| java.lang.Object | |
| ↳ | java.util.concurrent.CyclicBarrier |
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
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.
| Public Constructors | |||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
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.
| |||||||||||
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.
| |||||||||||
| Public Methods | |||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
Waits until all
parties have invoked
await on this barrier, or the specified waiting time elapses. | |||||||||||
Waits until all
parties have invoked
await on this barrier. | |||||||||||
Returns the number of parties currently waiting at the barrier.
| |||||||||||
Returns the number of parties required to trip this barrier.
| |||||||||||
Queries if this barrier is in a broken state.
| |||||||||||
Resets the barrier to its initial state.
| |||||||||||
|
[Expand]
Inherited Methods | |||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
From class java.lang.Object
| |||||||||||
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.
| 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 |
| IllegalArgumentException | if parties is less than 1
|
|---|
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.
| parties | the number of threads that must invoke await()
before the barrier is tripped |
|---|
| IllegalArgumentException | if parties is less than 1
|
|---|
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:
interrupts
the current thread; or
interrupts
one of the other waiting threads; or
reset() on this barrier.
If the current thread:
interrupted while waiting
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 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.
| timeout | the time to wait for the barrier |
|---|---|
| unit | the time unit of the timeout parameter |
getParties() - 1 indicates the first
to arrive and zero indicates the last to arrive| 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 await was called, or the barrier action (if
present) failed due an exception
|
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:
interrupts
the current thread; or
interrupts
one of the other waiting threads; or
reset() on this barrier.
If the current thread:
interrupted while waiting
InterruptedException is thrown and the current thread's
interrupted status is cleared.
If the barrier is 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.
getParties() - 1 indicates the first
to arrive and zero indicates the last to arrive| 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 an exception.
|
Returns the number of parties currently waiting at the barrier. This method is primarily useful for debugging and assertions.
await()
Returns the number of parties required to trip this barrier.
Queries if this barrier is in a broken state.
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.
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.