Java线程中的定时器

Question

I have a thread which is in charge of doing some processes. I want make it so that these processing would be done every 3 seconds. I've used the code below but when the thread starts, nothing happens. I assumed that when I define a task for my timer it automatically execute the ScheduledTask within time interval but it doesn't do anything at all. What am I missing?

class temperatureUp extends Thread 
{
    @Override
    public void run()
    {
    TimerTask increaseTemperature = new TimerTask(){

        public void run() {
        try {
            //do the processing 
        } catch (InterruptedException ex) {}
        }
    };

    Timer increaserTimer = new Timer("MyTimer");
    increaserTimer.schedule(increaseTemperature, 3000);

    }
};

Answer 1

A few errors in your code snippet:

• You extend the Thread class, which is not really good practice
• You have a Timer within a Thread ? That doesnt make sense as the a Timer runs on its own Thread .

You should rather (when/where necessary), implement a Runnable see here for a short example, however I cannot see the need for both a Thread and Timer in the snippet you gave.

Please see the below example of a working Timer which will simply increment the counter by one each time it is called (every 3seconds):

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

public class Test {

    static int counter = 0;

    public static void main(String[] args) {

        TimerTask timerTask = new TimerTask() {

            @Override
            public void run() {
                System.out.println("TimerTask executing counter is: " + counter);
                counter++;//increments the counter
            }
        };

        Timer timer = new Timer("MyTimer");//create a new Timer

        timer.scheduleAtFixedRate(timerTask, 30, 3000);//this line starts the timer at the same time its executed
    }
}

Addendum:

I did a short example of incorporating a Thread into the mix. So now the TimerTask will merely increment counter by 1 every 3 seconds, and the Thread will display counter s value sleeping for 1 seconds every time it checks counter (it will terminate itself and the timer after counter==3 ):

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

public class Test {

    static int counter = 0;
    static Timer timer;

    public static void main(String[] args) {

        //create timer task to increment counter
        TimerTask timerTask = new TimerTask() {

            @Override
            public void run() {
                // System.out.println("TimerTask executing counter is: " + counter);
                counter++;
            }
        };

        //create thread to print counter value
        Thread t = new Thread(new Runnable() {

            @Override
            public void run() {
                while (true) {
                    try {
                        System.out.println("Thread reading counter is: " + counter);
                        if (counter == 3) {
                            System.out.println("Counter has reached 3 now will terminate");
                            timer.cancel();//end the timer
                            break;//end this loop
                        }
                        Thread.sleep(1000);
                    } catch (InterruptedException ex) {
                        ex.printStackTrace();
                    }
                }
            }
        });

        timer = new Timer("MyTimer");//create a new timer
        timer.scheduleAtFixedRate(timerTask, 30, 3000);//start timer in 30ms to increment  counter

        t.start();//start thread to display counter
    }
}

Answer 2

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

public class ThreadTimer extends TimerTask{
    static int counter = 0;

    public static void main(String [] args) {
        Timer timer = new Timer("MyTimer");
        timer.scheduleAtFixedRate(new ThreadTimer(), 30, 3000);
    }

    @Override
    public void run() {
        // TODO Auto-generated method stub
        System.out.println("TimerTask executing counter is: " + counter);
        counter++;
   }

}

Answer 3

In order to do something every three seconds you should use scheduleAtFixedRate (see javadoc ).

However your code really does nothing because you create a thread in which you start a timer just before the thread's run stops (there is nothing more to do). When the timer (which is a single shoot one) triggers, there is no thread to interrupt (run finished before).

class temperatureUp extends Thread 
{
    @Override
    public void run()
    {
    TimerTask increaseTemperature = new TimerTask(){

        public void run() {
        try {
            //do the processing 
        } catch (InterruptedException ex) {}
        }
    };

    Timer increaserTimer = new Timer("MyTimer");
    //start a 3 seconds timer 10ms later
    increaserTimer.scheduleAtFixedRate(increaseTemperature, 3000, 10);

    while(true) {
         //give it some time to see timer triggering
         doSomethingMeaningful();
    }
}

Answer 4

I think the method you've used has the signature schedule(TimerTask task, long delay) . So in effect you're just delaying the start time of the ONLY execution.

To schedule it to run every 3 seconds you need to go with this method schedule(TimerTask task, long delay, long period) where the third param is used to give the period interval.

You can refer the Timer class definition here to be of further help

http://docs.oracle.com/javase/1.4.2/docs/api/java/util/Timer.html

Answer 5

Timer & TimerTask are legacy

The Timer & TimerTask classes are now legacy . To run code at a certain time, or to run code repeatedly, use a scheduled executor service .

To quote the Timer class Javadoc:

Java 5.0 introduced the java.util.concurrent package and one of the concurrency utilities therein is the ScheduledThreadPoolExecutor which is a thread pool for repeatedly executing tasks at a given rate or delay. It is effectively a more versatile replacement for the Timer/TimerTask combination, as it allows multiple service threads, accepts various time units, and doesn't require subclassing TimerTask (just implement Runnable). Configuring ScheduledThreadPoolExecutor with one thread makes it equivalent to Timer.

Executor framework

In modern Java, we use the Executors framework rather than directly addressing the Thread class.

Define your task as a Runnable or Callable . You can use compact lambda syntax seen below. Or you can use conventional syntax to define a class implementing the Runnable (or Callable ) interface.

Ask a ScheduledExecutorService object to execute your Runnable object's code every so often.

ScheduledExecutorService scheduledExecutorService = Executors.newSingleThreadScheduledExecutor() ;

Runnable task = () -> { 
    System.out.println( "Doing my thing at: " + Instant.now() );
};

long initialDelay = 0L ; 
long period = 3L ;
TimeUnit timeUnit = TimeUnit.SECONDS ;
scheduledExecutorService.submit( task , initialDelay, period , timeUnit ) ;

…
scheduledExecutorService.shutdown() ;  // Stops any more tasks from being scheduled.
scheduledExecutorService.awaitTermination() ;  // Waits until all currently running tasks are done/failed/canceled. 

Notice that we are not directly managing any Thread objects in the code above. Managing threads is the job of the executor service.

Tips:

• Always shutdown your executor service gracefully when no longer needed, or when your app exits. Otherwise the backing thread pool may continue indefinitely like a zombie 🧟‍♂️.
• Consider wrapping your task's working code in a try-catch. Any uncaught exception or error reaching the scheduled executor service results in silently halting the further scheduling of any more runs.