Class CyclicBarrier
- All Implemented Interfaces:
Barrier
CyclicBarriers use an all-or-none breakage model
for failed synchronization attempts: If threads
leave a barrier point prematurely because of timeout
or interruption, others will also leave abnormally
(via BrokenBarrierException), until
the barrier is restart
ed. This is usually
the simplest and best strategy for sharing knowledge
about failures among cooperating threads in the most
common usages contexts of Barriers.
This implementation has the property that interruptions
among newly arriving threads can cause as-yet-unresumed
threads from a previous barrier cycle to return out
as broken. This transmits breakage
as early as possible, but with the possible byproduct that
only some threads returning out of a barrier will realize
that it is newly broken. (Others will not realize this until a
future cycle.) (The Rendezvous class has a more uniform, but
sometimes less desirable policy.)
Barriers support an optional Runnable command that is run once per barrier point.
Sample usage Here is a code sketch of 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.barrier(); } catch (InterruptedException ex) { return; } catch (BrokenBarrierException ex) { return; } } } } public Solver(float[][] matrix) { data = matrix; N = matrix.length; barrier = new CyclicBarrier(N); barrier.setBarrierCommand(new Runnable() { public void run() { mergeRows(...); } }); for (int i = 0; i < N; ++i) { new Thread(new Worker(i)).start(); waitUntilDone(); } }
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Field Summary
Fields -
Constructor Summary
ConstructorsConstructorDescriptionCyclicBarrier
(int parties) Create a CyclicBarrier for the indicated number of parties, and no command to run at each barrier.CyclicBarrier
(int parties, Runnable command) Create a CyclicBarrier for the indicated number of parties. -
Method Summary
Modifier and TypeMethodDescriptionint
attemptBarrier
(long msecs) Enter barrier and wait at most msecs for the other parties()-1 threads.int
barrier()
Enter barrier and wait for the other parties()-1 threads.boolean
broken()
Returns true if the barrier has been compromised by threads leaving the barrier before a synchronization point (normally due to interruption or timeout).protected int
doBarrier
(boolean timed, long msecs) int
parties()
Return the number of parties that must meet per barrier point.void
restart()
Reset to initial state.setBarrierCommand
(Runnable command) Set the command to run at the point at which all threads reach the barrier.
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Field Details
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parties_
protected final int parties_ -
broken_
protected boolean broken_ -
barrierCommand_
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count_
protected int count_ -
resets_
protected int resets_
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Constructor Details
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CyclicBarrier
public CyclicBarrier(int parties) Create a CyclicBarrier for the indicated number of parties, and no command to run at each barrier.- Throws:
IllegalArgumentException
- if parties less than or equal to zero.
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CyclicBarrier
Create a CyclicBarrier for the indicated number of parties. and the given command to run at each barrier point.- Throws:
IllegalArgumentException
- if parties less than or equal to zero.
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Method Details
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setBarrierCommand
Set the command to run at the point at which all threads reach the barrier. This command is run exactly once, by the thread that trips the barrier. The command is not run if the barrier is broken.- Parameters:
command
- the command to run. If null, no command is run.- Returns:
- the previous command
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broken
public boolean broken()Description copied from interface:Barrier
Returns true if the barrier has been compromised by threads leaving the barrier before a synchronization point (normally due to interruption or timeout). Barrier methods in implementation classes throw throw BrokenBarrierException upon detection of breakage. Implementations may also support some means to clear this status. -
restart
public void restart()Reset to initial state. Clears both the broken status and any record of waiting threads, and releases all currently waiting threads with indeterminate return status. This method is intended only for use in recovery actions in which it is somehow known that no thread could possibly be relying on the the synchronization properties of this barrier. -
parties
public int parties()Description copied from interface:Barrier
Return the number of parties that must meet per barrier point. The number of parties is always at least 1. -
barrier
Enter barrier and wait for the other parties()-1 threads.- Returns:
- the arrival index: the number of other parties that were still waiting upon entry. This is a unique value from zero to parties()-1. If it is zero, then the current thread was the last party to hit barrier point and so was responsible for releasing the others.
- Throws:
BrokenBarrierException
- if any other thread in any previous or current barrier since either creation or the lastrestart
operation left the barrier prematurely due to interruption or time-out. (If so, thebroken
status is also set.) Threads that are notified to have been interrupted after being released are not considered to have broken the barrier. In all cases, the interruption status of the current thread is preserved, so can be tested by checkingThread.interrupted
.InterruptedException
- if this thread was interrupted during the barrier, and was the one causing breakage. If so,broken
status is also set.
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attemptBarrier
public int attemptBarrier(long msecs) throws InterruptedException, TimeoutException, BrokenBarrierException Enter barrier and wait at most msecs for the other parties()-1 threads.- Returns:
- if not timed out, the arrival index: the number of other parties that were still waiting upon entry. This is a unique value from zero to parties()-1. If it is zero, then the current thread was the last party to hit barrier point and so was responsible for releasing the others.
- Throws:
BrokenBarrierException
- if any other thread in any previous or current barrier since either creation or the lastrestart
operation left the barrier prematurely due to interruption or time-out. (If so, thebroken
status is also set.) Threads that are noticed to have been interrupted after being released are not considered to have broken the barrier. In all cases, the interruption status of the current thread is preserved, so can be tested by checkingThread.interrupted
.InterruptedException
- if this thread was interrupted during the barrier. If so,broken
status is also set.TimeoutException
- if this thread timed out waiting for the barrier. If the timeout occured while already in the barrier,broken
status is also set.
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doBarrier
protected int doBarrier(boolean timed, long msecs) throws InterruptedException, TimeoutException, BrokenBarrierException
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