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。