Timer与TimerTask的使用

java.util 套件之中有两个类别与工作排程非常有关系，他们分别是Timer与TimerTask。其中，Timer 是一个定时器，可以设定成特定时间或特定时间周期产生讯号。不过，只有Timer 是没有用的，必须配合TimerTask 才有作用。Timer一旦与某个TimerTask 产生关联，就会在产生讯号的同时，连带执行TimerTask所定义的工作。

TimerTask 的制作非常容易，任何一个类别只要继承TimerTask 类别，并实作其run()方法即可。Run()方法就是我们自行定义的工作，一旦Timer 在特定时间或特定时间周期产生讯号，run()方法就会被执行。我们会透过Timer 的schedule()方法来设定特定时间或特定时间周期，并将它与某个TimerTask 联系(进行TimerTask 的排程)。最后，您可以使用Timer 的cancel()方法来停止Timer，叫用cancel()之后，Timer 就会和TimerTask 脱离关系。TimerTask 本身也有cancel()方法。

假设我们希望在3 秒之后执行某些工作，范例程序如下:

//MyTask1.java
import javax.microedition.midlet.*;
import javax.microedition.lcdui.*;
import java.util.*;

public class MyTask1 extends TimerTask {
    public void run() {
        //印出run()被叫用的时间
        System.out.println("Task1 Fire Time:");
        System.out.println(System.currentTimeMillis());
    }
}


//TimerTest1.java
import javax.microedition.midlet.*;
import javax.microedition.lcdui.*;
import java.util.*;

public class TimerTest1 extends MIDlet {
    public TimerTest1() {
    }

    public void startApp() {
        Timer timer = new Timer();
        //规定3000 毫秒之后(3 秒)启动MyTask1
        timer.schedule(new MyTask1(), 3000);
        //印出排程后的时间
        System.out.println("Task Schedule Time:");
        System.out.println(System.currentTimeMillis());
    }

    public void pauseApp() {
    }

    public void destroyApp(boolean unconditional) {
    }
}

//MyTask2.java
import javax.microedition.midlet.*;
import javax.microedition.lcdui.*;
import java.util.*;

public class MyTask2 extends TimerTask {
    public void run() {
        System.out.println("Task2 Fire Time:");
        System.out.println(System.currentTimeMillis());
    }
}


//TimerTest2.java
import javax.microedition.midlet.*;
import javax.microedition.lcdui.*;
import java.util.*;

public class TimerTest2 extends MIDlet {
    public TimerTest2() {
    }

    public void startApp() {
        Timer timer = new Timer();
        Date after5sec = new Date(System.currentTimeMillis() + 5000);
        timer.schedule(new MyTask2(), after5sec);
        System.out.println("Task Schedule Time:");
        System.out.println(System.currentTimeMillis());
    }

    public void pauseApp() {
    }

    public void destroyApp(boolean unconditional) {
    }
}

//MyTask3.java
import javax.microedition.midlet.*;
import javax.microedition.lcdui.*;
import java.util.*;

public class MyTask3 extends TimerTask {
    long lasttime;
    public void run() {
        long now = System.currentTimeMillis();
        System.out.print("Task3 Fire Time: ");
        System.out.print(now);
        System.out.print(" ; Intervel :");
        //计算出与上一次的时间差
        System.out.println(now - lasttime);
        //重设上次发生时间为现在
        lasttime = now;
    }
}


//TimerTest3.java
import javax.microedition.midlet.*;
import javax.microedition.lcdui.*;
import java.util.*;

public class TimerTest3 extends MIDlet {
    Timer timer;
    public TimerTest3() {
    }

    public void startApp() {
        Form f = new Form("Test");
        Display.getDisplay(this).setCurrent(f);
        timer = new Timer();
        //Date after5sec = new Date(System.currentTimeMillis()+5000) ;
        MyTask3 mt = new MyTask3();
        mt.lasttime = System.currentTimeMillis();
        //timer.schedule(mt,after5sec,4000);
        timer.schedule(mt, 5000, 4000);
        System.out.println("Task Schedule Time:");
        System.out.println(System.currentTimeMillis());
    }

    public void pauseApp() {
    }

    public void destroyApp(boolean unconditional) {
        timer.cancel();
    }
}

//MyTask3.java
import javax.microedition.midlet.*;
import javax.microedition.lcdui.*;
import java.util.*;

public class MyTask3 extends TimerTask {
    long lasttime;
    public void run() {
        System.out.print("Task3 Fire Time: ");
        long now = scheduledExecutionTime();
        System.out.print(now);
        System.out.print(" ; Intervel :");
        System.out.println(now - lasttime);
        lasttime = now;
    }
}

//MyTask4.java
import javax.microedition.midlet.*;
import javax.microedition.lcdui.*;
import java.util.*;

public class MyTask4 extends TimerTask {
    long lasttime;
    public void run() {
        long now = System.currentTimeMillis();
        System.out.print("Task4 Fire Time: ");
        System.out.print(now);
        System.out.print(" ; Intervel :");
        System.out.println(now - lasttime);
        lasttime = now;
        doSomething();
    }

    public void doSomething() {
        //用随机数来模拟工作所需时间,有时超过4000 豪秒,有时少于4000 豪秒
        try {
            Random rdm = new Random();
            int i = rdm.nextInt() % 2;
            if (i == 0) {
                Thread.sleep(2500);
                System.out.println("doSomething : 2500 millisecond");
            } else {
                Thread.sleep(5500);
                System.out.println("doSomething : 5500 millisecond");
            }
        } catch (Exception e) {}
    }
}


//TimerTest4.java
import javax.microedition.midlet.*;
import javax.microedition.lcdui.*;
import java.util.*;

public class TimerTest4 extends MIDlet {
    Timer timer;
    public TimerTest4() {
    }

    public void startApp() {
        Form f = new Form("Test");
        Display.getDisplay(this).setCurrent(f);
        timer = new Timer();
        //Date after5sec = new Date(System.currentTimeMillis()+5000) ;
        MyTask4 mt = new MyTask4();
        mt.lasttime = System.currentTimeMillis();
        //timer.schedule(mt,after5sec,4000);
        timer.schedule(mt, 5000, 4000);
        System.out.println("Task Schedule Time:");
        System.out.println(System.currentTimeMillis());
    }

    public void pauseApp() {
    }

    public void destroyApp(boolean unconditional) {
        timer.cancel();
    }
}

从这个范例可以看出schedule()方法的运作属于一种称为fixed-delay execution 的模式。意思就是说，如果run()在时间周期之内完成，那么下一次的讯号就会在下一次预定的时间产生。但是，如果run()无法在时间周期之内完成，那么下一次的讯号就延迟，而且上一个run 执行之后就会立刻产生讯号。所以会有上述的结果。另外一个可以在固定周期产生讯号的方法为scheduleAtFixedRate()，但是其特性和schedule()不太相同。。范例如下(TimerTask 的部分我们仍采用MyTask4)

//TimerTest5.java
import javax.microedition.midlet.*;
import javax.microedition.lcdui.*;
import java.util.*;

public class TimerTest5 extends MIDlet {
    Timer timer;
    public TimerTest5() {
    }

    public void startApp() {
        Form f = new Form("Test");
        Display.getDisplay(this).setCurrent(f);
        timer = new Timer();
        //Date after5sec = new Date(System.currentTimeMillis()+5000) ;
        MyTask4 mt = new MyTask4();
        mt.lasttime = System.currentTimeMillis();
        //timer.scheduleAtFixedRate(mt,after5sec,4000);
        timer.scheduleAtFixedRate(mt, 5000, 4000);
        System.out.println("Task Schedule Time:");
        System.out.println(System.currentTimeMillis());
    }

    public void pauseApp() {
    }

    public void destroyApp(boolean unconditional) {
        timer.cancel();
    }
}