java.util.concurrent

Класс CountDownLatch

java.lang. Объект

java.util.concurrent. CountDownLatch

public class CountDownLatch
extends Object

Средство синхронизации, которое позволяет одному или более потокам ожидать до ряда операций, выполняемых в других потоках, завершается.

A CountDownLatch инициализируется с данным количеством. await блок методов до текущего количества достигает нуля из-за вызовов countDown() метод, после которого все потоки ожидания выпускаются и любые последующие вызовы await возвратитесь сразу. Это - явление с одним выстрелом - количество не может быть сброшено. Если Вы нуждаетесь в версии, которая сбрасывает количество, рассмотрите использование a CyclicBarrier.

A CountDownLatch универсальный инструмент синхронизации и может использоваться во многих целях. A CountDownLatch инициализированный с количеством каждый служит, поскольку простое вкл\выкл фиксируется, или логический элемент: весь вызов потоков await ожидайте в логическом элементе, пока он не открывается вызовом потока countDown(). A CountDownLatch инициализированный к N может использоваться, чтобы заставить один поток ожидать, пока N потоки не завершили некоторое действие, или некоторое действие было завершено времена N.

Полезное свойство a CountDownLatch это, это не требует что вызов потоков countDown ожидайте количества, чтобы достигнуть нуля перед продолжением, это просто препятствует тому, чтобы любой поток продолжился мимо await пока все потоки не могли передать.

Демонстрационное использование: Вот пара классов, в которых группа рабочих потоков используют два фиксатора обратного отсчета:

• Первым является сигнал запуска, который препятствует тому, чтобы любой рабочий продолжился, пока драйвер не готов к ним продолжиться;
• Вторым является сигнал завершения, который позволяет драйверу ожидать, пока все рабочие не завершились.

 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: Until the count reaches zero, actions in a thread prior to calling countDown() happen-before actions following a successful return from a corresponding await() in another thread.

Since:

1.5

Constructor Summary

Constructors

Constructor and Description
CountDownLatch(int count) Constructs a CountDownLatch initialized with the given count.

Method Summary

Methods

Modifier and Type	Method and 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, 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.
Строка	toString() Returns a string identifying this latch, as well as its state.

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, wait

Constructor Detail

CountDownLatch

public CountDownLatch(int count)

Constructs a CountDownLatch initialized with the given count.

Parameters:

count - the number of times countDown() must be invoked before threads can pass through await()

Throws:

IllegalArgumentException - if count is negative

Method Detail

await

public void await()
           throws 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,

then InterruptedException is thrown and the current thread's interrupted status is cleared.

Throws:

InterruptedException - if the current thread is interrupted while waiting

await

public boolean await(long timeout,
            TimeUnit unit)
              throws 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,

then 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 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 - if the current thread is interrupted while waiting

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.

getCount

public long getCount()

Returns the current count.

This method is typically used for debugging and testing purposes.

Returns:

the current count

toString

public 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 class Object

Returns:

a string identifying this latch, as well as its state