Home » concurrent-sources » EDU.oswego.cs.dl.util.concurrent » [javadoc | source]
public class: ClockDaemon [javadoc | source]
A general-purpose time-based daemon, vaguely similar in functionality to common system-level utilities such as at (and the associated crond) in Unix. Objects of this class maintain a single thread and a task queue that may be used to execute Runnable commands in any of three modes -- absolute (run at a given time), relative (run after a given delay), and periodic (cyclically run with a given delay).

All commands are executed by the single background thread. The thread is not actually started until the first request is encountered. Also, if the thread is stopped for any reason, one is started upon encountering the next request, or restart() is invoked.

If you would instead like commands run in their own threads, you can use as arguments Runnable commands that start their own threads (or perhaps wrap within ThreadedExecutors).

You can also use multiple daemon objects, each using a different background thread. However, one of the reasons for using a time daemon is to pool together processing of infrequent tasks using a single background thread.

Background threads are created using a ThreadFactory. The default factory does not automatically setDaemon status.

The class uses Java timed waits for scheduling. These can vary in precision across platforms, and provide no real-time guarantees about meeting deadlines.

[ Introduction to this package. ]
Nested Class Summary:
protected static class  ClockDaemon.TaskNode   
protected class  ClockDaemon.RunLoop  The runloop is isolated in its own Runnable class just so that the main class need not implement Runnable, which would allow others to directly invoke run, which is not supported. * 
Field Summary
protected final  Heap heap_    tasks are maintained in a standard priority queue 
protected  Thread thread_    The thread used to process commands * 
protected final  RunLoop runLoop_     
Fields inherited from EDU.oswego.cs.dl.util.concurrent.ThreadFactoryUser:
 public ClockDaemon() 
Method from EDU.oswego.cs.dl.util.concurrent.ClockDaemon Summary:
cancel,   clearThread,   executeAfterDelay,   executeAt,   executePeriodically,   getThread,   nextTask,   restart,   shutDown
Methods from EDU.oswego.cs.dl.util.concurrent.ThreadFactoryUser:
getThreadFactory,   setThreadFactory
Methods from java.lang.Object:
clone,   equals,   finalize,   getClass,   hashCode,   notify,   notifyAll,   toString,   wait,   wait,   wait
Method from EDU.oswego.cs.dl.util.concurrent.ClockDaemon Detail:
 public static  void cancel(Object taskID) 
    Cancel a scheduled task that has not yet been run. The task will be cancelled upon the next opportunity to run it. This has no effect if this is a one-shot task that has already executed. Also, if an execution is in progress, it will complete normally. (It may however be interrupted via getThread().interrupt()). But if it is a periodic task, future iterations are cancelled.
 protected synchronized  void clearThread() 
    set thread_ to null to indicate termination *
 public Object executeAfterDelay(long millisecondsToDelay,
    Runnable command) 
    Excecute the given command after waiting for the given delay.

    Sample Usage. You can use a ClockDaemon to arrange timeout callbacks to break out of stuck IO. For example (code sketch):

    class X {   ...
      ClockDaemon timer = ...
      Thread readerThread;
      FileInputStream datafile;
      void startReadThread() {
        datafile = new FileInputStream("data", ...);
        readerThread = new Thread(new Runnable() {
         public void run() {
           for(;;) {
             // try to gracefully exit before blocking
            if (Thread.currentThread().isInterrupted()) {
            else {
              try {
                int c = datafile.read();
                if (c == -1) break;
                else process(c);
              catch (IOException ex) {
        } };
       // establish callback to cancel after 60 seconds
       timer.executeAfterDelay(60000, new Runnable() {
         readerThread.interrupt();    // try to interrupt thread
         datafile.close(); // force thread to lose its input file 
 public Object executeAt(Date date,
    Runnable command) 
    Execute the given command at the given time.
 public Object executePeriodically(long period,
    Runnable command,
    boolean startNow) 
    Execute the given command every period milliseconds. If startNow is true, execution begins immediately, otherwise, it begins after the first period delay.

    Sample Usage. Here is one way to update Swing components acting as progress indicators for long-running actions.

    class X {
      JLabel statusLabel = ...;
      int percentComplete = 0;
      synchronized int  getPercentComplete() { return percentComplete; }
      synchronized void setPercentComplete(int p) { percentComplete = p; }
      ClockDaemon cd = ...;
      void startWorking() {
        Runnable showPct = new Runnable() {
          public void run() {
             SwingUtilities.invokeLater(new Runnable() {
               public void run() {
                 statusLabel.setText(getPercentComplete() + "%");
        final Object updater = cd.executePeriodically(500, showPct, true);
        Runnable action = new Runnable() {
          public void run() {
            for (int i = 0; i < 100; ++i) {
        new Thread(action).start();
 public synchronized Thread getThread() 
    Return the thread being used to process commands, or null if there is no such thread. You can use this to invoke any special methods on the thread, for example, to interrupt it.
 protected synchronized TaskNode nextTask() 
    Return the next task to execute, or null if thread is interrupted
 public synchronized  void restart() 
    Start (or restart) a thread to process commands, or wake up an existing thread if one is already running. This method can be invoked if the background thread crashed due to an unrecoverable exception in an executed command. *
 public synchronized  void shutDown() 
    Cancel all tasks and interrupt the background thread executing the current task, if any. A new background thread will be started if new execution requests are encountered. If the currently executing task does not repsond to interrupts, the current thread may persist, even if a new thread is started via restart().