Spring3系列9-Spring AOP——Advice

Spring AOP即Aspect-oriented programming，面向切面编程，是作为面向对象编程的一种补充，专门用于处理系统中分布于各个模块（不同方法）中的交叉关注点的问题。简单地说，就是一个拦截器（interceptor）拦截一些处理过程。例如，当一个method被执行，Spring AOP能够劫持正在运行的method，在method执行前或者后加入一些额外的功能。

在Spring AOP中，支持4中类型的通知（Advice）

Before advice      ——method执行前通知

After returning advice ——method返回一个结果后通知

After throwing advice – method抛出异常后通知

Around advice – 环绕通知，结合了以上三种

下边这个例子解释Spring AOP怎样工作。

首先一个简单的不使用AOP的例子。

先创建一个简单的Service，为了稍后演示，这个类中加了几个简单的打印method。

CustomerService.java如下：

package com.lei.demo.aop.advice;

public class CustomerService {

    private String name;
    private String url;
 
    public void setName(String name) {
        this.name = name;
    }
 
    public void setUrl(String url) {
        this.url = url;
    }
 
    public void printName() {
        System.out.println("Customer name : " + this.name);
    }
 
    public void printURL() {
        System.out.println("Customer website : " + this.url);
    }
 
    public void printThrowException() {
        throw new IllegalArgumentException();
    }

}

Xml配置文件Apring-AOP-Advice.xml如下：

<beans xmlns="http://www.springframework.org/schema/beans"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://www.springframework.org/schema/beans
    http://www.springframework.org/schema/beans/spring-beans-2.5.xsd">
 
    <bean id="customerService" class="com.lei.demo.aop.advice.CustomerService">
        <property name="name" value="LeiOOLei" />
        <property name="url" value="http://www.cnblogs.com/leiOOlei/" />
    </bean>
 
</beans>

 运行以下代码App.java：

package com.lei.demo.aop.advice;

import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;

public class App {

    public static void main(String[] args) {
        ApplicationContext appContext = new ClassPathXmlApplicationContext(
                new String[] { "Spring-AOP-Advice.xml" });
 
        CustomerService cust = (CustomerService) appContext.getBean("customerService");
 
        System.out.println("*************************");
        cust.printName();
        System.out.println("*************************");
        cust.printURL();
        System.out.println("*************************");
        try {
            cust.printThrowException();
        } catch (Exception e) {
 
        }
 
    }

}

运行结果：

*************************

Customer name : LeiOOLei

*************************

Customer website : http://www.cnblogs.com/leiOOlei/

*************************

1.      Before Advice

创建一个实现了接口MethodBeforeAdvice的class，method运行前，将运行下边的代码

HijackBeforeMethod.java如下:

package com.lei.demo.aop.advice;

import java.lang.reflect.Method;
import org.springframework.aop.MethodBeforeAdvice;

public class HijackBeforeMethod implements MethodBeforeAdvice {

    public void before(Method arg0, Object[] args, Object target)
            throws Throwable {
        System.out.println("HijackBeforeMethod : Before method hijacked!");
        
    }

}

 在配置文件中加入新的bean配置HijackBeforeMethod，然后创建一个新的代理（proxy），命名为customerServiceProxy。

“target”定义你想劫持哪个bean；

“interceptorNames”定义你想用哪个class(advice)劫持target。

Apring-AOP-Advice.xml如下：

<beans xmlns="http://www.springframework.org/schema/beans"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://www.springframework.org/schema/beans
    http://www.springframework.org/schema/beans/spring-beans-2.5.xsd">
 
    <bean id="customerService" class="com.lei.demo.aop.advice.CustomerService">
        <property name="name" value="LeiOOLei" />
        <property name="url" value="http://www.cnblogs.com/leiOOlei/" />
    </bean>
    
    <bean id="hijackBeforeMethodBean" class="com.lei.demo.aop.advice.HijackBeforeMethod" />
 
    <bean id="customerServiceProxy" class="org.springframework.aop.framework.ProxyFactoryBean">
        <property name="target" ref="customerService" />
        <property name="interceptorNames">
            <list>
                <value>hijackBeforeMethodBean</value>
            </list>
        </property>
    </bean>
 
</beans>

注意：

用Spring proxy之前，必须添加CGLIB2类库，，以下是pom.xml依赖

　　<dependency>
        <groupId>cglib</groupId>
        <artifactId>cglib</artifactId>
        <version>2.2.2</version>
    </dependency>

 

运行如下代码，注意代理

App.java如下

package com.lei.demo.aop.advice;

import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;

public class App {

    public static void main(String[] args) {
        ApplicationContext appContext = new ClassPathXmlApplicationContext(
                new String[] { "Spring-AOP-Advice.xml" });
 
        CustomerService cust = (CustomerService) appContext.getBean("customerServiceProxy");
 
        System.out.println("使用Spring AOP 如下");
        System.out.println("*************************");
        cust.printName();
        System.out.println("*************************");
        cust.printURL();
        System.out.println("*************************");
        
        try {
            cust.printThrowException();
        } catch (Exception e) {
 
        }
 
    }

}

输出结果：

使用Spring AOP 如下

*************************

HijackBeforeMethod : Before method hijacked!

Customer name : LeiOOLei

*************************

HijackBeforeMethod : Before method hijacked!

Customer website : http://www.cnblogs.com/leiOOlei/

*************************

HijackBeforeMethod : Before method hijacked!

每一个customerService的method运行前，都将先执行HijackBeforeMethod的before方法。

2.      After Returning Advice

创建一个实现了接口AfterReturningAdvice的class，method运行后，直到返回结果后，才运行下边的代码，如果没有返回结果，将不运行切入的代码。

HijackAfterMethod.java如下：

package com.lei.demo.aop.advice;

import java.lang.reflect.Method;
import org.springframework.aop.AfterReturningAdvice;

public class HijackAfterMethod implements AfterReturningAdvice {

    public void afterReturning(Object returnValue, Method method, Object[] args,
            Object target) throws Throwable {
        System.out.println("HijackAfterMethod : After method hijacked!");

    }

}

修改bean配置文件，加入hijackAfterMethodBean配置，Apring-AOP-Advice.xml如下：

<beans xmlns="http://www.springframework.org/schema/beans"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://www.springframework.org/schema/beans
    http://www.springframework.org/schema/beans/spring-beans-2.5.xsd">
 
    <bean id="customerService" class="com.lei.demo.aop.advice.CustomerService">
        <property name="name" value="LeiOOLei" />
        <property name="url" value="http://www.cnblogs.com/leiOOlei/" />
    </bean>
    
    <bean id="hijackBeforeMethodBean" class="com.lei.demo.aop.advice.HijackBeforeMethod" />
    <bean id="hijackAfterMethodBean" class="com.lei.demo.aop.advice.HijackAfterMethod" />
 
    <bean id="customerServiceProxy" class="org.springframework.aop.framework.ProxyFactoryBean">
        <property name="target" ref="customerService" />
        <property name="interceptorNames">
            <list>
                <value>hijackAfterMethodBean</value>
            </list>
        </property>
    </bean>
 
</beans>

现在再运行App.java后输出如下：

使用Spring AOP 如下

*************************

Customer name : LeiOOLei

HijackAfterMethod : After method hijacked!

*************************

Customer website : http://www.cnblogs.com/leiOOlei/

HijackAfterMethod : After method hijacked!

*************************

可以看到输出结果，每一个customerService的method运行返回结果后，都将再执行HijackAfterMethod的afterReturning方法。但是执行到cust.printThrowException()后，直接抛出异常，方法没有正常执行完毕（或者说没有返回结果），所以不运行切入的afterReturning方法。

3.      Afetr Throwing Advice

创建一个实现了ThrowsAdvice接口的class，劫持IllegalArgumentException异常，目标method运行时，抛出IllegalArgumentException异常后，运行切入的方法。

HijackThrowException.java如下：

package com.lei.demo.aop.advice;

import org.springframework.aop.ThrowsAdvice;

import sun.awt.SunToolkit.IllegalThreadException;

public class HijackThrowException implements ThrowsAdvice {

    public void afterThrowing(IllegalArgumentException e) throws Throwable {
        System.out.println("HijackThrowException : Throw exception hijacked!");
    }

}

修改bean配置文件，加入了hijackThrowExceptionBean，Apring-AOP-Advice.xml如下：

<beans xmlns="http://www.springframework.org/schema/beans"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://www.springframework.org/schema/beans
    http://www.springframework.org/schema/beans/spring-beans-2.5.xsd">
 
    <bean id="customerService" class="com.lei.demo.aop.advice.CustomerService">
        <property name="name" value="LeiOOLei" />
        <property name="url" value="http://www.cnblogs.com/leiOOlei/" />
    </bean>
    
    <bean id="hijackBeforeMethodBean" class="com.lei.demo.aop.advice.HijackBeforeMethod" />
    <bean id="hijackAfterMethodBean" class="com.lei.demo.aop.advice.HijackAfterMethod" />
    <bean id="hijackThrowExceptionBean" class="com.lei.demo.aop.advice.HijackThrowException" />
 
    <bean id="customerServiceProxy" class="org.springframework.aop.framework.ProxyFactoryBean">
        <property name="target" ref="customerService" />
        <property name="interceptorNames">
            <list>
                <value>hijackThrowExceptionBean</value>
            </list>
        </property>
    </bean>
 
</beans>

运行结果如下:

使用Spring AOP 如下

*************************

Customer name : LeiOOLei

*************************

Customer website : http://www.cnblogs.com/leiOOlei/

*************************

HijackThrowException : Throw exception hijacked!

当运行CustomerService中的printThrowException方法时，认为的抛出IllegalArgumentException异常，被HijackThrowException截获，运行其中的afterThrowing方法。注意，如果抛出异常不是IllegalArgumentException，则不能被截获。

4.      Around Advice

结合了以上3种形式的Advice，创建一个实现了接口MethodInterceptor的class，你必须通过methodInvocation.proceed()来调用原来的方法，即通过调用methodInvocation.proceed()来调用CustomerService中的每一个方法，当然也可以不调用原方法。

HijackAroundMethod.java如下：

package com.lei.demo.aop.advice;

import java.util.Arrays;
import org.aopalliance.intercept.MethodInterceptor;
import org.aopalliance.intercept.MethodInvocation;

public class HijackAroundMethod implements MethodInterceptor {

    public Object invoke(MethodInvocation methodInvocation) throws Throwable {
        System.out.println("Method name : "
                + methodInvocation.getMethod().getName());
        System.out.println("Method arguments : "
                + Arrays.toString(methodInvocation.getArguments()));
 
        // 相当于  MethodBeforeAdvice
        System.out.println("HijackAroundMethod : Before method hijacked!");
 
        try {
            // 调用原方法，即调用CustomerService中的方法
            Object result = methodInvocation.proceed();
 
            // 相当于 AfterReturningAdvice
            System.out.println("HijackAroundMethod : After method hijacked!");
 
            return result;
 
        } catch (IllegalArgumentException e) {
            // 相当于 ThrowsAdvice
            System.out.println("HijackAroundMethod : Throw exception hijacked!");
            throw e;
        }
    }

}

修改bean配置文件，加入了hijackAroundMethodBean，Apring-AOP-Advice.xml如下：

<beans xmlns="http://www.springframework.org/schema/beans"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://www.springframework.org/schema/beans
    http://www.springframework.org/schema/beans/spring-beans-2.5.xsd">
 
    <bean id="customerService" class="com.lei.demo.aop.advice.CustomerService">
        <property name="name" value="LeiOOLei" />
        <property name="url" value="http://www.cnblogs.com/leiOOlei/" />
    </bean>
    
    <bean id="hijackBeforeMethodBean" class="com.lei.demo.aop.advice.HijackBeforeMethod" />
    <bean id="hijackAfterMethodBean" class="com.lei.demo.aop.advice.HijackAfterMethod" />
    <bean id="hijackThrowExceptionBean" class="com.lei.demo.aop.advice.HijackThrowException" />
    <bean id="hijackAroundMethodBean" class="com.lei.demo.aop.advice.HijackAroundMethod" />
 
    <bean id="customerServiceProxy" class="org.springframework.aop.framework.ProxyFactoryBean">
        <property name="target" ref="customerService" />
        <property name="interceptorNames">
            <list>
                <value>hijackAroundMethodBean</value>
            </list>
        </property>
    </bean>
 
</beans>

执行App.java，输出结果：

使用Spring AOP 如下

*************************

Method name : printName

Method arguments : []

HijackAroundMethod : Before method hijacked!

Customer name : LeiOOLei

HijackAroundMethod : After method hijacked!

*************************

Method name : printURL

Method arguments : []

HijackAroundMethod : Before method hijacked!

Customer website : http://www.cnblogs.com/leiOOlei/

HijackAroundMethod : After method hijacked!

*************************

Method name : printThrowException

Method arguments : []

HijackAroundMethod : Before method hijacked!

HijackAroundMethod : Throw exception hijacked!

CustomerService中每一个方法的调用，都会执行HijackAroundMethod中的invoke方法，可以看到整个切入点将目标around。

大多数的Spring开发者只用Around Advice，因为它能够实现所有类型的Advice。在实际的项目开发中，我们还是要尽量选择适合的Advice。

       在以上的例子中，CustomerService中的所有方法都被自动拦截，但是大多数情况下，我们不需要拦截一个class中的所有方法，而是拦截符合条件的方法。这时，我们就需要用到Pointcut and Advice，即切入点和通知，以后的章节中会逐渐介绍。