Spring+Maven学习实验- Spring AOP面向切面编程（一）

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 种类型的通知（ 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 。

1.pom.xml

<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
  <modelVersion>4.0.0</modelVersion>

  <groupId>com.shiyanlou.spring</groupId>
  <artifactId>aop</artifactId>
  <version>0.0.1-SNAPSHOT</version>
  <packaging>jar</packaging>

  <name>aop</name>
  <url>http://maven.apache.org</url>

  <properties>
    <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
  </properties>

  <dependencies>
    <dependency>
      <groupId>junit</groupId>
      <artifactId>junit</artifactId>
      <version>3.8.1</version>
      <scope>test</scope>
    </dependency>
        <dependency>
       <groupId>org.springframework</groupId>
       <artifactId>spring-core</artifactId>
       <version>4.2.0.RELEASE</version>
    </dependency>
    <dependency>
       <groupId>org.springframework</groupId>
       <artifactId>spring-context</artifactId>
       <version>4.2.0.RELEASE</version>
    </dependency>
  </dependencies>
</project>

2.CustomerService.java

package com.shiyanlou.spring.aop;

public class CustomerService {
    private String name;
    private String url;
    
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public String getUrl() {
        return url;
    }
    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 url:"+this.url);
    }
    
    public void printThrowException() {
        throw new IllegalArgumentException();
    }

}

3.SrpingAOP.xml

<?xml version="1.0" encoding="UTF-8"?>
<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.xsd">
    
    <bean id="customerService" class="com.shiyanlou.spring.aop.CustomerService">
        <property name="name" value="zoey"/>
        <property name="url" value="http://shiyanlou.com"/>
    </bean>
</beans>

4.App.java

package com.shiyanlou.spring.aop;

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[]{"SpringAOP.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) {
        }
    }
}

4 种类型的通知（ Advice ）

1.Before Advice

method 运行前，将运行下面的代码

HijackBeforeMethod.java 如下:

package com.shiyanlou.spring.aop;

import java.lang.reflect.Method;

import org.springframework.aop.MethodBeforeAdvice;

public class HijackBeforeMethod implements MethodBeforeAdvice{

    public void before(Method arg0, Object[] arg1, Object arg2) 
            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 定义想用哪个 class ( advice )劫持 target 。 ApringAOP.xml 如下：

<?xml version="1.0" encoding="UTF-8"?>
<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.xsd">
    
    <bean id="customerService" class="com.shiyanlou.spring.aop.CustomerService">
        <property name="name" value="zoey"/>
        <property name="url" value="http://shiyanlou.com"/>
    </bean>
    
    <bean id="hijackBeforeMethodBean" class="com.shiyanlou.spring.aop.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>

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

    <dependency>
        <groupId>org.glassfish.hk2.external</groupId>
        <artifactId>cglib</artifactId>
        <version>2.2.0-b23</version>
    </dependency>

app.java

package com.shiyanlou.spring.aop;

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[]{"SpringAOP.xml"});
        CustomerService cust = (CustomerService)appContext.getBean("customerServiceProxy");
        System.out.println("*************************");
        cust.printName();
        System.out.println("*************************");
        cust.printUrl();      
        System.out.println("*************************");
        try {
            cust.printThrowException();
        } catch (Exception e) {
        }
    }
}

运行结果：

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.shiyanlou.spring.aop;

import java.lang.reflect.Method;

import org.springframework.aop.AfterReturningAdvice;

public class HijackAfterMethod implements AfterReturningAdvice {

    public void afterReturning(Object arg0, Method arg1, Object[] arg2,
            Object arg3) throws Throwable {
        System.out.println("After method hijack");

    }

}

SpringAOP.xml

<?xml version="1.0" encoding="UTF-8"?>
<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.xsd">
    
    <bean id="customerService" class="com.shiyanlou.spring.aop.CustomerService">
        <property name="name" value="zoey"/>
        <property name="url" value="http://shiyanlou.com"/>
    </bean>
    
    <bean id="hijackBeforeMethodBean" class="com.shiyanlou.spring.aop.HijackBeforeMethod"/>
    <bean id="hijackAfterMethodBean" class="com.shiyanlou.spring.aop.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>

App.java

package com.shiyanlou.spring.aop;

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[]{"SpringAOP.xml"});
        CustomerService cust = (CustomerService)appContext.getBean("customerServiceProxy");
        System.out.println("*************************");
        cust.printName();
        System.out.println("*************************");
        cust.printUrl();      
        System.out.println("*************************");
        try {
            cust.printThrowException();
        } catch (Exception e) {
        }
    }
}

运行结果：

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.shiyanlou.spring.aop;

import org.springframework.aop.ThrowsAdvice;

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 ，ApringAdvice.xml 如下：

<?xml version="1.0" encoding="UTF-8"?>
<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.xsd">
    
    <bean id="customerService" class="com.shiyanlou.spring.aop.CustomerService">
        <property name="name" value="zoey"/>
        <property name="url" value="http://shiyanlou.com"/>
    </bean>
    
    <bean id="hijackBeforeMethodBean" class="com.shiyanlou.spring.aop.HijackBeforeMethod"/>
    <bean id="hijackAfterMethodBean" class="com.shiyanlou.spring.aop.HijackAfterMethod"/>
    <bean id="hijackThrowExceptionBean" class="com.shiyanlou.spring.aop.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：

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.shiyanlou.spring.aop;

import java.lang.reflect.Method;
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 ，ApringAOP.xml如下：

<?xml version="1.0" encoding="UTF-8"?>
<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.xsd">
    
    <bean id="customerService" class="com.shiyanlou.spring.aop.CustomerService">
        <property name="name" value="zoey"/>
        <property name="url" value="http://shiyanlou.com"/>
    </bean>
    
    <bean id="hijackBeforeMethodBean" class="com.shiyanlou.spring.aop.HijackBeforeMethod"/>
    <bean id="hijackAfterMethodBean" class="com.shiyanlou.spring.aop.HijackAfterMethod"/>
    <bean id="hijackThrowExceptionBean" class="com.shiyanlou.spring.aop.HijackThrowException"/>
    <bean id="hijackAroundMethodBean" class="com.shiyanlou.spring.aop.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>

运行app.java结果：

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