在應(yīng)用開(kāi)發(fā)中，經(jīng)常需要一些周期性的操作，比如每5分鐘檢查一下新郵件等。對(duì)于這樣的操作最方便、高效的實(shí)現(xiàn)方式就是使用java.util.Timer工具類。比如下面的代碼每5分鐘檢查一遍是否有新郵件：

???????? private ?java.util.Timer?timer;?
????????timer? = ? new ?Timer( true );?
????????timer.schedule( new ?java.util.TimerTask()? {?
???????????? public ? void ?run()? {?
???????????????????? // server.checkNewMail();?檢查新郵件?
????????????} ?
????????} ,? 0 ,? 5 * 60 * 1000 );?

?

使用這幾行代碼之后，Timer本身會(huì)每隔5分鐘調(diào)用一遍server.checkNewMail()方法，不需要自己?jiǎn)?dòng)線程。Timer本身也是多線程同步的，多個(gè)線程可以共用一個(gè)Timer，不需要外部的同步代碼。
??? 在《The Java Tutorial》中有更完整的例子：

public ? class ?AnnoyingBeep? {?
????Toolkit?toolkit;?
????Timer?timer;?
??
???? public ?AnnoyingBeep()? {?
????toolkit? = ?Toolkit.getDefaultToolkit();?
????????timer? = ? new ?Timer();?
????????timer.schedule( new ?RemindTask(),?
??????????????????? 0 ,???????? // initial?delay?
??????????????????? 1 * 1000 );?? // subsequent?rate?
????} ?
??
???? class ?RemindTask? extends ?TimerTask? {?
???? int ?numWarningBeeps? = ? 3 ;?
??
???????? public ? void ?run()? {?
???????? if ?(numWarningBeeps? > ? 0 )? {?
????????????toolkit.beep();?
????????System.out.println( " Beep! " );?
????????numWarningBeeps -- ;?
????????} ? else ? {?
????????????toolkit.beep();??
????????????????System.out.println( " Time′s?up! " );?
???????????? // timer.cancel();? // Not?necessary?because?we?call?System.exit?
????????????System.exit( 0 );??? // Stops?the?AWT?thread?(and?everything?else)?
????????} ?
????????} ?
????} ?
?????
} ?

這段程序，每隔3秒響鈴一聲，并打印出一行消息。循環(huán)3次。程序輸出如下：
Task scheduled.
Beep!?????
Beep!????? //one second after the first beep
Beep!????? //one second after the second beep
Time′s up! //one second after the third beep

Timer類也可以方便地用來(lái)作為延遲執(zhí)行，比如下面的代碼延遲指定的時(shí)間（以秒為單位）執(zhí)行某操作。類似電視的延遲關(guān)機(jī)功能。

?
public ? class ?ReminderBeep? {?
?????
???? public ?ReminderBeep( int ?seconds)? {?
????toolkit? = ?Toolkit.getDefaultToolkit();?
????????timer? = ? new ?Timer();?
????????timer.schedule( new ?RemindTask(),?seconds * 1000 );?
????} ?
??
???? class ?RemindTask? extends ?TimerTask? {?
???????? public ? void ?run()? {?
????????????System.out.println( " Time′s?up! " );?
????????toolkit.beep();?
???????? // timer.cancel();? // Not?necessary?because?we?call?System.exit?
????????System.exit( 0 );??? // Stops?the?AWT?thread?(and?everything?else)?
????????} ?
????} ?
?????
} ?

?