AOP 面向方面编程

最新推荐文章于 2024-10-08 21:53:06 发布

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文章标签：

#编程 #AOP #Java #Spring #Apache

本文介绍了Spring框架中AOP（面向切面编程）的概念及其动态代理机制的实现原理。通过具体的代码示例展示了如何使用Java Dynamic Proxy和CGLib为类创建动态代理，以及这些代理如何在方法调用前后执行预处理、后处理和异常处理。

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Dynamic Proxy 与Spring AOP

Spring中提供的内置AOP支持，是基于动态AOP机制实现。从技术角度来讲，所谓动
态AOP，即通过动态Proxy模式，在目标对象的方法调用前后插入相应的处理代码。
而Spring AOP中的动态Proxy模式，则是基于Java Dynamic Proxy（面向Interface）
和CGLib（面向Class）实现。
前面曾经提及，Spring Framework中的“事务管理”服务，实际上是借助AOP机制
完成。我们这里就以“事务管理”为例，对动态AOP的实现加以探讨，一方面对动态AOP
的实现原理加以探究，另一方面，也可以加深对Spring中事务管理机制的理解。

以下是Dynamic Proxy的例子：

AOPHandler.java

package aop;

import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;
import java.util.ArrayList;
import java.util.List;

import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;


public class AOPHandler implements InvocationHandler {

	private static Log logger = LogFactory.getLog(AOPHandler.class);
	private List interceptors = null;
	private Object originalObject;
	/**
	* 返回动态代理实例
	* @param obj
	* @return
	*/
	public Object bind(Object obj) {
	this.originalObject = obj;
	return Proxy.newProxyInstance(obj.getClass().getClassLoader(),
	obj
	.getClass().getInterfaces(), this);
	}
	/**
	* 在Invoke方法中，加载对应的Interceptor，并进行
	* 预处理(before)、后处理(after)以及异常处理（exceptionThrow）过程
	*/
	public Object invoke(Object proxy, Method method, Object[] args)
	throws Throwable {
	Object result = null;
	Throwable ex = null;
	InvocationInfo invInfo = new InvocationInfo(proxy, method, args,
	result, ex);
	logger.debug("Invoking Before Intercetpors!");
	System.out.println("Invoking Before Intercetpors!");
	invokeInterceptorsBefore(invInfo);
	try {
		logger.debug("Invoking Proxy Method!");
		System.out.println("Invoking Proxy Method!");
		result = method.invoke(originalObject, args);
		invInfo.setResult(result);
		logger.debug("Invoking After Method!");
		System.out.println("Invoking After Method!");
		invokeInterceptorsAfter(invInfo);
		} catch (Throwable tr) {
		invInfo.setException(tr);
		logger.debug("Invoking exceptionThrow Method!");
		System.out.println("Invoking exceptionThrow Method!");
		invokeInterceptorsExceptionThrow(invInfo);
		throw new AOPRuntimeException(tr);
		}
		return result;
		}
		/**
		* 加载Interceptor
		* @return
		*/
		private synchronized List getIntercetors() {
		if (null == interceptors) {
		interceptors = new ArrayList();
		//Todo：读取配置，加载Interceptor实例
		interceptors.add(new MyInterceptor());	
		}
		return interceptors;
		}
		/**
		* 执行预处理方法
		* @param invInfo
		*/
		private void invokeInterceptorsBefore(InvocationInfo invInfo) {
		List interceptors = getIntercetors();
		int len = interceptors.size();
		for (int i = 0; i < len; i++) {
			((Interceptor) interceptors.get(i)).before(invInfo);
			}
			}
			/**
			* 执行后处理方法
			* @param invInfo
			*/
			private void invokeInterceptorsAfter(InvocationInfo invInfo) {
			List interceptors = getIntercetors();
			int len = interceptors.size();
			for (int i = len - 1; i >= 0; i--) {
			((Interceptor) interceptors.get(i)).after(invInfo);
			}
			}
			/**
			* 执行异常处理方法
			* @param invInfo
			*/
			private void invokeInterceptorsExceptionThrow(InvocationInfo
			invInfo) {
			List interceptors = getIntercetors();
			int len = interceptors.size();
			for (int i = len - 1; i >= 0; i--) {
			((Interceptor)
			interceptors.get(i)).exceptionThrow(invInfo);
			}
			}
			

}

AOPRuntimeException.java 异常处理

package aop;

public class AOPRuntimeException extends RuntimeException {
	
	AOPRuntimeException(Throwable e)
	{
		
	}

}

Interceptor.java拦截器接口

package aop;

public interface Interceptor {
	public void before(InvocationInfo invInfo);
	public void after(InvocationInfo invInfo);
	public void exceptionThrow(InvocationInfo invInfo);
	}

MyInterceptor.java拦截器的实现：

package aop;

import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;

public class MyInterceptor implements Interceptor{
	
	private static Log logger = LogFactory.getLog(MyInterceptor.class);
	public void before(InvocationInfo invInfo) {
	logger.debug("Pre-processing");
	System.out.println("Pre-processing");
	System.out.println("beginTransaction");
	}
	public void after(InvocationInfo invInfo) {
	logger.debug("Post-processing");
	System.out.println("Post-processing");
	System.out.println("endTransaction");
	}
	public void exceptionThrow(InvocationInfo invInfo) {
	logger.debug("Exception-processing");
	System.out.println("Exception-processing");
	System.out.println("rollback();");
	}

}

AOPFactory.java 生成代理类的工厂

package aop;

import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;

public class AOPFactory {
	
	private static Log logger = LogFactory.getLog(AOPFactory.class);
	/**
	* 根据类名创建类实例
	* @param clzName
	* @return
	* @throws ClassNotFoundException
	*/
	public static Object getClassInstance(String clzName){
	Class cls;
	try {
	cls = Class.forName(clzName);
	return (Object)cls.newInstance();
	} catch (ClassNotFoundException e) {
	logger.debug(e);
	throw new AOPRuntimeException(e);
	} catch (InstantiationException e) {
	logger.debug(e);
	throw new AOPRuntimeException(e);
	} catch (IllegalAccessException e) {
	logger.debug(e);
	throw new AOPRuntimeException(e);
	}
	}
	/**
	* 根据传入的类名，返回AOP代理对象
	* @param clzName
	* @return
	*/
	public static Object getAOPProxyedObject(String clzName){
		AOPHandler txHandler = new AOPHandler();
		Object obj = getClassInstance(clzName);
		return txHandler.bind(obj);
		
	}

}

InvocationInfo.java

package aop;

import java.lang.reflect.Method;

public class InvocationInfo {
	
	Object proxy;
	Method method;
	Object[] args;
	Object result;
	Throwable Exception;
	public InvocationInfo(Object proxy, Method method, Object[] args,
	Object result, Throwable exception) {
	super();
	this.proxy = proxy;
	this.method = method;
	this.args = args;
	this.result = result;
	Exception = exception;
	}
	public Object getResult() {
	return result;
	}
	public void setResult(Object result) {
	this.result = result;
	}
	public Object[] getArgs() {
	return args;
	}
	public void setArgs(Object[] args) {
	this.args = args;
	}
	public Throwable getException() {
	return Exception;
	}
	public void setException(Throwable exception) {
	Exception = exception;
	}
	public Method getMethod() {
	return method;
	}
	public void setMethod(Method method) {
	this.method = method;
	}
	public Object getProxy() {
	return proxy;
	}
	
	public void setProxy(Object proxy) {
		this.proxy = proxy;
		}

}

UserDAO.java

需要代理的接口

package aop;

public interface UserDAO {
	
	public void saveUser();

}

UserDAOImp.java

package aop;

public class UserDAOImp implements UserDAO {

	public void saveUser() {
		// TODO Auto-generated method stub
        System.out.println("UserDAOImp saveUser");
	}
 
}

TestAOP.java测试类

package aop;

public class TestAOP {
	
	public static void main(String[] args){
		
		Object ob = AOPFactory.getAOPProxyedObject("aop.UserDAOImp");
		UserDAO dao = (UserDAO)ob;
		dao.saveUser();
		
	}

}

执行结果：

Invoking Before Intercetpors!
Pre-processing
beginTransaction
Invoking Proxy Method!
UserDAOImp saveUser
Invoking After Method!
Post-processing
endTransaction

显示了代理的效果

CGLib 与Spring AOP

Spring中，引入了CGLib作为无接口情况下的动态代理实现。
CGLib与Dynamic Proxy的代理机制基本类似，只是其动态生成的代理对象并非某个
接口的实现，而是针对目标类扩展的子类。
换句话说，Dynamic Proxy返回的动态代理类，是目标类所实现的接口的另一个实现
版本，它实现了对目标类的代理（如同UserDAOProxy与UserDAOImp的关系）。而CGLib
返回的动态代理类，则是目标代理类的一个子类（代理类扩展了UserDAOImp类）。

引用CGLIB.工程需要导入几个jar包：antlr-2.7.2.jar，asm.jar，asm-attrs.jar，aspectj-1.6.1.jar，aspectjrt.jar，aspectjweaver.jar

AOPInstrumenter.java

package aop;

import java.lang.reflect.Method;

import net.sf.cglib.proxy.Enhancer;
import net.sf.cglib.proxy.MethodInterceptor;
import net.sf.cglib.proxy.MethodProxy;

import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;


public class AOPInstrumenter implements MethodInterceptor{
 
 private static Log logger =
  LogFactory.getLog(AOPInstrumenter.class);
  private Enhancer enhancer = new Enhancer();
  public Object getInstrumentedClass(Class clz) {
  enhancer.setSuperclass(clz);
  enhancer.setCallback(this);
  return enhancer.create();
  }
  
  public Object intercept(
    Object o,
    Method method,
    Object[] methodParameters,
    MethodProxy methodProxy)
    throws Throwable {
    logger.debug("Before Method =>"+method.getName());
    System.out.println("Before Method =>"+method.getName());
    Object result = methodProxy.invokeSuper(o, methodParameters);
    logger.debug("After Method =>"+method.getName());
    System.out.println("After Method =>"+method.getName());
    return result;
    }

}

UserDAOImp.java

package aop;

public class UserDAOImp {
 
 public void saveUser()
 {
  System.out.println("UserDAOImp method saveUser");
 }

}

TestAOPCGLib.java

package aop;

public class TestAOPCGLib {

 /**
  * @param args
  */
 public static void main(String[] args) {
  // TODO Auto-generated method stub
  
  AOPInstrumenter aopInst = new AOPInstrumenter();
  UserDAOImp userDAO =
  (UserDAOImp) aopInst.getInstrumentedClass(UserDAOImp.class);
 
  userDAO.saveUser();

 }

}

TestAOPCGLib的运行结果是：

Before Method =>saveUser
UserDAOImp method saveUser
After Method =>saveUser