多个线程顺序执行

日常总结都在github，大家可以看一下： https://github.com/nocoders/java-everything.git,本文示例代码在src.main.test.com.javaEverything.java.concurrent目录下

今天面试遇到这个问题,A、B、C三个线程，如何让他们按照顺序执行。当时没想起来怎么说，后来百度了下，发现实现有好几种方式，可以使用Thread.join()、使用ReentrantLock、使用Semaphore。

Thread.join()

Thread.join()：阻塞当前线程，直到执行线程执行结束。

定义三个线程，A线程直接start，B线程要等A线程执行结束才能执行，C线程要等A B线程执行结束才能执行。

示例代码

@Test
public void threadSortJoin() {Thread A = new Thread(() -> {System.out.println("Thread A run");});Thread B = new Thread(() -> {try {A.join();} catch (InterruptedException e) {e.printStackTrace();}System.out.println("Thread B run");});Thread C = new Thread(() -> {try {A.join();B.join();} catch (InterruptedException e) {e.printStackTrace();}System.out.println("Thread C run");});A.start();B.start();C.start();
}

Semaphore

Semaphore相关见参考链接2,给线程B、C定义两个信号量为0的Semaphore，A线程中执行完毕释放B线程对应的信号量，B线程执行完毕释放C线程的信号量。

示例代码

@Test
public void threadSortSemaphore() {Semaphore sb = new Semaphore(0);Semaphore sc = new Semaphore(0);Thread A = new Thread(()->{System.out.println("Thread A run");sb.release();});Thread B = new Thread(()->{try {sb.acquire();} catch (InterruptedException e) {e.printStackTrace();}System.out.println("Thread B run");sc.release();});Thread C = new Thread(() -> {try {sc.acquire();} catch (InterruptedException e) {e.printStackTrace();}System.out.println("Thread C run");});A.start();B.start();C.start();
}

ReentrantLock

定义一个number指定线程执行顺序，定义三个condition，number等于几，唤醒第几个线程执行。

@Testpublic void threadSortReentrantLock() {ReentrantLock lock = new ReentrantLock();Condition conditionA = lock.newCondition();Condition conditionB = lock.newCondition();Condition conditionC = lock.newCondition();AtomicInteger number = new AtomicInteger(1);Thread A = new Thread(() -> {System.out.println(1);lock.lock();try {while (number.get() != 1) {conditionA.await();}System.out.println("Thread A run");number.set(2);conditionB.signal();} catch (Exception e) {} finally {lock.unlock();}});Thread B = new Thread(() -> {System.out.println(2);lock.lock();try {while (number.get() != 2) {conditionB.await();}System.out.println("Thread B run");number.set(3);conditionC.signal();} catch (Exception e) {} finally {lock.unlock();}});Thread C = new Thread(() -> {System.out.println(3);while (number.get()!=2){}lock.lock();try {while (number.get() != 3) {conditionC.await();}System.out.println("Thread C run");number.set(1);conditionA.signal();} catch (Exception e) {}finally {lock.unlock();}});A.start();B.start();C.start();}

参考链接

1. 多线程中Thread的join方法
2. Semaphore 使用及原理
3. JAVA实现多个线程顺序执行的几种方式
4. [让线程按顺序执行8种方法](