深入理解Spring系列之六：bean初始化

《深入理解Spring系列之四：BeanDefinition装载前奏曲》中提到，对于非延迟单例bean的初始化在finishBeanFactoryInitialization(beanFactory)中完成。进入这个方法，代码如下。

protected void finishBeanFactoryInitialization(ConfigurableListableBeanFactory beanFactory) {
// Initialize conversion service for this context.
if (beanFactory.containsBean(CONVERSION_SERVICE_BEAN_NAME) &&
beanFactory.isTypeMatch(CONVERSION_SERVICE_BEAN_NAME, ConversionService.class)) {
beanFactory.setConversionService(
beanFactory.getBean(CONVERSION_SERVICE_BEAN_NAME, ConversionService.class));
}
// Initialize LoadTimeWeaverAware beans early to allow for registering their transformers early.
String[] weaverAwareNames = beanFactory.getBeanNamesForType(LoadTimeWeaverAware.class, false, false);
for (String weaverAwareName : weaverAwareNames) {
getBean(weaverAwareName);
}
// Stop using the temporary ClassLoader for type matching.beanFactory.setTempClassLoader(null);
// Allow for caching all bean definition metadata, not expecting further changes.
beanFactory.freezeConfiguration();
// Instantiate all remaining (non-lazy-init) singletons.
beanFactory.preInstantiateSingletons();
}

关注最后一行代码，

beanFactory.preInstantiateSingletons()完成初始化所有非延迟的单例bean，进入这个方法的具体实现，代码如下。
public void preInstantiateSingletons() throws BeansException {
if (this.logger.isDebugEnabled()) {
this.logger.debug("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.
List beanNames = new ArrayList(this.beanDefinitionNames);
// Trigger initialization of all non-lazy singletonbeans...
for (String beanName : beanNames) {
RootBeanDefinition bd = getMergedLocalBeanDefinition(beanName);
if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) {
if (isFactoryBean(beanName)) {
final FactoryBean factory = (FactoryBean) getBean(FACTORY_BEAN_PREFIX + beanName);
boolean isEagerInit;
if (System.getSecurityManager() != null && factory instanceof SmartFactoryBean) {
isEagerInit = AccessController.doPrivileged(new PrivilegedAction() {
@Override
public Boolean run() {
return ((SmartFactoryBean) factory).isEagerInit();
}
}, getAccessControlContext());
}else {
isEagerInit = (factory instanceof SmartFactoryBean &&
((SmartFactoryBean) factory).isEagerInit());
}
if (isEagerInit) {
getBean(beanName);
}
}else {
getBean(beanName);
}
}
}
// Trigger post-initialization callback for all applicable beans...
for (String beanName : beanNames) {
Object singletonInstance = getSingleton(beanName);
if (singletonInstance instanceof SmartInitializingSingleton) {
final SmartInitializingSingleton smartSingleton = (SmartInitializingSingleton) singletonInstance;
if (System.getSecurityManager() != null) {
AccessController.doPrivileged(new PrivilegedAction() {
@Override
public Object run() {
smartSingleton.afterSingletonsInstantiated();
return null;
}
}, getAccessControlContext());
}else {
smartSingleton.afterSingletonsInstantiated();
}
}
}
}

从上面的代码中看到，只会对非延迟单例bean进行初始化，scope为其它值的bean会在使用到的时候进行初始化，如prototype。这里关注getBean方法，这个方法看着很眼熟，其实就是《深入理解Spring系列之一：开篇》示例代码中用到的getBean，Spring对这个方法做了重复使用。getBean方法的具体实现在doGetBean方法中，这个方法的代码很长就不贴代码了。在doGetBean中，首先会初始化其依赖的bean，然后进行自身的初始化，这个方法里关注如下的代码段。

// Create bean instance.
if (mbd.isSingleton()) {
sharedInstance = getSingleton(beanName, new ObjectFactory() {
@Override
public Object getObject() throws BeansException {
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 receiveda temporary reference to the bean.
destroySingleton(beanName);
throw ex;
}
}
});
bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
}

这段代码完成了单例bean的初始化，追踪代码进入doCreateBean方法中，在这个方法中进行bean实例的创建、属性填充、将bean实例加入单例bean实例的缓存中。doCreateBean方法中有如下代码段。

if (instanceWrapper == null) {
instanceWrapper = createBeanInstance(beanName, mbd, args);
}

createBeanInstance方法里完成bean实例的创建，具体过程可继续追踪代码查看，其实就是使用反射进行实例对象的创建。

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