1. Bean的实例化概述

前一篇分析了BeanDefinition的封装过程，最终将beanName与BeanDefinition以一对一映射关系放到beanDefinitionMap容器中，这一篇重点分析如何利用bean的定义信息BeanDefinition实例化bean。

2. 流程概览

其实bean的实例化过程比较复杂，中间细节很多，为了抓住重点，先将核心流程梳理出来，主要包含以下几个流程：

step1: 通过反射创建实例；
step2：给实例属性赋初始值；
step3：如果Bean类实现BeanNameAware接口，则将通过传递Bean的名称来调用setBeanName()方法；如果Bean类实现BeanClassLoaderAware接口，则将通过传递加载此Bean的ClassLoader对象的实例来调用setBeanClassLoader()方法；如果Bean类实现BeanFactoryAware接口，则将通过传递BeanFactory对象的实例来调用setBeanFactory()方法；
step4: 如果有类实现BeanPostProcessors接口，则将在初始化之前调用postProcessBeforeInitialization()方法；
step5：如果Bean类实现了InitializingBean接口，将调用afterPropertiesSet()方法，如果配置文件中的Bean定义包含init-method属性，则该属性的值将解析为Bean类中的方法名称，并将调用该方法；
step6: 如果有类实现BeanPostProcessors接口，则将在初始化之后调用postProcessAfterInitialization()方法；
step7：如果Bean类实现DisposableBean接口，则当Application不再需要Bean引用时，将调用destroy()方法；如果配置文件中的Bean定义包含destroy-method属性，那么将调用Bean类中的相应方法定义。

3. 源码分析

进入AbstractApplicationContext中的fresh()方法，找到finishBeanFactoryInitialization(beanFactory)方法，该类是bean的实例化的入口，具体的实例化由preInstantiateSingletons()方法触发，见如下代码：

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解析时，把所有beanName都缓存到beanDefinitionNames了 List<String> beanNames = new ArrayList<>(this.beanDefinitionNames);
 // Trigger initialization of all non-lazy singleton beans... for (String beanName : beanNames) { //把父BeanDefinition里面的属性拿到子BeanDefinition中 RootBeanDefinition bd = getMergedLocalBeanDefinition(beanName); //如果不是抽象的，单例的，非懒加载的就实例化 if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) { //判断bean是否实现了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 { // 实例化过程 getBean(beanName); } } }

上述代码主要看getBean方法，随后进入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;
 // 从缓存中获取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 // 创建bean实例 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); }...}

由上述代码可知，先从缓存中获取bean，如果没有，则创建bean，最重要的方法就是getSingleton，该方法第二个参数是个函数式接口，进入getSingleton方法，当调用singletonObject = singletonFactory.getObject()时，会触发函数式接口中的createBean方法，随后一路进入doCreateBean，这个方法里面完成了所有实例化所需的步骤:

 protected Object doCreateBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args) throws BeanCreationException {
 // Instantiate the bean. // 真正开始创建bean的实例. 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 { // 属性赋值 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. // 有必要时，注册bean的销毁 try { registerDisposableBeanIfNecessary(beanName, bean, mbd); } catch (BeanDefinitionValidationException ex) { throw new BeanCreationException( mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex); }
 return exposedObject; }

从上述源码中看出bean的实例化主要分为以下三步：
step1:bean的创建；
step2:给bean的属性赋值；
step3:bean的初始化；
接着得到exposedObject这个已经完全实例化后的bean返回，其中当有必要时，注册bean的销毁，后面再详细看，先抓住主要流程。其中step3也是比较重要的方法，进入该方法:

 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()) { // 初始化前处理的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()) { // 初始化后处理的beanPostProcessor wrappedBean = applyBeanPostProcessorsAfterInitialization(wrappedBean, beanName); }
 return wrappedBean; }

从上面源码可知，梳理出主要的四个步骤：
step1:激活aware接口，完成aware接口的相关操作；
step2:初始化前处理的beanPostProcessor；
step3:完成init-method方法；
step4:初始化后处理的beanPostProcessor;
BeanPostProcessor作用是对初始化后的bean进行增强处理，在该阶段 BeanPostProcessor 会处理当前容器内所有符合条件的实例化后的 bean 对象。它主要是对 Spring 容器提供的 bean 实例对象进行有效的扩展，允许Spring在初始化 bean 阶段对其进行定制化修改，如处理标记接口或者为其提供代理实现。

4. 演示

定义一个MyBeanPostProcessor实现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 被调用..."); return bean; }
 @Override public Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException { System.out.println("post Process after Initialization 被调用..."); return bean; }}

定义一个LifeCycleBean类，实现如下接口：

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("属性注入...."); this.property = property; }
 public LifeCycleBean(){ System.out.println("构造函数调用..."); } @Override public void setBeanClassLoader(ClassLoader classLoader) { System.out.println("BeanClassLoaderAware 被调用..."); }
 @Override public void setBeanFactory(BeanFactory beanFactory) throws BeansException { System.out.println("BeanFactoryAware 被调用..."); }
 @Override public void setBeanName(String name) { System.out.println("BeanNameAware 被调用..."); }
 @Override public void destroy() throws Exception { System.out.println("DisposableBean destroy 被调用..."); }
 @Override public void afterPropertiesSet() throws Exception { System.out.println("InitializingBean afterPropertiesSet 被调用..."); }

 public void initMethod(){ System.out.println("init-method 被调用..."); }
 public void destroyMethod(){ System.out.println("destroy-method 被调用..."); }
 public void display(){ System.out.println("方法调用..."); }
}

指定配置文件spring.xml，配置init-method与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>

测试类如下：

@RunWith(SpringJUnit4ClassRunner.class)@ContextConfiguration(locations = {"classpath:spring.xml"})public class TestSpring {
 @Test public void testLifeCycleBean() { ClassPathXmlApplicationContext context = new ClassPathXmlApplicationContext("spring.xml");
 }

执行结果：

构造函数调用...属性注入....BeanNameAware 被调用...BeanClassLoaderAware 被调用...BeanFactoryAware 被调用...post Process Before Initialization 被调用...InitializingBean afterPropertiesSet 被调用...init-method 被调用...post Process after Initialization 被调用...DisposableBean destroy 被调用...destroy-method 被调用...