Spring Aop原理之Advised接口
通过之前我们介绍的ProxyFactory我们知道，Spring Aop是通过ProxyFactory来创建代理对象的。ProxyFactory在创建代理对象时会委托给DefaultAopProxyFactory.createAopProxy(AdvisedSupport config)，DefaultAopProxyFactory内部会分情况返回基于JDK的JdkDynamicAopProxy或基于CGLIB的ObjenesisCglibAopProxy，它俩都实现了Spring的AopProxy接口。AopProxy接口中只定义了一个方法，getProxy()方法，Spring Aop创建的代理对象也就是该接口方法的返回结果。
我们先来看一下基于JDK代理的JdkDynamicAopProxy的getProxy()的逻辑。
@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);
findDefinedEqualsAndHashCodeMethods(proxiedInterfaces);
return Proxy.newProxyInstance(classLoader, proxiedInterfaces, this);
}我们可以看到它最终是通过JDK的Proxy来创建的代理，使用的InvocationHandler实现类是它本身，而使用的接口是AopProxyUtils.completeProxiedInterfaces(this.advised)的返回结果。而这个this.advised对象是AdvisedSupport类型，它是ProxyFactory的父类（间接通过ProxyCreatorSupport继承，ProxyFactory的直接父类是ProxyCreatorSupport，ProxyCreatorSupport的父类是AdvisedSupport），AdvisedSupport的父类是ProxyConfig，ProxyConfig中包含创建代理对象时的一些配置项信息。以下是AopProxyUtils.completeProxiedInterfaces(this.advised)的内部逻辑。
public static Class<?>[] completeProxiedInterfaces(AdvisedSupport advised) {
Class<?>[] specifiedInterfaces = advised.getProxiedInterfaces();
if (specifiedInterfaces.length == 0) {
// No user-specified interfaces: check whether target class is an interface.
Class<?> targetClass = advised.getTargetClass();
if (targetClass != null && targetClass.isInterface()) {
specifiedInterfaces = new Class<?>[] {targetClass};
}
}
boolean addSpringProxy = !advised.isInterfaceProxied(SpringProxy.class);
boolean addAdvised = !advised.isOpaque() && !advised.isInterfaceProxied(Advised.class);
int nonUserIfcCount = 0;
if (addSpringProxy) {
nonUserIfcCount++;
}
if (addAdvised) {
nonUserIfcCount++;
}
Class<?>[] proxiedInterfaces = new Class<?>[specifiedInterfaces.length + nonUserIfcCount];
System.arraycopy(specifiedInterfaces, 0, proxiedInterfaces, 0, specifiedInterfaces.length);
if (addSpringProxy) {
proxiedInterfaces[specifiedInterfaces.length] = SpringProxy.class;
}
if (addAdvised) {
proxiedInterfaces[proxiedInterfaces.length - 1] = Advised.class;
}
return proxiedInterfaces;
}我们可以看到其会在!advised.isOpaque() && !advised.isInterfaceProxied(Advised.class)返回true的情况下加上本文的主角Advised接口。isOpaque()是ProxyConfig中的一个方法，对应的是opaque属性，表示是否禁止将代理对象转换为Advised对象，默认是false。!advised.isInterfaceProxied(Advised.class)表示将要代理的目标对象类没有实现Advised接口，对于我们自己应用的Class来说，一般都不会自己去实现Advised接口的，所以这个通常也是返回true，所以通常创建Aop代理对象时是会创建包含Advised接口的代理对象的，即上述的proxiedInterfaces[proxiedInterfaces.length - 1] = Advised.class会被执行。
前面我们已经提到，JdkDynamicAopProxy创建代理对象应用的InvocationHandler是其自身，所以我们在调用JdkDynamicAopProxy创建的代理对象的任何方法时都将调用JdkDynamicAopProxy实现的InvocationHandler接口的invoke(Object proxy, Method method, Object[] args)方法。该方法实现如下：
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);
}
}
}其中关于Advised接口方法调用最核心的一句是如下这句。我们可以看到，当我们调用的目标方法是定义自Advised接口时，对应方法的调用将委托给AopUtils.invokeJoinpointUsingReflection(this.advised, method, args)，invokeJoinpointUsingReflection方法的逻辑比较简单，是通过Java反射来调用目标方法。在这里invokeJoinpointUsingReflection传递的目标对象正是AdvisedSupport类型的this.advised对象。
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);
}AdvisedSupport类是实现了Advised接口的，所以Spring Aop创建了基于Advised接口的代理对象后在调用Advised接口方法时可以把它委托给AdvisedSupport。而我们知道Spring Aop代理对象的创建正是基于AdvisedSupport的配置进行的（配置项主要都定义在AdvisedSupport的父类ProxyConfig类中）。创建代理对象时应用AdvisedSupport，调用Advised接口方法也用同一个实现了Advised接口的AdvisedSupport对象，所以这个过程在Spring Aop内部就可以很好的衔接。接着我们来看一下Advised接口的定义。
public interface Advised extends TargetClassAware {
boolean isFrozen();
boolean isProxyTargetClass();
Class<?>[] getProxiedInterfaces();
boolean isInterfaceProxied(Class<?> intf);
void setTargetSource(TargetSource targetSource);
TargetSource getTargetSource();
void setExposeProxy(boolean exposeProxy);
boolean isExposeProxy();
void setPreFiltered(boolean preFiltered);
boolean isPreFiltered();
Advisor[] getAdvisors();
void addAdvisor(Advisor advisor) throws AopConfigException;
void addAdvisor(int pos, Advisor advisor) throws AopConfigException;
boolean removeAdvisor(Advisor advisor);
void removeAdvisor(int index) throws AopConfigException;
int indexOf(Advisor advisor);
boolean replaceAdvisor(Advisor a, Advisor b) throws AopConfigException;
void addAdvice(Advice advice) throws AopConfigException;
void addAdvice(int pos, Advice advice) throws AopConfigException;
boolean removeAdvice(Advice advice);
int indexOf(Advice advice);
String toProxyConfigString();
}Advised接口中定义的方法还是非常多的，通过它我们可以在运行时了解我们的代理对象是基于CGLIB的还是基于JDK代理的；可以了解我们的代理对应应用了哪些Advisor；也可以在运行时给我们的代理对象添加和删除Advisor/Advise。本文旨在描述Spring Aop在创建代理对象时是如何基于Advised接口创建代理的，以及我们能够应用Advised接口做哪些事。文中应用的是Spring创建基于JDK代理对象的过程为示例讲解的，其实基于CGLIB的代理也是一样的。关于CGLIB的代理过程、本文中描述的一些核心类以及本文的核心——Advised接口的接口方法说明等请有兴趣的朋友参考Spring的API文档和相关的源代码。
（注：本文是基于Spring4.1.0所写，Elim写于2017年5月15日）
