- /*
- * @(#)CyclicBarrier.java 1.7 04/07/13
- *
- * Copyright 2004 Sun Microsystems, Inc. All rights reserved.
- * SUN PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
- */
-
- package java.util.concurrent;
- import java.util.concurrent.locks.*;
-
- /**
- * A synchronization aid that allows a set of threads to all wait for
- * each other to reach a common barrier point. CyclicBarriers are
- * useful in programs involving a fixed sized party of threads that
- * must occasionally wait for each other. The barrier is called
- * <em>cyclic</em> because it can be re-used after the waiting threads
- * are released.
- *
- * <p>A <tt>CyclicBarrier</tt> supports an optional {@link Runnable} command
- * that is run once per barrier point, after the last thread in the party
- * arrives, but before any threads are released.
- * This <em>barrier action</em> is useful
- * for updating shared-state before any of the parties continue.
- *
- * <p><b>Sample usage:</b> Here is an example of
- * using a barrier in a parallel decomposition design:
- * <pre>
- * class Solver {
- * final int N;
- * final float[][] data;
- * final CyclicBarrier barrier;
- *
- * class Worker implements Runnable {
- * int myRow;
- * Worker(int row) { myRow = row; }
- * public void run() {
- * while (!done()) {
- * processRow(myRow);
- *
- * try {
- * barrier.await();
- * } catch (InterruptedException ex) {
- * return;
- * } catch (BrokenBarrierException ex) {
- * return;
- * }
- * }
- * }
- * }
- *
- * public Solver(float[][] matrix) {
- * data = matrix;
- * N = matrix.length;
- * barrier = new CyclicBarrier(N,
- * new Runnable() {
- * public void run() {
- * mergeRows(...);
- * }
- * });
- * for (int i = 0; i < N; ++i)
- * new Thread(new Worker(i)).start();
- *
- * waitUntilDone();
- * }
- * }
- * </pre>
- * Here, each worker thread processes a row of the matrix then waits at the
- * barrier until all rows have been processed. When all rows are processed
- * the supplied {@link Runnable} barrier action is executed and merges the
- * rows. If the merger
- * determines that a solution has been found then <tt>done()</tt> will return
- * <tt>true</tt> and each worker will terminate.
- *
- * <p>If the barrier action does not rely on the parties being suspended when
- * it is executed, then any of the threads in the party could execute that
- * action when it is released. To facilitate this, each invocation of
- * {@link #await} returns the arrival index of that thread at the barrier.
- * You can then choose which thread should execute the barrier action, for
- * example:
- * <pre> if (barrier.await() == 0) {
- * // log the completion of this iteration
- * }</pre>
- *
- * <p>The <tt>CyclicBarrier</tt> uses a fast-fail all-or-none breakage
- * model for failed synchronization attempts: If a thread leaves a
- * barrier point prematurely because of interruption, failure, or
- * timeout, all other threads, even those that have not yet resumed
- * from a previous {@link #await}, will also leave abnormally via
- * {@link BrokenBarrierException} (or <tt>InterruptedException</tt> if
- * they too were interrupted at about the same time).
- *
- * @since 1.5
- * @see CountDownLatch
- *
- * @author Doug Lea
- */
- public class CyclicBarrier {
- /** The lock for guarding barrier entry */
- private final ReentrantLock lock = new ReentrantLock();
- /** Condition to wait on until tripped */
- private final Condition trip = lock.newCondition();
- /** The number of parties */
- private final int parties;
- /* The command to run when tripped */
- private final Runnable barrierCommand;
-
- /**
- * The generation number. Incremented upon barrier trip.
- * Retracted upon reset.
- */
- private long generation;
-
- /**
- * Breakage indicator.
- */
- private boolean broken;
-
- /**
- * Number of parties still waiting. Counts down from parties to 0
- * on each cycle.
- */
- private int count;
-
- /**
- * Updates state on barrier trip and wake up everyone.
- */
- private void nextGeneration() {
- count = parties;
- ++generation;
- trip.signalAll();
- }
-
- /**
- * Sets barrier as broken and wake up everyone
- */
- private void breakBarrier() {
- broken = true;
- trip.signalAll();
- }
-
- /**
- * Main barrier code, covering the various policies.
- */
- private int dowait(boolean timed, long nanos)
- throws InterruptedException, BrokenBarrierException, TimeoutException {
- final ReentrantLock lock = this.lock;
- lock.lock();
- try {
- int index = --count;
- long g = generation;
-
- if (broken)
- throw new BrokenBarrierException();
-
- if (Thread.interrupted()) {
- breakBarrier();
- throw new InterruptedException();
- }
-
- if (index == 0) { // tripped
- nextGeneration();
- boolean ranAction = false;
- try {
- Runnable command = barrierCommand;
- if (command != null)
- command.run();
- ranAction = true;
- return 0;
- } finally {
- if (!ranAction)
- breakBarrier();
- }
- }
-
- for (;;) {
- try {
- if (!timed)
- trip.await();
- else if (nanos > 0L)
- nanos = trip.awaitNanos(nanos);
- } catch (InterruptedException ie) {
- breakBarrier();
- throw ie;
- }
-
- if (broken ||
- g > generation) // true if a reset occurred while waiting
- throw new BrokenBarrierException();
-
- if (g < generation)
- return index;
-
- if (timed && nanos <= 0L) {
- breakBarrier();
- throw new TimeoutException();
- }
- }
-
- } finally {
- lock.unlock();
- }
- }
-
- /**
- * Creates a new <tt>CyclicBarrier</tt> that will trip when the
- * given number of parties (threads) are waiting upon it, and which
- * will execute the given barrier action when the barrier is tripped,
- * performed by the last thread entering the barrier.
- *
- * @param parties the number of threads that must invoke {@link #await}
- * before the barrier is tripped.
- * @param barrierAction the command to execute when the barrier is
- * tripped, or <tt>null</tt> if there is no action.
- *
- * @throws IllegalArgumentException if <tt>parties</tt> is less than 1.
- */
- public CyclicBarrier(int parties, Runnable barrierAction) {
- if (parties <= 0) throw new IllegalArgumentException();
- this.parties = parties;
- this.count = parties;
- this.barrierCommand = barrierAction;
- }
-
- /**
- * Creates a new <tt>CyclicBarrier</tt> that will trip when the
- * given number of parties (threads) are waiting upon it, and
- * does not perform a predefined action upon each barrier.
- *
- * @param parties the number of threads that must invoke {@link #await}
- * before the barrier is tripped.
- *
- * @throws IllegalArgumentException if <tt>parties</tt> is less than 1.
- */
- public CyclicBarrier(int parties) {
- this(parties, null);
- }
-
- /**
- * Returns the number of parties required to trip this barrier.
- * @return the number of parties required to trip this barrier.
- **/
- public int getParties() {
- return parties;
- }
-
- /**
- * Waits until all {@link #getParties parties} have invoked <tt>await</tt>
- * on this barrier.
- *
- * <p>If the current thread is not the last to arrive then it is
- * disabled for thread scheduling purposes and lies dormant until
- * one of following things happens:
- * <ul>
- * <li>The last thread arrives; or
- * <li>Some other thread {@link Thread#interrupt interrupts} the current
- * thread; or
- * <li>Some other thread {@link Thread#interrupt interrupts} one of the
- * other waiting threads; or
- * <li>Some other thread times out while waiting for barrier; or
- * <li>Some other thread invokes {@link #reset} on this barrier.
- * </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.
- *
- * <p>If the barrier is {@link #reset} while any thread is waiting, or if
- * the barrier {@link #isBroken is broken} when <tt>await</tt> is invoked,
- * or while any thread is waiting,
- * then {@link BrokenBarrierException} is thrown.
- *
- * <p>If any thread is {@link Thread#interrupt interrupted} while waiting,
- * then all other waiting threads will throw
- * {@link BrokenBarrierException} and the barrier is placed in the broken
- * state.
- *
- * <p>If the current thread is the last thread to arrive, and a
- * non-null barrier action was supplied in the constructor, then the
- * current thread runs the action before allowing the other threads to
- * continue.
- * If an exception occurs during the barrier action then that exception
- * will be propagated in the current thread and the barrier is placed in
- * the broken state.
- *
- * @return the arrival index of the current thread, where index
- * <tt>{@link #getParties()} - 1</tt> indicates the first to arrive and
- * zero indicates the last to arrive.
- *
- * @throws InterruptedException if the current thread was interrupted
- * while waiting
- * @throws BrokenBarrierException if <em>another</em> thread was
- * interrupted while the current thread was waiting, or the barrier was
- * reset, or the barrier was broken when <tt>await</tt> was called,
- * or the barrier action (if present) failed due an exception.
- */
- public int await() throws InterruptedException, BrokenBarrierException {
- try {
- return dowait(false, 0L);
- } catch (TimeoutException toe) {
- throw new Error(toe); // cannot happen;
- }
- }
-
- /**
- * Waits until all {@link #getParties parties} have invoked <tt>await</tt>
- * on this barrier.
- *
- * <p>If the current thread is not the last to arrive then it is
- * disabled for thread scheduling purposes and lies dormant until
- * one of the following things happens:
- * <ul>
- * <li>The last thread arrives; or
- * <li>The specified timeout elapses; or
- * <li>Some other thread {@link Thread#interrupt interrupts} the current
- * thread; or
- * <li>Some other thread {@link Thread#interrupt interrupts} one of the
- * other waiting threads; or
- * <li>Some other thread times out while waiting for barrier; or
- * <li>Some other thread invokes {@link #reset} on this barrier.
- * </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.
- *
- * <p>If the specified waiting time elapses then {@link TimeoutException}
- * is thrown. If the time is less than or equal to zero, the
- * method will not wait at all.
- *
- * <p>If the barrier is {@link #reset} while any thread is waiting, or if
- * the barrier {@link #isBroken is broken} when <tt>await</tt> is invoked,
- * or while any thread is waiting,
- * then {@link BrokenBarrierException} is thrown.
- *
- * <p>If any thread is {@link Thread#interrupt interrupted} while waiting,
- * then all other waiting threads will throw
- * {@link BrokenBarrierException} and the barrier is placed in the broken
- * state.
- *
- * <p>If the current thread is the last thread to arrive, and a
- * non-null barrier action was supplied in the constructor, then the
- * current thread runs the action before allowing the other threads to
- * continue.
- * If an exception occurs during the barrier action then that exception
- * will be propagated in the current thread and the barrier is placed in
- * the broken state.
- *
- * @param timeout the time to wait for the barrier
- * @param unit the time unit of the timeout parameter
- * @return the arrival index of the current thread, where index
- * <tt>{@link #getParties()} - 1</tt> indicates the first to arrive and
- * zero indicates the last to arrive.
- *
- * @throws InterruptedException if the current thread was interrupted
- * while waiting
- * @throws TimeoutException if the specified timeout elapses.
- * @throws BrokenBarrierException if <em>another</em> thread was
- * interrupted while the current thread was waiting, or the barrier was
- * reset, or the barrier was broken when <tt>await</tt> was called,
- * or the barrier action (if present) failed due an exception.
- */
- public int await(long timeout, TimeUnit unit)
- throws InterruptedException,
- BrokenBarrierException,
- TimeoutException {
- return dowait(true, unit.toNanos(timeout));
- }
-
- /**
- * Queries if this barrier is in a broken state.
- * @return <tt>true</tt> if one or more parties broke out of this
- * barrier due to interruption or timeout since construction or
- * the last reset, or a barrier action failed due to an exception;
- * and <tt>false</tt> otherwise.
- */
- public boolean isBroken() {
- final ReentrantLock lock = this.lock;
- lock.lock();
- try {
- return broken;
- } finally {
- lock.unlock();
- }
- }
-
- /**
- * Resets the barrier to its initial state. If any parties are
- * currently waiting at the barrier, they will return with a
- * {@link BrokenBarrierException}. Note that resets <em>after</em>
- * a breakage has occurred for other reasons can be complicated to
- * carry out; threads need to re-synchronize in some other way,
- * and choose one to perform the reset. It may be preferable to
- * instead create a new barrier for subsequent use.
- */
- public void reset() {
- final ReentrantLock lock = this.lock;
- lock.lock();
- try {
- /*
- * Retract generation number enough to cover threads
- * currently waiting on current and still resuming from
- * previous generation, plus similarly accommodating spans
- * after the reset.
- */
- generation -= 4;
- broken = false;
- trip.signalAll();
- } finally {
- lock.unlock();
- }
- }
-
- /**
- * Returns the number of parties currently waiting at the barrier.
- * This method is primarily useful for debugging and assertions.
- *
- * @return the number of parties currently blocked in {@link #await}
- **/
- public int getNumberWaiting() {
- final ReentrantLock lock = this.lock;
- lock.lock();
- try {
- return parties - count;
- } finally {
- lock.unlock();
- }
- }
-
- }