在我们编程过程中如果需要执行一些简单的定时任务,无须做复杂的控制,我们可以考虑使用JDK中的Timer定时任务来实现。下面LZ就其原理、实例以及Timer缺陷三个方面来解析java Timer定时器。
一、简介
在java中一个完整定时任务需要由Timer、TimerTask两个类来配合完成。 API中是这样定义他们的,Timer:一种工具,线程用其安排以后在后台线程中执行的任务。可安排任务执行一次,或者定期重复执行。由TimerTask:Timer 安排为一次执行或重复执行的任务。我们可以这样理解Timer是一种定时器工具,用来在一个后台线程计划执行指定任务,而TimerTask一个抽象类,它的子类代表一个可以被Timer计划的任务。
Timer类
在工具类Timer中,提供了四个构造方法,每个构造方法都启动了计时器线程,同时Timer类可以保证多个线程可以共享单个Timer对象而无需进行外部同步,所以Timer类是线程安全的。但是由于每一个Timer对象对应的是单个后台线程,用于顺序执行所有的计时器任务,一般情况下我们的线程任务执行所消耗的时间应该非常短,但是由于特殊情况导致某个定时器任务执行的时间太长,那么他就会“独占”计时器的任务执行线程,其后的所有线程都必须等待它执行完,这就会延迟后续任务的执行,使这些任务堆积在一起,具体情况我们后面分析。
当程序初始化完成Timer后,定时任务就会按照我们设定的时间去执行,Timer提供了schedule方法,该方法有多中重载方式来适应不同的情况,如下:
schedule(TimerTask task, Date time):安排在指定的时间执行指定的任务。
schedule(TimerTask task, Date firstTime, long period) :安排指定的任务在指定的时间开始进行重复的固定延迟执行。
schedule(TimerTask task, long delay) :安排在指定延迟后执行指定的任务。
schedule(TimerTask task, long delay, long period) :安排指定的任务从指定的延迟后开始进行重复的固定延迟执行。
同时也重载了scheduleAtFixedRate方法,scheduleAtFixedRate方法与schedule相同,只不过他们的侧重点不同,区别后面分析。
scheduleAtFixedRate(TimerTask task, Date firstTime, long period):安排指定的任务在指定的时间开始进行重复的固定速率执行。
scheduleAtFixedRate(TimerTask task, long delay, long period):安排指定的任务在指定的延迟后开始进行重复的固定速率执行。
TimerTask
TimerTask类是一个抽象类,由Timer 安排为一次执行或重复执行的任务。它有一个抽象方法run()方法,该方法用于执行相应计时器任务要执行的操作。因此每一个具体的任务类都必须继承TimerTask,然后重写run()方法。
另外它还有两个非抽象的方法:
boolean cancel():取消此计时器任务。
long scheduledExecutionTime():返回此任务最近实际执行的安排执行时间。
二、实例
2.1、指定延迟时间执行定时任务
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public class TimerTest01 {
Timer timer;
public TimerTest01( int time){
timer = new Timer();
timer.schedule( new TimerTaskTest01(), time * 1000 );
}
</span><span style= "color: #0000ff" > public </span> <span style= "color: #0000ff" > static </span> <span style= "color: #0000ff" > void </span><span style= "color: #000000" > main(String[] args) {
System.out.println(</span> "timer begin...." <span style= "color: #000000" >);
</span><span style= "color: #0000ff" > new </span> TimerTest01( 3 <span style= "color: #000000" >);
}
}
public class TimerTaskTest01 extends TimerTask{
</span><span style= "color: #0000ff" > public </span> <span style= "color: #0000ff" > void </span><span style= "color: #000000" > run() {
System.out.println(</span> "Time's up!!!!" <span style= "color: #000000" >);
}
}
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运行结果:
首先打印:timer begin....
3秒后打印:Time's up!!!!
2.2、在指定时间执行定时任务 [code]public class TimerTest02 { Timer timer;
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public class TimerTest02 {
Timer timer;
</span><span style= "color: #0000ff" > public </span><span style= "color: #000000" > TimerTest02(){
Date time </span>=<span style= "color: #000000" > getTime();
System.out.println(</span> "指定时间time=" +<span style= "color: #000000" > time);
timer </span>= <span style= "color: #0000ff" > new </span><span style= "color: #000000" > Timer();
timer.schedule(</span><span style= "color: #0000ff" > new </span><span style= "color: #000000" > TimerTaskTest02(), time);
}
</span><span style= "color: #0000ff" > public </span><span style= "color: #000000" > Date getTime(){
Calendar calendar </span>=<span style= "color: #000000" > Calendar.getInstance();
calendar.set(Calendar.HOUR_OF_DAY, </span> 11 <span style= "color: #000000" >);
calendar.set(Calendar.MINUTE, </span> 39 <span style= "color: #000000" >);
calendar.set(Calendar.SECOND, </span> 00 <span style= "color: #000000" >);
Date time </span>=<span style= "color: #000000" > calendar.getTime();
</span><span style= "color: #0000ff" > return </span><span style= "color: #000000" > time;
}
</span><span style= "color: #0000ff" > public </span> <span style= "color: #0000ff" > static </span> <span style= "color: #0000ff" > void </span><span style= "color: #000000" > main(String[] args) {
</span><span style= "color: #0000ff" > new </span><span style= "color: #000000" > TimerTest02();
}
}
public class TimerTaskTest02 extends TimerTask{
@Override
</span><span style= "color: #0000ff" > public </span> <span style= "color: #0000ff" > void </span><span style= "color: #000000" > run() {
System.out.println(</span> "指定时间执行线程任务..." <span style= "color: #000000" >);
}
}
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当时间到达11:39:00时就会执行该线程任务,当然大于该时间也会执行!!执行结果为:
指定时间time=Tue Jun 10 11:39:00 CST 2014
指定时间执行线程任务...
2.3、在延迟指定时间后以指定的间隔时间循环执行定时任务
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public class TimerTest03 {
Timer timer;
</span><span style= "color: #0000ff" > public </span><span style= "color: #000000" > TimerTest03(){
timer </span>= <span style= "color: #0000ff" > new </span><span style= "color: #000000" > Timer();
timer.schedule(</span><span style= "color: #0000ff" > new </span> TimerTaskTest03(), 1000 , 2000 <span style= "color: #000000" >);
}
</span><span style= "color: #0000ff" > public </span> <span style= "color: #0000ff" > static </span> <span style= "color: #0000ff" > void </span><span style= "color: #000000" > main(String[] args) {
</span><span style= "color: #0000ff" > new </span><span style= "color: #000000" > TimerTest03();
}
}
public class TimerTaskTest03 extends TimerTask{
@Override
</span><span style= "color: #0000ff" > public </span> <span style= "color: #0000ff" > void </span><span style= "color: #000000" > run() {
Date date </span>= <span style= "color: #0000ff" > new </span> Date(<span style= "color: #0000ff" > this </span><span style= "color: #000000" >.scheduledExecutionTime());
System.out.println(</span> "本次执行该线程的时间为:" +<span style= "color: #000000" > date);
}
}
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运行结果:
本次执行该线程的时间为:Tue Jun 10 21:19:47 CST 2014
本次执行该线程的时间为:Tue Jun 10 21:19:49 CST 2014
本次执行该线程的时间为:Tue Jun 10 21:19:51 CST 2014
本次执行该线程的时间为:Tue Jun 10 21:19:53 CST 2014
本次执行该线程的时间为:Tue Jun 10 21:19:55 CST 2014
本次执行该线程的时间为:Tue Jun 10 21:19:57 CST 2014
.................
对于这个线程任务,如果我们不将该任务停止,他会一直运行下去。
对于上面三个实例,LZ只是简单的演示了一下,同时也没有讲解scheduleAtFixedRate方法的例子,其实该方法与schedule方法一样!
2.4、分析schedule和scheduleAtFixedRate
1、schedule(TimerTask task, Date time)、schedule(TimerTask task, long delay)
对于这两个方法而言,如果指定的计划执行时间scheduledExecutionTime<= systemCurrentTime,则task会被立即执行。scheduledExecutionTime不会因为某一个task的过度执行而改变。
2、schedule(TimerTask task, Date firstTime, long period)、schedule(TimerTask task, long delay, long period)
这两个方法与上面两个就有点儿不同的,前面提过Timer的计时器任务会因为前一个任务执行时间较长而延时。在这两个方法中,每一次执行的task的计划时间会随着前一个task的实际时间而发生改变,也就是scheduledExecutionTime(n+1)=realExecutionTime(n)+periodTime。也就是说如果第n个task由于某种情况导致这次的执行时间过程,最后导致systemCurrentTime>= scheduledExecutionTime(n+1),这是第n+1个task并不会因为到时了而执行,他会等待第n个task执行完之后再执行,那么这样势必会导致n+2个的执行实现scheduledExecutionTime放生改变即scheduledExecutionTime(n+2) = realExecutionTime(n+1)+periodTime。所以这两个方法更加注重保存间隔时间的稳定。
3、scheduleAtFixedRate(TimerTask task, Date firstTime, long period)、scheduleAtFixedRate(TimerTask task, long delay, long period)
在前面也提过scheduleAtFixedRate与schedule方法的侧重点不同,schedule方法侧重保存间隔时间的稳定,而scheduleAtFixedRate方法更加侧重于保持执行频率的稳定。为什么这么说,原因如下。在schedule方法中会因为前一个任务的延迟而导致其后面的定时任务延时,而scheduleAtFixedRate方法则不会,如果第n个task执行时间过长导致systemCurrentTime>= scheduledExecutionTime(n+1),则不会做任何等待他会立即执行第n+1个task,所以scheduleAtFixedRate方法执行时间的计算方法不同于schedule,而是scheduledExecutionTime(n)=firstExecuteTime +n*periodTime,该计算方法永远保持不变。所以scheduleAtFixedRate更加侧重于保持执行频率的稳定。
三、Timer的缺陷
3.1、Timer的缺陷
Timer计时器可以定时(指定时间执行任务)、延迟(延迟5秒执行任务)、周期性地执行任务(每隔个1秒执行任务),但是,Timer存在一些缺陷。首先Timer对调度的支持是基于绝对时间的,而不是相对时间,所以它对系统时间的改变非常敏感。其次Timer线程是不会捕获异常的,如果TimerTask抛出的了未检查异常则会导致Timer线程终止,同时Timer也不会重新恢复线程的执行,他会错误的认为整个Timer线程都会取消。同时,已经被安排单尚未执行的TimerTask也不会再执行了,新的任务也不能被调度。故如果TimerTask抛出未检查的异常,Timer将会产生无法预料的行为。
1、Timer管理时间延迟缺陷
前面Timer在执行定时任务时只会创建一个线程任务,如果存在多个线程,若其中某个线程因为某种原因而导致线程任务执行时间过长,超过了两个任务的间隔时间,会发生一些缺陷:
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public class TimerTest04 {
private Timer timer;
public long start;
</span><span style= "color: #0000ff" > public </span><span style= "color: #000000" > TimerTest04(){
</span><span style= "color: #0000ff" > this </span>.timer = <span style= "color: #0000ff" > new </span><span style= "color: #000000" > Timer();
start </span>=<span style= "color: #000000" > System.currentTimeMillis();
}
</span><span style= "color: #0000ff" > public </span> <span style= "color: #0000ff" > void </span><span style= "color: #000000" > timerOne(){
timer.schedule(</span><span style= "color: #0000ff" > new </span><span style= "color: #000000" > TimerTask() {
</span><span style= "color: #0000ff" > public </span> <span style= "color: #0000ff" > void </span><span style= "color: #000000" > run() {
System.out.println(</span> "timerOne invoked ,the time:" + (System.currentTimeMillis() -<span style= "color: #000000" > start));
</span><span style= "color: #0000ff" > try </span><span style= "color: #000000" > {
Thread.sleep(</span> 4000 ); <span style= "color: #008000" > //</span><span style="color: #008000">线程休眠3000</span>
} <span style= "color: #0000ff" > catch </span><span style= "color: #000000" > (InterruptedException e) {
e.printStackTrace();
}
}
}, </span> 1000 <span style= "color: #000000" >);
}
</span><span style= "color: #0000ff" > public </span> <span style= "color: #0000ff" > void </span><span style= "color: #000000" > timerTwo(){
timer.schedule(</span><span style= "color: #0000ff" > new </span><span style= "color: #000000" > TimerTask() {
</span><span style= "color: #0000ff" > public </span> <span style= "color: #0000ff" > void </span><span style= "color: #000000" > run() {
System.out.println(</span> "timerOne invoked ,the time:" + (System.currentTimeMillis() -<span style= "color: #000000" > start));
}
}, </span> 3000 <span style= "color: #000000" >);
}
</span><span style= "color: #0000ff" > public </span> <span style= "color: #0000ff" > static </span> <span style= "color: #0000ff" > void </span> main(String[] args) <span style= "color: #0000ff" > throws </span><span style= "color: #000000" > Exception {
TimerTest04 test </span>= <span style= "color: #0000ff" > new </span><span style= "color: #000000" > TimerTest04();
test.timerOne();
test.timerTwo();
}
}
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按照我们正常思路,timerTwo应该是在3s后执行,其结果应该是:
timerOne invoked ,the time:1001
timerOne invoked ,the time:3001
但是事与愿违,timerOne由于sleep(4000),休眠了4S,同时Timer内部是一个线程,导致timeOne所需的时间超过了间隔时间,结果:
timerOne invoked ,the time:1000
timerOne invoked ,the time:5000
2、Timer抛出异常缺陷
如果TimerTask抛出RuntimeException,Timer会终止所有任务的运行。如下:
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public class TimerTest04 {
private Timer timer;
</span><span style= "color: #0000ff" > public </span><span style= "color: #000000" > TimerTest04(){
</span><span style= "color: #0000ff" > this </span>.timer = <span style= "color: #0000ff" > new </span><span style= "color: #000000" > Timer();
}
</span><span style= "color: #0000ff" > public </span> <span style= "color: #0000ff" > void </span><span style= "color: #000000" > timerOne(){
timer.schedule(</span><span style= "color: #0000ff" > new </span><span style= "color: #000000" > TimerTask() {
</span><span style= "color: #0000ff" > public </span> <span style= "color: #0000ff" > void </span><span style= "color: #000000" > run() {
</span><span style= "color: #0000ff" > throw </span> <span style= "color: #0000ff" > new </span><span style= "color: #000000" > RuntimeException();
}
}, </span> 1000 <span style= "color: #000000" >);
}
</span><span style= "color: #0000ff" > public </span> <span style= "color: #0000ff" > void </span><span style= "color: #000000" > timerTwo(){
timer.schedule(</span><span style= "color: #0000ff" > new </span><span style= "color: #000000" > TimerTask() {
</span><span style= "color: #0000ff" > public </span> <span style= "color: #0000ff" > void </span><span style= "color: #000000" > run() {
System.out.println(</span> "我会不会执行呢??" <span style= "color: #000000" >);
}
}, </span> 1000 <span style= "color: #000000" >);
}
</span><span style= "color: #0000ff" > public </span> <span style= "color: #0000ff" > static </span> <span style= "color: #0000ff" > void </span><span style= "color: #000000" > main(String[] args) {
TimerTest04 test </span>= <span style= "color: #0000ff" > new </span><span style= "color: #000000" > TimerTest04();
test.timerOne();
test.timerTwo();
}
}
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运行结果:timerOne抛出异常,导致timerTwo任务终止。
Exception in thread "Timer-0" java.lang.RuntimeException
at com.chenssy.timer.TimerTest04$1.run(TimerTest04.java:25)
at java.util.TimerThread.mainLoop(Timer.java:555)
at java.util.TimerThread.run(Timer.java:505)
对于Timer的缺陷,我们可以考虑 ScheduledThreadPoolExecutor 来替代。Timer是基于绝对时间的,对系统时间比较敏感,而ScheduledThreadPoolExecutor 则是基于相对时间;Timer是内部是单一线程,而ScheduledThreadPoolExecutor内部是个线程池,所以可以支持多个任务并发执行。
3.2、用ScheduledExecutorService替代Timer
1、解决问题一:
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public class ScheduledExecutorTest {
private ScheduledExecutorService scheduExec;
</span><span style= "color: #0000ff" > public </span> <span style= "color: #0000ff" > long </span><span style= "color: #000000" > start;
ScheduledExecutorTest(){
</span><span style= "color: #0000ff" > this </span>.scheduExec = Executors.newScheduledThreadPool( 2 <span style= "color: #000000" >);
</span><span style= "color: #0000ff" > this </span>.start =<span style= "color: #000000" > System.currentTimeMillis();
}
</span><span style= "color: #0000ff" > public </span> <span style= "color: #0000ff" > void </span><span style= "color: #000000" > timerOne(){
scheduExec.schedule(</span><span style= "color: #0000ff" > new </span><span style= "color: #000000" > Runnable() {
</span><span style= "color: #0000ff" > public </span> <span style= "color: #0000ff" > void </span><span style= "color: #000000" > run() {
System.out.println(</span> "timerOne,the time:" + (System.currentTimeMillis() -<span style= "color: #000000" > start));
</span><span style= "color: #0000ff" > try </span><span style= "color: #000000" > {
Thread.sleep(</span> 4000 <span style= "color: #000000" >);
} </span><span style= "color: #0000ff" > catch </span><span style= "color: #000000" > (InterruptedException e) {
e.printStackTrace();
}
}
},</span> 1000 <span style= "color: #000000" >,TimeUnit.MILLISECONDS);
}
</span><span style= "color: #0000ff" > public </span> <span style= "color: #0000ff" > void </span><span style= "color: #000000" > timerTwo(){
scheduExec.schedule(</span><span style= "color: #0000ff" > new </span><span style= "color: #000000" > Runnable() {
</span><span style= "color: #0000ff" > public </span> <span style= "color: #0000ff" > void </span><span style= "color: #000000" > run() {
System.out.println(</span> "timerTwo,the time:" + (System.currentTimeMillis() -<span style= "color: #000000" > start));
}
},</span> 2000 <span style= "color: #000000" >,TimeUnit.MILLISECONDS);
}
</span><span style= "color: #0000ff" > public </span> <span style= "color: #0000ff" > static </span> <span style= "color: #0000ff" > void </span><span style= "color: #000000" > main(String[] args) {
ScheduledExecutorTest test </span>= <span style= "color: #0000ff" > new </span><span style= "color: #000000" > ScheduledExecutorTest();
test.timerOne();
test.timerTwo();
}
}
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运行结果:
timerOne,the time:1003
timerTwo,the time:2005
2、解决问题二
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public class ScheduledExecutorTest {
private ScheduledExecutorService scheduExec;
</span><span style= "color: #0000ff" > public </span> <span style= "color: #0000ff" > long </span><span style= "color: #000000" > start;
ScheduledExecutorTest(){
</span><span style= "color: #0000ff" > this </span>.scheduExec = Executors.newScheduledThreadPool( 2 <span style= "color: #000000" >);
</span><span style= "color: #0000ff" > this </span>.start =<span style= "color: #000000" > System.currentTimeMillis();
}
</span><span style= "color: #0000ff" > public </span> <span style= "color: #0000ff" > void </span><span style= "color: #000000" > timerOne(){
scheduExec.schedule(</span><span style= "color: #0000ff" > new </span><span style= "color: #000000" > Runnable() {
</span><span style= "color: #0000ff" > public </span> <span style= "color: #0000ff" > void </span><span style= "color: #000000" > run() {
</span><span style= "color: #0000ff" > throw </span> <span style= "color: #0000ff" > new </span><span style= "color: #000000" > RuntimeException();
}
},</span> 1000 <span style= "color: #000000" >,TimeUnit.MILLISECONDS);
}
</span><span style= "color: #0000ff" > public </span> <span style= "color: #0000ff" > void </span><span style= "color: #000000" > timerTwo(){
scheduExec.scheduleAtFixedRate(</span><span style= "color: #0000ff" > new </span><span style= "color: #000000" > Runnable() {
</span><span style= "color: #0000ff" > public </span> <span style= "color: #0000ff" > void </span><span style= "color: #000000" > run() {
System.out.println(</span> "timerTwo invoked ....." <span style= "color: #000000" >);
}
},</span> 2000 , 500 <span style= "color: #000000" >,TimeUnit.MILLISECONDS);
}
</span><span style= "color: #0000ff" > public </span> <span style= "color: #0000ff" > static </span> <span style= "color: #0000ff" > void </span><span style= "color: #000000" > main(String[] args) {
ScheduledExecutorTest test </span>= <span style= "color: #0000ff" > new </span><span style= "color: #000000" > ScheduledExecutorTest();
test.timerOne();
test.timerTwo();
}
}
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运行结果:
timerTwo invoked .....
timerTwo invoked .....
timerTwo invoked .....
timerTwo invoked .....
timerTwo invoked .....
timerTwo invoked .....
timerTwo invoked .....
timerTwo invoked .....
timerTwo invoked .....
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原文链接:https://www.cnblogs.com/chenssy/p/3788407.html