Spring框架系列(10) - Spring AOP实现原理详解之AOP代理的创建

上文我们介绍了Spring AOP原理解析的切面实现过程(将切面类的所有切面方法根据使用的注解生成对应Advice，并将Advice连同切入点匹配器和切面类等信息一并封装到Advisor)。本文在此基础上继续介绍，代理（cglib代理和JDK代理）的创建过程。@pdai

Spring框架系列(10) - Spring AOP实现原理详解之AOP代理的创建

引入

前文主要Spring AOP原理解析的切面实现过程(加载配置，将切面类的所有切面方法根据使用的注解生成对应Advice，并将Advice连同切入点匹配器和切面类等信息一并封装到Advisor)。

同时我们也总结了Spring AOP初始化的过程，具体如下：

由IOC Bean加载方法栈中找到parseCustomElement方法，找到parse aop:aspectj-autoproxy的handler(org.springframework.aop.config.AopNamespaceHandler)
AopNamespaceHandler注册了<aop:aspectj-autoproxy/>的解析类是AspectJAutoProxyBeanDefinitionParser
AspectJAutoProxyBeanDefinitionParser的parse 方法通过AspectJAwareAdvisorAutoProxyCreator类去创建
AspectJAwareAdvisorAutoProxyCreator实现了两类接口，BeanFactoryAware和BeanPostProcessor；根据Bean生命周期方法找到两个核心方法：postProcessBeforeInstantiation和postProcessAfterInitialization
1. postProcessBeforeInstantiation：主要是处理使用了@Aspect注解的切面类，然后将切面类的所有切面方法根据使用的注解生成对应Advice，并将Advice连同切入点匹配器和切面类等信息一并封装到Advisor
2. postProcessAfterInitialization：主要负责将Advisor注入到合适的位置，创建代理（cglib或jdk)，为后面给代理进行增强实现做准备。

本文接着介绍postProcessAfterInitialization的方法，即Spring AOP的代理（cglib或jdk)的创建过程。

代理的创建

创建代理的方法是postProcessAfterInitialization：如果bean被子类标识为代理，则使用配置的拦截器创建一个代理

/**

  * Create a proxy with the configured interceptors if the bean is

  * identified as one to proxy by the subclass.

  * @see #getAdvicesAndAdvisorsForBean

  */

@Override

public Object postProcessAfterInitialization(@Nullable Object bean, String beanName) {

  if (bean != null) {

    Object cacheKey = getCacheKey(bean.getClass(), beanName);

    // 如果不是提前暴露的代理

    if (this.earlyProxyReferences.remove(cacheKey) != bean) {

      return wrapIfNecessary(bean, beanName, cacheKey);

    }

  }

  return bean;

}

wrapIfNecessary方法主要用于判断是否需要创建代理，如果Bean能够获取到advisor才需要创建代理

/**

  * Wrap the given bean if necessary, i.e. if it is eligible for being proxied.

  * @param bean the raw bean instance

  * @param beanName the name of the bean

  * @param cacheKey the cache key for metadata access

  * @return a proxy wrapping the bean, or the raw bean instance as-is

  */

protected Object wrapIfNecessary(Object bean, String beanName, Object cacheKey) {

   // 如果bean是通过TargetSource接口获取

   if (beanName != null && this.targetSourcedBeans.contains(beanName)) {

      return bean;

   }

   // 如果bean是切面类

   if (Boolean.FALSE.equals(this.advisedBeans.get(cacheKey))) {

      return bean;

   }

   // 如果是aop基础类？是否跳过？

   if (isInfrastructureClass(bean.getClass()) || shouldSkip(bean.getClass(), beanName)) {

      this.advisedBeans.put(cacheKey, Boolean.FALSE);

      return bean;

   }

  // 重点：获取所有advisor，如果没有获取到，那说明不要进行增强，也就不需要代理了。

  Object[] specificInterceptors = getAdvicesAndAdvisorsForBean(bean.getClass(), beanName, null);

  if (specificInterceptors != DO_NOT_PROXY) {

    this.advisedBeans.put(cacheKey, Boolean.TRUE);

    // 重点：创建代理

    Object proxy = createProxy(

        bean.getClass(), beanName, specificInterceptors, new SingletonTargetSource(bean));

    this.proxyTypes.put(cacheKey, proxy.getClass());

    return proxy;

  }

  this.advisedBeans.put(cacheKey, Boolean.FALSE);

  return bean;

}

获取所有的Advisor

我们看下获取所有advisor的方法getAdvicesAndAdvisorsForBean

@Override

@Nullable

protected Object[] getAdvicesAndAdvisorsForBean(

    Class<?> beanClass, String beanName, @Nullable TargetSource targetSource) {

  List<Advisor> advisors = findEligibleAdvisors(beanClass, beanName);

  if (advisors.isEmpty()) {

    return DO_NOT_PROXY;

  }

  return advisors.toArray();

}

通过findEligibleAdvisors方法获取advisor，如果获取不到返回DO_NOT_PROXY（不需要创建代理），findEligibleAdvisors方法如下

/**

  * Find all eligible Advisors for auto-proxying this class.

  * @param beanClass the clazz to find advisors for

  * @param beanName the name of the currently proxied bean

  * @return the empty List, not {@code null},

  * if there are no pointcuts or interceptors

  * @see #findCandidateAdvisors

  * @see #sortAdvisors

  * @see #extendAdvisors

  */

protected List<Advisor> findEligibleAdvisors(Class<?> beanClass, String beanName) {

  // 和上文一样，获取所有切面类的切面方法生成Advisor

  List<Advisor> candidateAdvisors = findCandidateAdvisors();

  // 找到这些Advisor中能够应用于beanClass的Advisor

  List<Advisor> eligibleAdvisors = findAdvisorsThatCanApply(candidateAdvisors, beanClass, beanName);

  // 如果需要，交给子类拓展

  extendAdvisors(eligibleAdvisors);

  // 对Advisor排序

  if (!eligibleAdvisors.isEmpty()) {

    eligibleAdvisors = sortAdvisors(eligibleAdvisors);

  }

  return eligibleAdvisors;

}

获取所有切面类的切面方法生成Advisor

/**

  * Find all candidate Advisors to use in auto-proxying.

  * @return the List of candidate Advisors

  */

protected List<Advisor> findCandidateAdvisors() {

  Assert.state(this.advisorRetrievalHelper != null, "No BeanFactoryAdvisorRetrievalHelper available");

  return this.advisorRetrievalHelper.findAdvisorBeans();

}

找到这些Advisor中能够应用于beanClass的Advisor

/**

  * Determine the sublist of the {@code candidateAdvisors} list

  * that is applicable to the given class.

  * @param candidateAdvisors the Advisors to evaluate

  * @param clazz the target class

  * @return sublist of Advisors that can apply to an object of the given class

  * (may be the incoming List as-is)

  */

public static List<Advisor> findAdvisorsThatCanApply(List<Advisor> candidateAdvisors, Class<?> clazz) {

  if (candidateAdvisors.isEmpty()) {

    return candidateAdvisors;

  }

  List<Advisor> eligibleAdvisors = new ArrayList<>();

  for (Advisor candidate : candidateAdvisors) {

    // 通过Introduction实现的advice

    if (candidate instanceof IntroductionAdvisor && canApply(candidate, clazz)) {

      eligibleAdvisors.add(candidate);

    }

  }

  boolean hasIntroductions = !eligibleAdvisors.isEmpty();

  for (Advisor candidate : candidateAdvisors) {

    if (candidate instanceof IntroductionAdvisor) {

      // already processed

      continue;

    }

    // 是否能够应用于clazz的Advice

    if (canApply(candidate, clazz, hasIntroductions)) {

      eligibleAdvisors.add(candidate);

    }

  }

  return eligibleAdvisors;

}

创建代理的入口方法

获取所有advisor后，如果有advisor，则说明需要增强，即需要创建代理，创建代理的方法如下：

/**

  * Create an AOP proxy for the given bean.

  * @param beanClass the class of the bean

  * @param beanName the name of the bean

  * @param specificInterceptors the set of interceptors that is

  * specific to this bean (may be empty, but not null)

  * @param targetSource the TargetSource for the proxy,

  * already pre-configured to access the bean

  * @return the AOP proxy for the bean

  * @see #buildAdvisors

  */

protected Object createProxy(Class<?> beanClass, @Nullable String beanName,

    @Nullable Object[] specificInterceptors, TargetSource targetSource) {

  if (this.beanFactory instanceof ConfigurableListableBeanFactory) {

    AutoProxyUtils.exposeTargetClass((ConfigurableListableBeanFactory) this.beanFactory, beanName, beanClass);

  }

  ProxyFactory proxyFactory = new ProxyFactory();

  proxyFactory.copyFrom(this);

  if (proxyFactory.isProxyTargetClass()) {

    // Explicit handling of JDK proxy targets (for introduction advice scenarios)

    if (Proxy.isProxyClass(beanClass)) {

      // Must allow for introductions; can't just set interfaces to the proxy's interfaces only.

      for (Class<?> ifc : beanClass.getInterfaces()) {

        proxyFactory.addInterface(ifc);

      }

    }

  }

  else {

    // No proxyTargetClass flag enforced, let's apply our default checks...

    if (shouldProxyTargetClass(beanClass, beanName)) {

      proxyFactory.setProxyTargetClass(true);

    }

    else {

      evaluateProxyInterfaces(beanClass, proxyFactory);

    }

  }

  Advisor[] advisors = buildAdvisors(beanName, specificInterceptors);

  proxyFactory.addAdvisors(advisors);

  proxyFactory.setTargetSource(targetSource);

  customizeProxyFactory(proxyFactory);

  proxyFactory.setFrozen(this.freezeProxy);

  if (advisorsPreFiltered()) {

    proxyFactory.setPreFiltered(true);

  }

  // Use original ClassLoader if bean class not locally loaded in overriding class loader

  ClassLoader classLoader = getProxyClassLoader();

  if (classLoader instanceof SmartClassLoader && classLoader != beanClass.getClassLoader()) {

    classLoader = ((SmartClassLoader) classLoader).getOriginalClassLoader();

  }

  return proxyFactory.getProxy(classLoader);

}

proxyFactory.getProxy(classLoader)

/**

  * Create a new proxy according to the settings in this factory.

  * <p>Can be called repeatedly. Effect will vary if we've added

  * or removed interfaces. Can add and remove interceptors.

  * <p>Uses the given class loader (if necessary for proxy creation).

  * @param classLoader the class loader to create the proxy with

  * (or {@code null} for the low-level proxy facility's default)

  * @return the proxy object

  */

public Object getProxy(@Nullable ClassLoader classLoader) {

  return createAopProxy().getProxy(classLoader);

}

依据条件创建代理(jdk或cglib)

DefaultAopProxyFactory.createAopProxy

@Override

public AopProxy createAopProxy(AdvisedSupport config) throws AopConfigException {

  if (!NativeDetector.inNativeImage() &&

      (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() || Proxy.isProxyClass(targetClass)) {

      return new JdkDynamicAopProxy(config);

    }

    return new ObjenesisCglibAopProxy(config);

  }

  else {

    return new JdkDynamicAopProxy(config);

  }

}

几个要点

config.isOptimize() 是通过optimize设置，表示配置是自定义的，默认是false；
config.isProxyTargetClass()是通过<aop:config proxy-target-class="true" /> 来配置的，表示优先使用cglib代理，默认是false；
hasNoUserSuppliedProxyInterfaces(config) 表示是否目标类实现了接口

由此我们可以知道：

Spring默认在目标类实现接口时是通过JDK代理实现的，只有非接口的是通过Cglib代理实现的。当设置proxy-target-class为true时在目标类不是接口或者代理类时优先使用cglib代理实现。

巴特西

Spring框架系列(10) - Spring AOP实现原理详解之AOP代理的创建

引入

代理的创建

获取所有的Advisor

创建代理的入口方法

依据条件创建代理(jdk或cglib)

更多文章

最新文章

热门文章