/* * 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.*; /** * 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 * cyclic because it can be re-used after the waiting threads * are released. * *

A CyclicBarrier 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 barrier action is useful * for updating shared-state before any of the parties continue. * *

Sample usage: Here is an example of * using a barrier in a parallel decomposition design: * 

 * 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();
 *   }
 * }
 *
* 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 done() will return * true and each worker will terminate. * *

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: * 

  if (barrier.await() == 0) {
 *     // log the completion of this iteration
 *   }
* *

The CyclicBarrier 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 InterruptedException 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; /** * Update state on barrier trip and wake up everyone. */ private void nextGeneration() { count = parties; ++generation; trip.signalAll(); } /** * Set 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 > 0) 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 <= 0) { breakBarrier(); throw new TimeoutException(); } } } finally { lock.unlock(); } } /** * Create a new CyclicBarrier 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 null if there is no action. * * @throws IllegalArgumentException if parties 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; } /** * Create a new CyclicBarrier 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 parties is less than 1. */ public CyclicBarrier(int parties) { this(parties, null); } /** * Return the number of parties required to trip this barrier. * @return the number of parties required to trip this barrier. **/ public int getParties() { return parties; } /** * Wait until all {@link #getParties parties} have invoked await * on this barrier. * *

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: *

*

If the current thread: *

* then {@link InterruptedException} is thrown and the current thread's * interrupted status is cleared. * *

If the barrier is {@link #reset} while any thread is waiting, or if * the barrier {@link #isBroken is broken} when await is invoked, * or while any thread is waiting, * then {@link BrokenBarrierException} is thrown. * *

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. * *

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 * {@link #getParties()} - 1 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 another thread was * interrupted while the current thread was waiting, or the barrier was * reset, or the barrier was broken when await was called, * or the barrier action (if present) failed due an exception. */ public int await() throws InterruptedException, BrokenBarrierException { try { return dowait(false, 0); } catch (TimeoutException toe) { throw new Error(toe); // cannot happen; } } /** * Wait until all {@link #getParties parties} have invoked await * on this barrier. * *

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: *

*

If the current thread: *

* then {@link InterruptedException} is thrown and the current thread's * interrupted status is cleared. * *

If the barrier is {@link #reset} while any thread is waiting, or if * the barrier {@link #isBroken is broken} when await is invoked, * or while any thread is waiting, * then {@link BrokenBarrierException} is thrown. * *

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. * *

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 * {@link #getParties()} - 1 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 another thread was * interrupted while the current thread was waiting, or the barrier was * reset, or the barrier was broken when await 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)); } /** * Query if this barrier is in a broken state. * @return true 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 false otherwise. */ public boolean isBroken() { final ReentrantLock lock = this.lock; lock.lock(); try { return broken; } finally { lock.unlock(); } } /** * Reset 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 after * 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(); } } /** * Return 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(); } } }