spring aop分析(一)

一、简介

    Spring Aop模块采用了代理（Proxy）机制来实现AOP功能，在运行过程中aop框架大致可以分成三个阶段：创建代理、计算拦截器链、处理调用。

    在“创建代理”阶段，Spring AOP可以根据目标类的实现或设置采用JDK自动代理或CGLIB来创建目标对象的代理；对于静态的Pointcut，框架在目标方法第一次调用时将计算应用于该方法之上的“拦截器链”；在方法调用时，框架将依次调用拦截器并最终执行方法的调用。

二、创建代理

    ProxyFactoryBean是Spring AOP内置的工厂类，它能配置为spring容器中的bean，并为目标类创建代理：

public class ProxyFactoryBean extends AdvisedSupport
    implements ... {

	public Object getObject() throws BeansException {
		// 初始化advisor列表
		initializeAdvisorChain();
		
		if (isSingleton()) { // 单件代理
			return getSingletonInstance();
		}
		else {
			...
			return newPrototypeInstance();
		}
	}

  ...
}

    在initializeAdvisorChain()方法中，Spring AOP将初始化ProxyFactoryBean配置中的Advisor:

public class ProxyFactoryBean extends AdvisedSupport
    implements ... {

	private synchronized void initializeAdvisorChain() throws ... {
		...
               // interceptorNames是ProxyFactoryBean在配置时设定的拦截器列表
		if (!ObjectUtils.isEmpty(this.interceptorNames)) {
			...

			for (int i = 0; i < this.interceptorNames.length; i++) {
				String name = this.interceptorNames[i];
	
				if (name.endsWith(GLOBAL_SUFFIX)) {
					...
				} else {	
					Object advice = null;
                                       // 若代理和拦截器都是单件
					if (this.singleton || this.beanFactory.isSingleton(this.interceptorNames[i])) {
						// 从Spring Ioc容器中寻找
						advice = this.beanFactory.getBean(this.interceptorNames[i]);
					}
					else { // 处理prototype的情形
						...
					}
                                       // 将advice加入拦截器列表
					addAdvisorOnChainCreation(advice, this.interceptorNames[i]);
				}
			}
		}

		this.advisorChainInitialized = true;
	}

  ...
}

    拦截器链初始化完成之后将创建代理：

public class ProxyFactoryBean extends AdvisedSupport
    implements ... {

	private synchronized Object getSingletonInstance() {
		if (this.singletonInstance == null) {
			this.targetSource = freshTargetSource();
			if (...) { //自动探测目标对象实现的接口
				// Rely on AOP infrastructure to tell us what interfaces to proxy.
				setInterfaces(ClassUtils.getAllInterfacesForClass(this.targetSource.getTargetClass()));
			}
		        ...
			this.singletonInstance = getProxy(createAopProxy());
			...
		}
		return this.singletonInstance;
	}

	protected Object getProxy(AopProxy aopProxy) {
		return aopProxy.getProxy(this.beanClassLoader);
	}

  ...
}

    createAopProxy()方法来自ProxyFactoryBean的父类AdvisedSupport， 该方法将根据环境创建一个JDK或CGLIB代理

public class AdvisedSupport extends ProxyConfig implements Advised {

	protected synchronized AopProxy createAopProxy() {
		if (!this.isActive) {
			activate();
		}
		// 使用AopProxyFactory来创建AopProxy
                // 默认的factory是DefaultAopProxyFactory
                return getAopProxyFactory().createAopProxy(this);
	}

  ...
}

public class DefaultAopProxyFactory implements AopProxyFactory {

	public AopProxy createAopProxy(AdvisedSupport advisedSupport) throws AopConfigException {
		if (advisedSupport.isOptimize() || advisedSupport.isProxyTargetClass() ||
		    advisedSupport.getProxiedInterfaces().length == 0) { // 使用CGLIB创建proxy
			...
			return CglibProxyFactory.createCglibProxy(advisedSupport);
		}
		else {
			return new JdkDynamicAopProxy(advisedSupport);
		}
	}

  ...
}

    其中JdkDynamicAopProxy类使用JDK动态代理来创建AOP功能：

final class JdkDynamicAopProxy implements AopProxy, InvocationHandler, Serializable {

	public Object getProxy(ClassLoader classLoader) {
		...
               // 得到要代理的接口，系统自动追加Advised接口
		Class[] proxiedInterfaces = AopProxyUtils.completeProxiedInterfaces(this.advised);
               // 检查equals()和hashCode()方法
		findDefinedEqualsAndHashCodeMethods(proxiedInterfaces);
               // 使用java.lang.reflect.Proxy来创建JDK代理
		return Proxy.newProxyInstance(classLoader, proxiedInterfaces, this);
	}

  ...
}

   调用的UML图：