh() throws BeansException, IllegalStateException {
		synchronized (this.startupShutdownMonitor) {
			//第一步：刷新前的预处理
			prepareRefresh();

			// 第二步：获取BeanFactory；加载BeanDefition并注册到BeanDefitionRegistry
			ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();

			// 第三步：BeanFactory的预准备工作
			prepareBeanFactory(beanFactory);

			try {
				// 第四步：BeanFactory准备工作完成之后进行的后置处理
				postProcessBeanFactory(beanFactory);

				// 第五步：实例化并调用实现了BeanFactoryPostProcessor接口的Bean
				invokeBeanFactoryPostProcessors(beanFactory);

				// 第六步：注册BeanPostProcessor，在创建bean的前后等执行。
				registerBeanPostProcessors(beanFactory);

				// 第七步：初始化MessageSource组件（做国际化功能；消息绑定，消息解析）
				initMessageSource();

				// 第八步：初始化事件派发器
				initApplicationEventMulticaster();

				//第九步：提供给子类重写，容器刷新的适合可以自定义逻辑
				onRefresh();

				// 第十步：注册应用监听器。就是注册实现了ApplicationListener接口的监听器bean
				registerListeners();

				// 第十一步：初始化所有剩下的非懒加载的单例bean，
				//填充属性
				//初始化方法调用
				//调用BeanPostProcessor对bean进行后置处理
				finishBeanFactoryInitialization(beanFactory);

				// 第十二步：完成context的刷新。主要是调用LifecycleProcessor的onRefresh方法，并且发布ContextRefreshedEvent事件
				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();
			}
		}
	}

接下来我们分析第二步中的两个子流程：

2.1. 获取BeanFactory子流程

时序图如下：

2.2. BeanDefinition加载解析及注册⼦流程

BeanDefinition加载流程入口在AbstractRefreshableApplicationContext#refreshBeanFactory ⽅法中，会调用到loadBeanDefinitions方法，这个方法最终会由XmlBeanDefinitionReader的loadBeanDefinitions方法来完成。然后会调用它自己的doLoadBeanDefinitions方法。

protected int doLoadBeanDefinitions(InputSource inputSource, Resource resource)
			throws BeanDefinitionStoreException {

		try {
			//读取xml信息，将xml信息保存到Document中
			Document doc = doLoadDocument(inputSource, resource);
			//解析doc，封装到BeanDefinition对象并进行注册
			int count = registerBeanDefinitions(doc, resource);
			if (logger.isDebugEnabled()) {
				logger.debug("Loaded " + count + " bean definitions from " + resource);
			}
			return count;
		}
	//省略部分代码....

继续跟踪registerBeanDefinitions方法。

public int registerBeanDefinitions(Document doc, Resource resource) throws BeanDefinitionStoreException {
		BeanDefinitionDocumentReader documentReader = createBeanDefinitionDocumentReader();
		//获取已有的BeanDefinition的数量
		int countBefore = getRegistry().getBeanDefinitionCount();
		//注册BeanDefinitions
		documentReader.registerBeanDefinitions(doc, createReaderContext(resource));
		//返回新注册的BeanDefinition的数量
		return getRegistry().getBeanDefinitionCount() - countBefore;
	}

它又交由documentReader来执行registerBeanDefinitions方法。documentReader接收后调用本类的doRegisterBeanDefinitions来执行。这个方法里我们需要关注parseBeanDefinitions.preProcessXml和postProcessXml都是留给子类实现的。

protected void doRegisterBeanDefinitions(Element root) {
	    //省略部分代码...
		preProcessXml(root);
		parseBeanDefinitions(root, this.delegate);
		postProcessXml(root);

		this.delegate = parent;
	}

跟踪parseBeanDefinitions方法，可以看到由DefaultBeanDefinitionDocumentReader#parseDefaultElement在进行解析

private void parseDefaultElement(Element ele, BeanDefinitionParserDelegate delegate) {
		//import处理
		if (delegate.nodeNameEquals(ele, IMPORT_ELEMENT)) {
			importBeanDefinitionResource(ele);
		}
		//alias处理
		else if (delegate.nodeNameEquals(ele, ALIAS_ELEMENT)) {
			processAliasRegistration(ele);
		}
		//解析bean元素
		else if (delegate.nodeNameEquals(ele, BEAN_ELEMENT)) {
			processBeanDefinition(ele, delegate);
		}
		//嵌套bean处理
		else if (delegate.nodeNameEquals(ele, NESTED_BEANS_ELEMENT)) {
			// recurse
			doRegisterBeanDefinitions(ele);
		}
	}

最后跟踪processBeanDefinition的代码

protected void processBeanDefinition(Element ele, BeanDefinitionParserDelegate delegate) {
		//解析bean元素为BeanDefinition,但是此时使用BeanDefinitionHolder又包装了一层
		BeanDefinitionHolder bdHolder = delegate.parseBeanDefinitionElement(ele);
		if (bdHolder != null) {
			bdHolder = delegate.decorateBeanDefinitionIfRequired(ele, bdHolder);
			try {
				// 完成BeanDefinition的注册
				BeanDefinitionReaderUtils.registerBeanDefinition(bdHolder, getReaderContext().getRegistry());
			}
			catch (BeanDefinitionStoreException ex) {
				getReaderContext().error("Failed to register bean definition with name '" +
						bdHolder.getBeanName() + "'", ele, ex);
			}
			// Send registration event.
			getReaderContext().fireComponentRegistered(new BeanComponentDefinition(bdHolder));
		}
	}

到这里就完成了BeanDefinition的注册。我们发现，所谓的注册就是把 XML 中定义的Bean信息封装为BeanDefinition对象之后放⼊⼀个Map中。

this.beanDefinitionMap.put(beanName, beanDefinition);

简略的时序图如下：

接下来我们分析一下Bean创建的流程。Bean创建是在refresh主流程中的第十一步里面。

11.1 Bean创建子流程

我们进入finishBeanFactoryInitialization方法。

protected void finishBeanFactoryInitialization(ConfigurableListableBeanFactory beanFactory) {
		//省略部分代码...
		String[] weaverAwareNames = beanFactory.getBeanNamesForType(LoadTimeWeaverAware.class, false, false);
		for (String weaverAwareName : weaverAwareNames) {
			getBean(weaverAwareName);
		}

		// Stop using the temporary ClassLoader for type matching.
		beanFactory.setTempClassLoader(null);

		// Allow for caching all bean definition metadata, not expecting further changes.
		beanFactory.freezeConfiguration();

		// 实例化所有非懒加载的bean
		beanFactory.preInstantiateSingletons();
	}

跟踪preInstantiateSingletons方法

public void preInstantiateSingletons() throws BeansException {
		if (logger.isTraceEnabled()) {
			logger.trace("Pre-instantiating singletons in " + this);
		}

		// Iterate over a copy to allow for init methods which in turn register new bean definitions.
		// While this may not be part of the regular factory bootstrap, it does otherwise work fine.
		List<String> beanNames = new ArrayList<>(this.beanDefinitionNames);

		// 遍历所有的beanName
		for (String beanName : beanNames) {
			RootBeanDefinition bd = getMergedLocalBeanDefinition(beanName);
			if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) {
				//如果是工厂bean，即实现了FactoryBean接口
				if (isFactoryBean(beanName)) {
					Object bean = getBean(FACTORY_BEAN_PREFIX + beanName);
					if (bean instanceof FactoryBean) {
						FactoryBean<?> factory = (FactoryBean<?>) bean;
						boolean isEagerInit;
						if (System.getSecurityManager() != null && factory instanceof SmartFactoryBean) {
							isEagerInit = AccessController.doPrivileged(
									(PrivilegedAction<Boolean>) ((SmartFactoryBean<?>) factory)::isEagerInit,
									getAccessControlContext());
						}
						else {
							isEagerInit = (factory instanceof SmartFactoryBean &&
									((SmartFactoryBean<?>) factory).isEagerInit());
						}
						if (isEagerInit) {
							getBean(beanName);
						}
					}
				}
				else {
					//实例化当前bean
					getBean(beanName);
				}
			}
		}
    //省略...
	}

我们发现不管是工厂Bean，还是普通Bean都会调用getBean(beanName);方法。然后调用到doGetBean方法。

protected <T> T doGetBean(
			String name, @Nullable Class<T> requiredType, @Nullable Object[] args, boolean typeCheckOnly)
			throws BeansException {

		String beanName = transformedBeanName(name);
		Object bean;

		// 从缓存中拿
		Object sharedInstance = getSingleton(beanName);
		if (sharedInstance != null && args == null) {
			if (logger.isTraceEnabled()) {
				if (isSingletonCurrentlyInCreation(beanName)) {
					logger.trace("Returning eagerly cached instance of singleton bean '" + beanName +
							"' that is not fully initialized yet - a consequence of a circular reference");
				}
				else {
					logger.trace("Returning cached instance of singleton bean '" + beanName + "'");
				}
			}
			bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
		}

		else {
			//如果已经有创建过程中的同beanName的bean，抛异常
			if (isPrototypeCurrentlyInCreation(beanName)) {
				throw new BeanCurrentlyInCreationException(beanName);
			}

			// 从parentBeanFactory里找对应的bean
			BeanFactory parentBeanFactory = getParentBeanFactory();
			if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
				// Not found -> check parent.
				String nameToLookup = originalBeanName(name);
				if (parentBeanFactory instanceof AbstractBeanFactory) {
					return ((AbstractBeanFactory) parentBeanFactory).doGetBean(
							nameToLookup, requiredType, args, typeCheckOnly);
				}
				else if (args != null) {
					// Delegation to parent with explicit args.
					return (T) parentBeanFactory.getBean(nameToLookup, args);
				}
				else if (requiredType != null) {
					// No args -> delegate to standard getBean method.
					return parentBeanFactory.getBean(nameToLookup, requiredType);
				}
				else {
					return (T) parentBeanFactory.getBean(nameToLookup);
				}
			}

			if (!typeCheckOnly) {
				//打一个已创建的标记
				markBeanAsCreated(beanName);
			}

			try {
				RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
				checkMergedBeanDefinition(mbd, beanName, args);

				// 保证当前bean所依赖的bean的初始化
				String[] dependsOn = mbd.getDependsOn();
				if (dependsOn != null) {
					for (String dep : dependsOn) {
						if (isDependent(beanName, dep)) {
							throw new BeanCreationException(mbd.getResourceDescription(), beanName,
									"Circular depends-on relationship between '" + beanName + "' and '" + dep + "'");
						}
						registerDependentBean(dep, beanName);
						try {
							getBean(dep);
						}
						catch (NoSuchBeanDefinitionException ex) {
							throw new BeanCreationException(mbd.getResourceDescription(), beanName,
									"'" + beanName + "' depends on missing bean '" + dep + "'", ex);
						}
					}
				}

				//创建bean
				// 单例bean
				if (mbd.isSingleton()) {
					sharedInstance = getSingleton(beanName, () -> {
						try {
							//具体的创建bean逻辑
							return createBean(beanName, mbd, args);
						}
						catch (BeansException ex) {
							// Explicitly remove instance from singleton cache: It might have been put there
							// eagerly by the creation process, to allow for circular reference resolution.
							// Also remove any beans that received a temporary reference to the bean.
							destroySingleton(beanName);
							throw ex;
						}
					});
					bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
				}
				//多例bean
				else if (mbd.isPrototype()) {
					// It's a prototype -> create a new instance.
					Object prototypeInstance = null;
					try {
						beforePrototypeCreation(beanName);
						prototypeInstance = createBean(beanName, mbd, args);
					}
					finally {
						afterPrototypeCreation(beanName);
					}
					bean = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);
				}

				else {
					String scopeName = mbd.getScope();
					if (!StringUtils.hasLength(scopeName)) {
						throw new IllegalStateException("No scope name defined for bean ´" + beanName + "'");
					}
					Scope scope = this.scopes.get(scopeName);
					if (scope == null) {
						throw new IllegalStateException("No Scope registered for scope name '" + scopeName + "'");
					}
					try {
						Object scopedInstance = scope.get(beanName, () -> {
							beforePrototypeCreation(beanName);
							try {
								return createBean(beanName, mbd, args);
							}
							finally {
								afterPrototypeCreation(beanName);
							}
						});
						bean = getObjectForBeanInstance(scopedInstance, name, beanName, mbd);
					}
					catch (IllegalStateException ex) {
						throw new BeanCreationException(beanName,
								"Scope '" + scopeName + "' is not active for the current thread; consider " +
								"defining a scoped proxy for this bean if you intend to refer to it from a singleton",
								ex);
					}
				}
			}
			catch (BeansException ex) {
				cleanupAfterBeanCreationFailure(beanName);
				throw ex;
			}
		}

		// 检查所需类型是否与实际bean实例的类型匹配。
		if (requiredType != null && !requiredType.isInstance(bean)) {
			try {
				T convertedBean = getTypeConverter().convertIfNecessary(bean, requiredType);
				if (convertedBean == null) {
					throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
				}
				return convertedBean;
			}
			catch (TypeMismatchException ex) {
				if (logger.isTraceEnabled()) {
					logger.trace("Failed to convert bean '" + name + "' to required type '" +
							ClassUtils.getQualifiedName(requiredType) + "'", ex);
				}
				throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
			}
		}
		return (T) bean;
	}

可以看到经过一些判断和校验，会调createBean方法去创建bean，然后调用doCreateBean去完成创建。

protected Object doCreateBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)
			throws BeanCreationException {

		// Instantiate the bean.
		BeanWrapper instanceWrapper = null;
		if (mbd.isSingleton()) {
			instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
		}
		if (instanceWrapper == null) {
			//创建bean实例，但是尚未设置属性
			instanceWrapper = createBeanInstance(beanName, mbd, args);
		}
		Object bean = instanceWrapper.getWrappedInstance();
		Class<?> beanType = instanceWrapper.getWrappedClass();
		if (beanType != NullBean.class) {
			mbd.resolvedTargetType = beanType;
		}

		//允许后处理器修改合并的bean定义
		synchronized (mbd.postProcessingLock) {
			if (!mbd.postProcessed) {
				try {
					applyMergedBeanDefinitionPostProcessors(mbd, beanType, beanName);
				}
				catch (Throwable ex) {
					throw new BeanCreationException(mbd.getResourceDescription(), beanName,
							"Post-processing of merged bean definition failed", ex);
				}
				mbd.postProcessed = true;
			}
		}

		//急切地缓存单例，以便能够解析循环引用
		//即使由BeanFactoryAware等生命周期接口触发。
		boolean earlySingletonExposure = (mbd.isSingleton() && this.allowCircularReferences &&
				isSingletonCurrentlyInCreation(beanName));
		if (earlySingletonExposure) {
			if (logger.isTraceEnabled()) {
				logger.trace("Eagerly caching bean '" + beanName +
						"' to allow for resolving potential circular references");
			}
			addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
		}

		// Initialize the bean instance.
		Object exposedObject = bean;
		try {
			//设置属性
			populateBean(beanName, mbd, instanceWrapper);
			//应用初始化方法，调用BeanPostProcessor后置处理器
			exposedObject = initializeBean(beanName, exposedObject, mbd);
		}
		catch (Throwable ex) {
			if (ex instanceof BeanCreationException && beanName.equals(((BeanCreationException) ex).getBeanName())) {
				throw (BeanCreationException) ex;
			}
			else {
				throw new BeanCreationException(
						mbd.getResourceDescription(), beanName, "Initialization of bean failed", ex);
			}
		}

		if (earlySingletonExposure) {
			Object earlySingletonReference = getSingleton(beanName, false);
			if (earlySingletonReference != null) {
				if (exposedObject == bean) {
					exposedObject = earlySingletonReference;
				}
				else if (!this.allowRawInjectionDespiteWrapping && hasDependentBean(beanName)) {
					String[] dependentBeans = getDependentBeans(beanName);
					Set<String> actualDependentBeans = new LinkedHashSet<>(dependentBeans.length);
					for (String dependentBean : dependentBeans) {
						if (!removeSingletonIfCreatedForTypeCheckOnly(dependentBean)) {
							actualDependentBeans.add(dependentBean);
						}
					}
					if (!actualDependentBeans.isEmpty()) {
						throw new BeanCurrentlyInCreationException(beanName,
								"Bean with name '" + beanName + "' has been injected into other beans [" +
								StringUtils.collectionToCommaDelimitedString(actualDependentBeans) +
								"] in its raw version as part of a circular reference, but has eventually been " +
								"wrapped. This means that said other beans do not use the final version of the " +
								"bean. This is often the result of over-eager type matching - consider using " +
								"'getBeanNamesForType' with the 'allowEagerInit' flag turned off, for example.");
					}
				}
			}
		}

		// Register bean as disposable.
		try {
			registerDisposableBeanIfNecessary(beanName, bean, mbd);
		}
		catch (BeanDefinitionValidationException ex) {
			throw new BeanCreationException(
					mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex);
		}

		return exposedObject;
	}

在上述的方法中会先实例化一个bean，然后注入property里面配置的属性，最后执行初始化相关方法。

简单时序图：