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POHow to wake up RecursiveTask<V> with notify?
 

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  1. POHow to wake up RecursiveTask<V> with notify?
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    <p>I cannot figure it out, how can I wake up <strong>RecursiveTasks</strong>, invoked by <strong>ForkJoinPool</strong>, when these tasks are on hold by <strong>wait</strong> method. Here is my simple example with the method <em>MainRecursionClass.resume</em> which is incorrect (does not wakes up RecursiveTasks).</p> <pre><code>public class Program { public static void main(String[] args) { Program p = new Program(); final MainRecursionClass mrc = p.new MainRecursionClass(); //Thread outputs integers to simulate work new Thread() { public void run() { mrc.doJob(); } }.start(); //Thread performs wait and notify on MainRecursionClass object p.new PauseResume(mrc).start(); } /** * * This class performs suspend and resume operations to the MainRecursionClass class object * */ private class PauseResume extends Thread { private MainRecursionClass rv; public PauseResume(MainRecursionClass rv) { this.rv = rv; } @Override public void run() { while(!isInterrupted()) { try { sleep(4000); rv.suspend(); sleep(8000); rv.resume(); } catch (InterruptedException e) { e.printStackTrace(); } } } } private class MainRecursionClass { private boolean pause = false; private MyRecursive rv; public void doJob() { rv = new MyRecursive(0, 100000); ForkJoinPool pool = new ForkJoinPool(); pool.invoke(rv); } public void suspend() { pause = true; System.out.println("Suspended"); } /** * This method is incorrect. It should wake up all MyRecursive instances to continue their work. */ public synchronized void resume() { pause = false; notifyAll(); System.out.println("Resumed"); } private class MyRecursive extends RecursiveTask&lt;Object&gt; { private static final long serialVersionUID = 1L; private int start; private int length; private int threshold = 15; public MyRecursive(int start, int length) { super(); this.start = start; this.length = length; } protected void computeDirectly() throws Exception { for (int index = start; index &lt; start + length; index++) { //PAUSE synchronized (this) { try { while(pause) { wait(); } } catch (InterruptedException e) { e.printStackTrace(); } } //PAUSE //some output to simulate work... System.out.println(index); Thread.sleep(1000); } } /** * Recursion */ @Override protected Object compute() { if (length &lt;= threshold) { try { computeDirectly(); } catch (Exception e) { return e; } return null; } int split = length / 2; invokeAll(new MyRecursive(start, split), new MyRecursive(start + split, length - split)); return null; } } } } </code></pre>
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    1. USRobin Green
    1. USpolis
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    1. COYou have to `notify()` the `MyRecursive` instance.
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      1. POHow to wake up RecursiveTask<V> with notify?
      1. USSotirios Delimanolis
    2. CODo you mean I have to do `notify();` on all `MyRecursive` instances, which will be created recursively? FYI: I tried to change `resume()` method like this: `rv.notify();` and I got `java.lang.IllegalMonitorStateException`.
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      1. POHow to wake up RecursiveTask<V> with notify?
      1. USpolis
    3. COYou need to understand what `synchronized`, `wait`, and `notify` do. If you call `wait()` on an object, here `this.wait()`, then the current `Thread` will be blocked until another `Thread` calls `notify` on the same instance.
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      1. POHow to wake up RecursiveTask<V> with notify?
      1. USSotirios Delimanolis
 

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