Java Thread中， join() 方法是让调用该方法的主线程执行run()时暂时卡住，等run()执行完成后， 主线程再调用执行join()后面的代码。示例：

class ThreadTesterA implements Runnable {

private int counter;

@Override

public void run() {

while (counter <= 10) {

System.out.print("Counter = " + counter + " ");

counter++;

}

System.out.println();

}

}

class ThreadTesterB implements Runnable {

private int i;

@Override

public void run() {

while (i <= 10) {

System.out.print("i = " + i + " ");

i++;

}

System.out.println();

}

}

public class ThreadTester {

public static void main(String[] args) throws InterruptedException {

Thread t1 = new Thread(new ThreadTesterA());

Thread t2 = new Thread(new ThreadTesterB());

t1.start();

t1.join(); // wait t1 to be finished

t2.start();

t2.join(); // in this program, this may be removed

}

}

如果去掉t1.join()这个语句过后， 输出的顺序是乱的， 加上这个语句后， 就会按照顺序输出，  从某种意义上说实现了同步。