Spring @Async之一：实现异步调用示例

什么是“异步调用”？

“异步调用”对应的是“同步调用”，同步调用指程序按照定义顺序依次执行，每一行程序都必须等待上一行程序执行完成之后才能执行；异步调用指程序在顺序执行时，不等待异步调用的语句返回结果就执行后面的程序。

同步调用

下面通过一个简单示例来直观的理解什么是同步调用：

定义Task类，创建三个处理函数分别模拟三个执行任务的操作，操作消耗时间随机取（10秒内）

package com.dxz.demo1;

import java.util.Random;
import org.springframework.stereotype.Component;
import org.springframework.web.bind.annotation.RequestMapping;

/**
 * 定义3个任务
 */
@Component
public class Task1 {

    // 定义一个随机对象.
    public static Random random = new Random();

    // 任务一;
    public void doTaskOne() throws Exception {
        System.out.println("开始做任务一");
        long start = System.currentTimeMillis();
        Thread.sleep(random.nextInt(10000));
        long end = System.currentTimeMillis();
        System.out.println("完成任务一，耗时：" + (end - start) + "毫秒");
    }

    // 任务二;
    public void doTaskTwo() throws Exception {
        System.out.println("开始做任务二");
        long start = System.currentTimeMillis();
        Thread.sleep(random.nextInt(10000));
        long end = System.currentTimeMillis();
        System.out.println("完成任务二，耗时：" + (end - start) + "毫秒");
    }

    // 任务3;
    public void doTaskThree() throws Exception {
        System.out.println("开始做任务三");
        long start = System.currentTimeMillis();
        Thread.sleep(random.nextInt(10000));
        long end = System.currentTimeMillis();
        System.out.println("完成任务三，耗时：" + (end - start) + "毫秒");
    }

}

编写一个访问方法：

package com.dxz.demo1;

import org.junit.Test;
import org.junit.runner.RunWith;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.SpringApplicationConfiguration;
import org.springframework.test.context.junit4.SpringJUnit4ClassRunner;

import com.dxz.HelloApplication;

@RunWith(SpringJUnit4ClassRunner.class)
@SpringApplicationConfiguration(classes = HelloApplication.class)
public class Task1Test {

    @Autowired
    private Task1 task1;

    //测试task1.
    @Test
    public void task1() throws Exception{
       task1.doTaskOne();
       task1.doTaskTwo();
       task1.doTaskThree();
    }
}

运行可以看到类似如下输出：

开始做任务一
2017-04-28 18:02:57.397  WARN 11016 --- [cTaskExecutor-1] o.s.a.r.l.SimpleMessageListenerContainer : Consumer raised exception, processing can restart if the connection factory supports it. Exception summary: org.springframework.amqp.AmqpConnectException: java.net.ConnectException: Connection refused: connect
2017-04-28 18:02:57.398  INFO 11016 --- [cTaskExecutor-1] o.s.a.r.l.SimpleMessageListenerContainer : Restarting Consumer: tags=[{}], channel=null, acknowledgeMode=AUTO local queue size=0
完成任务一，耗时：7740毫秒
开始做任务二
完成任务二，耗时：723毫秒
开始做任务三
2017-04-28 18:03:03.415  WARN 11016 --- [cTaskExecutor-2] o.s.a.r.l.SimpleMessageListenerContainer : Consumer raised exception, processing can restart if the connection factory supports it. Exception summary: org.springframework.amqp.AmqpConnectException: java.net.ConnectException: Connection refused: connect
2017-04-28 18:03:03.415  INFO 11016 --- [cTaskExecutor-2] o.s.a.r.l.SimpleMessageListenerContainer : Restarting Consumer: tags=[{}], channel=null, acknowledgeMode=AUTO local queue size=0
完成任务三，耗时：5047毫秒

异步调用

上述的同步调用虽然顺利的执行完了三个任务，但是可以看到执行时间比较长，若这三个任务本身之间不存在依赖关系，可以并发执行的话，同步调用在执行效率方面就比较差，可以考虑通过异步调用的方式来并发执行。

在Spring Boot中，我们只需要通过使用@Async注解就能简单的将原来的同步函数变为异步函数，Task类改在为如下模式：

package com.dxz.demo1;

import java.util.Random;

import org.springframework.scheduling.annotation.Async;
import org.springframework.stereotype.Component;
import org.springframework.web.bind.annotation.RequestMapping;

/**
 * 定义3个任务
 */
@Component
public class Task2 {

    // 定义一个随机对象.
    public static Random random = new Random();

    // 任务一;
    @Async
    public void doTaskOne() throws Exception {
        System.out.println("开始做任务一");
        long start = System.currentTimeMillis();
        Thread.sleep(random.nextInt(10000));
        long end = System.currentTimeMillis();
        System.out.println("完成任务一，耗时：" + (end - start) + "毫秒");
    }

    // 任务二;
    @Async
    public void doTaskTwo() throws Exception {
        System.out.println("开始做任务二");
        long start = System.currentTimeMillis();
        Thread.sleep(random.nextInt(10000));
        long end = System.currentTimeMillis();
        System.out.println("完成任务二，耗时：" + (end - start) + "毫秒");
    }

    // 任务3;
    @Async
    public void doTaskThree() throws Exception {
        System.out.println("开始做任务三");
        long start = System.currentTimeMillis();
        Thread.sleep(random.nextInt(10000));
        long end = System.currentTimeMillis();
        System.out.println("完成任务三，耗时：" + (end - start) + "毫秒");
    }

}

为了让@Async注解能够生效，还需要在Spring Boot的主程序中配置@EnableAsync，如下所示：

package com.dxz;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.scheduling.annotation.EnableAsync;
@EnableAsync
@SpringBootApplication
public class HelloApplication {
    public static void main(String[] args) {
        SpringApplication.run(HelloApplication.class, args);
    }
}

编写测试方法：

package com.dxz.demo1;

import org.junit.Test;
import org.junit.runner.RunWith;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.SpringApplicationConfiguration;
import org.springframework.test.context.junit4.SpringJUnit4ClassRunner;
import org.springframework.web.bind.annotation.RequestMapping;

import com.dxz.HelloApplication;

@RunWith(SpringJUnit4ClassRunner.class)
@SpringApplicationConfiguration(classes = HelloApplication.class)
public class Task2Test {

    @Autowired
    private Task2 task2;

    //测试task1.
    @Test
    public void task1() throws Exception{
       task2.doTaskOne();
       task2.doTaskTwo();
       task2.doTaskThree();
    }
}

此时可以反复执行单元测试，您可能会遇到各种不同的结果，比如：

开始做任务一
开始做任务二
开始做任务三

修改下测试类：

package com.dxz.demo1;

import java.util.concurrent.TimeUnit;

import org.junit.Test;
import org.junit.runner.RunWith;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.SpringApplicationConfiguration;
import org.springframework.test.context.junit4.SpringJUnit4ClassRunner;
import org.springframework.web.bind.annotation.RequestMapping;

import com.dxz.HelloApplication;

@RunWith(SpringJUnit4ClassRunner.class)
@SpringApplicationConfiguration(classes = HelloApplication.class)
public class Task2Test {

    @Autowired
    private Task2 task2;

    // 测试task1.
    @Test
    public void task1() throws Exception {
        task2.doTaskOne();
        task2.doTaskTwo();
        task2.doTaskThree();

        System.out.println("i'm here");
        TimeUnit.SECONDS.sleep(15);
        System.out.println("over");
    }

}

jieguo:

i'm here
开始做任务二
开始做任务一
开始做任务三
完成任务三，耗时：1280毫秒
2017-04-28 18:25:36.936 WARN 17848 --- [cTaskExecutor-1] o.s.a.r.l.SimpleMessageListenerContainer : Consumer raised exception, processing can restart if the connection factory supports it. Exception summary: org.springframework.amqp.AmqpConnectException: java.net.ConnectException: Connection refused: connect
2017-04-28 18:25:36.938 INFO 17848 --- [cTaskExecutor-1] o.s.a.r.l.SimpleMessageListenerContainer : Restarting Consumer: tags=[{}], channel=null, acknowledgeMode=AUTO local queue size=0
完成任务一，耗时：4951毫秒
完成任务二，耗时：7451毫秒
2017-04-28 18:25:42.971 WARN 17848 --- [cTaskExecutor-2] o.s.a.r.l.SimpleMessageListenerContainer : Consumer raised exception, processing can restart if the connection factory supports it. Exception summary: org.springframework.amqp.AmqpConnectException: java.net.ConnectException: Connection refused: connect
2017-04-28 18:25:42.972 INFO 17848 --- [cTaskExecutor-2] o.s.a.r.l.SimpleMessageListenerContainer : Restarting Consumer: tags=[{}], channel=null, acknowledgeMode=AUTO local queue size=0
over