【spring原始碼系列】之【Bean的生命週期】

為原始碼付出的每一分努力都不會白費。

1. Bean的例項化概述

前一篇分析了BeanDefinition的封裝過程，最終將beanName與BeanDefinition以一對一對映關係放到beanDefinitionMap容器中，這一篇重點分析如何利用bean的定義資訊BeanDefinition例項化bean。

2. 流程概覽

其實bean的例項化過程比較複雜，中間細節很多，為了抓住重點，先將核心流程梳理出來，主要包含以下幾個流程：

step1: 通過反射建立例項；
step2：給例項屬性賦初始值；
step3：如果Bean類實現BeanNameAware介面，則將通過傳遞Bean的名稱來呼叫setBeanName()方法；如果Bean類實現BeanClassLoaderAware介面，則將通過傳遞載入此Bean的ClassLoader物件的例項來呼叫setBeanClassLoader()方法；如果Bean類實現BeanFactoryAware介面，則將通過傳遞BeanFactory物件的例項來呼叫setBeanFactory()方法；
step4: 如果有類實現BeanPostProcessors介面，則將在初始化之前呼叫postProcessBeforeInitialization()方法；
step5：如果Bean類實現了InitializingBean介面，將呼叫afterPropertiesSet()方法，如果配置檔案中的Bean定義包含init-method屬性，則該屬性的值將解析為Bean類中的方法名稱，並將呼叫該方法；
step6: 如果有類實現BeanPostProcessors介面，則將在初始化之後呼叫postProcessAfterInitialization()方法；
step7：如果Bean類實現DisposableBean介面，則當Application不再需要Bean引用時，將呼叫destroy()方法；如果配置檔案中的Bean定義包含destroy-method屬性，那麼將呼叫Bean類中的相應方法定義。

3. 原始碼分析

進入AbstractApplicationContext中的fresh()方法，找到finishBeanFactoryInitialization(beanFactory)方法，該類是bean的例項化的入口，具體的例項化由preInstantiateSingletons()方法觸發，見如下程式碼：

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.
		//xml解析時，把所有beanName都快取到beanDefinitionNames了
		List<String> beanNames = new ArrayList<>(this.beanDefinitionNames);

		// Trigger initialization of all non-lazy singleton beans...
		for (String beanName : beanNames) {
			//把父BeanDefinition裡面的屬性拿到子BeanDefinition中
			RootBeanDefinition bd = getMergedLocalBeanDefinition(beanName);
			//如果不是抽象的，單例的，非懶載入的就例項化
			if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) {
				//判斷bean是否實現了FactoryBean介面
				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 {
					// 例項化過程
					getBean(beanName);
				}
			}
		}

上述程式碼主要看getBean方法，隨後進入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;

		// 從快取中獲取bean.
		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 {
			// Fail if we're already creating this bean instance:
			// We're assumably within a circular reference.
			if (isPrototypeCurrentlyInCreation(beanName)) {
				throw new BeanCurrentlyInCreationException(beanName);
			}

			// Check if bean definition exists in this factory.
			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) {
				markBeanAsCreated(beanName);
			}

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

				// Guarantee initialization of beans that the current bean depends on.
				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);
						}
					}
				}

				// Create bean instance
                                // 建立bean例項
				if (mbd.isSingleton()) {
					sharedInstance = getSingleton(beanName, () -> {
						try {
							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);
				}
...
}

由上述程式碼可知，先從快取中獲取bean，如果沒有，則建立bean，最重要的方法就是getSingleton，該方法第二個引數是個函式式介面，進入getSingleton方法，當呼叫singletonObject = singletonFactory.getObject()時，會觸發函式式介面中的createBean方法，隨後一路進入doCreateBean，這個方法裡面完成了所有例項化所需的步驟:

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

		// Instantiate the bean.
		// 真正開始建立bean的例項.
		BeanWrapper instanceWrapper = null;
		if (mbd.isSingleton()) {
			instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
		}
		if (instanceWrapper == null) {
			instanceWrapper = createBeanInstance(beanName, mbd, args);
		}
		Object bean = instanceWrapper.getWrappedInstance();
		Class<?> beanType = instanceWrapper.getWrappedClass();
		if (beanType != NullBean.class) {
			mbd.resolvedTargetType = beanType;
		}

		// Allow post-processors to modify the merged bean definition.
		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;
			}
		}

		// Eagerly cache singletons to be able to resolve circular references
		// even when triggered by lifecycle interfaces like BeanFactoryAware.
		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 {
			// 屬性賦值
			populateBean(beanName, mbd, instanceWrapper);
			// 初始化bean
			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.
                // 有必要時，註冊bean的銷燬
		try {
			registerDisposableBeanIfNecessary(beanName, bean, mbd);
		}
		catch (BeanDefinitionValidationException ex) {
			throw new BeanCreationException(
					mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex);
		}

		return exposedObject;
	}

從上述原始碼中看出bean的例項化主要分為以下三步：
step1:bean的建立；
step2:給bean的屬性賦值；
step3:bean的初始化；
接著得到exposedObject這個已經完全例項化後的bean返回，其中當有必要時，註冊bean的銷燬，後面再詳細看，先抓住主要流程。其中step3也是比較重要的方法，進入該方法:

	protected Object initializeBean(String beanName, Object bean, @Nullable RootBeanDefinition mbd) {
		if (System.getSecurityManager() != null) {
			AccessController.doPrivileged((PrivilegedAction<Object>) () -> {
				invokeAwareMethods(beanName, bean);
				return null;
			}, getAccessControlContext());
		}
		else {
			// 啟用aware介面
			invokeAwareMethods(beanName, bean);
		}

		Object wrappedBean = bean;
		if (mbd == null || !mbd.isSynthetic()) {
			// 初始化前處理的beanPostProcessor
			wrappedBean = applyBeanPostProcessorsBeforeInitialization(wrappedBean, beanName);
		}

		try {
			// 啟用 init-method方法
			invokeInitMethods(beanName, wrappedBean, mbd);
		}
		catch (Throwable ex) {
			throw new BeanCreationException(
					(mbd != null ? mbd.getResourceDescription() : null),
					beanName, "Invocation of init method failed", ex);
		}
		if (mbd == null || !mbd.isSynthetic()) {
			// 初始化後處理的beanPostProcessor
			wrappedBean = applyBeanPostProcessorsAfterInitialization(wrappedBean, beanName);
		}

		return wrappedBean;
	}

從上面原始碼可知，梳理出主要的四個步驟：
step1:啟用aware介面，完成aware介面的相關操作；
step2:初始化前處理的beanPostProcessor；
step3:完成init-method方法；
step4:初始化後處理的beanPostProcessor;
BeanPostProcessor作用是對初始化後的bean進行增強處理，在該階段 BeanPostProcessor 會處理當前容器內所有符合條件的例項化後的 bean 物件。它主要是對 Spring 容器提供的 bean 例項物件進行有效的擴充套件，允許Spring在初始化 bean 階段對其進行定製化修改，如處理標記介面或者為其提供代理實現。

4. 演示

定義一個MyBeanPostProcessor實現BeanPostProcessor介面

public class MyBeanPostProcessor implements BeanPostProcessor {


    @Override
    public Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException {
        System.out.println("post Process Before Initialization 被呼叫...");
        return bean;
    }

    @Override
    public Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException {
        System.out.println("post Process after Initialization 被呼叫...");
        return bean;
    }
}

定義一個LifeCycleBean類，實現如下介面：

public class LifeCycleBean implements BeanNameAware, BeanFactoryAware, BeanClassLoaderAware,
        InitializingBean, DisposableBean {

    private String property;

    public String getProperty() {
        return property;
    }

    public void setProperty(String property) {
        System.out.println("屬性注入....");
        this.property = property;
    }

    public LifeCycleBean(){
        System.out.println("建構函式呼叫...");
    }
    @Override
    public void setBeanClassLoader(ClassLoader classLoader) {
        System.out.println("BeanClassLoaderAware 被呼叫...");
    }

    @Override
    public void setBeanFactory(BeanFactory beanFactory) throws BeansException {
        System.out.println("BeanFactoryAware 被呼叫...");
    }

    @Override
    public void setBeanName(String name) {
        System.out.println("BeanNameAware 被呼叫...");
    }

    @Override
    public void destroy() throws Exception {
        System.out.println("DisposableBean destroy 被呼叫...");
    }

    @Override
    public void afterPropertiesSet() throws Exception {
        System.out.println("InitializingBean afterPropertiesSet 被呼叫...");
    }


    public void initMethod(){
        System.out.println("init-method 被呼叫...");
    }

    public void destroyMethod(){
        System.out.println("destroy-method 被呼叫...");
    }

    public void display(){
        System.out.println("方法呼叫...");
    }

}

指定配置檔案spring.xml，配置init-method與destroy-method方法

	<bean id="lifeCycle" class="com.wzj.bean.LifeCycleBean"
		  init-method="initMethod" destroy-method="destroyMethod">
		<property name="property" value="property"/>
	</bean>
	<bean id="myBeanPostProcessor" class="com.wzj.bean.MyBeanPostProcessor" >
	</bean>

測試類如下：

@RunWith(SpringJUnit4ClassRunner.class)
@ContextConfiguration(locations = {"classpath:spring.xml"})
public class TestSpring {

    @Test
    public void testLifeCycleBean() {
	    ClassPathXmlApplicationContext context = new ClassPathXmlApplicationContext("spring.xml");

    }

執行結果：

建構函式呼叫...
屬性注入....
BeanNameAware 被呼叫...
BeanClassLoaderAware 被呼叫...
BeanFactoryAware 被呼叫...
post Process Before Initialization 被呼叫...
InitializingBean afterPropertiesSet 被呼叫...
init-method 被呼叫...
post Process after Initialization 被呼叫...
DisposableBean destroy 被呼叫...
destroy-method 被呼叫...

4. 總結

本篇從一個初學者的角度概覽了bean的整個生命週期，並描述了其中的主要流程，閱讀原始碼的初始階段，優先抓住主要流程，別陷入細節，並通過跑案例、寫註解、畫流程圖等方式加深理解，後續將繼續分析bean例項化中的核心流程、設計思想等。