Spring+Maven學習實驗- Spring AOP面向切面編程（一）

Spring AOP 即 Aspect-oriented programming，面向切面編程，是作為面向對象編程的一種補充，專門用於處理系統中分布於各個模塊（不同方法）中的交叉關注點的問題。簡單地說，就是一個攔截器（ interceptor ）攔截一些處理過程。

例如，當一 個method 被執行，Spring AOP 能夠劫持正在運行的 method ，在 method 執行前或者后加入一些額外的功能。

在 Spring AOP 中，支持 4 種類型的通知（ Advice ）：

• Before advice - method 執行前通知
• After returning advice - method 返回一個結果后通知
• After throwing advice - method 拋出異常后通知
• Around advice - 環繞通知，結合了以上三種        

下邊這個例子解釋 Spring AOP 怎樣工作。首先一個簡單的不使用 AOP 的例子。先創建一個簡單的 Service ，為了稍后演示，這個類中加了幾個簡單的打印 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 種類型的通知（ Advice ）

1.Before Advice

method 運行前，將運行下面的代碼

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 ，然后創建一個新的代理（ proxy ），命名為 customerServiceProxy 。target 定義你想劫持哪個 bean； interceptorNames 定義想用哪個 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，必須添加 CGLIB2 類庫，，以下是 pom.xml 依賴：

    <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) {
        }
    }
}

運行結果：

每一個 customerService 的 method 運行前，都將先執行 HijackBeforeMethod 的 before 方法。

2 After Returning Advice

創建一個實現了接口 AfterReturningAdvice 的 class ，method 運行后，直到返回結果后，才運行下邊的代碼，如果沒有返回結果，將不運行切入的代碼。

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) {
        }
    }
}

運行結果：

3 Afetr Throwing Advice

創建一個實現了 ThrowsAdvice 接口的 class ，劫持 IllegalArgumentException 異常，目標 method 運行時，拋出 IllegalArgumentException 異常后，運行切入的方法。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>

運行結果：

4 Around Advice

結合了以上 3 種形式的 Advice ，創建一個實現了接口 MethodInterceptor 的 class ，你必須通過 methodInvocation.proceed() 來調用原來的方法，即通過調用 methodInvocation.proceed() 來調用 CustomerService 中的每一個方法，當然也可以不調用原方法 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()));

        // 相當於  MethodBeforeAdvice
        System.out.println("HijackAroundMethod : Before method hijacked!");

        try {
            // 調用原方法，即調用CustomerService中的方法
            Object result = methodInvocation.proceed();

            // 相當於 AfterReturningAdvice
            System.out.println("HijackAroundMethod : After method hijacked!");

            return result;

        } catch (IllegalArgumentException e) {
            // 相當於 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>

運行app.java結果：

CustomerService 中每一個方法的調用，都會執行 HijackAroundMethod 中的 invoke 方法，可以看到整個切入點將目標 around 。

大多數的 Spring 開發者只用 Around Advice ，因為它能夠實現所有類型的 Advice 。

在實際的項目開發中，我們還是要盡量選擇適合的 Advice 。

在以上的例子中，CustomerService 中的所有方法都被自動攔截，但是大多數情況下，我們不需要攔截一個 class 中的所有方法，而是攔截符合條件的方法。

這時，我們就需要用到 Pointcut and Advice ，即切入點和通知