延遲任務與周期任務-Timer&ScheduledThread定時器使用

背景：

Timer類負責延遲任務和周期任務，但是Timer存在一些缺陷，是以應該考慮使用ScheduledThreadPoolExecutor來代替它。

Timer缺陷：

1、Timer在執行所有定時任務時隻會建立一個線程。如果某個任務的執行時間過長，那麼将破壞其他TimerTask的定時精确性。

2、如果TimerTask抛出了一個未檢查異常，那麼Timer将表現出糟糕行為。Timer線程并不捕獲異常，是以TimerTask抛出未檢查異常時将終止定時線程。

時鐘排程差別：

Timer支援基于絕對時間而不是相對時間的排程機制，是以任務的執行對系統時鐘變化很敏感，而ScheduledThreadPoolExecutor 隻支援基于相對時間的排程。

代碼示例：

package com.csdn.test.timer;

import java.util.Timer;
import java.util.TimerTask;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.Executors;
import java.util.concurrent.ScheduledExecutorService;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicBoolean;


public class TestScheTask
{
    private static final int TRY_SEND_DATA_PERIOD = 2000;

    // 重試間隔時間(毫秒)
    private static final int TIMEOUT = 300000;

    // 機關統一為毫秒
    private static final TimeUnit UNIT = TimeUnit.MILLISECONDS;
    
    private static final int MAX_TIME = 5;

    private volatile AtomicBoolean isFinish = new AtomicBoolean(false);

    private volatile CountDownLatch done = new CountDownLatch(1);
    
    private int scheduledES_count1 = 0;
    
    private int scheduledES_count2 = 0;
    
    private int timer_count1 = 0;
    
    private int timer_count2 = 0;

    public void scheduleTaskByScheduledES()
    {
        // 單線程線程池
        ScheduledExecutorService exec = Executors.newScheduledThreadPool(1);
        exec.scheduleAtFixedRate(new Runnable()
        {

            @Override
            public void run()
            {
                boolean isOk = isFinish.get();
                if (!isOk)
                {
                    
                    System.out.println("ScheduleTaskByScheduledES: try start ... count1 is: " + scheduledES_count1);
                    scheduledES_count1 ++;
                    if (scheduledES_count1 > 1)
                    {
                        throw new RuntimeException("current count1 is: " + scheduledES_count1);
                    }
                    if (scheduledES_count1 > MAX_TIME)
                    {
                        isFinish.getAndSet(true);
                        done.countDown();
                        System.out.println("Scheduled thread: end success");
                    }
                    
                }
            }

        }, 0, TRY_SEND_DATA_PERIOD, UNIT);

        exec.scheduleAtFixedRate(new Runnable()
        {

            @Override
            public void run()
            {
                boolean isOk = isFinish.get();
                if (!isOk)
                {
                    
                    System.out.println("ScheduleTaskByScheduledES: try start ... count2 is: " + scheduledES_count2);
                    scheduledES_count2 ++;
                    if (scheduledES_count2 > MAX_TIME)
                    {
                        isFinish.getAndSet(true);
                        done.countDown();
                        System.out.println("Scheduled thread: end success");
                    }
                    
                }
            }

        }, 0, TRY_SEND_DATA_PERIOD, UNIT);
        
        System.out.println("Watch countDownLatch thread start");
        new Thread(new Runnable()
        {

            @Override
            public void run()
            {
                try
                {
                    // 等待上報資料成功
                    done.await();
                    System.out.println("Watch countDownLatch thread: try shutdown scheduled executor");
                    // 安全關閉線程池
                    exec.shutdown();
                    // 逾時退出
                    exec.awaitTermination(TIMEOUT, UNIT);
                    System.out.println("Watch countDownLatch thread: end shutdown scheduled executor");
                }
                catch (InterruptedException e)
                {
                    System.out.println("Watch countDownLatch thread is interrupted, exception is: "
                            + e);
                }

            }

        }).start();
        System.out.println("Scheduled thread and watch countDownLatch thread both start");
    }
    
    public void scheduleTaskByTimer()
    {
        Timer timer = new Timer();
        timer.schedule(new TimerTask(){

            @Override
            public void run()
            {
                System.out.println("ScheduleTaskByTimer1: try start ... count1 is: " + timer_count1);
                timer_count1++;
                if (timer_count1 > 1)
                {
                    throw new RuntimeException("current count is: " + timer_count1);
                }
            }
            
        }, 0, TRY_SEND_DATA_PERIOD);
        timer.schedule(new TimerTask(){

            @Override
            public void run()
            {
                System.out.println("ScheduleTaskByTimer2: try start ... count2 is: " + timer_count2);
                timer_count2++;
            }
            
        }, 0, TRY_SEND_DATA_PERIOD);
    }
    
    public static void main(String args[])
    {
        TestScheTask test1 = new TestScheTask();
        new Thread(new Runnable(){

            @Override
            public void run()
            {
                test1.scheduleTaskByScheduledES();
            }
            
        }).start();
        TestScheTask test2 = new TestScheTask();
        new Thread(new Runnable(){

            @Override
            public void run()
            {
                test2.scheduleTaskByTimer();
            }
            
        }).start();
    }
}
           

輸出：

ScheduleTaskByTimer1: try start ... count1 is: 0

ScheduleTaskByTimer2: try start ... count2 is: 0

ScheduleTaskByScheduledES: try start ... count1 is: 0

Watch countDownLatch thread start

ScheduleTaskByScheduledES: try start ... count2 is: 0

Scheduled thread and watch countDownLatch thread both start

ScheduleTaskByTimer2: try start ... count2 is: 1

ScheduleTaskByTimer1: try start ... count1 is: 1

Exception in thread "Timer-0" ScheduleTaskByScheduledES: try start ... count1 is: 1   ------異常捕獲

ScheduleTaskByScheduledES: try start ... count2 is: 1

java.lang.RuntimeException: current count is: 2                                                          -------異常抛出 線程終止 導緻Timer cannel

    at com.csdn.test.timer.TestScheTask$4.run(TestScheTask.java:138)

    at java.util.TimerThread.mainLoop(Unknown Source)

    at java.util.TimerThread.run(Unknown Source)

ScheduleTaskByScheduledES: try start ... count2 is: 2                                                ------另一線程繼續執行 

ScheduleTaskByScheduledES: try start ... count2 is: 3

ScheduleTaskByScheduledES: try start ... count2 is: 4

ScheduleTaskByScheduledES: try start ... count2 is: 5

Scheduled thread: end success

Watch countDownLatch thread: try shutdown scheduled executor                    

Watch countDownLatch thread: end shutdown scheduled executor

                                                                                                                                   ---------- Timer的另一TimerTask不再執行

結果分析：

1、同一Timer對象不适合啟動多個TimerTask任務，一是因為單線程執行這些任務，二是因為任何一個TimerTask抛出未受檢異常，Timer不會進行捕獲，會導緻Timer cannel，後續任務無法繼續進行。

2、延遲或定時任務推薦使用 ScheduledExecutorService 對象的 scheduleAtFixedRate 方法，安全高效。

參考書籍：

《Java并發程式設計實戰》