最近被问到如何让多线程按顺序执行，想到了join，但是不知道join内部具体的实现，翻看源码记录下。

先看个小例子，目标是让线程按照T3,T2,T1的顺序执行

public class T1 implements Runnable {
    private String threadName;
    public T1(String name){
        this.threadName = name;
    }
    public void run() {
        Thread t2 = new Thread(new T2("Thread2"));
        t2.start();
        try {
            t2.join();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println("T1 is running:"+threadName);
    }
}

public class T2 implements Runnable{
    private String threadName;
    public T2(String name){
        this.threadName = name;
    }
    public void run() {
        Thread t3 = new Thread(new T3("Thread3"));
        t3.start();
        try {
            t3.join();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println("T2 is runing:"+threadName);
    }
}

public class T3 implements Runnable {
    private String threadName;
    public T3(String name){
        this.threadName = name;
    }
    public void run() {

        System.out.println("T3 is running:"+threadName);
    }
}

输出结果：T1调用T2的join后，T2子线程正常运行，T2调用T3的join后，T3子线程正常运行，T2等待T3运行结束后再运行，T1等待T2运行结束后再运行

join作用：让主线程等待子线程执行完成后再执行，源码如下：

 public final void join() throws InterruptedException {
        join(0);
    }

public final synchronized void join(long millis)
    throws InterruptedException {
        long base = System.currentTimeMillis();
        long now = 0;

        if (millis < 0) {
            throw new IllegalArgumentException("timeout value is negative");
        }

        if (millis == 0) {
            while (isAlive()) {
                wait(0);
            }
        } else {
            while (isAlive()) {
                long delay = millis - now;
                if (delay <= 0) {
                    break;
                }
                wait(delay);
                now = System.currentTimeMillis() - base;
            }
        }
    }

 public final synchronized void join(long millis, int nanos)
    throws InterruptedException {

        if (millis < 0) {
            throw new IllegalArgumentException("timeout value is negative");
        }

        if (nanos < 0 || nanos > 999999) {
            throw new IllegalArgumentException(
                                "nanosecond timeout value out of range");
        }

        if (nanos >= 500000 || (nanos != 0 && millis == 0)) {
            millis++;
        }

        join(millis);
    }

public final native boolean isAlive();

最终实际调用的是 public final synchronized void join(long millis) throws InterruptedException 方法，下面看下这个方法：以mils参数为0来解释

主线程A中调用子线程B的join（）方法，同时获取到了子线程B实例的锁（其他线程可以进入子线程B的join（）方法，但是无法进入子线程B的join（0）方法），join（0）方法一直在调用native方法isAlive()检测子线程B的状态，如果子线程B是active的状态，调用子线程B的wait方法，此时主线程A释放了锁（wait方法会释放锁），其他线程可以竞争子线程B的实例锁。如果检测到子线程B的状态不是active,主线程A继续运行。