CountDownLatch

CountDownLatch是我目前使用比較多的類，CountDownLatch初始化時(shí)會(huì)給定一個(gè)計(jì)數(shù)，然后每次調(diào)用countDown() 計(jì)數(shù)減1，

當(dāng)計(jì)數(shù)未到達(dá)0之前調(diào)用await() 方法會(huì)阻塞直到計(jì)數(shù)減到0；

使用場(chǎng)景：多用于劃分任務(wù)由多個(gè)線程執(zhí)行，例如：最近寫個(gè)豆瓣爬蟲，需要爬取每個(gè)電影的前五頁短評(píng)，可以劃分成五個(gè)線程來處理數(shù)據(jù)。通過latch.await()保證全部完成再返回。

????public?void?latch()?throws?InterruptedException?{
????????int?count=?5;
????????CountDownLatch?latch?=?new?CountDownLatch(count);
????????for?(int?x=0;x????????????new?Worker(x*20,latch).start();
????????}
????????latch.await();
????????System.out.println("全部執(zhí)行完畢");
????}
????
????class?Worker?extends?Thread{
????????Integer?start;
????????CountDownLatch?latch;
????????public?Worker(Integer?start,CountDownLatch?latch){
????????????this.start=start;
????????????this.latch=latch;
????????}????????@Override
????????public?void?run()?{
????????????System.out.println(start+"?已執(zhí)行");
????????????latch.countDown();
????????}
????}

輸出如下：

20?已執(zhí)行
0?已執(zhí)行
40?已執(zhí)行
60?已執(zhí)行
80?已執(zhí)行
全部執(zhí)行完畢

CyclicBarrier

它允許一組線程互相等待，直到到達(dá)某個(gè)公共屏障點(diǎn) (common barrier point)也就是阻塞在調(diào)用cyclicBarrier.await()的地方。

看上去CyclicBarrier 跟CountDownLatch 功能上類似，在官方doc上CountDownLatch的描述上就說明了，CountDownLatch 的計(jì)數(shù)無法被重置，

如果需要重置計(jì)數(shù)，請(qǐng)考慮使用CyclicBarrier。

CyclicBarrier初始時(shí)還可添加一個(gè)Runnable的參數(shù)， 此Runnable在CyclicBarrier的數(shù)目達(dá)到后，所有其它線程被喚醒前被最后一個(gè)進(jìn)入 CyclicBarrier 的線程執(zhí)行

使用場(chǎng)景：類似CyclicBarrier，但是 CyclicBarrier提供了幾個(gè)countdownlatch 沒有的方法以應(yīng)付更復(fù)雜的場(chǎng)景，例如：

getNumberWaiting() 獲取阻塞線程數(shù)量，

isBroken() 用來知道阻塞的線程是否被中斷等方法。

reset() 將屏障重置為其初始狀態(tài)。如果所有參與者目前都在屏障處等待，則它們將返回，同時(shí)拋出一個(gè) BrokenBarrierException。

????public?void?latch()?throws?InterruptedException?{
????????int?count?=?5;
????????CyclicBarrier?cb?=?new?CyclicBarrier(count,?new?Runnable()?{
????????????@Override
????????????public?void?run()?{
????????????????System.out.println("全部執(zhí)行完畢");
????????????}
????????});
????????ExecutorService?executorService?=?Executors.newFixedThreadPool(count);
????????while?(true){
????????????for?(int?x=0;x????????????????executorService.execute(new?Worker(x,cb));
????????????}
????????}
????}????
????
????class?Worker?extends?Thread?{
????????Integer?start;
????????CyclicBarrier?cyclicBarrier;????????public?Worker(Integer?start,?CyclicBarrier?cyclicBarrier)?{
????????????this.start?=?start;
????????????this.cyclicBarrier?=?cyclicBarrier;
????????}????????@Override
????????public?void?run()?{
????????????System.out.println(start?+?"?已執(zhí)行");
????????????try?{
????????????????cyclicBarrier.await();
????????????}?catch?(InterruptedException?e)?{
????????????????e.printStackTrace();
????????????}?catch?(BrokenBarrierException?e)?{
????????????????e.printStackTrace();
????????????}
????????}
????}

輸出如下：

0?已執(zhí)行
3?已執(zhí)行
4?已執(zhí)行
2?已執(zhí)行
1?已執(zhí)行
全部執(zhí)行完畢
0?已執(zhí)行
1?已執(zhí)行
2?已執(zhí)行
3?已執(zhí)行
4?已執(zhí)行
全部執(zhí)行完畢

Semaphore

Semaphore 信號(hào)量維護(hù)了一個(gè)許可集,每次使用時(shí)執(zhí)行acquire()從Semaphore獲取許可，如果沒有則會(huì)阻塞，每次使用完執(zhí)行release()釋放許可。

使用場(chǎng)景：Semaphore對(duì)用于對(duì)資源的控制，比如數(shù)據(jù)連接有限，使用Semaphore限制訪問數(shù)據(jù)庫的線程數(shù)。

????public?void?latch()?throws?InterruptedException,?IOException?{
????????int?count?=?5;
????????Semaphore?semaphore?=?new?Semaphore(1);
????????ExecutorService?executorService?=?Executors.newFixedThreadPool(count);
????????????for?(int?x=0;x????????????????executorService.execute(new?Worker(x,semaphore));
????????????}
????????System.in.read();
????}????
????
????class?Worker?extends?Thread?{
????????Integer?start;
????????Semaphore?semaphore;????????public?Worker(Integer?start,?Semaphore?semaphore)?{
????????????this.start?=?start;
????????????this.semaphore?=?semaphore;
????????}????????@Override
????????public?void?run()?throws?IllegalArgumentException?{
????????????try?{
????????????????System.out.println(start?+?"?準(zhǔn)備執(zhí)行");
????????????????TimeUnit.SECONDS.sleep(1);
????????????????semaphore.acquire();
????????????????System.out.println(start?+?"?已經(jīng)執(zhí)行");
????????????????semaphore.release();
????????????????System.out.println(start?+?"?已經(jīng)釋放");
????????????}?catch?(InterruptedException?e)?{
????????????????e.printStackTrace();
????????????}????????}
????}

輸出如下：

0?準(zhǔn)備執(zhí)行
2?準(zhǔn)備執(zhí)行
1?準(zhǔn)備執(zhí)行
3?準(zhǔn)備執(zhí)行
4?準(zhǔn)備執(zhí)行
2?已經(jīng)執(zhí)行
2?已經(jīng)釋放
4?已經(jīng)執(zhí)行
4?已經(jīng)釋放
1?已經(jīng)執(zhí)行
1?已經(jīng)釋放
0?已經(jīng)執(zhí)行
0?已經(jīng)釋放
3?已經(jīng)執(zhí)行
3?已經(jīng)釋放

Exchanger

Exchanger 用于兩個(gè)線程間的數(shù)據(jù)交換，它提供一個(gè)同步點(diǎn)，在這個(gè)同步點(diǎn)兩個(gè)線程可以交換彼此的數(shù)據(jù)。

使用場(chǎng)景：兩個(gè)線程相互等待處理結(jié)果并進(jìn)行數(shù)據(jù)傳遞。

????public?void?latch()?throws?InterruptedException,?IOException?{
????????int?count?=?5;
????????Exchanger?exchanger?=?new?Exchanger<>();
????????ExecutorService?executorService?=?Executors.newFixedThreadPool(count);
????????????for?(int?x=0;x????????????????executorService.execute(new?Worker(x,exchanger));
????????????}
????????System.in.read();
????}????
????
????class?Worker?extends?Thread?{
????????Integer?start;
????????Exchanger??exchanger;????????public?Worker(Integer?start,?Exchanger?exchanger)?{
????????????this.start?=?start;
????????????this.exchanger?=?exchanger;
????????}????????@Override
????????public?void?run()?throws?IllegalArgumentException?{
????????????try?{
????????????????System.out.println(Thread.currentThread().getName()?+?"?準(zhǔn)備執(zhí)行");
????????????????TimeUnit.SECONDS.sleep(start);
????????????????System.out.println(Thread.currentThread().getName()?+?"?等待交換");
????????????????String?value?=?exchanger.exchange(Thread.currentThread().getName());
????????????????System.out.println(Thread.currentThread().getName()?+?"?交換得到數(shù)據(jù)為："+value);
????????????}?catch?(InterruptedException?e)?{
????????????????e.printStackTrace();
????????????}????????}
????}

輸出如下：

pool-1-thread-1?準(zhǔn)備執(zhí)行
pool-1-thread-1?等待交換
pool-1-thread-3?準(zhǔn)備執(zhí)行
pool-1-thread-2?準(zhǔn)備執(zhí)行
pool-1-thread-5?準(zhǔn)備執(zhí)行
pool-1-thread-4?準(zhǔn)備執(zhí)行
pool-1-thread-2?等待交換
pool-1-thread-1 交換得到數(shù)據(jù)為：pool-1-thread-2
pool-1-thread-2 交換得到數(shù)據(jù)為：pool-1-thread-1
pool-1-thread-3?等待交換
pool-1-thread-4?等待交換
pool-1-thread-4 交換得到數(shù)據(jù)為：pool-1-thread-3
pool-1-thread-3 交換得到數(shù)據(jù)為：pool-1-thread-4
pool-1-thread-5?等待交換

Exchanger必須成對(duì)出現(xiàn)，否則會(huì)像上面代碼執(zhí)行結(jié)果那樣，pool-1-thread-5一直阻塞等待與其交換數(shù)據(jù)的線程，為了避免這一現(xiàn)象，可以使用exchange(V x, long timeout, TimeUnit unit)設(shè)置最大等待時(shí)長