Timer 和 ScheduleExecutorService 区别 (转)

原文: http://sunnylocus.iteye.com/blog/530969

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java.util.Timer计时器有管理任务延迟执行("如1000ms后 执行任务")以及周期性执行("如每500ms执行一次该任务")。但是，Timer存在一些缺陷，因此你应该考虑使用 ScheduledThreadPoolExecutor作为代替品,Timer对调度的支持是基于绝对时间,而不是相对时间的，由此任务对系统时钟的改 变是敏感的;ScheduledThreadExecutor只支持相对时间。

    Timer的另一个问题在于，如果TimerTask抛出未检查的异常，Timer将会产生无法预料的行为。Timer线程并不捕获异常，所以 TimerTask抛出的未检查的异常会终止timer线程。这种情况下，Timer也不会再重新恢复线程的执行了;它错误的认为整个Timer都被取消 了。此时，已经被安排但尚未执行的TimerTask永远不会再执行了，新的任务也不能被调度了。

　测试Timer的例子

 

package threadDemo.TimerAndScheduledExecutorDemo;

import java.util.Timer;
import java.util.TimerTask;

public class TimerTest {
    private Timer timer = new Timer();
    
    public void launchTimer() {
        timer.schedule(new TimerTask(){
            @Override
            public void run() {
                throw new RuntimeException();
            }
        }, 1000*3, 500);
    }
    
    public void addOneTask() {
        timer.schedule(new TimerTask(){
            @Override
            public void run() {
                System.out.println("hello world");
            }
        }, 1000*1, 1000*5);
    }
    
    public static void main(String[] args) throws InterruptedException {
        TimerTest test = new TimerTest();
        test.launchTimer();
        Thread.sleep(5000);
        test.addOneTask();
    }
}

 

运行该程序，Timer会抛出一个RumtimeException和java.lang.IllegalStateException:Timer already cancelled.

 

常言道，真是祸不单行，Timer还将它的问题传染给下一个倒霉的调用者，这个调用者原本试图提交一个TimerTask的，你可能希望程序会一直运行下去，然而实际情况如程序所示5秒钟后就中止了，还伴随着一个异常，异常的消息是"Timer already cancelled"。ScheduledThreadPoolExector妥善地处理了这个异常的任务，所以说在java5.0或更高的JDK中，几乎没有理由再使用Timer了。

 

Exception in thread "Timer-0" java.lang.RuntimeException
    at threadDemo.TimerAndScheduledExecutorDemo.TimerTest$1.run(TimerTest.java:14)
    at java.util.TimerThread.mainLoop(Timer.java:512)
    at java.util.TimerThread.run(Timer.java:462)
Exception in thread "main" java.lang.IllegalStateException: Timer already cancelled.
    at java.util.Timer.sched(Timer.java:354)
    at java.util.Timer.schedule(Timer.java:222)
    at threadDemo.TimerAndScheduledExecutorDemo.TimerTest.addOneTask(TimerTest.java:21)
    at threadDemo.TimerAndScheduledExecutorDemo.TimerTest.main(TimerTest.java:35)

用ScheduledThreadPoolExector改进后的例子

 

package threadDemo.TimerAndScheduledExecutorDemo;

import java.util.concurrent.Executors;
import java.util.concurrent.ScheduledExecutorService;
import java.util.concurrent.TimeUnit;

public class ScheduledExecutorTest {
    public ScheduledExecutorService scheduledExec = Executors.newScheduledThreadPool(1);
    int i = 0;
    int j = 0;
    public void launchTimer() {
        Runnable task = new Runnable(){
            @Override
            public void run() {
                System.out.println("launch: " + ++j);
                throw new RuntimeException();
            }
            
        };
        scheduledExec.scheduleWithFixedDelay(task, 1000*5, 1000*10, TimeUnit.MILLISECONDS);
    }
    
    public void addOneTask() {
        Runnable task = new Runnable(){
            @Override
            public void run() {
                System.out.println("hello world: " + ++i);
            }
        };
        scheduledExec.scheduleWithFixedDelay(task, 1000*1, 1000, TimeUnit.MILLISECONDS);
    }
    
    public static void main(String[] args) throws InterruptedException {
        ScheduledExecutorTest test = new ScheduledExecutorTest();
        test.launchTimer();
        Thread.sleep(5000);
        test.addOneTask();
    }
}

结果:

 

launch: 1
hello world: 1
hello world: 2
hello world: 3
hello world: 4
hello world: 5
hello world: 6
...