/**
* Drains the queue as {@link BlockingQueue#drainTo(Collection, int)}, but if the requested {@code
* numElements} elements are not available, it will wait for them up to the specified timeout.
*
* @param q the blocking queue to be drained
* @param buffer where to add the transferred elements
* @param numElements the number of elements to be waited for
* @param timeout how long to wait before giving up, in units of {@code unit}
* @param unit a {@code TimeUnit} determining how to interpret the timeout parameter
* @return the number of elements transferred
* @throws InterruptedException if interrupted while waiting
*/
@Beta
public static <E> int drain(
BlockingQueue<E> q,
Collection<? super E> buffer,
int numElements,
long timeout,
TimeUnit unit)
throws InterruptedException {
Preconditions.checkNotNull(buffer);
/*
* This code performs one System.nanoTime() more than necessary, and in return, the time to
* execute Queue#drainTo is not added *on top* of waiting for the timeout (which could make
* the timeout arbitrarily inaccurate, given a queue that is slow to drain).
*/
long deadline = System.nanoTime() + unit.toNanos(timeout);
int added = 0;
while (added < numElements) {
// we could rely solely on #poll, but #drainTo might be more efficient when there are multiple
// elements already available (e.g. LinkedBlockingQueue#drainTo locks only once)
added += q.drainTo(buffer, numElements - added);
if (added < numElements) { // not enough elements immediately available; will have to poll
E e = q.poll(deadline - System.nanoTime(), TimeUnit.NANOSECONDS);
if (e == null) {
break; // we already waited enough, and there are no more elements in sight
}
buffer.add(e);
added++;
}
}
return added;
}
/**
* Drains the queue as {@linkplain #drain(BlockingQueue, Collection, int, long, TimeUnit)}, but
* with a different behavior in case it is interrupted while waiting. In that case, the operation
* will continue as usual, and in the end the thread's interruption status will be set (no {@code
* InterruptedException} is thrown).
*
* @param q the blocking queue to be drained
* @param buffer where to add the transferred elements
* @param numElements the number of elements to be waited for
* @param timeout how long to wait before giving up, in units of {@code unit}
* @param unit a {@code TimeUnit} determining how to interpret the timeout parameter
* @return the number of elements transferred
*/
@Beta
public static <E> int drainUninterruptibly(
BlockingQueue<E> q,
Collection<? super E> buffer,
int numElements,
long timeout,
TimeUnit unit) {
Preconditions.checkNotNull(buffer);
long deadline = System.nanoTime() + unit.toNanos(timeout);
int added = 0;
boolean interrupted = false;
try {
while (added < numElements) {
// we could rely solely on #poll, but #drainTo might be more efficient when there are
// multiple elements already available (e.g. LinkedBlockingQueue#drainTo locks only once)
added += q.drainTo(buffer, numElements - added);
if (added < numElements) { // not enough elements immediately available; will have to poll
E e; // written exactly once, by a successful (uninterrupted) invocation of #poll
while (true) {
try {
e = q.poll(deadline - System.nanoTime(), TimeUnit.NANOSECONDS);
break;
} catch (InterruptedException ex) {
interrupted = true; // note interruption and retry
}
}
if (e == null) {
break; // we already waited enough, and there are no more elements in sight
}
buffer.add(e);
added++;
}
}
} finally {
if (interrupted) {
Thread.currentThread().interrupt();
}
}
return added;
}