作者：zuoxiaolong8810（左潇龙），转载请注明出处。
上一章和各位一起看了一下springAOP的工作流程，当我们给出AOP相关的配置以后，直接从IOC容器中拿出来的就是已经加强过的bean。这说明spring在这个过程中一定做了什么。
本章我们就一起来看一下spring是如何完成对bean的增强的，首先我们来看一下，FactoryBean接口中一个方法的定义。
public interface FactoryBean<T> {
/**
* Return an instance (possibly shared or independent) of the object
* managed by this factory.
* <p>As with a {@link BeanFactory}, this allows support for both the
* Singleton and Prototype design pattern.
* <p>If this FactoryBean is not fully initialized yet at the time of
* the call (for example because it is involved in a circular reference),
* throw a corresponding {@link FactoryBeanNotInitializedException}.
* <p>As of Spring 2.0, FactoryBeans are allowed to return <code>null</code>
* objects. The factory will consider this as normal value to be used; it
* will not throw a FactoryBeanNotInitializedException in this case anymore.
* FactoryBean implementations are encouraged to throw
* FactoryBeanNotInitializedException themselves now, as appropriate.
* @return an instance of the bean (can be <code>null</code>)
* @throws Exception in case of creation errors
* @see FactoryBeanNotInitializedException
*/
T getObject() throws Exception;
getObject这个方法，就是用来获取被这个factorybean加强后的对象的，上一章测试的过程中，最终就是调用了这个方法，来完成了对bean的加强。我们来跟踪一下上一次测试的代码，看看到底是在什么地方调用的。这里再次贴出来上次测试的代码，方便解释。
public class TestAOP {
public static void main(String[] args) {
ApplicationContext applicationContext = new FileSystemXmlApplicationContext("classpath:beans.xml");
TestTarget target = (TestTarget) applicationContext.getBean("testAOP");
target.test();
System.out.println("------无敌分割线-----");
target.test2();
}
}
其实整个过程也就两行代码，第一行代码，是我们对IOC容器的初始化，这时其实并没有发生对bean的增强，原因就是这个时候只是完成了对ProxyFactoryBean的初始化，也就是相当于我们已经new出来了一个ProxyFactoryBean，但是此时并没有调用接口方法，去获得加强后的bean。
下面我们去跟进第二行获取testAOP的代码，来看一下究竟。首先我们会找到AbstractApplicationContext中的getBean方法，但是这个类并不负责bean的实例化工作，而是交给了bean工厂，我们跟踪bean工厂的方法，能找到上述第二行其实是调用了如下这个方法。
@SuppressWarnings("unchecked")
protected <T> T doGetBean(
final String name, final Class<T> requiredType, final Object[] args, boolean typeCheckOnly)
throws BeansException {
final String beanName = transformedBeanName(name);
Object bean;
// Eagerly check singleton cache for manually registered singletons.
Object sharedInstance = getSingleton(beanName);
if (sharedInstance != null && args == null) {
if (logger.isDebugEnabled()) {
if (isSingletonCurrentlyInCreation(beanName)) {
logger.debug("Returning eagerly cached instance of singleton bean '" + beanName +
"' that is not fully initialized yet - a consequence of a circular reference");
}
else {
logger.debug("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 (args != null) {
// Delegation to parent with explicit args.
return (T) parentBeanFactory.getBean(nameToLookup, args);
}
else {
// No args -> delegate to standard getBean method.
return parentBeanFactory.getBean(nameToLookup, requiredType);
}
}
if (!typeCheckOnly) {
markBeanAsCreated(beanName);
}
final 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 dependsOnBean : dependsOn) {
getBean(dependsOnBean);
registerDependentBean(dependsOnBean, beanName);
}
}
// Create bean instance.
if (mbd.isSingleton()) {
sharedInstance = getSingleton(beanName, new ObjectFactory() {
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 received a temporary reference to the bean.
destroySingleton(beanName);
throw ex;
}
}
});
bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
}
else if (mbd.isPrototype()) {
// It's a prototype -> create a new instance.
Object prototypeInstance = null;
try {
beforePrototypeCreation(beanName);
prototypeInstance = createBean(beanName, mbd, args);
}
finally {
afterPrototypeCreation(beanName);
}
bean = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);
}
else {
String scopeName = mbd.getScope();
final Scope scope = this.scopes.get(scopeName);
if (scope == null) {
throw new IllegalStateException("No Scope registered for scope '" + scopeName + "'");
}
try {
Object scopedInstance = scope.get(beanName, new ObjectFactory() {
public Object getObject() throws BeansException {
beforePrototypeCreation(beanName);
try {
return createBean(beanName, mbd, args);
}
finally {
afterPrototypeCreation(beanName);
}
}
});
bean = getObjectForBeanInstance(scopedInstance, name, beanName, mbd);
}
catch (IllegalStateException ex) {
throw new BeanCreationException(beanName,
"Scope '" + scopeName + "' is not active for the current thread; " +
"consider defining a scoped proxy for this bean if you intend to refer to it from a singleton",
ex);
}
}
}
// Check if required type matches the type of the actual bean instance.
if (requiredType != null && bean != null && !requiredType.isAssignableFrom(bean.getClass())) {
throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
}
return (T) bean;
}
这是一个重载方法，后面三个参数两个为null，一个为false。下面注意，在这面这一行的时候，我们已经获取到了实例。
Object sharedInstance = getSingleton(beanName);
所以分支在碰到第一个if判断时，会直接进入if块而不是else块，在这里提醒一下，这个是获取的单例的bean实例，而这个sharedInstance并不是TestTarget，而是ProxyFactoryBean的实例。好了，接下来相信你已经明白了，我们该进入getObjectForBeanInstance这个方法了，来看这个方法。
protected Object getObjectForBeanInstance(
Object beanInstance, String name, String beanName, RootBeanDefinition mbd) {
// Don't let calling code try to dereference the factory if the bean isn't a factory.
if (BeanFactoryUtils.isFactoryDereference(name) && !(beanInstance instanceof FactoryBean)) {
throw new BeanIsNotAFactoryException(transformedBeanName(name), beanInstance.getClass());
}
// Now we have the bean instance, which may be a normal bean or a FactoryBean.
// If it's a FactoryBean, we use it to create a bean instance, unless the
// caller actually wants a reference to the factory.
if (!(beanInstance instanceof FactoryBean) || BeanFactoryUtils.isFactoryDereference(name)) {
return beanInstance;
}
Object object = null;
if (mbd == null) {
object = getCachedObjectForFactoryBean(beanName);
}
if (object == null) {
// Return bean instance from factory.
FactoryBean factory = (FactoryBean) beanInstance;
// Caches object obtained from FactoryBean if it is a singleton.
if (mbd == null && containsBeanDefinition(beanName)) {
mbd = getMergedLocalBeanDefinition(beanName);
}
boolean synthetic = (mbd != null && mbd.isSynthetic());
object = getObjectFromFactoryBean(factory, beanName, !synthetic);
}
return object;
}
方法的刚开始是两个卫语句，第一个判断如果是想获得factorybean本身，却又不是factorybean则抛出异常，第二个则是正常的获得factorybean。但是我们都不属于这两种情况。所以在经过getCachedObjectForFactoryBean获取无果和getCachedObjectForFactoryBean获取到bean定义以后，就进入了getObjectFromFactoryBean方法。
protected Object getObjectFromFactoryBean(FactoryBean factory, String beanName, boolean shouldPostProcess) {
if (factory.isSingleton() && containsSingleton(beanName)) {
synchronized (getSingletonMutex()) {
Object object = this.factoryBeanObjectCache.get(beanName);
if (object == null) {
object = doGetObjectFromFactoryBean(factory, beanName, shouldPostProcess);
this.factoryBeanObjectCache.put(beanName, (object != null ? object : NULL_OBJECT));
}
return (object != NULL_OBJECT ? object : null);
}
}
else {
return doGetObjectFromFactoryBean(factory, beanName, shouldPostProcess);
}
}
进入以后，由于proxyFactorybean是单例bean，所以会进入到if块不是else块，接下来系统再次尝试从cache中获得，自然是无果。接下来依然会进入到和else一样的方法doGetObjectFromFactoryBean，先不说这个方法，看后面，获得以后就会放入cache，然后直接将对象返回。所以如果重复调用，下一次就会从cache当中取出来直接返回。好了，接下来我们进去doGetObjectFromFactoryBean方法。
private Object doGetObjectFromFactoryBean(
final FactoryBean factory, final String beanName, final boolean shouldPostProcess)
throws BeanCreationException {
Object object;
try {
if (System.getSecurityManager() != null) {
AccessControlContext acc = getAccessControlContext();
try {
object = AccessController.doPrivileged(new PrivilegedExceptionAction<Object>() {
public Object run() throws Exception {
return factory.getObject();
}
}, acc);
}
catch (PrivilegedActionException pae) {
throw pae.getException();
}
}
else {
object = factory.getObject();
}
}
catch (FactoryBeanNotInitializedException ex) {
throw new BeanCurrentlyInCreationException(beanName, ex.toString());
}
catch (Throwable ex) {
throw new BeanCreationException(beanName, "FactoryBean threw exception on object creation", ex);
}
// Do not accept a null value for a FactoryBean that's not fully
// initialized yet: Many FactoryBeans just return null then.
if (object == null && isSingletonCurrentlyInCreation(beanName)) {
throw new BeanCurrentlyInCreationException(
beanName, "FactoryBean which is currently in creation returned null from getObject");
}
if (object != null && shouldPostProcess) {
try {
object = postProcessObjectFromFactoryBean(object, beanName);
}
catch (Throwable ex) {
throw new BeanCreationException(beanName, "Post-processing of the FactoryBean's object failed", ex);
}
}
return object;
}
此处判断了一下当前是否设置了安全管理器，我们并没有设置，所以将直接调用ProxyFactoryBean的getObject方法，也就是对bean增强的地方。下面我们着重来看一下是如何对bean进行增强的。首先我们进入到ProxyFactoryBean的getObject方法来看一下。
public Object getObject() throws BeansException {
initializeAdvisorChain();
if (isSingleton()) {
return getSingletonInstance();
}
else {
if (this.targetName == null) {
logger.warn("Using non-singleton proxies with singleton targets is often undesirable. " +
"Enable prototype proxies by setting the 'targetName' property.");
}
return newPrototypeInstance();
}
}
此处主要是先初始化了一下通知器链，然后就会根据是否单例做相应的动作，我们看一下初始化通知器链的进行。
private synchronized void initializeAdvisorChain() throws AopConfigException, BeansException {
if (this.advisorChainInitialized) {
return;
}
if (!ObjectUtils.isEmpty(this.interceptorNames)) {
if (this.beanFactory == null) {
throw new IllegalStateException("No BeanFactory available anymore (probably due to serialization) " +
"- cannot resolve interceptor names " + Arrays.asList(this.interceptorNames));
}
// Globals can't be last unless we specified a targetSource using the property...
if (this.interceptorNames[this.interceptorNames.length - 1].endsWith(GLOBAL_SUFFIX) &&
this.targetName == null && this.targetSource == EMPTY_TARGET_SOURCE) {
throw new AopConfigException("Target required after globals");
}
// Materialize interceptor chain from bean names.
for (String name : this.interceptorNames) {
if (logger.isTraceEnabled()) {
logger.trace("Configuring advisor or advice '" + name + "'");
}
if (name.endsWith(GLOBAL_SUFFIX)) {
if (!(this.beanFactory instanceof ListableBeanFactory)) {
throw new AopConfigException(
"Can only use global advisors or interceptors with a ListableBeanFactory");
}
addGlobalAdvisor((ListableBeanFactory) this.beanFactory,
name.substring(0, name.length() - GLOBAL_SUFFIX.length()));
}
else {
// If we get here, we need to add a named interceptor.
// We must check if it's a singleton or prototype.
Object advice;
if (this.singleton || this.beanFactory.isSingleton(name)) {
// Add the real Advisor/Advice to the chain.
advice = this.beanFactory.getBean(name);
}
else {
// It's a prototype Advice or Advisor: replace with a prototype.
// Avoid unnecessary creation of prototype bean just for advisor chain initialization.
advice = new PrototypePlaceholderAdvisor(name);
}
addAdvisorOnChainCreation(advice, name);
}
}
}
this.advisorChainInitialized = true;
}
可以看到，其中针对我们配置的interpretorNames进行了循环，我们并非是配置的全局通知器，所以会进入else块，然后因为我们配置的testAdvisor默认是单例的，所以会从bean工厂中去获取这个实例，此时TestAdvisor已经实例化完成的，我们只是去取一下而已。然后就会进入addAdvisorOnChainCreation方法。这个方法不再一一贴进来，各位有兴趣的可以自己去看一下，就是把通知器加到了通知链当中。
值得注意的是在这个过程中，触发了一个这样的方法this.advisorAdapterRegistry.wrap(next)。这个方法就是用来包装通知器的，如果不是advisor而是advice，就会包装一下返回。
好了，接着刚才的过程，初始化通知器链完成以后，就会进入getSingletonInstance方法，这是用来获取单例实例的，而真正的加强也是在这里发生的，我们来看一下。
private synchronized Object getSingletonInstance() {
if (this.singletonInstance == null) {
this.targetSource = freshTargetSource();
if (this.autodetectInterfaces && getProxiedInterfaces().length == 0 && !isProxyTargetClass()) {
// Rely on AOP infrastructure to tell us what interfaces to proxy.
Class targetClass = getTargetClass();
if (targetClass == null) {
throw new FactoryBeanNotInitializedException("Cannot determine target class for proxy");
}
setInterfaces(ClassUtils.getAllInterfacesForClass(targetClass, this.proxyClassLoader));
}
// Initialize the shared singleton instance.
super.setFrozen(this.freezeProxy);
this.singletonInstance = getProxy(createAopProxy());
}
return this.singletonInstance;
}
此时第一次获取，单例实例为null，所以会进入if块，首先刷新targetSource，因为我们的Target类没有实现targetSource接口，所以会由spring帮我们产生一个targetSource适配，这里是使用的适配器的模式，有兴趣可以进去看一下，我们此处不关注这个。接下来，会去判断代理接口，并且设置代理接口，但是我们的target未实现任何接口，所以此处interfaces仍然为空的，所以最后一步createAopProxy时，会帮我们创建cglib的proxy。最终由cglib生成代理返回。
执行下国际惯例，说完以后总要稍微总结一下，主要说几点：
1.在IOC容器初始化的过程中，并没有发生增强的动作，而是初始化了proxyFactoryBean。
2.如果配置中不指定，所有bean默认都是单例和非延迟加载的，也就是说所有的bean都将在第一次IOC容器初始化时全部实例化，所以上一章中所配置的三个bean都是在IOC容器初始化时进行的实例化。
3.springAOP代理有两种方式，一种是JDK提供的动态代理，一种是cglib字节码生成的技术，当要代理的类有实现的接口的时候，就会针对接口进行代理，否则就会采用cglib直接生成字节码产生子类。
到此处，我们已经基本上完全跟了一遍整个bean增强的过程，也大概了解了springAOP的大概原理，相信各位心中应该有个大概的印象了，其实springAOP增强的原理已经浮现出来了，接下来再研究下去，可能会收获甚微，还是要结合平时的应用和自己的兴趣去体会，始终不赞同一头扎进去就埋头苦干的作风。
好了，spring源码学习之路就圆满结束了，虽说时间不长，但收获甚大。各位如果有兴趣，相信现在也完全有能力自己去看源码了，以后有问题，不要找度娘了，找源码吧。