CountDownLatch是一個線程併發流程工具,它允許一個或者多個線程阻塞直到一些線程執行完畢,纔會繼續執行,CountDownLatch會存在一個計數器,當 await方法調用,那麼後面的線程都會被阻塞,直到由於countDown()方法的調用而導致當前計數達到零,後續被阻塞的線程纔會繼續執行,這個計數器是一次性的,不能重置,如果需要重置計數,請考慮使用CyclicBarrier
CountDownLatch內部是通過AQS實現併發流程控制,在的內部有一個AQS類
/**
* 用於控制 CountDownLatch.
* 用AQS的狀態代替計數器.
*/
private static final class Sync extends AbstractQueuedSynchronizer {
private static final long serialVersionUID = 4982264981922014374L;
//傳入計數值,將其設置爲AQS狀態, setState(count)是父類的一個方法,狀態變化整體是可見的
Sync(int count) {
setState(count);
}
//獲得當前的狀態。同樣調的是父類的方法
int getCount() {
return getState();
}
protected int tryAcquireShared(int acquires) {
return (getState() == 0) ? 1 : -1;
}
protected boolean tryReleaseShared(int releases) {
// 自旋的方式,判斷狀態如果爲0直接執行被阻塞的線程
for (;;) {
int c = getState();
if (c == 0)
return false;
int nextc = c-1;
//利用CAS更新狀態
if (compareAndSetState(c, nextc))
return nextc == 0;
}
}
}
//父類方法用於設置狀態
/**
* 設置AQS狀態
*
*/
protected final void setState(int newState) {
// private volatile int state;,保證可見行
state = newState;
}
//用於阻塞後續所有的線程
public void await() throws InterruptedException {
sync.acquireSharedInterruptibly(1);
}
//阻塞後續線程,直到計數器爲0或者超時
public boolean await(long timeout, TimeUnit unit)
throws InterruptedException {
return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout));
}
/**
*
* 計數器減一,當計數器爲0的時候後續阻塞線程會繼續執行
*
* 如果計數器已經爲0,調用此方法不會做任何事
*/
public void countDown() {
//利用AQS的狀態量去控制
sync.releaseShared(1);
}
/**
* 返回計數器
*/
public long getCount() {
return sync.getCount();
}
構造方法
/**
* 初始化一個對象,並設置計數器數量和AQS的狀態量
* 如果傳入一個負數回報參數異常
*/
public CountDownLatch(int count) {
if (count < 0) throw new IllegalArgumentException("count < 0");
this.sync = new Sync(count);
}
案例
/**
* 描述: 模擬一個連隊,所有人都到齊了才統一出發
*/
public class CountDownLatchDemo1 {
public static void main(String[] args) throws InterruptedException {
CountDownLatch latch = new CountDownLatch(12);
ExecutorService service = Executors.newFixedThreadPool(12);
for (int i = 0; i < 12; i++) {
final int no = i + 1;
Runnable runnable = new Runnable() {
@Override
public void run() {
try {
Thread.sleep((long) (Math.random() * 10000));
System.out.println("No." + no + "達到連隊。");
} catch (InterruptedException e) {
e.printStackTrace();
} finally {
latch.countDown();
}
}
};
service.submit(runnable);
}
System.out.println("所有人都到齊了.....");
latch.await();
System.out.println("所有人都到齊了,出發!!!!!。");
}
}
/**
* 描述: 模擬100米跑步,5名選手都準備好了,只等裁判員一聲令下,所有人同時開始跑步。當所有人都到終點後,比賽結束。
*/
public class CountDownLatchDemo1And2 {
public static void main(String[] args) throws InterruptedException {
CountDownLatch begin = new CountDownLatch(1);
CountDownLatch end = new CountDownLatch(5);
ExecutorService service = Executors.newFixedThreadPool(5);
for (int i = 0; i < 5; i++) {
final int no = i + 1;
Runnable runnable = new Runnable() {
@Override
public void run() {
System.out.println("No." + no + "準備完畢,等待發令槍");
try {
begin.await();
System.out.println("No." + no + "開始跑步了");
Thread.sleep((long) (Math.random() * 10000));
System.out.println("No." + no + "跑到終點了");
} catch (InterruptedException e) {
e.printStackTrace();
} finally {
end.countDown();
}
}
};
service.submit(runnable);
}
//裁判員檢查發令槍...
Thread.sleep(5000);
System.out.println("發令槍響,比賽開始!");
begin.countDown();
end.await();
System.out.println("所有人到達終點,比賽結束");
}
}