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