Spring AOP --JDK动态代理方式

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());
        // 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 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。
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