Module java.base
Package java.util.concurrent

Interface ForkJoinPool.ManagedBlocker

Enclosing class:
ForkJoinPool
public static interface ForkJoinPool.ManagedBlocker
Interface for extending managed parallelism for tasks running in ForkJoinPools.

A ManagedBlocker provides two methods. Method isReleasable() must return true if blocking is not necessary. Method block() blocks the current thread if necessary (perhaps internally invoking isReleasable before actually blocking). These actions are performed by any thread invoking ForkJoinPool.managedBlock(ManagedBlocker). The unusual methods in this API accommodate synchronizers that may, but don't usually, block for long periods. Similarly, they allow more efficient internal handling of cases in which additional workers may be, but usually are not, needed to ensure sufficient parallelism. Toward this end, implementations of method isReleasable must be amenable to repeated invocation. Neither method is invoked after a prior invocation of isReleasable or block returns true.

For example, here is a ManagedBlocker based on a ReentrantLock: 

 
 class ManagedLocker implements ManagedBlocker {
   final ReentrantLock lock;
   boolean hasLock = false;
   ManagedLocker(ReentrantLock lock) { this.lock = lock; }
   public boolean block() {
     if (!hasLock)
       lock.lock();
     return true;
   }
   public boolean isReleasable() {
     return hasLock || (hasLock = lock.tryLock());
   }
 }

Here is a class that possibly blocks waiting for an item on a given queue: 

 
 class QueueTaker<E> implements ManagedBlocker {
   final BlockingQueue<E> queue;
   volatile E item = null;
   QueueTaker(BlockingQueue<E> q) { this.queue = q; }
   public boolean block() throws InterruptedException {
     if (item == null)
       item = queue.take();
     return true;
   }
   public boolean isReleasable() {
     return item != null || (item = queue.poll()) != null;
   }
   public E getItem() { // call after pool.managedBlock completes
     return item;
   }
 }

  • Method Summary

    Modifier and Type
    Method
    Description
    boolean
    block()
    Possibly blocks the current thread, for example waiting for a lock or condition.
    boolean
    isReleasable()
    Returns true if blocking is unnecessary.

  • Method Details

    • block

      boolean block() throws InterruptedException
      Possibly blocks the current thread, for example waiting for a lock or condition.
      Returns:
      true if no additional blocking is necessary (i.e., if isReleasable would return true)
      Throws:
      InterruptedException - if interrupted while waiting (the method is not required to do so, but is allowed to)

    • isReleasable

      boolean isReleasable()
      Returns true if blocking is unnecessary.
      Returns:
      true if blocking is unnecessary