java線程中鎖存器CountDownLatch的使用
來自： http://blog.csdn.net//chenleixing/article/details/42173593
CountDownLatch類是一個同步計數器,構造時傳入int參數,該參數就是計數器的初始值，每調用一次countDown()方法，計數器減1,計數器大于0 時，await()方法會阻塞程序繼續執行。CountDownLatch可以看作是一個倒計數的鎖存器，當計數減至0時觸發特定的事件。利用這種特性，可以讓主線程等待子線程的結束。下面以一個模擬運動員比賽的例子加以說明。
CountDownLatch的一個非常典型的應用場景是：有一個任務想要往下執行，但必須要等到其他的任務執行完畢后才可以繼續往下執行。假如我們這個想要繼續往下執行的任務調用一個CountDownLatch對象的await()方法，其他的任務執行完自己的任務后調用同一個CountDownLatch對象上的countDown()方法，這個調用await()方法的任務將一直阻塞等待，直到這個CountDownLatch對象的計數值減到0為止。
2 import java.util.concurrent.Executor;
3 import java.util.concurrent.ExecutorService;
4 import java.util.concurrent.Executors;
5
6 public class CountDownLatchDemo {
7 private static final int PLAYER_AMOUNT = 5 ;
8 public CountDownLatchDemo() {
9 // TODO Auto-generated constructor stub
10 }
11 /**
12 * @param args
13 */
14 public static void main(String[] args) {
15 // TODO Auto-generated method stub
16 // 對于每位運動員，CountDownLatch減1后即結束比賽
17 CountDownLatch begin = new CountDownLatch( 1 );
18 // 對于整個比賽，所有運動員結束后才算結束
19 CountDownLatch end = new CountDownLatch(PLAYER_AMOUNT);
20 Player[] plays = new Player[PLAYER_AMOUNT];
21
22 for ( int i = 0 ;i < PLAYER_AMOUNT;i ++ )
23 plays[i] = new Player(i + 1 ,begin,end);
24
25 // 設置特定的線程池，大小為5
26 ExecutorService exe = Executors.newFixedThreadPool(PLAYER_AMOUNT);
27 for (Player p:plays)
28 exe.execute(p); // 分配線程
29 System.out.println( " Race begins! " );
30 begin.countDown();
31 try {
32 end.wait(); // 等待end狀態變為0，即為比賽結束
33 } catch (InterruptedException e) {
34 // TODO: handle exception
35 e.printStackTrace();
36 } finally {
37 System.out.println( " Race ends! " );
38 }
39 exe.shutdown();
40 }
41 }
接下來是Player類
注：countDown最好是在finally里面調用
2
3
4 public class Player implements Runnable {
5
6 private int id;
7 private CountDownLatch begin;
8 private CountDownLatch end;
9 public Player( int i, CountDownLatch begin, CountDownLatch end) {
10 // TODO Auto-generated constructor stub
11 super ();
12 this .id = i;
13 this .begin = begin;
14 this .end = end;
15 }
16
17 @Override
18 public void run() {
19 // TODO Auto-generated method stub
20 try {
21 begin.await(); // 等待begin的狀態為0
22 Thread.sleep(( long )(Math.random() * 100 )); // 隨機分配時間，即運動員完成時間
23 System.out.println( " Play " + id + " arrived. " );
24 } catch (InterruptedException e) {
25 // TODO: handle exception
26 e.printStackTrace();
27 } finally {
28 end.countDown(); // 使end狀態減1，最終減至0
29 }
30 }
31 }
下面是張孝祥老師所舉例子的程序代碼和運行結果：
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
public class CountDownLatchTest {
public static void main(String [] args){
ExecutorService service = Executors.newCachedThreadPool();
final CountDownLatch cdOrder = new CountDownLatch(1);
final CountDownLatch cdAnswer = new CountDownLatch(3);
for(int i=0;i<3;i++){
Runnable runnable=new Runnable(){
public void run(){
try {
System.out.println("線程"+Thread.currentThread().getName()+"正準備接受命令");
cdOrder.await();
System.out.println("線程"+Thread.currentThread().getName()+"已接受命令");
Thread.sleep((long)(Math.random()*10000));
System.out.println("線程"+Thread.currentThread().getName()+"回應命令處理結果");
cdAnswer.countDown();
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
};
service.execute(runnable);
}
try {
System.out.println("線程"+Thread.currentThread().getName()+"即將發布命令");
cdOrder.countDown();
System.out.println("線程"+Thread.currentThread().getName()+"已接受命令，正在等待結果");
cdAnswer.await();
Thread.sleep((long)(Math.random()*10000));
System.out.println("線程"+Thread.currentThread().getName()+"已收到所有響應結果");
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
service.shutdown();
}
}
運行結果：
線程pool-1-thread-1正準備接受命令
線程pool-1-thread-2正準備接受命令
線程pool-1-thread-3正準備接受命令
線程main即將發布命令
線程main已接受命令，正在等待結果
線程pool-1-thread-2已接受命令
線程pool-1-thread-1已接受命令
線程pool-1-thread-3已接受命令
線程pool-1-thread-2回應命令處理結果
線程pool-1-thread-1回應命令處理結果
線程pool-1-thread-3回應命令處理結果
線程main已收到所有響應結果