spring---aop（6）---Spring AOP中ProxyFactoryBean介绍

写在前面

　　这篇文章里面就要说说Spring自己的AOP，搞清楚哪种方式是Spring自己实现的AOP，哪种方式是Spring引入aspectj的AOP。 

简单例子

　　Spring自己的AOP实现在于ProxyFactoryBean。先看下使用案例（仍和之前的案例是一样的）：接口AService、实现类AServiceImpl、通知MyBeforeAdvice 。

public interface AService {
    public void barA(); 
    public void barB(); 
}

public class AServiceImpl implements AService{
    @Override
    public void barA() {
         System.out.println("AServiceImpl.barA()");  
         this.barB();
    }
    @Override
    public void barB() {
        System.out.println("AServiceImpl.barB()");  
    }
}

public class MyBeforeAdvice implements MethodBeforeAdvice{
    @Override
    public void before(Method method, Object[] args, Object target)
            throws Throwable {
        System.out.println("run my before advice");
    }
}

　　然后就是xml的配置： 

    <bean id="aServiceImpl" class="com.xxx.plus.aop.demo.AServiceImpl"/>
    <bean id="myBeforAdvice" class="com.xxx.plus.aop.demo.MyBeforeAdvice"/>
    <bean id="aServiceImplProxy" class="org.springframework.aop.framework.ProxyFactoryBean">
        <property name="interfaces" value="com.xxx.plus.aop.demo.AService"/>
        <property name="target">
            <ref bean="aServiceImpl"/>
        </property>
         <property name="interceptorNames">  
            <list>  
                <value>myBeforAdvice</value>  
            </list>  
        </property>  
    </bean>

　　然后就可以使用了： 

    @Resource
    private AService aService;
    
    @Test
    public void testAOP(){
        aService.barA();
    }

　　运行这个单元测试，然后你就会看到报如下错误： 

No qualifying bean of type [com.xxx.aop.service.AService] is defined: expected single matching bean but found 2: aServiceImpl,org.springframework.aop.framework.ProxyFactoryBean#0

　　原因就是对于接口AService，有两个实现类aServiceImpl和ProxyFactoryBean所生产的代理类。所以我们不能使用@Resource（它是按类型注入），所以要使用按名称注入，我们怎么获取ProxyFactoryBean所产生的代理类的名称呢？其实就是ProxyFactoryBean配置的名称。因为ProxyFactoryBean实现了FactoryBean接口，对于这种接口从容器中获取该bean，不是获取的本身而是获取他的getObject方法所返回的值，看FactoryBean的文档： 

/**
 * Interface to be implemented by objects used within a {@link BeanFactory}
 * which are themselves factories. If a bean implements this interface,
 * it is used as a factory for an object to expose, not directly as a bean
 * instance that will be exposed itself.
 *
 * <p><b>NB: A bean that implements this interface cannot be used as a
 * normal bean.</b> A FactoryBean is defined in a bean style, but the
 * object exposed for bean references ({@link #getObject()} is always
 * the object that it creates.
 *
 * <p>FactoryBeans can support singletons and prototypes, and can
 * either create objects lazily on demand or eagerly on startup.
 * The {@link SmartFactoryBean} interface allows for exposing
 * more fine-grained behavioral metadata.
 *
 * <p>This interface is heavily used within the framework itself, for
 * example for the AOP {@link org.springframework.aop.framework.ProxyFactoryBean}
 * or the {@link org.springframework.jndi.JndiObjectFactoryBean}.
 * It can be used for application components as well; however,
 * this is not common outside of infrastructure code.
 *
 * <p><b>NOTE:</b> FactoryBean objects participate in the containing
 * BeanFactory's synchronization of bean creation. There is usually no
 * need for internal synchronization other than for purposes of lazy
 * initialization within the FactoryBean itself (or the like).

 */
public interface FactoryBean<T> {}

　　所以通过beanName找到了ProxyFactoryBean，然而不是返回该对象，而是返回他的getObject方法的返回值，所以我们通过ProxyFactoryBean的id就可以获取到它所产生的代理对象，所以更改如下： 

<bean  id="aServiceImplProxy" class="org.springframework.aop.framework.ProxyFactoryBean">

@Resource(name="aServiceImplProxy")
private AService aService;

　　然后就可以正常运行了如下： 

run my before advice
AServiceImpl.barA()

源代码解读

　　然后我们就要源码分析下这一过程，先看下是如何产生代理对象的，在ProxyFactoryBean的getObject方法中：

public class ProxyFactoryBean extends ProxyCreatorSupport
        implements FactoryBean<Object>, BeanClassLoaderAware, BeanFactoryAware {
    @Override
    public Object getObject() throws BeansException {
   　　 //重点一
        initializeAdvisorChain();
        if (isSingleton()) {
        　　//重点二
            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();
        }
    }
}

　　重点1：就是根据我们配置的interceptorNames来获取对应的bean，并却转化成Advisor。

　　this.advisorChainInitialized：标示是否已进行过初始化，若以初始化则不再进行初始化。然后就是将interceptorNames转化成Advisor。根据interceptorNames所包含的字符串到容器中进行查找，如果含有*则，则表示进行一定的匹配，符合的都会纳入。

　　如下： 

    private synchronized void initializeAdvisorChain() throws AopConfigException, BeansException {
        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));
            }

            // Globals can't be last unless we specified a targetSource using the property...
            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");
            }

            // Materialize interceptor chain from bean names.
            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");
                    }
                    addGlobalAdvisor((ListableBeanFactory) this.beanFactory,
                            name.substring(0, name.length() - GLOBAL_SUFFIX.length()));
                }

                else {
                    // If we get here, we need to add a named interceptor.
                    // We must check if it's a singleton or prototype.
                    Object advice;
                    if (this.singleton || this.beanFactory.isSingleton(name)) {
                        // Add the real Advisor/Advice to the chain.
                        advice = this.beanFactory.getBean(name);
                    }
                    else {
                        // It's a prototype Advice or Advisor: replace with a prototype.
                        // Avoid unnecessary creation of prototype bean just for advisor chain initialization.
                        advice = new PrototypePlaceholderAdvisor(name);
                    }
                    addAdvisorOnChainCreation(advice, name);
                }
            }
        }

        this.advisorChainInitialized = true;
    }

　　这中间页经过了Advice到Advisor的转换，如下： 

    private void addAdvisorOnChainCreation(Object next, String name) {
        // We need to convert to an Advisor if necessary so that our source reference
        // matches what we find from superclass interceptors.
        Advisor advisor = namedBeanToAdvisor(next);
        if (logger.isTraceEnabled()) {
            logger.trace("Adding advisor with name '" + name + "'");
        }
        addAdvisor(advisor);
    }

    private Advisor namedBeanToAdvisor(Object next) {
        try {
            return this.advisorAdapterRegistry.wrap(next);
        }
        catch (UnknownAdviceTypeException ex) {
            // We expected this to be an Advisor or Advice,
            // but it wasn't. This is a configuration error.
            throw new AopConfigException("Unknown advisor type " + next.getClass() +
                    "; Can only include Advisor or Advice type beans in interceptorNames chain except for last entry," +
                    "which may also be target or TargetSource", ex);
        }
    }

    public Advisor wrap(Object adviceObject) throws UnknownAdviceTypeException {
        if (adviceObject instanceof Advisor) {
            return (Advisor) adviceObject;
        }
        if (!(adviceObject instanceof Advice)) {
            throw 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 (AdvisorAdapter adapter : this.adapters) {
            // Check that it is supported.
            if (adapter.supportsAdvice(advice)) {
                return new DefaultPointcutAdvisor(advice);
            }
        }
        throw new UnknownAdviceTypeException(advice);
    }

public class DefaultPointcutAdvisor extends AbstractGenericPointcutAdvisor implements Serializable {
    public DefaultPointcutAdvisor(Advice advice) {
        this(Pointcut.TRUE, advice);
    }
}

　　这个包裹过程已经见过很多遍了，采用了适配器的模式。

　　之后又是和其他的AOP方式接轨了，设置一些列要实现的接口和参数，使用DefaultAopProxyFactory先创建出AopProxy，要么是JdkDynamicAopProxy，要么是CglibAopProxy，然后就可以调用AopProxy的getProxy方法来获取代理对象了

　　这种方式实现的AOP还是比较麻烦的，同时配置一个ProxyFactoryBean仅能实现对一个目标对象的拦截，要想拦截多个目标对象，需要配置多个ProxyFactoryBean。所以大部分还是使用Spring引进的aspectj的AOP方式来进行AOP编程。