Java并发定时任务调度之Timer和ScheduledExecutorService

绝地灬酷狼 2022-07-10 11:28 260阅读 0赞

笔者这篇博文主要写的是Timer和ScheduledExecutorService完成定时任务调度的区别。

1)因为Timer是单线程执行的,如果第一个任务执行用掉了过多的时间,会导致第二个任务不能在指定延迟时间后得到调用。

我们来写一个demo

package org.hl.javaapi.scheduledapi;
    
    import java.util.Timer;
    import java.util.TimerTask;
    
    public class MyTimerTest {
        public static void main(String[] args) {
            final long startTime = System.currentTimeMillis();
            Timer timer = new Timer();
    
            TimerTask timerTask1 = new TimerTask() {
                @Override
                public void run() {
                    System.out.println("在这么多毫秒后,得到执行:" + (System.currentTimeMillis() - startTime));
                    try {
                        Thread.sleep(5000);
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                }
            };
    
            TimerTask timerTask2 = new TimerTask() {
                @Override
                public void run() {
                    System.out.println("在这么多毫秒后,得到执行:" + (System.currentTimeMillis() - startTime));
                }
            };
            //1秒后得到执行
            timer.schedule(timerTask1, 1000);
            //3秒后得到执行
            timer.schedule(timerTask2, 3000);
    
    
        }
    }

输出的结果为:

在这么多毫秒后,得到执行:1001
    在这么多毫秒后,得到执行:6002
    
    Process finished with exit code 1

Timer的第二个任务在6秒后才被执行,这是不符合要求的。所以我们考虑下使用ScheduledExecutorService来实现。

上代码

package org.hl.javaapi.scheduledapi;
    
    import java.util.TimerTask;
    import java.util.concurrent.Executors;
    import java.util.concurrent.ScheduledExecutorService;
    import java.util.concurrent.TimeUnit;
    
    public class MyScheduledExecutorServiceTest {
        private static long startTime;
    
        public static void main(String[] args) {
    
            ScheduledExecutorService scheduledExecutorService = Executors.newScheduledThreadPool(2);
    
            TimerTask timerTask1 = new TimerTask() {
                @Override
                public void run() {
                    System.out.println("task1 invoked in " + (System.currentTimeMillis() - startTime));
                    try {
                        //人为睡眠5秒
                        Thread.sleep(5000);
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                }
            };
    
            TimerTask timerTask2 = new TimerTask() {
                @Override
                public void run() {
                    System.out.println("task2 invoked in  "
                            + (System.currentTimeMillis() - startTime));
    
                }
            };
            startTime = System.currentTimeMillis();
            scheduledExecutorService.schedule(timerTask1, 1000, TimeUnit.MILLISECONDS);
            scheduledExecutorService.schedule(timerTask2, 3000, TimeUnit.MILLISECONDS);
        }
    }

得到的结果是

task1 invoked in 1000
    task2 invoked in  3001
    
    Process finished with exit code 1

这样符合我们的要求,因为ScheduledExecutorService内部是个线程池。有多个线程,所以就算第一个任务占用了较多的时间,第二个任务也会在指定的时间执行。

2)当Timer的第一个任务抛出了异常,第二个任务也就不执行了。而ScheduledExecutorService第二个任务还可以接着去执行。

package org.hl.javaapi.scheduledapi;
    
    import java.util.Timer;
    import java.util.TimerTask;
    
    public class MyTimerTest1 {
    
        private static long startTime;
        public static void main(String[] args) {
            Timer timer = new Timer();
    
            TimerTask timerTask1 = new TimerTask() {
                @Override
                public void run() {
                    throw new RuntimeException("人为的抛出异常");
                }
            };
            TimerTask timerTask2 = new TimerTask() {
                @Override
                public void run() {
                    System.out.println("task2 get invoked in" + (System.currentTimeMillis() - startTime));
                }
            };
    
            startTime = System.currentTimeMillis();
    
            timer.schedule(timerTask1,1000);
            timer.scheduleAtFixedRate(timerTask2,3000,1000);
        }
    }

运行结果如下:

Exception in thread "Timer-0" java.lang.RuntimeException: 人为的抛出异常
    	at org.hl.javaapi.scheduledapi.MyTimerTest1$1.run(MyTimerTest1.java:15)
    	at java.util.TimerThread.mainLoop(Timer.java:555)
    	at java.util.TimerThread.run(Timer.java:505)
    
    Process finished with exit code 0

接下来我们看一看ScheduledExecutorService的实现

package org.hl.javaapi.scheduledapi;
    
    import java.util.TimerTask;
    import java.util.concurrent.Executors;
    import java.util.concurrent.ScheduledExecutorService;
    import java.util.concurrent.TimeUnit;
    
    public class MyScheduledExecutorServiceTest1 {
        private static long startTime;
    
        public static void main(String[] args) {
            ScheduledExecutorService scheduledExecutorService = Executors.newScheduledThreadPool(2);
    
            TimerTask timerTask1 = new TimerTask() {
                @Override
                public void run() {
                    throw new RuntimeException("人为地抛出异常");
                }
            };
    
            TimerTask timerTask2 = new TimerTask() {
                @Override
                public void run() {
                    System.out.println("task get invoked in :" + (System.currentTimeMillis() - startTime));
                }
            };
    
            startTime = System.currentTimeMillis();
            scheduledExecutorService.schedule(timerTask1, 1000, TimeUnit.MILLISECONDS);
            scheduledExecutorService.scheduleAtFixedRate(timerTask2, 3000, 1000, TimeUnit.MILLISECONDS);
        }
    }

运行结果如下:

task get invoked in :3003  
task get invoked in :4002  
task get invoked in :5002  
task get invoked in :6003  
  
  
Process finished with exit code 1

Timer执行周期任务时依赖系统时间