File	Doc	Category	Size	Date	Package
CyclicBarrier.java	API Doc	Java SE 5 API	15844	Fri Aug 26 14:57:26 BST 2005	java.util.concurrent

CyclicBarrier

• java.lang.Object

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();
}
}

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

Fields Summary
private final ReentrantLock	lock The lock for guarding barrier entry
private final Condition	trip Condition to wait on until tripped
private final int	parties The number of parties
private final Runnable	barrierCommand
private long	generation The generation number. Incremented upon barrier trip. Retracted upon reset.
private boolean	broken Breakage indicator.
private int	count Number of parties still waiting. Counts down from parties to 0 on each cycle.

Constructors Summary
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. 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. if (parties <= 0) throw new IllegalArgumentException(); this.parties = parties; this.count = parties; this.barrierCommand = barrierAction;
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 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. this(parties, null);

Methods Summary
public int	await() Waits 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: The last thread arrives; or Some other thread {@link Thread#interrupt interrupts} the current thread; or Some other thread {@link Thread#interrupt interrupts} one of the other waiting threads; or Some other thread times out while waiting for barrier; or Some other thread invokes {@link #reset} on this barrier. If the current thread: has its interrupted status set on entry to this method; or is {@link Thread#interrupt interrupted} while waiting 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. try { return dowait(false, 0L); } catch (TimeoutException toe) { throw new Error(toe); // cannot happen; }
public int	await(long timeout, java.util.concurrent.TimeUnit unit) Waits 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: The last thread arrives; or The specified timeout elapses; or Some other thread {@link Thread#interrupt interrupts} the current thread; or Some other thread {@link Thread#interrupt interrupts} one of the other waiting threads; or Some other thread times out while waiting for barrier; or Some other thread invokes {@link #reset} on this barrier. If the current thread: has its interrupted status set on entry to this method; or is {@link Thread#interrupt interrupted} while waiting then {@link InterruptedException} is thrown and the current thread's interrupted status is cleared. 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. 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. return dowait(true, unit.toNanos(timeout));
private void	breakBarrier() Sets barrier as broken and wake up everyone broken = true; trip.signalAll();
private int	dowait(boolean timed, long nanos) Main barrier code, covering the various policies. 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(); }
public int	getNumberWaiting() 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} final ReentrantLock lock = this.lock; lock.lock(); try { return parties - count; } finally { lock.unlock(); }
public int	getParties() Returns the number of parties required to trip this barrier. return the number of parties required to trip this barrier. return parties;
public boolean	isBroken() Queries 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. final ReentrantLock lock = this.lock; lock.lock(); try { return broken; } finally { lock.unlock(); }
private void	nextGeneration() Updates state on barrier trip and wake up everyone. count = parties; ++generation; trip.signalAll();
public void	reset() 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 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. 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(); }