AOP就是面向切面编程，我们可以从几个层面来实现AOP。
- 性能监控，在方法调用前后记录调用时间，方法执行太长或超时报警。
- 缓存代理，缓存某方法的返回值，下次执行该方法时，直接从缓存里获取。
- 软件破解，使用AOP修改软件的验证类的判断逻辑。
- 记录日志，在方法执行前后记录系统日志。
- 工作流系统，工作流系统需要将业务代码和流程引擎代码混合在一起执行，那么我们可以使用AOP将其分离，并动态挂接业务。
- 权限验证，方法执行前验证是否有权限执行当前方法，没有则抛出没有权限执行异常，由业务代码捕捉。
本篇将结合动态代理的源代码讲解其实现原理：
1. 先分析Advice
before执行Cglib2AopProxy的intercept方法：
/** * General purpose AOP callback. Used when the target is dynamic or when the * proxy is not frozen. */ private static class DynamicAdvisedInterceptor implements MethodInterceptor, Serializable { private AdvisedSupport advised; public DynamicAdvisedInterceptor(AdvisedSupport advised) { this.advised = advised; } public Object intercept(Object proxy, Method method, Object[] args, MethodProxy methodProxy) throws Throwable { MethodInvocation invocation = null; Object oldProxy = null; boolean setProxyContext = false; Class targetClass = null; Object target = null; try { Object retVal = null; 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 chain = this.advised.getInterceptorsAndDynamicInterceptionAdvice(method, targetClass); // 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... invocation = new CglibMethodInvocation(proxy, target, method, args, targetClass, chain, methodProxy); // If we get here, we need to create a MethodInvocation. retVal = invocation.proceed(); } retVal = massageReturnTypeIfNecessary(proxy, target, method, retVal); return retVal; } finally { if (target != null) { releaseTarget(target); } if (setProxyContext) { // Restore old proxy. AopContext.setCurrentProxy(oldProxy); } } }复制代码 第一步：获取target
target.getClass();
第二步：获取拦截器和advice，返回定义好的
org.springframework.aop.framework.adapter.MethodBeforeAdviceInterceptor示例
/** * Determine a list of {@link org.aopalliance.intercept.MethodInterceptor} objects * for the given method, based on this configuration. * @param method the proxied method * @param targetClass the target class * @return List of MethodInterceptors (may also include InterceptorAndDynamicMethodMatchers) */ public List getInterceptorsAndDynamicInterceptionAdvice(Method method, Class targetClass) { MethodCacheKey cacheKey = new MethodCacheKey(method); List cached = (List) this.methodCache.get(cacheKey); if (cached == null) { cached = this.advisorChainFactory.getInterceptorsAndDynamicInterceptionAdvice( this, method, targetClass); this.methodCache.put(cacheKey, cached); } return cached; }复制代码 第三步创建一个方法的invocation
// We need to create a method invocation... invocation = new CglibMethodInvocation(proxy, target, method, args, targetClass, chain, methodProxy);复制代码
第四步 执行aop的before方法
public void before(Method method, Object[] args, Object target) throws Throwable { System.out.println(" Before method!"); }复制代码 第五步 触发MethodBeforeAdviceInterceptor的invoke方法
public Object invoke(MethodInvocation mi) throws Throwable { this.advice.before(mi.getMethod(), mi.getArguments(), mi.getThis() ); return mi.proceed(); }复制代码 第六步：触发ReflectiveMethodInvocation的process方法
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); } }复制代码 第七步，包装返回值Cglib2AopProxy
/** * Wrap a return of this if necessary to be the proxy */ private static Object massageReturnTypeIfNecessary(Object proxy, Object target, Method method, Object retVal) { // Massage return value if necessary if (retVal != null && retVal == target && !RawTargetAccess.class.isAssignableFrom(method.getDeclaringClass())) { // Special case: it returned "this". // Note that we can't help if the target sets a reference // to itself in another returned object. retVal = proxy; } return retVal; }复制代码 最后执行finanly方法
finally { if (target != null) { releaseTarget(target); } if (setProxyContext) { // Restore old proxy. AopContext.setCurrentProxy(oldProxy); }复制代码 before，after，around，throw基本相似，不一一赘述
2.PointCut和Advisor为例
2.1 创建代理的过程
首先是ProxyFactoryBean获取对象代理
/** * Return a proxy. Invoked when clients obtain beans from this factory bean. * Create an instance of the AOP proxy to be returned by this factory. * The instance will be cached for a singleton, and create on each call to * getObject() for a proxy. * @return a fresh AOP proxy reflecting the current state of this factory */ 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(); } }复制代码 获取过程如下：
/** * 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. 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; } 复制代码 父类创建代理的过程
/** * Subclasses should call this to get a new AOP proxy. They should not * create an AOP proxy with this as an argument. */ protected final synchronized AopProxy createAopProxy() { if (!this.active) { activate(); } return getAopProxyFactory().createAopProxy(this); }复制代码 调用代理工厂创建代理的过程
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); } if (!cglibAvailable) { throw new AopConfigException( "Cannot proxy target class because CGLIB2 is not available. " + "Add CGLIB to the class path or specify proxy interfaces."); } return CglibProxyFactory.createCglibProxy(config); } else { return new JdkDynamicAopProxy(config); } }复制代码