Added in API level 21

ManagedBlocker

interface ManagedBlocker

java.util.concurrent.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 java.util.concurrent.ForkJoinPool#managedBlock(java.util.concurrent.ForkJoinPool.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:

<code>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());
    }
  }</code>

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

<code>class QueueTaker&lt;E&gt; implements ManagedBlocker {
    final BlockingQueue&lt;E&gt; queue;
    volatile E item = null;
    QueueTaker(BlockingQueue&lt;E&gt; 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;
    }
  }</code>

Summary

Public methods
abstract Boolean	block() Possibly blocks the current thread, for example waiting for a lock or condition.
abstract Boolean	isReleasable() Returns true if blocking is unnecessary.

Public methods

block

Added in API level 21

abstract fun block(): Boolean

Possibly blocks the current thread, for example waiting for a lock or condition.

Return
Boolean	true if no additional blocking is necessary (i.e., if isReleasable would return true)

Exceptions
java.lang.InterruptedException	if interrupted while waiting (the method is not required to do so, but is allowed to)

isReleasable

Added in API level 21

abstract fun isReleasable(): Boolean

Returns true if blocking is unnecessary.

Return
Boolean	true if blocking is unnecessary