我们知道Spring是通过JDK或者CGLib实现动态代理的，今天我们讨论一下JDK实现动态代理的原理。
一、简述
Spring在解析Bean的定义之后会将Bean的定义生成一个BeanDefinition对象并且由BeanDefinitionHolder对象持有。在这个过程中，如果Bean需要被通知切入，BeanDefinition会被重新转换成一个proxyDefinition（其实也是一个BeanDefinition对象，只不过描述的是一个ProxyFactoryBean）。ProxyFactoryBean是一个实现了FactoryBean的接口，用来生成被被切入的对象。Spring AOP的实现基本上是通过ProxyFactoryBean实现的。我们今天讨论的重点也是这个类。
在讨论ProxyFactoryBean之前，我们先看一下一个BeanDefinition转换成proxyDefintion的过程。
public final BeanDefinitionHolder decorate(Node node, BeanDefinitionHolder definitionHolder, ParserContext parserContext) {
BeanDefinitionRegistry registry = parserContext.getRegistry();
// get the root bean name - will be the name of the generated proxy factory bean
String existingBeanName = definitionHolder.getBeanName();
BeanDefinition targetDefinition = definitionHolder.getBeanDefinition();
BeanDefinitionHolder targetHolder = new BeanDefinitionHolder(targetDefinition, existingBeanName + ".TARGET");
// delegate to subclass for interceptor definition
BeanDefinition interceptorDefinition = createInterceptorDefinition(node);
// generate name and register the interceptor
String interceptorName = existingBeanName + "." + getInterceptorNameSuffix(interceptorDefinition);
BeanDefinitionReaderUtils.registerBeanDefinition(
new BeanDefinitionHolder(interceptorDefinition, interceptorName), registry);
BeanDefinitionHolder result = definitionHolder;
if (!isProxyFactoryBeanDefinition(targetDefinition)) {
// create the proxy definition 这里创建proxyDefinition对象，并且从原来的BeanDefinition对象中复制属性
RootBeanDefinition proxyDefinition = new RootBeanDefinition();
// create proxy factory bean definition
proxyDefinition.setBeanClass(ProxyFactoryBean.class);
proxyDefinition.setScope(targetDefinition.getScope());
proxyDefinition.setLazyInit(targetDefinition.isLazyInit());
// set the target
proxyDefinition.setDecoratedDefinition(targetHolder);
proxyDefinition.getPropertyValues().add("target", targetHolder);
// create the interceptor names list
proxyDefinition.getPropertyValues().add("interceptorNames", new ManagedList<String>());
// copy autowire settings from original bean definition.
proxyDefinition.setAutowireCandidate(targetDefinition.isAutowireCandidate());
proxyDefinition.setPrimary(targetDefinition.isPrimary());
if (targetDefinition instanceof AbstractBeanDefinition) {
proxyDefinition.copyQualifiersFrom((AbstractBeanDefinition) targetDefinition);
}
// wrap it in a BeanDefinitionHolder with bean name
result = new BeanDefinitionHolder(proxyDefinition, existingBeanName);
}
addInterceptorNameToList(interceptorName, result.getBeanDefinition());
return result;
}
二、ProxyFactoryBean的原理
我们先来看一下ProxyFactoryBean的继承关系：
ProxyFactoryBean类图
ProxyFactoryBean实现了FactoryBean、BeanClassLoaderAware、BeanFactoryAware接口，这里就不多说了。ProxyCreatorSupport这个类则是创建代理对象的关键所在。
我们先来看看产生代理对象的方法：
public Object getObject() throws BeansException {
initializeAdvisorChain();
if (isSingleton()) {
//单例
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.");
}
//非单例
return newPrototypeInstance();
}
}
initializeAdvisorChain() 方法是将通知链实例化。然后判断对象是否要生成单例而选择调用不同的方法，这里我们只看生成单例对象的方法。
private synchronized Object getSingletonInstance() {
if (this.singletonInstance == null) {
this.targetSource = freshTargetSource();
//如果以接口的方式代理对象
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");
}
//获取目标类实现的所有接口，并注册给父类的interfaces属性，为jdk动态代理做准备
setInterfaces(ClassUtils.getAllInterfacesForClass(targetClass, this.proxyClassLoader));
}
// Initialize the shared singleton instance.
super.setFrozen(this.freezeProxy);
//这里产生代理对象
this.singletonInstance = getProxy(createAopProxy());
}
return this.singletonInstance;
}
我们可以看到，产生代理对象是通过getProxy()方法实现的，这个方法我们看一下：
protected Object getProxy(AopProxy aopProxy) {
return aopProxy.getProxy(this.proxyClassLoader);
}
AopProxy对象的getProxy()方法产生我们需要的代理对象，究竟AopProxy这个类是什么，我们接下来先看一下产生这个对象的方法createAopProxy()：
protected final synchronized AopProxy createAopProxy() {
if (!this.active) {
activate();
}
return getAopProxyFactory().createAopProxy(this);
}
createAopProxy方法：
public AopProxy createAopProxy(AdvisedSupport config) throws AopConfigException {
//目标对象不是接口类的实现或者没有提供代理接口
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.");
}
//代理对象自身是接口
if (targetClass.isInterface()) {
return new JdkDynamicAopProxy(config);
}
return new ObjenesisCglibAopProxy(config);
}
else {
return new JdkDynamicAopProxy(config);
}
}
在这里我们只看JdkDynamicAopProxy这个类的实现，我们前面提到，真正代理对象的生成是由AopProxy的getProxy方法完成的，这里我们看一下JdkDynamicAopProxy的getProxy方法，这也是本文讨论的重点：
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);
}
我们看可以很清楚的看到，代理对象的生成直接使用了jdk动态代理:Proxy.newProxyInstance(classLoader, proxiedInterfaces, this);而代理逻辑是通过实现了InvocationHandler接口的invoke方法实现的。而这里用到的实现了InvocationHandler接口的类就是JdkDynamicAopProxy自身。JdkDynamicAopProxy自身实现了InvocationHandler接口，完成了Spring AOP拦截器链拦截等一系列逻辑，我们看一下JdkDynamicAopProxy的invoke方法的具体实现：
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 {
//没有重写equals方法
if (!this.equalsDefined && AopUtils.isEqualsMethod(method)) {
// The target does not implement the equals(Object) method itself.
return equals(args[0]);
}
//没有重写hashCode方法
if (!this.hashCodeDefined && AopUtils.isHashCodeMethod(method)) {
// The target does not implement the hashCode() method itself.
return hashCode();
}
//代理的类是Advised，这里直接执行，不做任何代理
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.
//获得代理对象
target = targetSource.getTarget();
if (target != null) {
targetClass = target.getClass();
}
// Get the interception chain for this method.
//获得已经定义的拦截器链
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.
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.
//拦截器链是空的，直接执行需要代理的方法
retVal = AopUtils.invokeJoinpointUsingReflection(target, method, args);
}
else {
// We need to create a method invocation...
//这里是调用拦截器链的地方，先创建一个MethodInvocation对象，然后调用该对象的proceed方法完成拦截器链调用
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();
//这里处理返回值，判断返回值和方法需要的返回是否一致
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);
}
}
}
拦截器链的调用
从上面的代码和注释中我们可以看到spring实现aop的主要流程，具体如何调用拦截器链，我们来看一下MethodInvocation的proceed方法
public Object proceed() throws Throwable {
// We start with an index of -1 and increment early.
// currentInterceptorIndex是从-1开始的，所以拦截器链调用结束的时候index是 this.interceptorsAndDynamicMethodMatchers.size() - 1
// 调用链结束后执行目标方法
if (this.currentInterceptorIndex == this.interceptorsAndDynamicMethodMatchers.size() - 1) {
return invokeJoinpoint();
}
// 获得当前处理到的拦截器
Object interceptorOrInterceptionAdvice =
this.interceptorsAndDynamicMethodMatchers.get(++this.currentInterceptorIndex);
// 这里判断是否是InterceptorAndDynamicMethodMatcher，如果是，这要判断是否匹配methodMatcher，不匹配则此拦截器不生效
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);
}
}
proceed()方法是一个递归方法，我们可以根据代码的注释知道大体逻辑，InterceptorAndDynamicMethodMatcher的代码如下，我们可以看到，InterceptorAndDynamicMethodMatcher 持有一个MethodInterceptor 对象和一个MethodMatcher 对象，在拦截器链调用过程中，如果拦截器是InterceptorAndDynamicMethodMatcher ，则会先根据MethodMatcher 判断是否匹配，匹配MethodInterceptor 才会生效。
class InterceptorAndDynamicMethodMatcher {
final MethodInterceptor interceptor;
final MethodMatcher methodMatcher;
public InterceptorAndDynamicMethodMatcher(MethodInterceptor interceptor, MethodMatcher methodMatcher) {
this.interceptor = interceptor;
this.methodMatcher = methodMatcher;
}
}
至于MethodInterceptor 是什么，MethodInterceptor 的逻辑是怎么样的，我们可以看一下MethodInterceptor 的一个子类AfterReturningAdviceInterceptor的实现：
public class AfterReturningAdviceInterceptor implements MethodInterceptor, AfterAdvice, Serializable {
private final AfterReturningAdvice advice;
/**
* Create a new AfterReturningAdviceInterceptor for the given advice.
* @param advice the AfterReturningAdvice to wrap
*/
public AfterReturningAdviceInterceptor(AfterReturningAdvice advice) {
Assert.notNull(advice, "Advice must not be null");
this.advice = advice;
}
@Override
public Object invoke(MethodInvocation mi) throws Throwable {
Object retVal = mi.proceed();
this.advice.afterReturning(retVal, mi.getMethod(), mi.getArguments(), mi.getThis());
return retVal;
}
}
AfterReturningAdviceInterceptor的作用是在被代理的方法返回结果之后添加我们需要的处理逻辑，其实现方式我们可以看到，先调用MethodInvocation 的proceed，也就是先继续处理拦截器链，等调用完成后执行我们需要的逻辑：this.advice.afterReturning(retVal, mi.getMethod(), mi.getArguments(), mi.getThis());
到这里，spring使用jdk动态代理实现aop的分析基本上结束，其中拦截器链的调用比较难懂而且比较重要，需要的同学可以多看看这一块。
