| /* |
| * Written by Doug Lea with assistance from members of JCP JSR-166 |
| * Expert Group and released to the public domain, as explained at |
| * http://creativecommons.org/licenses/publicdomain |
| */ |
| |
| package java.util.concurrent; |
| import java.util.concurrent.locks.*; |
| import java.util.concurrent.atomic.*; |
| |
| /** |
| * A synchronization aid that allows one or more threads to wait until |
| * a set of operations being performed in other threads completes. |
| * |
| * <p>A <tt>CountDownLatch</tt> is initialized with a given |
| * <em>count</em>. The {@link #await await} methods block until the current |
| * {@link #getCount count} reaches zero due to invocations of the |
| * {@link #countDown} method, after which all waiting threads are |
| * released and any subsequent invocations of {@link #await await} return |
| * immediately. This is a one-shot phenomenon -- the count cannot be |
| * reset. If you need a version that resets the count, consider using |
| * a {@link CyclicBarrier}. |
| * |
| * <p>A <tt>CountDownLatch</tt> is a versatile synchronization tool |
| * and can be used for a number of purposes. A |
| * <tt>CountDownLatch</tt> initialized with a count of one serves as a |
| * simple on/off latch, or gate: all threads invoking {@link #await await} |
| * wait at the gate until it is opened by a thread invoking {@link |
| * #countDown}. A <tt>CountDownLatch</tt> initialized to <em>N</em> |
| * can be used to make one thread wait until <em>N</em> threads have |
| * completed some action, or some action has been completed N times. |
| * <p>A useful property of a <tt>CountDownLatch</tt> is that it |
| * doesn't require that threads calling <tt>countDown</tt> wait for |
| * the count to reach zero before proceeding, it simply prevents any |
| * thread from proceeding past an {@link #await await} until all |
| * threads could pass. |
| * |
| * <p><b>Sample usage:</b> Here is a pair of classes in which a group |
| * of worker threads use two countdown latches: |
| * <ul> |
| * <li>The first is a start signal that prevents any worker from proceeding |
| * until the driver is ready for them to proceed; |
| * <li>The second is a completion signal that allows the driver to wait |
| * until all workers have completed. |
| * </ul> |
| * |
| * <pre> |
| * class Driver { // ... |
| * void main() throws InterruptedException { |
| * CountDownLatch startSignal = new CountDownLatch(1); |
| * CountDownLatch doneSignal = new CountDownLatch(N); |
| * |
| * for (int i = 0; i < N; ++i) // create and start threads |
| * new Thread(new Worker(startSignal, doneSignal)).start(); |
| * |
| * doSomethingElse(); // don't let run yet |
| * startSignal.countDown(); // let all threads proceed |
| * doSomethingElse(); |
| * doneSignal.await(); // wait for all to finish |
| * } |
| * } |
| * |
| * class Worker implements Runnable { |
| * private final CountDownLatch startSignal; |
| * private final CountDownLatch doneSignal; |
| * Worker(CountDownLatch startSignal, CountDownLatch doneSignal) { |
| * this.startSignal = startSignal; |
| * this.doneSignal = doneSignal; |
| * } |
| * public void run() { |
| * try { |
| * startSignal.await(); |
| * doWork(); |
| * doneSignal.countDown(); |
| * } catch (InterruptedException ex) {} // return; |
| * } |
| * |
| * void doWork() { ... } |
| * } |
| * |
| * </pre> |
| * |
| * <p>Another typical usage would be to divide a problem into N parts, |
| * describe each part with a Runnable that executes that portion and |
| * counts down on the latch, and queue all the Runnables to an |
| * Executor. When all sub-parts are complete, the coordinating thread |
| * will be able to pass through await. (When threads must repeatedly |
| * count down in this way, instead use a {@link CyclicBarrier}.) |
| * |
| * <pre> |
| * class Driver2 { // ... |
| * void main() throws InterruptedException { |
| * CountDownLatch doneSignal = new CountDownLatch(N); |
| * Executor e = ... |
| * |
| * for (int i = 0; i < N; ++i) // create and start threads |
| * e.execute(new WorkerRunnable(doneSignal, i)); |
| * |
| * doneSignal.await(); // wait for all to finish |
| * } |
| * } |
| * |
| * class WorkerRunnable implements Runnable { |
| * private final CountDownLatch doneSignal; |
| * private final int i; |
| * WorkerRunnable(CountDownLatch doneSignal, int i) { |
| * this.doneSignal = doneSignal; |
| * this.i = i; |
| * } |
| * public void run() { |
| * try { |
| * doWork(i); |
| * doneSignal.countDown(); |
| * } catch (InterruptedException ex) {} // return; |
| * } |
| * |
| * void doWork() { ... } |
| * } |
| * |
| * </pre> |
| * |
| * @since 1.5 |
| * @author Doug Lea |
| */ |
| public class CountDownLatch { |
| /** |
| * Synchronization control For CountDownLatch. |
| * Uses AQS state to represent count. |
| */ |
| private static final class Sync extends AbstractQueuedSynchronizer { |
| Sync(int count) { |
| setState(count); |
| } |
| |
| int getCount() { |
| return getState(); |
| } |
| |
| public int tryAcquireShared(int acquires) { |
| return getState() == 0? 1 : -1; |
| } |
| |
| public boolean tryReleaseShared(int releases) { |
| // Decrement count; signal when transition to zero |
| for (;;) { |
| int c = getState(); |
| if (c == 0) |
| return false; |
| int nextc = c-1; |
| if (compareAndSetState(c, nextc)) |
| return nextc == 0; |
| } |
| } |
| } |
| |
| private final Sync sync; |
| /** |
| * Constructs a <tt>CountDownLatch</tt> 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 <tt>count</tt> is less than zero. |
| */ |
| public CountDownLatch(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 {@link Thread#interrupt interrupted}. |
| * |
| * <p>If the current {@link #getCount count} is zero then this method |
| * returns immediately. |
| * <p>If the current {@link #getCount count} is greater than zero then |
| * the current thread becomes disabled for thread scheduling |
| * purposes and lies dormant until one of two things happen: |
| * <ul> |
| * <li>The count reaches zero due to invocations of the |
| * {@link #countDown} method; or |
| * <li>Some other thread {@link Thread#interrupt interrupts} the current |
| * thread. |
| * </ul> |
| * <p>If the current thread: |
| * <ul> |
| * <li>has its interrupted status set on entry to this method; or |
| * <li>is {@link Thread#interrupt interrupted} while waiting, |
| * </ul> |
| * 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 {@link Thread#interrupt interrupted}, |
| * or the specified waiting time elapses. |
| * |
| * <p>If the current {@link #getCount count} is zero then this method |
| * returns immediately with the value <tt>true</tt>. |
| * |
| * <p>If the current {@link #getCount count} is greater than zero then |
| * the current thread becomes disabled for thread scheduling |
| * purposes and lies dormant until one of three things happen: |
| * <ul> |
| * <li>The count reaches zero due to invocations of the |
| * {@link #countDown} method; or |
| * <li>Some other thread {@link Thread#interrupt interrupts} the current |
| * thread; or |
| * <li>The specified waiting time elapses. |
| * </ul> |
| * <p>If the count reaches zero then the method returns with the |
| * value <tt>true</tt>. |
| * <p>If the current thread: |
| * <ul> |
| * <li>has its interrupted status set on entry to this method; or |
| * <li>is {@link Thread#interrupt interrupted} while waiting, |
| * </ul> |
| * then {@link InterruptedException} is thrown and the current thread's |
| * interrupted status is cleared. |
| * |
| * <p>If the specified waiting time elapses then the value <tt>false</tt> |
| * 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 <tt>timeout</tt> argument. |
| * @return <tt>true</tt> if the count reached zero and <tt>false</tt> |
| * if the waiting time elapsed before the count reached zero. |
| * |
| * @throws InterruptedException if the current thread is interrupted |
| * while waiting. |
| */ |
| public boolean await(long timeout, TimeUnit unit) |
| throws InterruptedException { |
| return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout)); |
| } |
| |
| /** |
| * Decrements the count of the latch, releasing all waiting threads if |
| * the count reaches zero. |
| * <p>If the current {@link #getCount 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. |
| * <p>If the current {@link #getCount count} equals zero then nothing |
| * happens. |
| */ |
| public void countDown() { |
| sync.releaseShared(1); |
| } |
| |
| /** |
| * Returns the current count. |
| * <p>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 |
| * "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() + "]"; |
| } |
| |
| } |