生成SingleTon代理对象在getSingleTonInstance方法中完成，这个方法时ProxyFactoryBean生成AopProxy对象的入口。代理对象会封装对target目标对象的调用，也就是说针对target对象的方法调用行为会被这里生成的代理对象所拦截。具体的生成过程是首先读取ProxyFactoryBean配置，为生成代理对象做好准备。Spring通过AopProxy类来具体生成代理对象。对于getSingleTonInstance方法中生成代理对象的过程如下：
/** * Return the singleton instance of this class's proxy object, * lazily creating it if it hasn't been created already. * @return the shared singleton proxy */
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.
// 根据AOP框架来判断需要代理的接口
Class targetClass = getTargetClass();
if (targetClass == null) {
throw new FactoryBeanNotInitializedException("Cannot determine target class for proxy");
}
// 这里设置代理对象的接口
setInterfaces(ClassUtils.getAllInterfacesForClass(targetClass, this.proxyClassLoader));
}
// Initialize the shared singleton instance.
super.setFrozen(this.freezeProxy);
// 这里会使用ProxyFactoryBean来生成需要的proxy对象
this.singletonInstance = getProxy(createAopProxy());
}
return this.singletonInstance;
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/** * Return the proxy object to expose. * <p>The default implementation uses a {@code getProxy} call with * the factory's bean class loader. Can be overridden to specify a * custom class loader. * @param aopProxy the prepared AopProxy instance to get the proxy from * @return the proxy object to expose * @see AopProxy#getProxy(ClassLoader) */
protected Object getProxy(AopProxy aopProxy) {
return aopProxy.getProxy(this.proxyClassLoader);
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这里出现了AopProxy对象类型，Spring利用AOPProxy接口类把AOP代理对象的实现与框架其他部分有效隔离开来。AopProxy接口有两个子类实现，一个Cglib2AopProxy，另一个是JdkDynamicProxy。
具体代理对象的生成是在ProxyFactoryBean的基类AdvisedSupport中实现，借助AopProxyFactory完成，这个对象要么从JDK中生成，要么借助CGLIB获得。下面看看ProxyCreatorSupport中是如何生成代理对象的。
/** * Subclasses should call this to get a new AOP proxy. They should <b>not</b> * create an AOP proxy with {@code this} as an argument. */
protected final synchronized AopProxy createAopProxy() {
if (!this.active) {
activate();
}
// 通过AopProxyFactory取得AopProxy，这个AopProxyFactory是在初始化函数中定义的，使用的是DefaultAopProxyFactory
return getAopProxyFactory().createAopProxy(this);
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AopProxy代理对象的生成有两种方式，如果目标对象是接口类使用JDK来生成，否则Spring会使用CGLIB来生成目标的代理对象。下面看看在DefaultAopProxyFactory是如何生成AopProxy目标代理对象的：
public AopProxy createAopProxy(AdvisedSupport config) throws AopConfigException {
if (config.isOptimize() || config.isProxyTargetClass() || hasNoUserSuppliedProxyInterfaces(config)) {
Class targetClass = config.getTargetClass();
if (targetClass == null) {
throw new AopConfigException("TargetSource cannot determine target class: " +
"Either an interface or a target is required for proxy creation.");
}
if (targetClass.isInterface()) {
return new JdkDynamicAopProxy(config);
}
return CglibProxyFactory.createCglibProxy(config);
}
else {
return new JdkDynamicAopProxy(config);
}
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在AopProxy代理对象的生成过程中，首先要从AdviseSupport对象中取得配置的目标对象，AOP完成的是切面应用对目标应用对象的增强。如果这里没有配置目标对象会直接抛出异常。一般而言，默认方式是使用JDK来产生AopProxy代理对象，但如果配置的目标对象不是接口类的实现，会使用CGLIB来产生AopProxy代理对象；在使用CGLIB来产生AopProxy代理对象时，因为CGLIB是第三方类库，本身不在JDK基类库中，所有需要在classPath中正确配置，以便能够加载和利用。在Spring中，使用JDK和CGLIB来生成AopProxy代理对象的工作，是由JdkDynamicAopProxy和CglibProxyFactory来完成。
4、JDK生成AopProxy对象（接口实现类）
通过上面我们已经知道生成AopProxy对象有两种方式，下面看下类图： 
我们先看下JdkDynamicAopProxy是如何生成AopProxy对象的：
public Object getProxy(ClassLoader classLoader) {
if (logger.isDebugEnabled()) {
logger.debug("Creating JDK dynamic proxy: target source is " + this.advised.getTargetSource());
}
Class<?>[] proxiedInterfaces = AopProxyUtils.completeProxiedInterfaces(this.advised);
findDefinedEqualsAndHashCodeMethods(proxiedInterfaces);
// 调用JDK生成Proxy
return Proxy.newProxyInstance(classLoader, proxiedInterfaces, this);
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5、CGLIB生成AopProxy对象（非接口实现类）
public Object getProxy(ClassLoader classLoader) {
if (logger.isDebugEnabled()) {
logger.debug("Creating CGLIB proxy: target source is " + this.advised.getTargetSource());
}
try {
Class<?> rootClass = this.advised.getTargetClass();
Assert.state(rootClass != null, "Target class must be available for creating a CGLIB proxy");
Class<?> proxySuperClass = rootClass;
if (ClassUtils.isCglibProxyClass(rootClass)) {
proxySuperClass = rootClass.getSuperclass();
Class<?>[] additionalInterfaces = rootClass.getInterfaces();
for (Class<?> additionalInterface : additionalInterfaces) {
this.advised.addInterface(additionalInterface);
}
}
// Validate the class, writing log messages as necessary.
validateClassIfNecessary(proxySuperClass);
// Configure CGLIB Enhancer...
// 来自advised的IOC配置，比如使用AOP的DynamicAdvisedInterceptor
Enhancer enhancer = createEnhancer();
if (classLoader != null) {
enhancer.setClassLoader(classLoader);
if (classLoader instanceof SmartClassLoader &&
((SmartClassLoader) classLoader).isClassReloadable(proxySuperClass)) {
enhancer.setUseCache(false);
}
}
enhancer.setSuperclass(proxySuperClass);
enhancer.setInterfaces(AopProxyUtils.completeProxiedInterfaces(this.advised));
enhancer.setNamingPolicy(SpringNamingPolicy.INSTANCE);
enhancer.setStrategy(new MemorySafeUndeclaredThrowableStrategy(UndeclaredThrowableException.class));
enhancer.setInterceptDuringConstruction(false);
// 通过设置DynamicAdvisedInterceptor拦截器来完成AOP功能，getCallBacks方法如下：
// Callback aopInterceptor = new DynamicAdvisedInterceptor(this.advised)
Callback[] callbacks = getCallbacks(rootClass);
Class<?>[] types = new Class<?>[callbacks.length];
for (int x = 0; x < types.length; x++) {
types[x] = callbacks[x].getClass();
}
enhancer.setCallbackFilter(new ProxyCallbackFilter(
this.advised.getConfigurationOnlyCopy(), this.fixedInterceptorMap, this.fixedInterceptorOffset));
enhancer.setCallbackTypes(types);
enhancer.setCallbacks(callbacks);
// Generate the proxy class and create a proxy instance.
Object proxy;
if (this.constructorArgs != null) {
proxy = enhancer.create(this.constructorArgTypes, this.constructorArgs);
}
else {
proxy = enhancer.create();
}
return proxy;
}
catch (CodeGenerationException ex) {
throw new AopConfigException("Could not generate CGLIB subclass of class [" +
this.advised.getTargetClass() + "]: " +
"Common causes of this problem include using a final class or a non-visible class",
ex);
}
catch (IllegalArgumentException ex) {
throw new AopConfigException("Could not generate CGLIB subclass of class [" +
this.advised.getTargetClass() + "]: " +
"Common causes of this problem include using a final class or a non-visible class",
ex);
}
catch (Exception ex) {
// TargetSource.getTarget() failed
throw new AopConfigException("Unexpected AOP exception", ex);
}
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四、Spring AOP拦截器调用的实现
1、设计原理
在Spring AOP通过JDK的Proxy方式或CGLIB方式生成代理对象的时候，相关的拦截器已经配置到代理对象中去了，拦截器在代理对象中起作用是通过对这些方法的回调来完成的。
如果使用JDK的Proxy来生成代理对象，那么需要InvocationHandler来设置拦截器回调，而如果使用CGLIB来生成代理对象，通过DynamicAdvisedInterceptor来完成回调。
2、JdkDynamicAopProxy的invoke拦截
在JDKDynamicAopProxy生成代理对象时，他的AopProxy代理对象生成调用：
Proxy.newProxyInstance(classLoader, proxiedInterfaces, this);- 1
this指的是InvocationHandler对象，InvocationHandler是JDK定义反射类的一个接口，这个接口定义了invoke方法，此方法为回调方法。通过invoke的具体实现，来完成对目标对象方法调用的拦截器或者功能增强工作。在这个方法中，包含一个完整的拦截器链对目标对象的拦截过程，比如获取拦截器链中的拦截器进行配置，逐个运行拦截器链里的拦截器增强，知道最后的目标对象方法的运行。下面看下invoke的源码
/** * Implementation of {@code InvocationHandler.invoke}. * <p>Callers will see exactly the exception thrown by the target, * unless a hook method throws an exception. */
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
MethodInvocation invocation;
Object oldProxy = null;
boolean setProxyContext = false;
TargetSource targetSource = this.advised.targetSource;
Class<?> targetClass = null;
Object target = null;
try {
if (!this.equalsDefined && AopUtils.isEqualsMethod(method)) {
// The target does not implement the equals(Object) method itself.
return equals(args[0]);
}
if (!this.hashCodeDefined && AopUtils.isHashCodeMethod(method)) {
// The target does not implement the hashCode() method itself.
return hashCode();
}
if (!this.advised.opaque && method.getDeclaringClass().isInterface() &&
method.getDeclaringClass().isAssignableFrom(Advised.class)) {
// Service invocations on ProxyConfig with the proxy config...
return AopUtils.invokeJoinpointUsingReflection(this.advised, method, args);
}
Object retVal;
if (this.advised.exposeProxy) {
// Make invocation available if necessary.
oldProxy = AopContext.setCurrentProxy(proxy);
setProxyContext = true;
}
// May be null. Get as late as possible to minimize the time we "own" the target,
// in case it comes from a pool.
target = targetSource.getTarget();
if (target != null) {
targetClass = target.getClass();
}
// Get the interception chain for this method.
// 获得定义好的拦截器
List<Object> chain = this.advised.getInterceptorsAndDynamicInterceptionAdvice(method, targetClass);
// Check whether we have any advice. If we don't, we can fallback on direct
// reflective invocation of the target, and avoid creating a MethodInvocation.
// 如果没有拦截器直接调用target的对应方法
if (chain.isEmpty()) {
// We can skip creating a MethodInvocation: just invoke the target directly
// Note that the final invoker must be an InvokerInterceptor so we know it does
// nothing but a reflective operation on the target, and no hot swapping or fancy proxying.
// 如果有拦截器设定，那么需要调用拦截器之后才调用目标对象相应的方法
retVal = AopUtils.invokeJoinpointUsingReflection(target, method, args);
}
else {
// We need to create a method invocation...
invocation = new ReflectiveMethodInvocation(proxy, target, method, args, targetClass, chain);
// Proceed to the joinpoint through the interceptor chain.
retVal = invocation.proceed();
}
// Massage return value if necessary.
Class<?> returnType = method.getReturnType();
if (retVal != null && retVal == target && returnType.isInstance(proxy) &&
!RawTargetAccess.class.isAssignableFrom(method.getDeclaringClass())) {
// Special case: it returned "this" and the return type of the method
// is type-compatible. Note that we can't help if the target sets
// a reference to itself in another returned object.
retVal = proxy;
} else if (retVal == null && returnType != Void.TYPE && returnType.isPrimitive()) {
throw new AopInvocationException("Null return value from advice does not match primitive return type for: " + method);
}
return retVal;
}
finally {
if (target != null && !targetSource.isStatic()) {
// Must have come from TargetSource.
targetSource.releaseTarget(target);
}
if (setProxyContext) {
// Restore old proxy.
AopContext.setCurrentProxy(oldProxy);
}
}
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3、CglibAopProxy的intercept拦截器
使用CglibAopProxy生成AopProxy对象时候，对于AOP拦截器调用，回调的是DynamicAdvisedInterceptor对象生成的。回调的方法时intercept，下面看看回调方法的源码：
public Object intercept(Object proxy, Method method, Object[] args, MethodProxy methodProxy) throws Throwable { Object oldProxy = null; boolean setProxyContext = false; Class<?> targetClass = null; Object target = null; try { if (this.advised.exposeProxy) { // Make invocation available if necessary. oldProxy = AopContext.setCurrentProxy(proxy); setProxyContext = true; } // May be null. Get as late as possible to minimize the time we // "own" the target, in case it comes from a pool... target = getTarget();
if (target != null) { targetClass = target.getClass(); }
List<Object> chain = this.advised.getInterceptorsAndDynamicInterceptionAdvice(method, targetClass);
Object retVal;
// Check whether we only have one InvokerInterceptor: that is,
// no real advice, but just reflective invocation of the target.
if (chain.isEmpty() && Modifier.isPublic(method.getModifiers())) { // We can skip creating a MethodInvocation: just invoke the target directly. // Note that the final invoker must be an InvokerInterceptor, so we know // it does nothing but a reflective operation on the target, and no hot // swapping or fancy proxying. retVal = methodProxy.invoke(target, args); } else { // We need to create a method invocation... retVal = new CglibMethodInvocation(proxy, target, method, args, targetClass, chain, methodProxy).proceed(); } retVal = processReturnType(proxy, target, method, retVal);
return retVal;
}
finally { if (target != null) { releaseTarget(target); }
if (setProxyContext) { // Restore old proxy. AopContext.setCurrentProxy(oldProxy); }
}
}