Spring源碼解析之Bean的生命周期

一、Bean的實例化概述

前一篇分析瞭BeanDefinition的封裝過程,最終將beanName與BeanDefinition以一對一映射關系放到beanDefinitionMap容器中,這一篇重點分析如何利用bean的定義信息BeanDefinition實例化bean。

二、流程概覽

其實bean的實例化過程比較復雜,中間細節很多,為瞭抓住重點,先將核心流程梳理出來,主要包含以下幾個流程:

  • step1: 通過反射創建實例;
  • step2:給實例屬性賦初始值;
  • step3:如果Bean類實現BeanNameAware接口,則將通過傳遞Bean的名稱來調用setBeanName()方法;如果Bean類實現BeanClassLoaderAware接口,則將通過傳遞加載此Bean的ClassLoader對象的實例來調用setBeanClassLoader()方法;如果Bean類實現BeanFactoryAware接口,則將通過傳遞BeanFactory對象的實例來調用setBeanFactory()方法;
  • step4: 如果有類實現BeanPostProcessors接口,則將在初始化之前調用postProcessBeforeInitialization()方法;
  • step5:如果Bean類實現瞭InitializingBean接口,將調用afterPropertiesSet()方法,如果配置文件中的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,並將調用該方法;
  • step6: 如果有類實現BeanPostProcessors接口,則將在初始化之後調用postProcessAfterInitialization()方法;
  • step7:如果Bean類實現DisposableBean接口,則當Application不再需要Bean引用時,將調用destroy()方法;如果配置文件中的Bean定義包含destroy-method屬性,那麼將調用Bean類中的相應方法定義。

三、源碼分析

進入AbstractApplicationContext中的fresh()方法,找到finishBeanFactoryInitialization(beanFactory)方法,該類是bean的實例化的入口,具體的實例化由preInstantiateSingletons()方法觸發,見如下代碼:

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都緩存到beanDefinitionNames瞭
		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,非懶加載的就實例化
			if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) {
				//判斷bean是否實現瞭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 {
					// 實例化過程
					getBean(beanName);
				}
			}
		}

上述代碼主要看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;

		// 從緩存中獲取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
                                // 創建bean實例
				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);
				}
...
}

由上述代碼可知,先從緩存中獲取bean,如果沒有,則創建bean,最重要的方法就是getSingleton,該方法第二個參數是個函數式接口,進入getSingleton方法,當調用singletonObject = singletonFactory.getObject()時,會觸發函數式接口中的createBean方法,隨後一路進入doCreateBean,這個方法裡面完成瞭所有實例化所需的步驟:

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

		// Instantiate the bean.
		// 真正開始創建bean的實例.
		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的銷毀
		try {
			registerDisposableBeanIfNecessary(beanName, bean, mbd);
		}
		catch (BeanDefinitionValidationException ex) {
			throw new BeanCreationException(
					mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex);
		}

		return exposedObject;
	}

從上述源碼中看出bean的實例化主要分為以下三步:

  • step1:bean的創建;
  • step2:給bean的屬性賦值;
  • step3:bean的初始化;

接著得到exposedObject這個已經完全實例化後的bean返回,其中當有必要時,註冊bean的銷毀,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 {
			// 激活aware接口
			invokeAwareMethods(beanName, bean);
		}

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

		try {
			// 激活 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;
	}

從上面源碼可知,梳理出主要的四個步驟:

  • step1:激活aware接口,完成aware接口的相關操作;
  • 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實例化後的 bean 對象。它主要是對 Spring 容器提供的 bean 實例對象進行有效的擴展,允許Spring在初始化 bean 階段對其進行定制化修改,如處理標記接口或者為其提供代理實現。

四、演示

定義一個MyBeanPostProcessor實現BeanPostProcessor接口

public class MyBeanPostProcessor implements BeanPostProcessor {


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

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

定義一個LifeCycleBean類,實現如下接口:

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("屬性註入....");
        this.property = property;
    }

    public LifeCycleBean(){
        System.out.println("構造函數調用...");
    }
    @Override
    public void setBeanClassLoader(ClassLoader classLoader) {
        System.out.println("BeanClassLoaderAware 被調用...");
    }

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

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

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

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


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

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

    public void display(){
        System.out.println("方法調用...");
    }

}

指定配置文件spring.xml,配置init-methoddestroy-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:

構造函數調用…
屬性註入….
BeanNameAware 被調用…
BeanClassLoaderAware 被調用…
BeanFactoryAware 被調用…
post Process Before Initialization 被調用…
InitializingBean afterPropertiesSet 被調用…
init-method 被調用…
post Process after Initialization 被調用…
DisposableBean destroy 被調用…
destroy-method 被調用…

五、總結

本篇從一個初學者的角度概覽瞭bean的整個生命周期,並描述瞭其中的主要流程,閱讀源碼的初始階段,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實例化中的核心流程、設計思想等。

到此這篇關於Spring源碼解析之Bean的生命周期的文章就介紹到這瞭,更多相關Bean的生命周期內容請搜索WalkonNet以前的文章或繼續瀏覽下面的相關文章希望大傢以後多多支持WalkonNet!

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