java多线程之join

java的Thread.Join，是把指定的线程加入到当前线程，也就是如果调用了某个线程的join方法，则后续线程必须等待该线程执行完成后才能执行。

假设有三个线程，同时执行并输出当前线程的名称，输出后让线程sleep100毫秒，由于是并发执行，我们无法控制其输出的顺序，代码如下：

public class ThreadJoinDemo {class MyThread extends Thread {private String threadName;public MyThread(String threadName) {this.threadName = threadName;}@Overridepublic void run() {System.out.println(threadName);try {Thread.sleep(100);} catch (InterruptedException e) {e.printStackTrace();}}}public static void main(String[] args) {ThreadJoinDemo.MyThread t1 = new ThreadJoinDemo().new MyThread("t1");ThreadJoinDemo.MyThread t2 = new ThreadJoinDemo().new MyThread("t2");ThreadJoinDemo.MyThread t3 = new ThreadJoinDemo().new MyThread("t3");t1.start();t2.start();t3.start();}}

执行结果：
Thread name: t2
Thread name: t1
Thread name: t3

如果在每个线程启动后调用join方法，则后续线程必须等待该线程执行完成后才可以执行，就可以实现顺序输出的要求了。

public class ThreadJoinDemo {class MyThread extends Thread {private String threadName;public MyThread(String threadName) {this.threadName = threadName;}@Overridepublic void run() {System.out.println("Thread name: " + threadName);try {Thread.sleep(100);} catch (InterruptedException e) {e.printStackTrace();}}}public static void main(String[] args) throws InterruptedException {ThreadJoinDemo.MyThread t1 = new ThreadJoinDemo().new MyThread("t1");ThreadJoinDemo.MyThread t2 = new ThreadJoinDemo().new MyThread("t2");ThreadJoinDemo.MyThread t3 = new ThreadJoinDemo().new MyThread("t3");t1.start();t1.join();t2.start();t2.join();t3.start();}}

执行结果：
Thread name: t1
Thread name: t2
Thread name: t3