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<div class="subTitle">edu.emory.mathcs.backport.java.util.concurrent</div>
<h2 title="Class CountDownLatch" class="title">Class CountDownLatch</h2>
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<pre>public class <span class="strong">CountDownLatch</span>
extends java.lang.Object</pre>
<div class="block">A synchronization aid that allows one or more threads to wait until
a set of operations being performed in other threads completes.
<p>A <code>CountDownLatch</code> is initialized with a given <em>count</em>.
The <a href="../../../../../../../edu/emory/mathcs/backport/java/util/concurrent/CountDownLatch.html#await()"><code>await</code></a> methods block until the current count reaches
zero due to invocations of the <a href="../../../../../../../edu/emory/mathcs/backport/java/util/concurrent/CountDownLatch.html#countDown()"><code>countDown()</code></a> method, after which
all waiting threads are released and any subsequent invocations of
<a href="../../../../../../../edu/emory/mathcs/backport/java/util/concurrent/CountDownLatch.html#await()"><code>await</code></a> return immediately. This is a one-shot phenomenon
-- the count cannot be reset. If you need a version that resets the
count, consider using a <a href="../../../../../../../edu/emory/mathcs/backport/java/util/concurrent/CyclicBarrier.html" title="class in edu.emory.mathcs.backport.java.util.concurrent"><code>CyclicBarrier</code></a>.
<p>A <code>CountDownLatch</code> is a versatile synchronization tool
and can be used for a number of purposes. A
<code>CountDownLatch</code> initialized with a count of one serves as a
simple on/off latch, or gate: all threads invoking <a href="../../../../../../../edu/emory/mathcs/backport/java/util/concurrent/CountDownLatch.html#await()"><code>await</code></a>
wait at the gate until it is opened by a thread invoking <a href="../../../../../../../edu/emory/mathcs/backport/java/util/concurrent/CountDownLatch.html#countDown()"><code>countDown()</code></a>. A <code>CountDownLatch</code> initialized to <em>N</em>
can be used to make one thread wait until <em>N</em> threads have
completed some action, or some action has been completed N times.
<p>A useful property of a <code>CountDownLatch</code> is that it
doesn't require that threads calling <code>countDown</code> wait for
the count to reach zero before proceeding, it simply prevents any
thread from proceeding past an <a href="../../../../../../../edu/emory/mathcs/backport/java/util/concurrent/CountDownLatch.html#await()"><code>await</code></a> until all
threads could pass.
<p><b>Sample usage:</b> Here is a pair of classes in which a group
of worker threads use two countdown latches:
<ul>
<li>The first is a start signal that prevents any worker from proceeding
until the driver is ready for them to proceed;
<li>The second is a completion signal that allows the driver to wait
until all workers have completed.
</ul>
<pre>
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() { ... }
}
</pre>
<p>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 <a href="../../../../../../../edu/emory/mathcs/backport/java/util/concurrent/CyclicBarrier.html" title="class in edu.emory.mathcs.backport.java.util.concurrent"><code>CyclicBarrier</code></a>.)
<pre>
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() { ... }
}
</pre>
<p>Memory consistency effects: Actions in a thread prior to calling
<code>countDown()</code>
<a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
actions following a successful return from a corresponding
<code>await()</code> in another thread.</div>
<dl><dt><span class="strong">Since:</span></dt>
<dd>1.5</dd>
<dt><span class="strong">Author:</span></dt>
<dd>Doug Lea</dd></dl>
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<td class="colOne"><code><strong><a href="../../../../../../../edu/emory/mathcs/backport/java/util/concurrent/CountDownLatch.html#CountDownLatch(int)">CountDownLatch</a></strong>(int count)</code>
<div class="block">Constructs a <code>CountDownLatch</code> initialized with the given count.</div>
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<td class="colFirst"><code>void</code></td>
<td class="colLast"><code><strong><a href="../../../../../../../edu/emory/mathcs/backport/java/util/concurrent/CountDownLatch.html#await()">await</a></strong>()</code>
<div class="block">Causes the current thread to wait until the latch has counted down to
zero, unless the thread is interrupted.</div>
</td>
</tr>
<tr class="rowColor">
<td class="colFirst"><code>boolean</code></td>
<td class="colLast"><code><strong><a href="../../../../../../../edu/emory/mathcs/backport/java/util/concurrent/CountDownLatch.html#await(long, edu.emory.mathcs.backport.java.util.concurrent.TimeUnit)">await</a></strong>(long timeout,
<a href="../../../../../../../edu/emory/mathcs/backport/java/util/concurrent/TimeUnit.html" title="class in edu.emory.mathcs.backport.java.util.concurrent">TimeUnit</a> unit)</code>
<div class="block">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.</div>
</td>
</tr>
<tr class="altColor">
<td class="colFirst"><code>void</code></td>
<td class="colLast"><code><strong><a href="../../../../../../../edu/emory/mathcs/backport/java/util/concurrent/CountDownLatch.html#countDown()">countDown</a></strong>()</code>
<div class="block">Decrements the count of the latch, releasing all waiting threads if
the count reaches zero.</div>
</td>
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<tr class="rowColor">
<td class="colFirst"><code>long</code></td>
<td class="colLast"><code><strong><a href="../../../../../../../edu/emory/mathcs/backport/java/util/concurrent/CountDownLatch.html#getCount()">getCount</a></strong>()</code>
<div class="block">Returns the current count.</div>
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<td class="colFirst"><code>java.lang.String</code></td>
<td class="colLast"><code><strong><a href="../../../../../../../edu/emory/mathcs/backport/java/util/concurrent/CountDownLatch.html#toString()">toString</a></strong>()</code>
<div class="block">Returns a string identifying this latch, as well as its state.</div>
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<pre>public CountDownLatch(int count)</pre>
<div class="block">Constructs a <code>CountDownLatch</code> initialized with the given count.</div>
<dl><dt><span class="strong">Parameters:</span></dt><dd><code>count</code> - the number of times <a href="../../../../../../../edu/emory/mathcs/backport/java/util/concurrent/CountDownLatch.html#countDown()"><code>countDown()</code></a> must be invoked
before threads can pass through <a href="../../../../../../../edu/emory/mathcs/backport/java/util/concurrent/CountDownLatch.html#await()"><code>await()</code></a></dd>
<dt><span class="strong">Throws:</span></dt>
<dd><code>java.lang.IllegalArgumentException</code> - if <code>count</code> is negative</dd></dl>
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<h4>await</h4>
<pre>public void await()
throws java.lang.InterruptedException</pre>
<div class="block">Causes the current thread to wait until the latch has counted down to
zero, unless the thread is interrupted.
<p>If the current count is zero then this method returns immediately.
<p>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:
<ul>
<li>The count reaches zero due to invocations of the
<a href="../../../../../../../edu/emory/mathcs/backport/java/util/concurrent/CountDownLatch.html#countDown()"><code>countDown()</code></a> method; or
<li>Some other thread interrupts
the current thread.
</ul>
<p>If the current thread:
<ul>
<li>has its interrupted status set on entry to this method; or
<li>is interrupted while waiting,
</ul>
then <code>InterruptedException</code> is thrown and the current thread's
interrupted status is cleared.</div>
<dl><dt><span class="strong">Throws:</span></dt>
<dd><code>java.lang.InterruptedException</code> - if the current thread is interrupted
while waiting</dd></dl>
</li>
</ul>
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<pre>public boolean await(long timeout,
<a href="../../../../../../../edu/emory/mathcs/backport/java/util/concurrent/TimeUnit.html" title="class in edu.emory.mathcs.backport.java.util.concurrent">TimeUnit</a> unit)
throws java.lang.InterruptedException</pre>
<div class="block">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.
<p>If the current count is zero then this method returns immediately
with the value <code>true</code>.
<p>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:
<ul>
<li>The count reaches zero due to invocations of the
<a href="../../../../../../../edu/emory/mathcs/backport/java/util/concurrent/CountDownLatch.html#countDown()"><code>countDown()</code></a> method; or
<li>Some other thread interrupts
the current thread; or
<li>The specified waiting time elapses.
</ul>
<p>If the count reaches zero then the method returns with the
value <code>true</code>.
<p>If the current thread:
<ul>
<li>has its interrupted status set on entry to this method; or
<li>is interrupted while waiting,
</ul>
then <code>InterruptedException</code> is thrown and the current thread's
interrupted status is cleared.
<p>If the specified waiting time elapses then the value <code>false</code>
is returned. If the time is less than or equal to zero, the method
will not wait at all.</div>
<dl><dt><span class="strong">Parameters:</span></dt><dd><code>timeout</code> - the maximum time to wait</dd><dd><code>unit</code> - the time unit of the <code>timeout</code> argument</dd>
<dt><span class="strong">Returns:</span></dt><dd><code>true</code> if the count reached zero and <code>false</code>
if the waiting time elapsed before the count reached zero</dd>
<dt><span class="strong">Throws:</span></dt>
<dd><code>java.lang.InterruptedException</code> - if the current thread is interrupted
while waiting</dd></dl>
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<h4>countDown</h4>
<pre>public void countDown()</pre>
<div class="block">Decrements the count of the latch, releasing all waiting threads if
the count reaches zero.
<p>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.
<p>If the current count equals zero then nothing happens.</div>
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<h4>getCount</h4>
<pre>public long getCount()</pre>
<div class="block">Returns the current count.
<p>This method is typically used for debugging and testing purposes.</div>
<dl><dt><span class="strong">Returns:</span></dt><dd>the current count</dd></dl>
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<pre>public java.lang.String toString()</pre>
<div class="block">Returns a string identifying this latch, as well as its state.
The state, in brackets, includes the String <code>"Count ="</code>
followed by the current count.</div>
<dl>
<dt><strong>Overrides:</strong></dt>
<dd><code>toString</code> in class <code>java.lang.Object</code></dd>
<dt><span class="strong">Returns:</span></dt><dd>a string identifying this latch, as well as its state</dd></dl>
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