CyclicBarrier JDK 1.6)

Description

public class CyclicBarrier

A synchronization aid that allows a set of threads to all wait for each other to reach a common barrier point.

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;
     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 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 CyclicBarrier.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.

See also: CountDownLatch

Methods

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public int await () throws InterruptedException BrokenBarrierException

Waits until all 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:

The last thread arrives; or
Some other thread interrupts the current thread; or
Some other thread interrupts one of the other waiting threads; or
Some other thread times out while waiting for barrier; or
Some other thread invokes CyclicBarrier.reset() on this barrier.

If the current thread:

has its interrupted status set on entry to this method; or
is interrupted while waiting

then InterruptedException is thrown and the current thread's interrupted status is cleared.

If the barrier is CyclicBarrier.reset() while any thread is waiting, or if the barrier is broken when await is invoked, or while any thread is waiting, then BrokenBarrierException is thrown.

If any thread is interrupted while waiting, then all other waiting threads will throw 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.
Returns: 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
- 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 {@code await} was called, or the barrier action (if present) failed due an exception.

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

Waits until all parties have invoked await on this barrier, or the specified waiting time elapses.

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:

The last thread arrives; or
The specified timeout elapses; or
Some other thread interrupts the current thread; or
Some other thread interrupts one of the other waiting threads; or
Some other thread times out while waiting for barrier; or
Some other thread invokes CyclicBarrier.reset() on this barrier.

If the current thread:

has its interrupted status set on entry to this method; or
is interrupted while waiting

then InterruptedException is thrown and the current thread's interrupted status is cleared.

If the specified waiting time elapses then TimeoutException is thrown. If the time is less than or equal to zero, the method will not wait at all.

If the barrier is CyclicBarrier.reset() while any thread is waiting, or if the barrier is broken when await is invoked, or while any thread is waiting, then BrokenBarrierException is thrown.

If any thread is interrupted while waiting, then all other waiting threads will throw 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.
Returns: the arrival index of the current thread, where index {@link #getParties()} - 1 indicates the first to arrive and zero indicates the last to arrive
Parameters:
- timeout - the time to wait for the barrier
- unit - the time unit of the timeout parameter
Throws:
- InterruptedException - if the current thread was interrupted while waiting
- TimeoutException - if the specified timeout elapses
- 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 {@code await} was called, or the barrier action (if present) failed due an exception

public int getNumberWaiting ()

Returns the number of parties currently waiting at the barrier.

public int getParties ()

Returns the number of parties required to trip this barrier.

public boolean isBroken ()

Queries if this barrier is in a broken state.

public void reset ()

Resets the barrier to its initial state.