Class ResettableCountDownLatch
- java.lang.Object
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- com.microstrategy.webapi.utils.ResettableCountDownLatch
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public class ResettableCountDownLatch extends java.lang.Object
A synchronization aid that allows one or more threads to wait until a set of operations being performed in other threads completes.A
CountDownLatch
is initialized with a given count. Theawait
methods block until the current count reaches zero due to invocations of thecountDown()
method, after which all waiting threads are released and any subsequent invocations ofawait
return immediately. This is a one-shot phenomenon -- the count cannot be reset. If you need a version that resets the count, consider using aCyclicBarrier
.A
CountDownLatch
is a versatile synchronization tool and can be used for a number of purposes. ACountDownLatch
initialized with a count of one serves as a simple on/off latch, or gate: all threads invokingawait
wait at the gate until it is opened by a thread invokingcountDown()
. ACountDownLatch
initialized to N can be used to make one thread wait until N threads have completed some action, or some action has been completed N times.A useful property of a
CountDownLatch
is that it doesn't require that threads callingcountDown
wait for the count to reach zero before proceeding, it simply prevents any thread from proceeding past anawait
until all threads could pass.Sample usage: Here is a pair of classes in which a group of worker threads use two countdown latches:
- The first is a start signal that prevents any worker from proceeding until the driver is ready for them to proceed;
- The second is a completion signal that allows the driver to wait until all workers have completed.
class Driver { // ... void main() throws InterruptedException { CountDownLatch startSignal = new CountDownLatch(1); CountDownLatch doneSignal = new CountDownLatch(N); for (int i = 0; i < N; ++i) // create and start threads new Thread(new Worker(startSignal, doneSignal)).start(); doSomethingElse(); // don't let run yet startSignal.countDown(); // let all threads proceed doSomethingElse(); doneSignal.await(); // wait for all to finish } } class Worker implements Runnable { private final CountDownLatch startSignal; private final CountDownLatch doneSignal; Worker(CountDownLatch startSignal, CountDownLatch doneSignal) { this.startSignal = startSignal; this.doneSignal = doneSignal; } public void run() { try { startSignal.await(); doWork(); doneSignal.countDown(); } catch (InterruptedException ex) {} // return; } void doWork() { ... } }
Another typical usage would be to divide a problem into N parts, describe each part with a Runnable that executes that portion and counts down on the latch, and queue all the Runnables to an Executor. When all sub-parts are complete, the coordinating thread will be able to pass through await. (When threads must repeatedly count down in this way, instead use a
CyclicBarrier
.)class Driver2 { // ... void main() throws InterruptedException { CountDownLatch doneSignal = new CountDownLatch(N); Executor e = ... for (int i = 0; i < N; ++i) // create and start threads e.execute(new WorkerRunnable(doneSignal, i)); doneSignal.await(); // wait for all to finish } } class WorkerRunnable implements Runnable { private final CountDownLatch doneSignal; private final int i; WorkerRunnable(CountDownLatch doneSignal, int i) { this.doneSignal = doneSignal; this.i = i; } public void run() { try { doWork(i); doneSignal.countDown(); } catch (InterruptedException ex) {} // return; } void doWork() { ... } }
Memory consistency effects: Actions in a thread prior to calling
countDown()
happen-before actions following a successful return from a correspondingawait()
in another thread.- Since:
- 1.5
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Constructor Summary
Constructors Constructor Description ResettableCountDownLatch(int count)
Constructs aCountDownLatch
initialized with the given count.
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Method Summary
All Methods Instance Methods Concrete Methods Modifier and Type Method Description void
await()
Causes the current thread to wait until the latch has counted down to zero, unless the thread is interrupted.boolean
await(long timeout, java.util.concurrent.TimeUnit unit)
Causes the current thread to wait until the latch has counted down to zero, unless the thread is interrupted, or the specified waiting time elapses.void
countDown()
Decrements the count of the latch, releasing all waiting threads if the count reaches zero.long
getCount()
Returns the current count.void
reset()
java.lang.String
toString()
Returns a string identifying this latch, as well as its state.
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Constructor Detail
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ResettableCountDownLatch
public ResettableCountDownLatch(int count)
Constructs aCountDownLatch
initialized with the given count.- Parameters:
count
- the number of timescountDown()
must be invoked before threads can pass throughawait()
- Throws:
java.lang.IllegalArgumentException
- ifcount
is negative
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Method Detail
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await
public void await() throws java.lang.InterruptedException
Causes the current thread to wait until the latch has counted down to zero, unless the thread is interrupted.If the current count is zero then this method returns immediately.
If the current count is greater than zero then the current thread becomes disabled for thread scheduling purposes and lies dormant until one of two things happen:
- The count reaches zero due to invocations of the
countDown()
method; or - Some other thread interrupts the current thread.
If the current thread:
- has its interrupted status set on entry to this method; or
- is interrupted while waiting,
InterruptedException
is thrown and the current thread's interrupted status is cleared.- Throws:
java.lang.InterruptedException
- if the current thread is interrupted while waiting
- The count reaches zero due to invocations of the
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reset
public void reset()
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await
public boolean await(long timeout, java.util.concurrent.TimeUnit unit) throws java.lang.InterruptedException
Causes the current thread to wait until the latch has counted down to zero, unless the thread is interrupted, or the specified waiting time elapses.If the current count is zero then this method returns immediately with the value
true
.If the current count is greater than zero then the current thread becomes disabled for thread scheduling purposes and lies dormant until one of three things happen:
- The count reaches zero due to invocations of the
countDown()
method; or - Some other thread interrupts the current thread; or
- The specified waiting time elapses.
If the count reaches zero then the method returns with the value
true
.If the current thread:
- has its interrupted status set on entry to this method; or
- is interrupted while waiting,
InterruptedException
is thrown and the current thread's interrupted status is cleared.If the specified waiting time elapses then the value
false
is returned. If the time is less than or equal to zero, the method will not wait at all.- Parameters:
timeout
- the maximum time to waitunit
- the time unit of thetimeout
argument- Returns:
true
if the count reached zero andfalse
if the waiting time elapsed before the count reached zero- Throws:
java.lang.InterruptedException
- if the current thread is interrupted while waiting
- The count reaches zero due to invocations of the
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countDown
public void countDown()
Decrements the count of the latch, releasing all waiting threads if the count reaches zero.If the current count is greater than zero then it is decremented. If the new count is zero then all waiting threads are re-enabled for thread scheduling purposes.
If the current count equals zero then nothing happens.
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getCount
public long getCount()
Returns the current count.This method is typically used for debugging and testing purposes.
- Returns:
- the current count
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toString
public java.lang.String toString()
Returns a string identifying this latch, as well as its state. The state, in brackets, includes the String"Count ="
followed by the current count.- Overrides:
toString
in classjava.lang.Object
- Returns:
- a string identifying this latch, as well as its state
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