Spring AOP 由浅入深

（分析基于Spring的版本：version = 4.1.6.RELEASE）

SpringAOP 和 AspectJ 的关系：
它们是两种不同的编程风格， SpringAOP 使用 xml 配置的形式配置 aop。而 AspectJ 使用 AspectJ 的注解来配置 aop

aspect、JoinPoint、Pointcut、Weaving、Advice
JoinPoint: 连接点。表示目标对象中的方法
Pointcut: 切点。表示连接点的集合
Weaving: 织入。把代理逻辑加入到目标对象上的过程叫织入
Advice: 通知。包括 “around”, “before” and “after 等

this、target
this: 产生的代理对象
target: 被代理的原始对象

JoinPoint、ProceedingJoinPoint
JoinPoint 接口可以拿到连接点的相关信息，比如：方法签名、方法参数、this、target
ProceedingJoinPoint 继承自 JoinPoint，它是用来支持环绕（around）通知的，多暴露了一个 proceed() 方法，用来执行目标对象的方法。

Spring AOP工厂： org.springframework.aop.framework.DefaultAopProxyFactory

　　@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()) {
                return new JdkDynamicAopProxy(config);
            }
            return new ObjenesisCglibAopProxy(config);
        }
        else {
            return new JdkDynamicAopProxy(config);
        }
    }

从代码可以看出：
1. 如果AdvisedSupport中没有进行自定义设置，Spring去创建代理的时候，会默认使用JDK动态代理。
2. 如果AdvisedSupport中进行了设置，那么就去判断被代理的类是不是接口，如果是接口的话就使用JDK动态代理，否则才使用Cglib的代理。

（2020.1.17 更新）
从源码看， proxyTargetClass=true 时，如果 targetClass 是接口 or targetClass 是一个代理类型，也有可能走 jdk 动态代理。但是绝大多数情况是不会的。
首先，在源码中找到 targetClass 赋值的地方，可以发现 targetClass 是 bean 的实例的 class，所以通常是一个实体类，不是接口。至于 targetClass 什么时候会是代理类，还不太清楚。

那既然 targetClass 是 bean 的实例的 class 的话，它并不是一个接口，那么当 proxyTargetClass=false 的时候， jdk 怎么来产生代理呢？
答案是，spring 会找到 targetClass 对应的接口来生成代理，如果这个类没有接口的话，会将 proxyTargetClass 设置成 true，从而使用 cglib 来生成代理

Spring 基于JDK动态代理的AOP实现解析：
我们知道，使用JDK的动态代理，主要是实现 java.lang.reflect.InvocationHandler#invoke(Object obj, Method method, Object[] aobj)方法。
Spring相关的源码如下：JdkDynamicAopProxy.class

　　@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)) {
                // 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);
            }
        }
    }

从上面可以看出，spring 会先拿到一个 MethodInterceptor 的 chain 。

如果chain为空的话，就直接反射调用原方法；否则，就会根据这个 chain 来生成一个 MethodInvocation ，然后去执行 invocation.proceed() 。
invocation.proceed() 方法的实现，是递归调用 chain 中的 MethodInterceptor。

也就是说，我们所有的AOP拦截，都是通过一连串的 MethodInterceptor 来实现的。
而这个 MethodInterceptor 的 chain 是通过 List<Object> chain = this.advised.getInterceptorsAndDynamicInterceptionAdvice(method, targetClass); 来生成的。
也就是基于 AdvisedSupport 来生成的。

Spring 对 AdvisedSupport 的注释是：Base class for AOP proxy configuration managers. 也就是说，AdvisedSupport 是产生AOP代理的配置管理者。

AdvisedSupport 中保存了一个List<Advisor> advisors 的集合。
满足条件的Advisor（即：能对此method进行拦截的Advisor），最终会被转换成 MethodInterceptor ，从而添加到chain中。

Spring 基于Cglib的AOP实现解析：
我们知道，使用Cglib实现AOP时，最主要的是要设置Callback来对Method进行拦截，即：void net.sf.cglib.proxy.Enhancer.setCallback(Callback callback) 。

spring生成Callback的源码如下：CglibAopProxy.class

private Callback[] getCallbacks(Class<?> rootClass) throws Exception {
        // Parameters used for optimisation choices...
        boolean exposeProxy = this.advised.isExposeProxy();
        boolean isFrozen = this.advised.isFrozen();
        boolean isStatic = this.advised.getTargetSource().isStatic();

        // Choose an "aop" interceptor (used for AOP calls).
        Callback aopInterceptor = new DynamicAdvisedInterceptor(this.advised);

            ..............
}

从上面的代码可以看出，spring在生成 Callback 时，也是基于 AdvisedSupport 来的（同JDK动态代理生成 MethodInterceptor 的chain一样）。

CglibAopProxy$DynamicAdvisedInterceptor.class

private static class DynamicAdvisedInterceptor implements MethodInterceptor, Serializable {

        private final AdvisedSupport advised;

        public DynamicAdvisedInterceptor(AdvisedSupport advised) {
            this.advised = advised;
        }

        @Override
        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);
                }
            }
        }
...............
}

不难看出，最终JDK动态代理与 Cglib代理对AOP的实现都是基于 AdvisedSupport 的 MethodInterceptor 链。

参考：

http://www.jianshu.com/p/0734ec855183

http://www.cnblogs.com/digdeep/p/4528353.html

http://my.oschina.net/robinyao/blog/649518

http://lgbolgger.iteye.com/blog/2180251

http://www.cnblogs.com/chanedi/p/4552555.html （原码说明）

public class DefaultAopProxyFactory implements AopProxyFactory, Serializable {

class CglibAopProxy implements AopProxy, Serializable {

Spring 异步@Async也是基于Spring AOP来做的：
在bean创建完成后，会调用 AsyncAnnotationBeanPostProcessor.postProcessAfterInitialization(Object bean, String beanName) 来生成一个代理bean

2018.07.23 add：
编程式使用aop https://blog.csdn.net/qq_26525215/article/details/52422395/

2019.01.11 add:
spring @Transactional 的原理解析：
TransactionAspectSupport#completeTransactionAfterThrowing 对 @Transactional 事务的处理，默认回滚 RuntimeException
https://www.ibm.com/developerworks/cn/java/j-master-spring-transactional-use/index.html
https://docs.spring.io/spring/docs/current/spring-framework-reference/data-access.html#tx-decl-explained
https://doanduyhai.wordpress.com/2011/11/20/spring-transactional-explained/