Spring源码学习之认识AOP

前言

作为Spring的重要特性之一，AOP的作用当然是毋庸置疑的。它使我们可以在代码执行过程中，横切代码，封装那些重复执行的操作。保证代码的模块化和管理时的便捷性。而这篇博文将从源码的角度解析AOP的工作原理和使用过程：

简单的AOP实现

<bean id="testadvice" class="chapter3_AOP.TestAdvice">bean>
<bean id="aop" class="org.springframework.aop.framework.ProxyFactoryBean">
        
        <property name="proxyInterfaces">
            <value>chapter3_AOP.OperateApivalue>
        property>
        <property name="target">
            <bean class="chapter3_AOP.OperateTarget">bean>
        property>
        <property name="interceptorNames">
            <list>
                <value>testadvicevalue>
            list>
        property>
    bean>

// OperateTarget.java 
public class OperateTarget implements OperateApi{
    @Override
    public void update() {
        System.out.println("update");
    }
}

// TestAdvice.java
public class TestAdvice implements MethodBeforeAdvice{

    @Override
    public void before(Method method, Object[] args, Object target) throws Throwable {
        System.out.println("prehandle");
    }

}

// client.java
public class client {
    public static void main(String []args){
        ApplicationContext ac = new ClassPathXmlApplicationContext("bean.xml");
        OperateApi api = (OperateApi)ac.getBean("aop");
        api.update();
    }
}

// 输出
prehandle
update

AOP流程

1.生成代理对象

ProxyFactoryBean的实现是Spring IoC环境中创建AOP的底层方法，也是最灵活的方法。因此通过ProxyFactoryBean的使用，来看看AOP的运行到底是怎样的过程。

// ProxyFactoryBean.java
// 获得生成的代理对象
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();
        }
    }

// 创建拦截器链
private synchronized void initializeAdvisorChain() throws AopConfigException, BeansException {
        // 如果已经初始化过了，那么直接返回
        if (this.advisorChainInitialized) {
            return;
        }

        if (!ObjectUtils.isEmpty(this.interceptorNames)) {
            if (this.beanFactory == null) {
                throw new IllegalStateException("No BeanFactory available anymore (probably due to serialization) " +
                        "- cannot resolve interceptor names " + Arrays.asList(this.interceptorNames));
            }
            if (this.interceptorNames[this.interceptorNames.length - 1].endsWith(GLOBAL_SUFFIX) &&
                    this.targetName == null && this.targetSource == EMPTY_TARGET_SOURCE) {
                throw new AopConfigException("Target required after globals");
            }

            // 遍历设置的拦截器
            for (String name : this.interceptorNames) {
                if (logger.isTraceEnabled()) {
                    logger.trace("Configuring advisor or advice '" + name + "'");
                }


                if (name.endsWith(GLOBAL_SUFFIX)) {
                    if (!(this.beanFactory instanceof ListableBeanFactory)) {
                        throw new AopConfigException(
                                "Can only use global advisors or interceptors with a ListableBeanFactory");
                    }
                  // 添加所有全局拦截器，主要过程是在beanfactory找出所有advisor和interceptor，
                  // 添加到advisors中
                    addGlobalAdvisor((ListableBeanFactory) this.beanFactory,
                            name.substring(0, name.length() - GLOBAL_SUFFIX.length()));
                }

                else {
                    Object advice;
                    if (this.singleton || this.beanFactory.isSingleton(name)) {
                        // 之前分析过的从beanfactory中直接获取对象
                        advice = this.beanFactory.getBean(name);
                    }
                    else {
                        advice = new PrototypePlaceholderAdvisor(name);
                    }
                    addAdvisorOnChainCreation(advice, name);
                }
            }
        }

        this.advisorChainInitialized = true;
    }

// 添加拦截器
private void addAdvisorOnChainCreation(Object next, String name) {
        // 将获得的类转换成advisor。如果是advice，将advice作为参数生成一个新的advisor
        Advisor advisor = namedBeanToAdvisor(next);
        if (logger.isTraceEnabled()) {
            logger.trace("Adding advisor with name '" + name + "'");
        }
        addAdvisor(advisor);
    }

// 获取单例实例
private synchronized Object getSingletonInstance() {
        if (this.singletonInstance == null) {
            // 根据设置的targetName获取实例并包裹在targetSource中
            this.targetSource = freshTargetSource();
            if (this.autodetectInterfaces && getProxiedInterfaces().length == 0 && !isProxyTargetClass()) {
                // 获得Class
                Class targetClass = getTargetClass();
                if (targetClass == null) {
                    throw new FactoryBeanNotInitializedException("Cannot determine target class for proxy");
                }
                // 获得所有超类的接口，并设置到接口属性中
                setInterfaces(ClassUtils.getAllInterfacesForClass(targetClass, this.proxyClassLoader));
            }
            // Initialize the shared singleton instance.
            super.setFrozen(this.freezeProxy);
            // 生成代理实例的关键
            this.singletonInstance = getProxy(createAopProxy());
        }
        return this.singletonInstance;
    }


private synchronized Object newPrototypeInstance() {
        // In the case of a prototype, we need to give the proxy
        // an independent instance of the configuration.
        // In this case, no proxy will have an instance of this object's configuration,
        // but will have an independent copy.
        if (logger.isTraceEnabled()) {
            logger.trace("Creating copy of prototype ProxyFactoryBean config: " + this);
        }

        ProxyCreatorSupport copy = new ProxyCreatorSupport(getAopProxyFactory());
        // The copy needs a fresh advisor chain, and a fresh TargetSource.
        TargetSource targetSource = freshTargetSource();
        copy.copyConfigurationFrom(this, targetSource, freshAdvisorChain());
        if (this.autodetectInterfaces && getProxiedInterfaces().length == 0 && !isProxyTargetClass()) {
            // Rely on AOP infrastructure to tell us what interfaces to proxy.
            copy.setInterfaces(
                    ClassUtils.getAllInterfacesForClass(targetSource.getTargetClass(), this.proxyClassLoader));
        }
        copy.setFrozen(this.freezeProxy);

        if (logger.isTraceEnabled()) {
            logger.trace("Using ProxyCreatorSupport copy: " + copy);
        }
        return getProxy(copy.createAopProxy());
    }

    protected final synchronized AopProxy createAopProxy() {
        if (!this.active) {
            activate();
        }
        return getAopProxyFactory().createAopProxy(this);
    }

    // DefaultAopProxyFactory.java
    @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.");
            }
            // 如果是接口类或是是代理类，使用jdk动态代理
            if (targetClass.isInterface() || Proxy.isProxyClass(targetClass)) {
                return new JdkDynamicAopProxy(config);
            }
            return new ObjenesisCglibAopProxy(config);
        }
        else {
            return new JdkDynamicAopProxy(config);
        }
    }

// 根据aopPorxy是哪种代理方式决定不同的生成object方式
protected Object getProxy(AopProxy aopProxy) {
        return aopProxy.getProxy(this.proxyClassLoader);
    }

    //AdvisorSupport.java
    //以下代码主要功能在于将获取的advisor添加到advisors链表中，辅以清除缓存
    @Override
    public void addAdvisor(Advisor advisor) {
        int pos = this.advisors.size();
        addAdvisor(pos, advisor);
    }

    @Override
    public void addAdvisor(int pos, Advisor advisor) throws AopConfigException {
        if (advisor instanceof IntroductionAdvisor) {
            validateIntroductionAdvisor((IntroductionAdvisor) advisor);
        }
        addAdvisorInternal(pos, advisor);
    }

    private void addAdvisorInternal(int pos, Advisor advisor) throws AopConfigException {
        Assert.notNull(advisor, "Advisor must not be null");
        if (isFrozen()) {
            throw new AopConfigException("Cannot add advisor: Configuration is frozen.");
        }
        if (pos > this.advisors.size()) {
            throw new IllegalArgumentException(
                    "Illegal position " + pos + " in advisor list with size " +         this.advisors.size());
        }
        this.advisors.add(pos, advisor);
        updateAdvisorArray();
        adviceChanged();
    }

    /**
     * Bring the array up to date with the list.
     */
    protected final void updateAdvisorArray() {
        this.advisorArray = this.advisors.toArray(new Advisor[this.advisors.size()]);
    }
    protected void adviceChanged() {
        this.methodCache.clear();
    }

下面就是jdk动态代理和cglib动态代理两种代理方式生成代理对象的过程：

• jdk动态代理

  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);
          // 根据类装载器和接口通过反射生成代理类
      return Proxy.newProxyInstance(classLoader, proxiedInterfaces, this);
  }

  ​

• cglib动态代理

  public Object getProxy(ClassLoader classLoader) {
      if (logger.isDebugEnabled()) {
          logger.debug("Creating CGLIB proxy: target source is " + this.advised.getTargetSource());
      }
  
      try {
          Class rootClass = this.advised.getTargetClass();
          Assert.state(rootClass != null, "Target class must be available for creating a CGLIB proxy");
  
          Class proxySuperClass = rootClass;
          if (ClassUtils.isCglibProxyClass(rootClass)) {
              proxySuperClass = rootClass.getSuperclass();
              Class[] additionalInterfaces = rootClass.getInterfaces();
              for (Class additionalInterface : additionalInterfaces) {
                  this.advised.addInterface(additionalInterface);
              }
          }
  
          // Validate the class, writing log messages as necessary.
          validateClassIfNecessary(proxySuperClass, classLoader);
  
          // Configure CGLIB Enhancer...
          Enhancer enhancer = createEnhancer();
          if (classLoader != null) {
              enhancer.setClassLoader(classLoader);
              if (classLoader instanceof SmartClassLoader &&
                      ((SmartClassLoader) classLoader).isClassReloadable(proxySuperClass)) {
                  enhancer.setUseCache(false);
              }
          }
          enhancer.setSuperclass(proxySuperClass);
          enhancer.setInterfaces(AopProxyUtils.completeProxiedInterfaces(this.advised));
          enhancer.setNamingPolicy(SpringNamingPolicy.INSTANCE);
          enhancer.setStrategy(new ClassLoaderAwareUndeclaredThrowableStrategy(classLoader));
  
          Callback[] callbacks = getCallbacks(rootClass);
          Class[] types = new Class[callbacks.length];
          for (int x = 0; x < types.length; x++) {
              types[x] = callbacks[x].getClass();
          }
          // fixedInterceptorMap only populated at this point, after getCallbacks call above
          enhancer.setCallbackFilter(new ProxyCallbackFilter(
                  this.advised.getConfigurationOnlyCopy(), this.fixedInterceptorMap, this.fixedInterceptorOffset));
          enhancer.setCallbackTypes(types);
  
          // Generate the proxy class and create a proxy instance.
          return createProxyClassAndInstance(enhancer, callbacks);
      }
      catch (CodeGenerationException ex) {
          throw new AopConfigException("Could not generate CGLIB subclass of class [" +
                  this.advised.getTargetClass() + "]: " +
                  "Common causes of this problem include using a final class or a non-visible class",
                  ex);
      }
      catch (IllegalArgumentException ex) {
          throw new AopConfigException("Could not generate CGLIB subclass of class [" +
                  this.advised.getTargetClass() + "]: " +
                  "Common causes of this problem include using a final class or a non-visible class",
                  ex);
      }
      catch (Exception ex) {
          // TargetSource.getTarget() failed
          throw new AopConfigException("Unexpected AOP exception", ex);
      }
  }

  以上是实现生成代理对象的过程，而拦截器的步骤在于两种代理的回调。那么接下来就深入分析执行回调代码达到拦截器的作用。

  2. 生成拦截链

  以jdk动态代理回调为例，来看怎么为目标对象生成拦截链

// jdk动态代理的回调方法
@Override
    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
        MethodInvocation invocation;
        Object oldProxy = null;
        boolean setProxyContext = false;
        // 获取目标代理对象
        TargetSource targetSource = this.advised.targetSource;
        Class targetClass = null;
        Object target = null;

        try {
            if (!this.equalsDefined && AopUtils.isEqualsMethod(method)) {
                // 如果目标类没有实现equal，这里实现。
                return equals(args[0]);
            }
            if (!this.hashCodeDefined && AopUtils.isHashCodeMethod(method)) {
                // 如果没有实现hashCode()
                return hashCode();
            }
            if (!this.advised.opaque && method.getDeclaringClass().isInterface() &&
                    method.getDeclaringClass().isAssignableFrom(Advised.class)) {
                // 通过反射调用方法并执行
                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();
            }

            // 这里获取对象所有的拦截链
            List