为源码付出的每一分努力都不会白费。

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...

4. 总结

本篇从一个初学者的角度概览了bean的整个生命周期，并描述了其中的主要流程，阅读源码的初始阶段，优先抓住主要流程，别陷入细节，并通过跑案例、写注解、画流程图等方式加深理解，后续将继续分析bean实例化中的核心流程、设计思想等。