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// GPars - Groovy Parallel Systems
//
// Copyright © 2008-11 The original author or authors
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
package groovyx.gpars.dataflow.impl;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.locks.AbstractQueuedSynchronizer;
/**
* CountDownLatch with the ability to change the number of waiting parties
*
* @author Vaclav Pech
* @author Doug Lea
*/
public final class ResizeableCountDownLatch {
/**
* Synchronization control For CountDownLatch.
* Uses AQS state to represent count.
*/
private static final class Sync extends AbstractQueuedSynchronizer {
private static final long serialVersionUID = 4982264981922014374L;
Sync(final int count) {
setState(count);
}
int getCount() {
return getState();
}
public int tryAcquireShared(final int acquires) {
return getState() == 0 ? 1 : -1;
}
public boolean tryReleaseShared(final int releases) {
// Decrement count; signal when transition to zero
//noinspection ForLoopWithMissingComponent
for (; ; ) {
final int c = getState();
if (c == 0)
return false;
final int nextc = c - 1;
if (compareAndSetState(c, nextc))
return nextc == 0;
}
}
public void increaseCount() {
boolean repeat = true;
while (repeat) {
final int state = getState();
repeat = !compareAndSetState(state, state + 1);
}
}
public void decreaseCount() {
boolean repeat = true;
while (repeat) {
final int state = getState();
repeat = !compareAndSetState(state, state - 1);
}
}
}
private final Sync sync;
/**
* Constructs a {@code CountDownLatch} initialized with the given count.
*
* @param count the number of times {@link #countDown} must be invoked
* before threads can pass through {@link #await}
* @throws IllegalArgumentException if {@code count} is negative
*/
public ResizeableCountDownLatch(final int count) {
if (count < 0) throw new IllegalArgumentException("count < 0");
this.sync = new Sync(count);
}
/**
* Causes the current thread to wait until the latch has counted down to
* zero, unless the thread is {@linkplain Thread#interrupt interrupted}.
*
* If the current count is zero then this method returns immediately.
*
* If the current count is greater than zero then the current
* thread becomes disabled for thread scheduling purposes and lies
* dormant until one of two things happen:
*
* - The count reaches zero due to invocations of the
* {@link #countDown} method; or
*
- Some other thread {@linkplain Thread#interrupt interrupts}
* the current thread.
*
*
* If the current thread:
*
* - has its interrupted status set on entry to this method; or
*
- is {@linkplain Thread#interrupt interrupted} while waiting,
*
* then {@link InterruptedException} is thrown and the current thread's
* interrupted status is cleared.
*
* @throws InterruptedException if the current thread is interrupted
* while waiting
*/
public void await() throws InterruptedException {
sync.acquireSharedInterruptibly(1);
}
/**
* Causes the current thread to wait until the latch has counted down to
* zero, unless the thread is {@linkplain Thread#interrupt interrupted},
* or the specified waiting time elapses.
*
* If the current count is zero then this method returns immediately
* with the value {@code true}.
*
* If the current count is greater than zero then the current
* thread becomes disabled for thread scheduling purposes and lies
* dormant until one of three things happen:
*
* - The count reaches zero due to invocations of the
* {@link #countDown} method; or
*
- Some other thread {@linkplain Thread#interrupt interrupts}
* the current thread; or
*
- The specified waiting time elapses.
*
*
* If the count reaches zero then the method returns with the
* value {@code true}.
*
* If the current thread:
*
* - has its interrupted status set on entry to this method; or
*
- is {@linkplain Thread#interrupt interrupted} while waiting,
*
* then {@link InterruptedException} is thrown and the current thread's
* interrupted status is cleared.
*
* If the specified waiting time elapses then the value {@code false}
* is returned. If the time is less than or equal to zero, the method
* will not wait at all.
*
* @param timeout the maximum time to wait
* @param unit the time unit of the {@code timeout} argument
* @return {@code true} if the count reached zero and {@code false}
* if the waiting time elapsed before the count reached zero
* @throws InterruptedException if the current thread is interrupted
* while waiting
*/
public boolean await(final long timeout, final TimeUnit unit)
throws InterruptedException {
return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout));
}
/**
* Attempts to atomically count down the latch and await release with a timeout.
* If the timeout expires, the count is increased and the latch is re-tested before reporting failed timeout.
*
* @param timeout The time in nanoseconds to await
* @return True, if successful, false, is the timeout elapses without the latch being released
* @throws InterruptedException If the thread gets interrupted while waiting for the release
*/
public boolean attemptToCountDownAndAwait(final long timeout) throws InterruptedException {
if (await(timeout, TimeUnit.NANOSECONDS)) return true;
increaseCount();
if (getCount() <= 1L) {
countDown();
return true;
}
return false;
}
public boolean isReleasedFlag() {
return sync.getCount() == 0;
}
/**
* Decrements the count of the latch, releasing all waiting threads if
* the count reaches zero.
*
* If the current count is greater than zero then it is decremented.
* If the new count is zero then all waiting threads are re-enabled for
* thread scheduling purposes.
*
* If the current count equals zero then nothing happens.
*/
public void countDown() {
sync.releaseShared(1);
}
public void increaseCount() {
sync.increaseCount();
}
public void decreaseCount() {
sync.decreaseCount();
}
/**
* Returns the current count.
*
* This method is typically used for debugging and testing purposes.
*
* @return the current count
*/
public long getCount() {
return sync.getCount();
}
/**
* Returns a string identifying this latch, as well as its state.
* The state, in brackets, includes the String {@code "Count ="}
* followed by the current count.
*
* @return a string identifying this latch, as well as its state
*/
public String toString() {
return super.toString() + "[Count = " + sync.getCount() + "]";
}
}