能想起來幾種呢？

先說下要求，就是三個線程，假設(shè)是線程 1，2，3， 現(xiàn)在的要求是：必須是線程 1 先執(zhí)行，然后線程 2 再執(zhí)行，最后是線程 3 執(zhí)行

然后有幾種實現(xiàn)方法呢？

其實它的本質(zhì)就是實現(xiàn)，讓線程 2，3 等待線程 1 執(zhí)行完畢，所以重點就是有哪些方法可以讓線程 2，3 等待

join

第一反應(yīng)應(yīng)該就是使用 join 方法，因為 join 本來就是支持這種機(jī)制的

比如，我在線程 B 中調(diào)用了線程 A 的 join 方法，那么線程 B 就會等線程 A 執(zhí)行結(jié)束之后再執(zhí)行

那么具體應(yīng)該怎么使用嘞？

別慌嘛，我這里有例子，你瞅瞅：

public?class?ThreadLoopOne?{
????public?static?void?main(String[]?args)?{
????????Thread?t1?=?new?Thread(new?Work(null));
????????Thread?t2?=?new?Thread(new?Work(t1));
????????Thread?t3?=?new?Thread(new?Work(t2));

????????t1.start();
????????t2.start();
????????t3.start();
????}

????static?class?Work?implements?Runnable?{
????????private?Thread?beforeThread;
????????public?Work(Thread?beforeThread){
????????????this.beforeThread?=?beforeThread;
????????}

????????@Override
????????public?void?run()?{
????????????//?如果有線程,就?join?進(jìn)來,沒有的話就直接輸出
????????????if?(beforeThread?!=?null?){
????????????????try?{
????????????????????beforeThread.join();
????????????????????System.out.println("thread?start?:?"?+?Thread.currentThread().getName());
????????????????}?catch?(InterruptedException?e)?{
????????????????????e.printStackTrace();
????????????????}
????????????}else{
????????????????System.out.println("thread?start?:?"?+?Thread.currentThread().getName());
????????????}
????????}
????}
}

CountDownLatch

剛才說了，本質(zhì)就是讓線程 B，C 等待線程 A 執(zhí)行完畢

那么信號量就是一個不錯的選擇

如果想要實現(xiàn)的話，那大概就是下面這樣：

public?class?ThreadLoopTwo?{
????public?static?void?main(String[]?args)?{
????????//?設(shè)置線程?1?的信號量為?0
????????CountDownLatch?cOne?=?new?CountDownLatch(0);
????????//?設(shè)置線程?2?的信號量為?1
????????CountDownLatch?cTwo?=?new?CountDownLatch(1);
????????//?設(shè)置線程?3?的信號量為?1
????????CountDownLatch?cThree?=?new?CountDownLatch(1);

????????//?因為?cOne?為?0?,故?t1?可以直接執(zhí)行
????????Thread?t1?=?new?Thread(new?Work(cOne,cTwo));
????????//?線程?t1?執(zhí)行完畢之后,此時的?cTwo?為?0?,?t2?開始執(zhí)行
????????Thread?t2?=?new?Thread(new?Work(cTwo,cThree));
????????//?線程?t2?執(zhí)行完畢,此時?cThree?為?0?,?t3?開始執(zhí)行
????????Thread?t3?=?new?Thread(new?Work(cThree,cThree));

????????t1.start();
????????t2.start();
????????t3.start();
????}

????static?class?Work?implements?Runnable{
????????CountDownLatch?cOne;
????????CountDownLatch?cTwo;

????????public?Work(CountDownLatch?cOne,?CountDownLatch?cTwo){
????????????super();
????????????this.cOne?=?cOne;
????????????this.cTwo?=?cTwo;
????????}
????????@Override
????????public?void?run()?{
????????????try?{
????????????????//?當(dāng)前一個線程信號量為?0?時,才執(zhí)行
????????????????cOne.await();
????????????????System.out.println("thread?start?:?"?+?Thread.currentThread().getName());
????????????????//?后一個線程信號量減?1
????????????????cTwo.countDown();
????????????}?catch?(InterruptedException?e)?{
????????????????e.printStackTrace();
????????????}
????????}
????}
}

使用單個線程池

之所以線程 1，2，3 的執(zhí)行順序無法保證，是因為在編譯器可能會去做一些優(yōu)化，導(dǎo)致沒有辦法按照順序執(zhí)行

如果我們使用單個線程池去執(zhí)行的話，那就沒有這樣的問題了

具體實現(xiàn)：

public?class?ThreadLoopThree?{
????public?static?void?main(String[]?args)?{
????????Thread?t1?=?new?Thread(new?Runnable()?{
????????????@Override
????????????public?void?run()?{
????????????????System.out.println("thread?start?:?"?+?Thread.currentThread().getName()?+?"?run?one");
????????????}
????????});

????????Thread?t2?=?new?Thread(new?Runnable()?{
????????????@Override
????????????public?void?run()?{
????????????????System.out.println("thread?start?:?"?+?Thread.currentThread().getName()?+?"?run?two");
????????????}
????????});

????????Thread?t3?=?new?Thread(new?Runnable()?{
????????????@Override
????????????public?void?run()?{
????????????????System.out.println("thread?start?:?"?+?Thread.currentThread().getName()?+?"?run?three");
????????????}
????????});

????????ExecutorService?executor?=?Executors.newSingleThreadExecutor();
????????//?將線程依次加入到線程池中
????????executor.submit(t1);
????????executor.submit(t2);
????????executor.submit(t3);
????????//?及時將線程池關(guān)閉
????????executor.shutdown();
????}
}

CompletableFuture

如果使用 CompletableFuture 來實現(xiàn)的話，代碼就非常簡潔了

public?class?ThreadLoopFour?{
????public?static?void?main(String[]?args)??{
????????Thread?t1?=?new?Thread(new?Work());
????????Thread?t2?=?new?Thread(new?Work());
????????Thread?t3?=?new?Thread(new?Work());

????????CompletableFuture.runAsync(()->?t1.start())
????????????????.thenRun(()->t2.start())
????????????????.thenRun(()->t3.start());
????}

????static?class?Work?implements?Runnable{
????????@Override
????????public?void?run()?{
????????????System.out.println("thread?start?:?"?+?Thread.currentThread().getName());
????????}
????}
}