Java ForkJoinPool class

The ForkJoinPool was introduced in Java 7. It is similar to the Executor framework but with one difference. ForkJoinPool acts recursively, unlike Executor threads, which splits the task and submits smaller chunks to worker Threads. ForkJoinPool takes a big task, splits it into smaller tasks, and those smaller tasks split themselves again into subtasks until each subtask is atomic or not divisible. So it works recursively.

Java ForkJoinPool class declaration

List of ForkJoinPool Methods

NO	Method	Description
1.	public boolean awaitQuiescence(long timeout, TimeUnit unit)	Execute pool until the pool is quiescent. Otherwise, assist performing tasks until specified time value and unit elapses or the pool is quiescent.
2.	public boolean awaitTermination(long timeout, TimeUnit unit) throws InterruptedException	Awaits termination after a shutdown request or timeout specified in the parameter as timeout elapses until program shutdown.
3.	public static ForkJoinPool commonPool()	It returns an instance of common Pool.
4.	public void execute(Runnable task)	This executes the given task in future it may execute in the separate thread or a new pool.
5.	public int getActiveThreadCount()	Estimate the number of active threads.
6.	public boolean getAsyncMode()	Checks if the pool uses First-in-First-out scheduling for forked tasks that are never joined.
7.	public static int getCommonPoolParallelism()	Returns the parallelism level of the common Pool.
8.	public ForkJoinPool.ForkJoinWorkerThreadFactory getFactory()	Checks the factory used for constructing new workers in the pool. Also, returns the factory used.
9.	public int getParallelism()	Returns the parallelism level of the common Pool.
10.	public int getPoolSize()	Returns the Pool size. The size of the pool depends on the number of workers that have started but not yet terminated.
11.	public int getQueuedSubmissionCount()	Returns the number of tasks submitted to pool but not yet begun to execute.
12.	public long getQueuedTaskCount()	Returns estimation of the number of tasks in the working queue. This does not include tasks that have not begun to execute.
13.	public int getRunningThreadCount()	Returns the number of thread that is currently running and not waiting to join the queue.
14.	public long getStealCount()	Returns estimation of the number of tasks stolen from one thread by another after completing execution of all tasks in own queue.
15.	public Thread.UncaughtExceptionHandler getUncaughtExceptionHandler()	Returns the exception handler for internal worker threads that throw the exception which has an unrecoverable error and terminate due to the uncaught exception.
16.	public boolean hasQueuedSubmissions()	Returns true if there are any tasks submitted to this pool that have not yet begun executing otherwise false.
17.	public <T> T invoke(ForkJoinTask<T> task)	Performs the task specified in parameter and returns the result upon its completion.
18.	public boolean isQuiescent()	Returns true if all worker threads are currently idle.
19.	public boolean isShutdown()	Returns true if the pool calling isShutdown() has been shut down.
20.	public boolean isTerminated()	Returns true if all the task in the pool has been shut down.
21.	public boolean isTerminating()	Returns true if the process of termination has started but not yet completed.
22.	public void shutdown()	Initiates an orderly shutdown in which previously submitted tasks are executed, but no new tasks will be accepted.
23.	public List<Runnable> shutdownNow()	Attempts to cancel and/or stop all tasks, and reject all subsequently submitted tasks.
24.	public ForkJoinTask<?> submit(Runnable task)	Submits a Runnable task for execution and returns a Future representing that task. The Future's get method will return null upon successful completion. And it is overloaded as follows:
25.	public String toString()	The toString() method returns a string identifying this pool, as well as its state, including indications of run state, parallelism level, and worker and task counts.

Java ForkJoinPool Class Example: Terminating()

import java.util.concurrent.ForkJoinPool;
import static java.util.concurrent.ForkJoinTask.invokeAll;
import java.util.concurrent.RecursiveTask;
import java.util.concurrent.TimeUnit;
public class JavaForkJoinPoolisTerminatingExample1   {
 public static void main(final String[] arguments) throws InterruptedException
      {
      
      int proc = Runtime.getRuntime().availableProcessors();
      System.out.println("numbers of core available in your processor:"  +proc);
      int[] n = {20 , 23 , 5 ,6 ,7,8,23,12,56,1}; 
      ForkJoinPool Pool = new ForkJoinPool(proc);
      Testl1 t=new Testl1(n,0,n.length);
      System.out.println("   Before awaitTermination (is terminating):" +Pool.isTerminating() );
      System.out.println("  is awaitQuiescent :" + Pool.awaitQuiescence(2, TimeUnit.SECONDS) );
     Pool.awaitTermination(3, TimeUnit.SECONDS);
     System.out.println("   After awaitTermination (is terminating):" +Pool.isTerminating() );
      Integer r = Pool.invoke(t);
      System.out.println("Pool.invoke :" +r);  
   }  
}

   class Testl1 extends RecursiveTask<Integer> {
      int st;
      int end;
      int[] arr;
      Testl1(int[] arr, int st, int end) {
         this.arr = arr;
         this.st  = st;
         this.end  = end;
      }

      protected Integer compute() {
         if(st - end <= 10) {
            int sum = 0;
            for(int i = st; i < end; ++i) 
               sum += arr[i];
              return sum;
         } else {	    	
            int mid = st + (end - st) / 2;
            Testl1 t1 = new Testl1(arr, st, mid);
            Testl1 t2 = new Testl1(arr, st, mid);
            invokeAll(t1 , t2);
            return (t1.join() + t2.join());
         }
      }
   }

Test it Now

Output:

numbers of core available in your processor:4
   Before awaitTermination (is terminating):false
  is awaitQuiescent :true
   After awaitTermination (is terminating):false
Pool.invoke :161

Java ForkJoinPool Class Example: getActiveThreadCount()

import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.ForkJoinPool;
import java.util.concurrent.RecursiveAction;
  class Task9 extends RecursiveAction {
     private long Load = 0;
 public Task9(long Load) {
         this.Load = Load;
    }
    @Override
    protected void compute() {
        //if work is above threshold, break tasks up into smaller tasks
            List<Task9> subtasks = new ArrayList<Task9>();
                    subtasks.addAll(createSubtasks());
            for(RecursiveAction subtask : subtasks){
                subtask.fork();
            } 
    }
   private List<Task9> createSubtasks() {
        List<Task9> subtasks =new ArrayList<Task9>();
     Task9 subtask1 = new Task9(this.Load / 2);
     Task9 subtask2 = new Task9(this.Load / 2);
      Task9 subtask3 = new Task9(this.Load / 2);
        subtasks.add(subtask1);
        subtasks.add(subtask2);
        subtasks.add(subtask3);
        return subtasks;
    }
}
public class JavaForkJoingetActivethreadcountExample1 {
   public static void main(final String[] arguments) throws InterruptedException
      {      
      int proc = Runtime.getRuntime().availableProcessors();
      System.out.println("numbers of core available in your processor:"  +proc);
      ForkJoinPool Pool = ForkJoinPool.commonPool();
      System.out.println(" Before invoking number of active thread   :" +Pool.getActiveThreadCount());
       Task9 t = new Task9(400);
       Pool.invoke(t); 
      System.out.println(" After invoking Numbers of active thread   :" +Pool.getActiveThreadCount());
      System.out.println("Common Pool Size   :" +Pool.getPoolSize());
   }  
}

Test it Now

Output:

numbers of core available in your processor:4
 Before invoking number of active thread   :0
 After invoking Numbers of active thread   :3
Common Pool Size   :3

Java ForkJoinPool Class Example: invoke()

           import java.util.Random;
	import java.util.concurrent.ForkJoinPool;
	import java.util.concurrent.RecursiveTask;
        public class JavaForkJoinPoolinvokeExample1 extends RecursiveTask<Integer> {
          private static final int var = 5;
              private final int[] value;
             private final int st;
             private final int ed;
             //parameterized constructor
       public JavaForkJoinPoolinvokeExample1(int[] value, int st, int ed) {
	    this.value = value;
	    this.st = st;
	    this.ed = ed;
	  }
       public JavaForkJoinPoolinvokeExample1(int[] value) {
	    this(value, 0, value.length);
	  }
          @Override
	  protected Integer compute() {
	    final int length = ed - st;
	    if (length < var) {
	      return computeDirectly();
	    }
	    final int split = length / 2;
            //new class object 
            ForkJoinPool pl = new  ForkJoinPool();
	    JavaForkJoinPoolinvokeExample1 left = new JavaForkJoinPoolinvokeExample1(value, st, st + split);
	    pl.invoke(left.fork());
             JavaForkJoinPoolinvokeExample1 right = new JavaForkJoinPoolinvokeExample1(value, st + split, ed);
                return Math.max(right.compute(), left.join()); 
	  }
	  private Integer computeDirectly() {
	    System.out.println(Thread.currentThread() + " computing: " + st + " to " + ed);
	    int max = Integer.MIN_VALUE;
	    for (int i = st; i < ed; i++) {
	      if (value[i] > max) {
	        max = value[i];
	      }
	    }
	    return max;
	  }

	  public static void main(String[] args) {
	    // create a random data set
	    final int[] value = new int[10];
	    final Random rand = new Random();
	    for (int i = 0; i < value.length; i++) {
	      value[i] = rand.nextInt(100);
	    }
           // submit the task to the poool
           ForkJoinPool pool = ForkJoinPool.commonPool();
           JavaForkJoinPoolinvokeExample1 finder = new JavaForkJoinPoolinvokeExample1(value);
	    System.out.println(pool.invoke(finder));
          //  pool.invokeAll(t);
           pool.invoke(finder);
}
	}

Test it Now

Output:

Thread[ForkJoinPool.commonPool-worker-2,5,main] computing: 0 to 2
Thread[ForkJoinPool.commonPool-worker-2,5,main] computing: 2 to 5
Thread[ForkJoinPool-3-worker-1,5,main] computing: 0 to 2
Thread[ForkJoinPool-2-worker-1,5,main] computing: 0 to 2
Thread[ForkJoinPool-1-worker-1,5,main] computing: 0 to 2
Thread[ForkJoinPool-1-worker-1,5,main] computing: 2 to 5
Thread[ForkJoinPool.commonPool-worker-2,5,main] computing: 5 to 7
Thread[ForkJoinPool.commonPool-worker-1,5,main] computing: 7 to 10
87

Next TopicForkjoinpool awaitTermination()