文章介绍4种方法，简单易懂，通过4个demo抛砖引玉。
目录
在子线程中通过join()方法指定顺序
在主线程中通过join()方法指定顺序
通过倒数计时器CountDownLatch实现
通过创建单一化线程池newSingleThreadExecutor()实现
在子线程中通过join()方法指定顺序
通过join()方法使当前线程“阻塞”，等待指定线程执行完毕后继续执行。举例：在线程thread2中，加上一句thread1.join()，其意义在于，当前线程2运行到此行代码时会进入阻塞状态，直到线程thread1执行完毕后，线程thread2才会继续运行，这就保证了线程thread1与线程thread2的运行顺序。
public class ThreadJoinDemo {public static void main(String[] args) throws InterruptedException {final Thread thread1 = new Thread(new Runnable() {@Overridepublic void run() {System.out.println("打开冰箱！");}});final Thread thread2 = new Thread(new Runnable() {@Overridepublic void run() {try {thread1.join();} catch (InterruptedException e) {e.printStackTrace();}System.out.println("拿出一瓶牛奶！");}});final Thread thread3 = new Thread(new Runnable() {@Overridepublic void run() {try {thread2.join();} catch (InterruptedException e) {e.printStackTrace();}System.out.println("关上冰箱！");}});//下面三行代码顺序可随意调整，程序运行结果不受影响，因为我们在子线程中通过“join()方法”已经指定了运行顺序。thread3.start();thread2.start();thread1.start();}}
运行结果：
打开冰箱！拿出一瓶牛奶！关上冰箱！
在主线程中通过join()方法指定顺序
简单说一下子线程与主线程的区别，子线程指的是发生在Thread内部的代码，主线程指的是发生在main函数中的代码，我们可以在main函数中通过join()方法让主线程阻塞等待以达到指定顺序执行的目的。
public class ThreadMainJoinDemo {public static void main(String[] args) throws InterruptedException {final Thread thread1 = new Thread(new Runnable() {@Overridepublic void run() {System.out.println("打开冰箱！");}});final Thread thread2 = new Thread(new Runnable() {@Overridepublic void run() {System.out.println("拿出一瓶牛奶！");}});final Thread thread3 = new Thread(new Runnable() {@Overridepublic void run() {System.out.println("关上冰箱！");}});thread1.start();thread1.join();thread2.start();thread2.join();thread3.start();}}
输出结果：
打开冰箱！拿出一瓶牛奶！关上冰箱！
通过倒数计时器CountDownLatch实现
CountDownLatch通过计数器提供了更灵活的控制，只要检测到计数器为0当前线程就可以往下执行而不用管相应的thread是否执行完毕。
public class ThreadCountDownLatchDemo {private static CountDownLatch countDownLatch1 = new CountDownLatch(1);private static CountDownLatch countDownLatch2 = new CountDownLatch(1);public static void main(String[] args) {final Thread thread1 = new Thread(new Runnable() {@Overridepublic void run() {System.out.println("打开冰箱！");countDownLatch1.countDown();}});final Thread thread2 = new Thread(new Runnable() {@Overridepublic void run() {try {countDownLatch1.await();System.out.println("拿出一瓶牛奶！");countDownLatch2.countDown();} catch (InterruptedException e) {e.printStackTrace();}}});final Thread thread3 = new Thread(new Runnable() {@Overridepublic void run() {try {countDownLatch2.await();System.out.println("关上冰箱！");} catch (InterruptedException e) {e.printStackTrace();}}});//下面三行代码顺序可随意调整，程序运行结果不受影响thread3.start();thread1.start();thread2.start();}}
输出结果：
打开冰箱！拿出一瓶牛奶！关上冰箱！
通过创建单一化线程池newSingleThreadExecutor()实现
单线程化线程池(newSingleThreadExecutor)的优点，串行执行所有任务。
public class ThreadPoolDemo {static ExecutorService executorService = Executors.newSingleThreadExecutor();public static void main(String[] args) {final Thread thread1 = new Thread(new Runnable() {@Overridepublic void run() {System.out.println("打开冰箱！");}});final Thread thread2 =new Thread(new Runnable() {@Overridepublic void run() {System.out.println("拿出一瓶牛奶！");}});final Thread thread3 = new Thread(new Runnable() {@Overridepublic void run() {System.out.println("关上冰箱！");}});executorService.submit(thread1);executorService.submit(thread2);executorService.submit(thread3);executorService.shutdown(); //使用完毕记得关闭线程池}}
输出结果：
打开冰箱！拿出一瓶牛奶！关上冰箱！
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