Java ForkJoinPool invoke() Method

The invoke() method of ForkJoinPool class performs the task specified as the parameter and returns the result on its completion. If the computation encounters an unchecked Exception or Error, it is thrown as the outcome of this invocation.

Syntax

Parameter

T - The type of the task's result

Returns

Task - the task

Throws

NullPointerException - if the task is null

RejectedExecutionException - if the task cannot be scheduled for execution

Example 1

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

Example 2

import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.ForkJoinPool;
import java.util.concurrent.RecursiveAction;
 class Taskk extends RecursiveAction {
     private long Load = 0;
  public Taskk(long Load) {
         this.Load = Load;
    }
    @Override
    protected void compute() {
        //if work is above threshold, break tasks up into smaller tasks
                  List<Taskk> subtasks = new ArrayList<Taskk>();
                  subtasks.addAll(createSubtasks());
              for(RecursiveAction subtask : subtasks){
                subtask.fork();
            }
     }
private List<Taskk> createSubtasks() {
        List<Taskk> subtasks =new ArrayList<Taskk>();
     Taskk subtask1 = new Taskk(this.Load / 2);
        subtasks.add(subtask1);
        return subtasks;
    }
}
public class JavaForkJoinPoolinvokeExample2  {
public static void main(final String[] arguments) throws InterruptedException
      {      
      ForkJoinPool Pool = ForkJoinPool.commonPool();
      int i= Pool.getPoolSize();
      System.out.println("Common Pool Size before  :" +i);
       Taskk t = new Taskk(1000000);
       Pool.invoke(t);
       Pool.execute(t);
      System.out.println("Common Pool Size after  :" +Pool.getPoolSize());
         }  
}

Test it Now

Output:

Common Pool Size before  :0
Common Pool Size after  :3

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