Class CyclicBarrier

public class CyclicBarrier
extends Object

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 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;
     Runnable barrierAction =
       new Runnable() { public void run() { mergeRows(...); }};
     barrier = new CyclicBarrier(N, barrierAction);

     List<Thread> threads = new ArrayList<Thread>(N);
     for (int i = 0; i < N; i++) {
       Thread thread = new Thread(new Worker(i));
       threads.add(thread);
       thread.start();
     }

     // wait until done
     for (Thread thread : threads)
       thread.join();
   }
 }

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 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 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 an 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 waiting at that barrier point will also leave abnormally via BrokenBarrierException (or InterruptedException if they too were interrupted at about the same time).

Memory consistency effects: Actions in a thread prior to calling await() happen-before actions that are part of the barrier action, which in turn happen-before actions following a successful return from the corresponding await() in other threads.

Constructors

CyclicBarrier

public CyclicBarrier(int parties,
                     Runnable barrierAction)

Creates 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.

Parameters:

parties - the number of threads that must invoke await() before the barrier is tripped

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

CyclicBarrier

public CyclicBarrier(int parties)

Creates a new CyclicBarrier that will trip when the given number of parties (threads) are waiting upon it, and does not perform a predefined action when the barrier is tripped.

Parameters:

parties - the number of threads that must invoke await() before the barrier is tripped

Throws:

IllegalArgumentException - if parties is less than 1

Methods

getParties

public int getParties()

Returns the number of parties required to trip this barrier.

Returns:

the number of parties required to trip this barrier

await

public int await()
          throws InterruptedException,
                 BrokenBarrierException

Returns:

the arrival index of the current thread, where index getParties() - 1 indicates the first to arrive and zero indicates the last to arrive

Throws:

InterruptedException - if the current thread was interrupted while waiting

BrokenBarrierException - if another thread was interrupted or timed out 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 to an exception

await

public int await(long timeout,
                 TimeUnit unit)
          throws InterruptedException,
                 BrokenBarrierException,
                 TimeoutException

Parameters:

timeout - the time to wait for the barrier

unit - the time unit of the timeout parameter

Returns:

the arrival index of the current thread, where index getParties() - 1 indicates the first to arrive and zero indicates the last to arrive

Throws:

InterruptedException - if the current thread was interrupted while waiting

TimeoutException - if the specified timeout elapses. In this case the barrier will be broken.

BrokenBarrierException - if another thread was interrupted or timed out 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 to an exception

isBroken

public boolean isBroken()

Queries if this barrier is in a broken state.

Returns:

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; false otherwise.

reset

public void reset()

Resets the barrier to its initial state. If any parties are currently waiting at the barrier, they will return with a 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.

getNumberWaiting

public int getNumberWaiting()

Returns the number of parties currently waiting at the barrier. This method is primarily useful for debugging and assertions.

Returns:

the number of parties currently blocked in await()