Spring AOP分析(2) -- JdkDynamicAopProxy实现AOP

上文介绍了代理类是由默认AOP代理工厂DefaultAopProxyFactory中createAopProxy方法产生的。如果代理对象是接口类型，则生成JdkDynamicAopProxy代理；否则生成ObjenesisCglibAopProxy代理，ObjenesisCglibAopProxy代理是继承于CglibAopProxy。下面先从熟悉的入手，选择JdkDynamicAopProxy分析。

构造器

查看源码，可以看到JdkDynamicAopProxy是一个final类，不能被继承和实现。其实现了AopProxy, InvocationHandler, Serializable接口，如下所示：

final class JdkDynamicAopProxy implements AopProxy, InvocationHandler, Serializable

下面看看JdkDynamicAopProxy 构造器，源码如下：

public JdkDynamicAopProxy(AdvisedSupport config) throws AopConfigException {
  Assert.notNull(config, "AdvisedSupport must not be null");
  if (config.getAdvisors().length == 0 && config.getTargetSource() == AdvisedSupport.EMPTY_TARGET_SOURCE) {
    throw new AopConfigException("No advisors and no TargetSource specified");
  }
  this.advised = config;
}

从构造器可以看出，JdkDynamicAopProxy依赖于AdvisedSupport，根据config配置信息创建动态代理对象。代码中config.getAdvisors()提供的是Advisor列表。

getProxy

getProxy 方法是实现AopProxy接口，源码如下：

	@Override
	public Object getProxy() {
		return getProxy(ClassUtils.getDefaultClassLoader());
	}

	@Override
	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, true);
		findDefinedEqualsAndHashCodeMethods(proxiedInterfaces);
		return Proxy.newProxyInstance(classLoader, proxiedInterfaces, this);
	}

	/**
	 * Finds any {@link #equals} or {@link #hashCode} method that may be defined
	 * on the supplied set of interfaces.
	 * @param proxiedInterfaces the interfaces to introspect
	 */
	private void findDefinedEqualsAndHashCodeMethods(Class<?>[] proxiedInterfaces) {
		for (Class<?> proxiedInterface : proxiedInterfaces) {
			Method[] methods = proxiedInterface.getDeclaredMethods();
			for (Method method : methods) {
				if (AopUtils.isEqualsMethod(method)) {
					this.equalsDefined = true;
				}
				if (AopUtils.isHashCodeMethod(method)) {
					this.hashCodeDefined = true;
				}
				if (this.equalsDefined && this.hashCodeDefined) {
					return;
				}
			}
		}
	}

在创建代理时，既可以采用默认的类加载器，也可以指定特定的类加载器。JDK动态代理的代理对象是接口类型，先获取被代理对象的完整接口、根据指定的类加载器以及实现的调用处理器应用静态方法Proxy.newProxyInstance创建代理对象。

invoke

上文介绍了InvocationHandler 接口，invoke该接口中唯一一个定义的方法。JdkDynamicAopProxy 是final类并且实现了InvocationHandler 接口，那么也必然实现了invoke方法，其源码如下：

	@Override
	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)) {
				return equals(args[0]);
			}
			else if (!this.hashCodeDefined && AopUtils.isHashCodeMethod(method)) {
				return hashCode();
			}
			else if (method.getDeclaringClass() == DecoratingProxy.class) {
				return AopProxyUtils.ultimateTargetClass(this.advised);
			}
			else if (!this.advised.opaque && method.getDeclaringClass().isInterface() &&
					method.getDeclaringClass().isAssignableFrom(Advised.class)) {
				return AopUtils.invokeJoinpointUsingReflection(this.advised, method, args);
			}

			Object retVal;

			if (this.advised.exposeProxy) {

				oldProxy = AopContext.setCurrentProxy(proxy);
				setProxyContext = true;
			}

			target = targetSource.getTarget();
			if (target != null) {
				targetClass = target.getClass();
			}

			List<Object> chain = this.advised.getInterceptorsAndDynamicInterceptionAdvice(method, targetClass);

			if (chain.isEmpty()) {
				Object[] argsToUse = AopProxyUtils.adaptArgumentsIfNecessary(method, args);
				retVal = AopUtils.invokeJoinpointUsingReflection(target, method, argsToUse);
			}
			else {
				invocation = new ReflectiveMethodInvocation(proxy, target, method, args, targetClass, chain);
				retVal = invocation.proceed();
			}

			Class<?> returnType = method.getReturnType();
			if (retVal != null && retVal == target &&
					returnType != Object.class && returnType.isInstance(proxy) &&
					!RawTargetAccess.class.isAssignableFrom(method.getDeclaringClass())) {
				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()) {
				targetSource.releaseTarget(target);
			}
			if (setProxyContext) {
				AopContext.setCurrentProxy(oldProxy);
			}
		}
	}

前面只是一些校验，直接省略，步入重点。通过target = targetSource.getTarget()得到被代理对象的类名。再根据被代理类名和方法名得到拦截链，也即通知链。如下所示：

List<Object> chain = this.advised.getInterceptorsAndDynamicInterceptionAdvice(method, targetClass);

如果拦截链为空，则直接反射调用被代理方法，否则需要创建代理方法，此代理方法中已经加入附加处理（通知）。如下：

			if (chain.isEmpty()) {
				//处理被代理方法参数
				Object[] argsToUse = AopProxyUtils.adaptArgumentsIfNecessary(method, args);
				//反射执行被代理方法
				retVal = AopUtils.invokeJoinpointUsingReflection(target, method, argsToUse);
			}
			else {
				// 创建代理方法
				invocation = new ReflectiveMethodInvocation(proxy, target, method, args, targetClass, chain);
				// 执行代理方法
				retVal = invocation.proceed();
			}

反射执行被代理方法是调用工具类AopUtils中方法invokeJoinpointUsingReflection实现的，具体如下：

	public static Object invokeJoinpointUsingReflection(Object target, Method method, Object[] args)
			throws Throwable {

		// Use reflection to invoke the method.
		try {
			ReflectionUtils.makeAccessible(method);
			return method.invoke(target, args);
		}
		catch (InvocationTargetException ex) {
			// Invoked method threw a checked exception.
			// We must rethrow it. The client won't see the interceptor.
			throw ex.getTargetException();
		}
		catch (IllegalArgumentException ex) {
			throw new AopInvocationException("AOP configuration seems to be invalid: tried calling method [" +
					method + "] on target [" + target + "]", ex);
		}
		catch (IllegalAccessException ex) {
			throw new AopInvocationException("Could not access method [" + method + "]", ex);
		}
	}

创建代理方法是通过ReflectiveMethodInvocation实现的，然后调用proceed()方法执行拦截链和被代理方法。ReflectiveMethodInvocation实现了Joinpoint接口，其构造器如下：

	protected ReflectiveMethodInvocation(
			Object proxy, Object target, Method method, Object[] arguments,
			Class<?> targetClass, List<Object> interceptorsAndDynamicMethodMatchers) {

		this.proxy = proxy;
		this.target = target;
		this.targetClass = targetClass;
		this.method = BridgeMethodResolver.findBridgedMethod(method);
		this.arguments = AopProxyUtils.adaptArgumentsIfNecessary(method, arguments);
		this.interceptorsAndDynamicMethodMatchers = interceptorsAndDynamicMethodMatchers;
	}

ReflectiveMethodInvocation调用proceed方法执行代理，proceed方法是在Joinpoint接口中定义的，ReflectiveMethodInvocation中进行了实现。具体实现如下：

	public Object proceed() throws Throwable {
		//	We start with an index of -1 and increment early.
		if (this.currentInterceptorIndex == this.interceptorsAndDynamicMethodMatchers.size() - 1) {
			return invokeJoinpoint();
		}

		Object interceptorOrInterceptionAdvice =
				this.interceptorsAndDynamicMethodMatchers.get(++this.currentInterceptorIndex);
		if (interceptorOrInterceptionAdvice instanceof InterceptorAndDynamicMethodMatcher) {
			// Evaluate dynamic method matcher here: static part will already have
			// been evaluated and found to match.
			InterceptorAndDynamicMethodMatcher dm =
					(InterceptorAndDynamicMethodMatcher) interceptorOrInterceptionAdvice;
			if (dm.methodMatcher.matches(this.method, this.targetClass, this.arguments)) {
				return dm.interceptor.invoke(this);
			}
			else {
				// Dynamic matching failed.
				// Skip this interceptor and invoke the next in the chain.
				return proceed();
			}
		}
		else {
			// It's an interceptor, so we just invoke it: The pointcut will have
			// been evaluated statically before this object was constructed.
			return ((MethodInterceptor) interceptorOrInterceptionAdvice).invoke(this);
		}
	}

从上面分析源码大致可以了解Spring AOP 动态代理的设计思想，采用类加载器根据接口产生代理方法，代理方法是在原方法的基础上加上通知链，以实现AOP功能。当执行方法时，判断该方法通知链是否为空，若为空，则通过反射直接调用原方法；若不为空，则产生代理方法，执行代理方法。下一节将继续探讨Spring AOP 的另一种实现方法CGLIB