Spring AOP 的proxy详解

spring 提供了多种不同的方案实现对 bean 的 aop proxy, 包括 ProxyFactoryBean, 便利的 TransactionProxyFactoryBean 以及 AutoProxyCreator 等,
下图是 proxy class diagram 以供参考

这里重点说一下最常用的 ProxyFactoryBean, TransactionProxyFactoryBean, BeanNameAutoProxyCreator, DefaultAdvisorAutoProxyCreator 的联系和区别

1. ProxyFactoryBean : 使用率最高的 proxy 方式, 它通过配置 interceptorNames 属性决定加入哪些 advisor (method interceptor 将会被自动包装成 advisor, 下文将描述这个细节),
注意是 "interceptorNames" 而不是 "interceptors",

原因是 ProxyFactoryBean 可能返回非 singleton 的 proxy 实例, 而 advisior 可能也是非 singleton 的,

因此不能通过 interceptor reference 来注入

2. TransactionProxyFactoryBean : 特定用于 transaction proxy, 注意其 super class 是 AbstractSingletonProxyFactoryBean, 也就是说,

TransactionProxyFactoryBean 永远无法返回非 singleton 的 proxy 实例 !!!

如果你需要非 singleton 的 proxy 实例, 请考虑使用 ProxyFactoryBean.

3. BeanNameAutoProxyCreator : 故名思义, 根据 bean name 进行 auto proxy, bean name 的 match 规则参见 org.springframework.util.PatternMatchUtils

4. DefaultAdvisorAutoProxyCreator : 更强大的 auto proxy creator, 强大之处在于它会 cahce 容器中所有注册的 advisor, 然后搜索容器中所有的 bean ,
如果某个 bean 满足 advisor 中的 Pointcut, 那么将会被自动代理, 与 BeanNameAutoProxyCreator 相比, 省去了配置 beanNames 的工作,

引用：
eg :      

   <bean class="org.springframework.aop.framework.autoproxy.DefaultAdvisorAutoProxyCreator" />
 <bean id="defaultPointcutAdvisor" class="org.springframework.aop.support.DefaultPointcutAdvisor" scope="prototype">
        <property name="pointcut" ref="fooPointcut"/>
        <property name="advice" ref="fooAdvice"/>
</bean>

<bean id="fooAdvice" class="com.mycompany.FooAdvice" scope="prototype" />

<bean id="fooPointcut" class="org.springframework.aop.support.JdkRegexpMethodPointcut" scope="prototype">
        <property name="patterns">
            <list>
                <value>com.mycompany.FooService.*</value>
            </list>
        </property>
    </bean>

   <bean class="org.springframework.aop.framework.autoproxy.DefaultAdvisorAutoProxyCreator" />
 <bean id="defaultPointcutAdvisor" class="org.springframework.aop.support.DefaultPointcutAdvisor" scope="prototype">
        <property name="pointcut" ref="fooPointcut"/>
        <property name="advice" ref="fooAdvice"/>
</bean>

<bean id="fooAdvice" class="com.mycompany.FooAdvice" scope="prototype" />

<bean id="fooPointcut" class="org.springframework.aop.support.JdkRegexpMethodPointcut" scope="prototype">
        <property name="patterns">
            <list>
                <value>com.mycompany.FooService.*</value>
            </list>
        </property>
    </bean>

以上配置将自动代理容器中所有 com.mycompany.FooService 类型的 bean, 并拦截其所有方法

深度话题

1. MethodInterceptor 如何被包装成 Advisor ?

在 AdvisorAdapterRegistry#wrap(Object) 方法中实现, code as below

public Advisor wrap(Object adviceObject) throws UnknownAdviceTypeException {
  if (adviceObject instanceof Advisor) {
    return (Advisor) adviceObject;
  }
  if (!(adviceObject instanceof Advice)) {
    hrow new UnknownAdviceTypeException(adviceObject);
  }
  Advice advice = (Advice) adviceObject;
  if (advice instanceof MethodInterceptor) {
   // So well-known it doesn't even need an adapter.
   return new DefaultPointcutAdvisor(advice);
  }
  for (int i = 0; i < this.adapters.size(); i++) {
   / Check that it is supported.
                               AdvisorAdapter adapter = (AdvisorAdapter) this.adapters.get(i);
if (adapter.supportsAdvice(advice)) {
                                 return new DefaultPointcutAdvisor(advice);
 }
}
  throw new UnknownAdviceTypeException(advice);
                    }

public Advisor wrap(Object adviceObject) throws UnknownAdviceTypeException {
  if (adviceObject instanceof Advisor) {
    return (Advisor) adviceObject;
  }
  if (!(adviceObject instanceof Advice)) {
    hrow new UnknownAdviceTypeException(adviceObject);
  }
  Advice advice = (Advice) adviceObject;
  if (advice instanceof MethodInterceptor) {
   // So well-known it doesn't even need an adapter.
   return new DefaultPointcutAdvisor(advice);
  }
  for (int i = 0; i < this.adapters.size(); i++) {
   / Check that it is supported.
                               AdvisorAdapter adapter = (AdvisorAdapter) this.adapters.get(i);
if (adapter.supportsAdvice(advice)) {
                                 return new DefaultPointcutAdvisor(advice);
 }
}
  throw new UnknownAdviceTypeException(advice);
                    }

从代码可以看到, 如果 adviceObject(也就是 interceptorNames 对应的 bean) 不是 advisor
而是 MethodInterceptor 或 Advice, 那么 spring 将其包装成 DefaultPointcutAdvisor,
而 DefaultPointcutAdvisor 中定义的 Pointcut 是 TruePointcut :

    class TruePointcut implements Pointcut, Serializable {

    public static final TruePointcut INSTANCE = new TruePointcut();

    /**
     * Enforce Singleton pattern.
     */
    private TruePointcut() {
    }

    public ClassFilter getClassFilter() {
        return ClassFilter.TRUE;
    }

    public MethodMatcher getMethodMatcher() {
        return MethodMatcher.TRUE;
    }

    /**
     * Required to support serialization. Replaces with canonical
     * instance on deserialization, protecting Singleton pattern.
     * Alternative to overriding <code>equals()</code>.
     */
    private Object readResolve() {
        return INSTANCE;
    }

    public String toString() {
        return "Pointcut.TRUE";
    }

}

    class TruePointcut implements Pointcut, Serializable {

    public static final TruePointcut INSTANCE = new TruePointcut();

    /**
     * Enforce Singleton pattern.
     */
    private TruePointcut() {
    }

    public ClassFilter getClassFilter() {
        return ClassFilter.TRUE;
    }

    public MethodMatcher getMethodMatcher() {
        return MethodMatcher.TRUE;
    }

    /**
     * Required to support serialization. Replaces with canonical
     * instance on deserialization, protecting Singleton pattern.
     * Alternative to overriding <code>equals()</code>.
     */
    private Object readResolve() {
        return INSTANCE;
    }

    public String toString() {
        return "Pointcut.TRUE";
    }

}

也就是说, MethodInterceptor 和 Advice 被包装成的 Advisor 将会匹配容器中的所有 bean,
所以, 永 远不要在 DefaultAdvisorAutoProxyCreator 的 interceptorNames 中引用一个 Advice, 那将会使容器中所有的 bean 被自动代理!!! 此时应该考虑使用 BeanNameAutoProxyCreator