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Java_闭锁_CountDownLatch应用
闭锁是一种同步工具类，可以延迟线程的进度直到其到达终止状态。闭锁的作用相当于一扇门：在闭锁到达结束状态之前，这扇门一直是关闭的，并且没有任何线程能通过，当到达结束状态时，这扇门会打开并允许所有的线程通过。当闭锁到达结束状态后，将不会在改变状态，因此这扇门将永远保持打开状态。闭锁可以用来确定某些活动直到其他活动都完成后才继续执行，例如：
确保某个计算在其需要的所有资源都被初始化之后才继续执行。二元闭锁（包括两个状态）可以用来表示“资源R已经被初始化”，而所有需要R的操作都必须在这个闭锁上等待
确保某个服务在其依赖的所有其他服务都已经启动之后才启动。每个服务都有一个相关的二元闭锁。当启动服务S时，将首先在S 依赖的其他服务的闭锁上 等待，在所有依赖的服务都启动后会释放闭锁S，这样其他依赖S 的服务才能继续执行。
等待直到某个操作的所有参与者都就绪再继续执行。在这种情况下，当所有玩家都准备就绪时，闭锁将到达结束状态。
CountDownLatch是一种灵活的闭锁实现，可以在上述情况中使用，他可以使一个或多个线程等待一组时间发生。闭锁状态包括一个计数器，该计数器初始化为一个正数，表示需要等待的事件数量。countDown方法递减计数器，表示有一个事件发生了，而await方法等待计数器达到零，这表示所有需要等待的事件都已经发生。如果计数器的值非零，那么await会一直阻塞直到计数器为零，或者等待中的线程中断，或者等待超时。
Causes the current thread to wait until the latch has counted down to zero
/**
* Causes the current thread to wait until the latch has counted down to
* zero, unless the thread is {@linkplain Thread#interrupt 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
* {@link #countDown} method; or
* <li>Some other thread {@linkplain Thread#interrupt 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 {@linkplain Thread#interrupt interrupted} while waiting,
* </ul>
* then {@link InterruptedException} is thrown and the current thread‘s
* interrupted status is cleared.
*
* @throws InterruptedException if the current thread is interrupted
* while waiting
*/
public void await() throws InterruptedException {
sync.acquireSharedInterruptibly(1);
}
Decrements the count of the latch, releasing all waiting threads if the count reaches zero
/**
* 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.
*/
public void countDown() {
sync.releaseShared(1);
}
在下面这个示例中，它使用两个闭锁，分别表示起始门，和 结束门。
起始门 计数器的初始值为 1，而结束门计数器 的初始值为工作线程的数量。每个工作线程首先要做的就是在启动门上等待，从而确保所有的线程都就绪后才开始执行。而每个线程要做的最后一件事就是将调用结束门的countDown 方法减一，这能使主线程高效的等待直到所有的工作线程都执行完成，因此可以统计所消耗的时间：
package com.lyx;
import java.util.concurrent.CountDownLatch;
public class TestHarness {
public static void main(String[] args) throws InterruptedException {
// TODO Auto-generated method stub
Runnable task = new Runnable() {
public void run() {
System.out.println("hello latch............");
}
};
System.out.println(timeTashs(100, task));
}
public static long timeTashs(int nThreads, final Runnable task)
throws InterruptedException {
final CountDownLatch startGate = new CountDownLatch(1);
final CountDownLatch endGate = new CountDownLatch(nThreads);
for (int i = 0; i < nThreads; i++) {
Thread t = new Thread() {
@Override
public void run() {
try {
startGate.await();
try {
task.run();
} finally {
endGate.countDown();
}
} catch (InterruptedException e) {
// TODO: handle exception
}
}
};
t.start();
}
long start = System.nanoTime();
startGate.countDown();
endGate.await();
long end = System.nanoTime();
return end - start;
}
}
下面这个示例来自http://lbxc.iteye.com/blog/1717482
public long runSample(int n, final Runnable runSport) throws InterruptedException {
final CountDownLatch gun = new CountDownLatch(1);
final CountDownLatch end = new CountDownLatch(n);
for (int i = 0; i < n; i++){
Thread t = new Thread() {
public void run(){
try{
gun.await();//等待开枪
try{
runSport.run();
}finally{
end.countDown();
}
}catch( InterruptedException e){
Thread.currentThread().interrupt();
}
}
};
t.start();
}
long startTime = System.nanoTime();
gun.countDown();//开枪
end.await();
long endTime = System.nanoTime();
return endTime - startTime;
}
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标签：style blog http io color ar os 使用 sp
原文地址：http://my.oschina.net/xinxingegeya/blog/345969
