# CountDownLatch

public class **CountDownLatch** extends [Object](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Object.html)  
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* .
The `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/CountDownLatch.html#await()` methods block until the current count reaches
zero due to invocations of the `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/CountDownLatch.html#countDown()` method, after which
all waiting threads are released and any subsequent invocations of
`https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/CountDownLatch.html#await()` 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 `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/CyclicBarrier.html`.

A `CountDownLatch` is a versatile synchronization tool
and can be used for a number of purposes. A
`CountDownLatch` initialized with a count of one serves as a
simple on/off latch, or gate: all threads invoking `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/CountDownLatch.html#await()`
wait at the gate until it is opened by a thread invoking `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/CountDownLatch.html#countDown()`. A `CountDownLatch` 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 calling `countDown` wait for
the count to reach zero before proceeding, it simply prevents any
thread from proceeding past an `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/CountDownLatch.html#await()` 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 `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/CyclicBarrier.html`.)

     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: Until the count reaches
zero, actions in a thread prior to calling
`countDown()`
[*happen-before*](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/package-summary#MemoryVisibility)
actions following a successful return from a corresponding
`await()` in another thread.

### Public Constructor Summary

|---|---|
|   | [CountDownLatch](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/CountDownLatch.html#CountDownLatch(int))(int count) Constructs a `CountDownLatch` initialized with the given count. |

### Public Method Summary

|---|---|
| void | [await](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/CountDownLatch.html#await())() Causes the current thread to wait until the latch has counted down to zero, unless the thread is {@linkplain Thread#interrupt interrupted}. |
| boolean | [await](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/CountDownLatch.html#await(long,%20java.util.concurrent.TimeUnit))(long timeout, [TimeUnit](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/TimeUnit.html) unit) Causes the current thread to wait until the latch has counted down to zero, unless the thread is {@linkplain Thread#interrupt interrupted}, or the specified waiting time elapses. |
| void | [countDown](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/CountDownLatch.html#countDown())() Decrements the count of the latch, releasing all waiting threads if the count reaches zero. |
| long | [getCount](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/CountDownLatch.html#getCount())() Returns the current count. |
| [String](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/String.html) | [toString](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/CountDownLatch.html#toString())() Returns a string identifying this latch, as well as its state. |

### Inherited Method Summary

From class [java.lang.Object](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Object.html)

|---|---|
| [Object](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Object.html) | [clone](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Object.html#clone())() Creates and returns a copy of this `Object`. |
| boolean | [equals](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Object.html#equals(java.lang.Object))([Object](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Object.html) obj) Compares this instance with the specified object and indicates if they are equal. |
| void | [finalize](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Object.html#finalize())() Invoked when the garbage collector has detected that this instance is no longer reachable. |
| final [Class](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Class.html)\<?\> | [getClass](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Object.html#getClass())() Returns the unique instance of `https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Class.html` that represents this object's class. |
| int | [hashCode](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Object.html#hashCode())() Returns an integer hash code for this object. |
| final void | [notify](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Object.html#notify())() Causes a thread which is waiting on this object's monitor (by means of calling one of the `wait()` methods) to be woken up. |
| final void | [notifyAll](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Object.html#notifyAll())() Causes all threads which are waiting on this object's monitor (by means of calling one of the `wait()` methods) to be woken up. |
| [String](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/String.html) | [toString](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Object.html#toString())() Returns a string containing a concise, human-readable description of this object. |
| final void | [wait](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Object.html#wait(long,%20int))(long timeout, int nanos) Causes the calling thread to wait until another thread calls the `notify()` or `notifyAll()` method of this object or until the specified timeout expires. |
| final void | [wait](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Object.html#wait(long))(long timeout) Causes the calling thread to wait until another thread calls the `notify()` or `notifyAll()` method of this object or until the specified timeout expires. |
| final void | [wait](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Object.html#wait())() Causes the calling thread to wait until another thread calls the `notify()` or `notifyAll()` method of this object. |

## Public Constructors

#### public
**CountDownLatch**
(int count)

Constructs a `CountDownLatch` initialized with the given count.

##### Parameters

| count | the number of times `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/CountDownLatch.html#countDown()` must be invoked before threads can pass through `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/CountDownLatch.html#await()` |
|---|---|

##### Throws

| [IllegalArgumentException](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/IllegalArgumentException.html) | if `count` is negative |
|---|---|

## Public Methods

#### public void
**await**
()

Causes the current thread to wait until the latch has counted down to
zero, unless the thread is {@linkplain Thread#interrupt 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 `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/CountDownLatch.html#countDown()` method; or
- Some other thread {@linkplain Thread#interrupt interrupts} the current thread.

If the current thread:

- has its interrupted status set on entry to this method; or
- is {@linkplain Thread#interrupt interrupted} while waiting,

then `https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/InterruptedException.html` is thrown and the current thread's interrupted status is cleared.

<br />

##### Throws

| [InterruptedException](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/InterruptedException.html) | if the current thread is interrupted while waiting |
|---|---|

#### public boolean
**await**
(long timeout, [TimeUnit](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/TimeUnit.html) unit)

Causes the current thread to wait until the latch has counted down to
zero, unless the thread is {@linkplain Thread#interrupt 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 `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/CountDownLatch.html#countDown()` method; or
- Some other thread {@linkplain Thread#interrupt 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 {@linkplain Thread#interrupt interrupted} while waiting,

then `https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/InterruptedException.html` 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 wait |
| unit | the time unit of the `timeout` argument |
|---|---|

##### Returns

- `true` if the count reached zero and `false` if the waiting time elapsed before the count reached zero

##### Throws

| [InterruptedException](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/InterruptedException.html) | if the current thread is interrupted while waiting |
|---|---|

#### 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.

#### public long
**getCount**
()

Returns the current count.

This method is typically used for debugging and testing purposes.

##### Returns

- the current count

#### public [String](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/String.html)
**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.

##### Returns

- a string identifying this latch, as well as its state