Spring AOP实现原理

AOP的实现者

AOP工具的设计目标是把横切的问题（如性能监视、事务管理）模块化。使用类似于OOP的方式进行切面的编程工作。位于AOP工具核心的是连接点模型，它提供了一种机制，可以识别出在哪里发生了横切。

AspectJ

AspectJ是语言级的AOP实现，2001年由Xerox PARC的AOP小组发布。AspectJ扩展了Java语言，定义了AOP语法，能够在编译器提供横切代码的织入，所以它有一个专门的编译器用来生成遵守Java字节编码规范的Class文件。

AspectWerkz

基于Java的简单、动态、轻量级的AOP框架，2002年发布。支持运行期或类装载期织入横切代码，所以它拥有一个特殊的类装载器。现在已与AspectJ项目合并，合作的第一个版本是AspectJ 5，扩展AspectJ语言，以基于注解的方式支持类似AspectJ的代码风格。

JBoss AOP

2004作为JBoss应用服务器的扩展功能发布。

Spring AOP

Spring AOP 使用纯Java实现，它不需要专门的编译过程，不需要特殊的类装载器，它在运行期通过代理方式想目标类织入增强代码。Spring并不尝试提供最完整的AOP实现，相反，它侧重于提供一种和IoC容器整合的AOP实现，用以解决企业级开发中的常见问题。在Spring中，我们可以无缝地将Spring AOP、IoC和AspectJ整合在一起。

AOP联盟

AOP联盟(aopalliance.sourceforge.net)是众多开源AOP项目的联合组织，该组织的目的是为了制定一套规范描述AOP的标准，定义标准的AOP接口，以便各种遵守标准的具体实现可以相互调用。现以成为事实上的标准。

Spring对AOP的支持

Spring在2.0以后对AOP功能进行了重要的增强，主要表现在以下几个方面：

- 新增了基于Schema的配置支持，为AOP专门提供了aop命名空间。

- 新增了对AspectJ切点表达式语言支持。使用和@AspectJ相同风格的注解，并通过AspectJ提供的注解库和解析库处理切点。当然，由于Spring只支持方法级的切点，所以仅对@AspectJ提供了有限的支持。

- 可以无缝地继承AspectJ，AspectJ提供了语言级切面的实现。

我们所说的Spring AOP，它包括基于XML配置的AOP和基于@AspectJ注解的AOP，这两种方法虽然在配置切面时的表现方式不同，但底层都是采用动态代理技术（JDK代理或CGLib代理）。Spring可以集成AspectJ，但AspectJ本身并不属于Spring AOP的范畴。

在一般情况下，对于开发JAVAEE企业应用的开发者而言，Spring AOP已经可以满足使用的要求，虽然AspectJ提供对AOP更为细致的实现，但像实例化切面、属性访问切面、条件切面等功能在实际应用中并不常用。

如果是基于JDK5.0的项目，推荐使用Spring提供的@AspectJ配置方式，因为这种方式能以更简单的更直接的方式应用切面。

 

ProxyFactoryBean(aspectj方案则是AspectJProxyFactory)

 

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();
		}
	}

 ProxyCreatorSupport

 

 

protected final synchronized AopProxy createAopProxy() {
		if (!this.active) {
			activate();
		}
		return getAopProxyFactory().createAopProxy(this);
	}

 DefaultAopProxyFactory

 

 

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);
		}
	}

我们回忆一下我们做过的jdk代理的例子：

 

 

UserMgr mgr = new UserMgrImpl();
InvocationHandler h = new TransactionHandler(mgr);
UserMgr u = (UserMgr) Proxy.newProxyInstance(UserMgr.class, h);

 生成的代理是比如$Proxy34，h是$Proxy34的成员变量，

 

public class Proxy implements java.io.Serializable {
  /** prefix for all proxy class names */
    private final static String proxyClassNamePrefix = "$Proxy";
  /**
     * the invocation handler for this proxy instance.
     * @serial
     */
    protected InvocationHandler h;
}

 

在spring aop中正是JdkDynamicAopProxy。那么重点来了，我们就从JdkDynamicAopProxy的invoke方法看起：

 

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.
		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.
		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.
		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);
		}
	}
}

 ReflectiveMethodInvocation implements ProxyMethodInvocation extends MethodInvocation extends Invocation extends Joinpoint

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 { // eg. ExposeInvocationInterceptor
		// 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);
	}
}

 

package org.aopalliance.intercept; // Aop联盟

import java.lang.reflect.AccessibleObject;

public interface Joinpoint {
    Object proceed() throws Throwable;

    Object getThis();

    AccessibleObject getStaticPart();
}

 interceptor.invoker - eg. MethodBeforeAdviceInterceptor

public Object invoke(MethodInvocation mi) throws Throwable {
	this.advice.before(mi.getMethod(), mi.getArguments(), mi.getThis() );
	return mi.proceed(); // 又回归到了上面的调用
}

 值得注意的是，虽然切面可以只用到一个类的部分方法上，但我们调用其他方法时，仍然会经历上面的逻辑，此时拦截器链里只有一个interceptor - ExposeInvocationInterceptor

public Object invoke(MethodInvocation mi) throws Throwable {
	MethodInvocation oldInvocation = invocation.get();
	invocation.set(mi);
	try {
		return mi.proceed();
	}
	finally {
		invocation.set(oldInvocation);
	}
}

 

 Aop应用参考：

一、分库分表http://wely.iteye.com/blog/2275725

二、方法性能监控

package com.itlong.bjxizhan.support.web.service.monitor;

import org.aspectj.lang.ProceedingJoinPoint;
import org.aspectj.lang.annotation.Around;
import org.aspectj.lang.annotation.Aspect;

/**
 * Created by shenhongxi on 2016/8/10.
 */
@Aspect
public class MonitorAspect {

    private String tagPrefix;

    @Around(
            value = "execution(* *(..)) && @annotation(monitor)",
            argNames = "pjp,monitor"
    )
    public Object doUmpLogging(ProceedingJoinPoint pjp, Monitor monitor) throws Throwable {
        // String tag = monitor.tag();
        // boolean heart = monitor.heart();
        long start = System.currentTimeMillis();
        // record invocation (times)
        Object obj = null;
        try {
            obj = pjp.proceed();
        } catch (Exception e) {
            // record error
            throw e;
        } finally {
            long end = System.currentTimeMillis();
            // record time -> end - start
        }
        return obj;
    }

    public String getTagPrefix() {
        return tagPrefix;
    }

    public void setTagPrefix(String tagPrefix) {
        this.tagPrefix = tagPrefix;
    }
}

@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.METHOD, ElementType.CONSTRUCTOR})
public @interface Monitor {
    String DEFAULT_TAG_NAME = "@@USE_METHOD_NAME";

    String tag() default "@@USE_METHOD_NAME";

    String message() default "";

    boolean heart() default false;

}

 另外，性能监控拦截器可参考org.springframework.aop.interceptor.PerformanceMonitorInterceptor