动态代理3--Spring AOP分析

Spring AOP的基本实现方式

    ​Spring AOP，一种模块化机制，能够动态的对切点添加行为，而不破坏原有的代码结构。

这是一个非常好地动态代理的应用方式。Spring AOP实现依赖于JDK的动态代理库和CGLIB字节码库技术两种来分别实现。

    ​在Spring AOP中，JdkDynamicAopProxy实现基于JDK动态代理生成代理对象，CglibAopProxy来实现基于CGLIB的动态代理对象生成。并通过DefaultAopProxyFactory进行调用。此处採用策略模式。针对不同场景。调用不同的实现。

例如以下我们对详细实现进行分析。假设对SpringAOP有疑惑的话，能够參考例如以下文章，一篇是我的博客：http://blog.csdn.net/mergades/article/details/46841079  还有IBM Devloper社区的一篇：http://www.ibm.com/developerworks/cn/java/j-lo-springaopcglib/

DefaultAopProxyFactory

    ​Spring AOP内部採用AopProxy对使用不同的代理实现机制进行了适度的抽象，针对不同的代理实现机制提供相应的AopProxy子类实现。DefaultAopProxyFactory实现了AopProxyFactory。当中AopProxyFactory接口定义createAopProxy方法。来决定依据哪种详细的策略来实现代理类。详细实现则由DefaultAopProxyFactory实现，我们查看其相应的createAopProxy方法。

    ​

public class DefaultAopProxyFactory implements AopProxyFactory, Serializable {
	@Override
	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()) {//假设是接口，则通过JDK的实现，否则通过CGLIB
				return new JdkDynamicAopProxy(config);
			}
			return new ObjenesisCglibAopProxy(config);
		}
		else {
			return new JdkDynamicAopProxy(config);
		}
	}
	/**
	 * Determine whether the supplied {@link AdvisedSupport} has only the
	 * {@link org.springframework.aop.SpringProxy} interface specified
	 * (or no proxy interfaces specified at all).
	 */
	private boolean hasNoUserSuppliedProxyInterfaces(AdvisedSupport config) {
		Class<?>[] interfaces = config.getProxiedInterfaces();
		return (interfaces.length == 0 || (interfaces.length == 1 && SpringProxy.class.equals(interfaces[0])));
	}
}

JdkDynamicAopProxy

    ​基于JDK动态代理的实现。该类实现了InvocationHandler接口。那么我们依据动态代理的知识能够知道，不管调用目标类的什么方法，都会运行该类的Invoke方法，Invoke方法就是Spring AOP增加切面的主要方法。

    ​我们查看相应的Invoke方法，尽管Invoke方法总体看起来非常长非常复杂，可是仅仅要我们包握住几个重点就可以了解。

@Override
	public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
MethodInvocation invocation;
		Object oldProxy = null;
		boolean setProxyContext = false;
//1。获取目标对象。
TargetSource targetSource = this.advised.targetSource;
		Class<?> targetClass = null;
		Object target = null;
//2，推断对JDK原生方法
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;
//3，设置代理对象
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.
//4,获取目标类
			target = targetSource.getTarget();
			if (target != null) {
				targetClass = target.getClass();
			}
			// Get the interception chain for this method.
//5，获取通知链
			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.
//6。推断是否存在通知链。并运行相应方法，获取返回值
			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.
				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.
//7，对返回值进行处理
			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);
			}
		}
	}

以上是invoke方法的实现。这种方法是动态代理机制较为核心的方法。

以下我们查看在该类中的getProxy方法，查看SpringAOP是怎样获取一个代理对象的。

@Override
	public Object getProxy(ClassLoader classLoader) {
		if (logger.isDebugEnabled()) {
			logger.debug("Creating JDK dynamic proxy: target source is " + this.advised.getTargetSource());
		}
//获代替理接口
		Class<?>[] proxiedInterfaces = AopProxyUtils.completeProxiedInterfaces(this.advised);
//获取是否定义equals和hashCode方法
		findDefinedEqualsAndHashCodeMethods(proxiedInterfaces);
//调用JDK Proxy生成代理
		return Proxy.newProxyInstance(classLoader, proxiedInterfaces, this);
	}

以上两个方法。我们抛开Spring详细各种细节的实现。全然能够看做是一个简单的动态代理模式的应用。

ObjenesisCglibAopProxy

    ​ObjenesisCglibAopProxy基于CGLIb的AOP代理对象的生成。在DefaultAopProxyFactory类中，通过调用此方法来实现CGLIB生成代理对象。

	@Override
	@SuppressWarnings("unchecked")
	protected Object createProxyClassAndInstance(Enhancer enhancer, Callback[] callbacks) {
		try {
//详细生成代理,详细实现源代码没有公开
			Factory factory = (Factory) this.objenesis.newInstance(enhancer.createClass());
			factory.setCallbacks(callbacks);
			return factory;
		}
		catch (ObjenesisException ex) {
			// Fallback to regular proxy construction on unsupported JVMs
			if (logger.isDebugEnabled()) {
				logger.debug("Unable to instantiate proxy using Objenesis, falling back to regular proxy construction", ex);
			}
			return super.createProxyClassAndInstance(enhancer, callbacks);
		}
	}

总结

    ​如上的各种实现即为Spring AOP对动态代理的应用。我们通过查看以上代码能够看到动态代理的作用，能够不改变原有代码而动态的增加我们自己的操作。这样的方式能够实现对我们代码全然的解耦。

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