背景:
Timer类负责延迟任务和周期任务,但是Timer存在一些缺陷,因此应该考虑使用ScheduledThreadPoolExecutor来代替它。
Timer缺陷:
1、Timer在执行所有定时任务时只会创建一个线程。如果某个任务的执行时间过长,那么将破坏其他TimerTask的定时精确性。
2、如果TimerTask抛出了一个未检查异常,那么Timer将表现出糟糕行为。Timer线程并不捕获异常,因此TimerTask抛出未检查异常时将终止定时线程。
时钟调度区别:
Timer支持基于绝对时间而不是相对时间的调度机制,因此任务的执行对系统时钟变化很敏感,而ScheduledThreadPoolExecutor 只支持基于相对时间的调度。
代码示例:
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;
// 重试间隔时间(毫秒)
private static final int TIMEOUT = 300000;
// 单位统一为毫秒
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()
{
// 单线程线程池
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
{
// 等待上报数据成功
done.await();
System.out.println("Watch countDownLatch thread: try shutdown scheduled executor");
// 安全关闭线程池
exec.shutdown();
// 超时退出
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();
}
}
输出:
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 ------异常捕获
ScheduleTaskByScheduledES: try start ... count2 is: 1
java.lang.RuntimeException: current count is: 2 -------异常抛出 线程终止 导致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 ------另一线程继续执行
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不再执行
结果分析:
1、同一Timer对象不适合启动多个TimerTask任务,一是因为单线程执行这些任务,二是因为任何一个TimerTask抛出未受检异常,Timer不会进行捕获,会导致Timer cannel,后续任务无法继续进行。
2、延迟或定时任务推荐使用 ScheduledExecutorService 对象的 scheduleAtFixedRate 方法,安全高效。
参考书籍:
《Java并发编程实战》