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 . 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 implements ManagedBlocker {
final BlockingQueue queue;
volatile E item = null;
QueueTaker(BlockingQueue 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;
}
}`

Inheritors