Java Thread join()的使用场景和原理详解

Waiting for the finalization of a thread

In some situations, we will have to wait for the finalization of a thread. For example, we may have a program that will begin initializing the resources it needs before proceeding with the rest of the execution. 
We can run the initialization tasks as threads and wait for its finalization before continuing with the rest of the program. 
For this purpose, we can use the join() method of the Thread class. When we call this method using a thread object, it suspends the execution of the calling thread until the object called finishes its execution.

public class JoinDemo {

    public static void main(String[] args) throws InterruptedException {
        //获取当前线程信息
       Thread previousThread= Thread.currentThread();
        for(int i=0;i<10;i++){
            Thread thread=new Thread(new Domino(previousThread));
            thread.start();
            previousThread=thread;
        }
        TimeUnit.SECONDS.sleep(5);
        System.out.println("Thread.currentThread().getName()+\" terminate.\" = " + Thread.currentThread().getName()+" terminate.");

    }

    static class Domino implements Runnable{
        private Thread thread;
        public Domino(Thread thread){
            this.thread=thread;
        }
        @Override
        public void run() {
            try {
                thread.join();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println(Thread.currentThread().getName() + " terminate.");
        }
    }
}

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 void join() throws InterruptedException;

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

package com.dxz.join;
class BThread extends Thread {
    public BThread() {
        super("[BThread] Thread");
    };
    public void run() {
        String threadName = Thread.currentThread().getName();
        System.out.println(threadName + " start.");
        try {
            for (int i = 0; i < 5; i++) {
                System.out.println(threadName + " loop at " + i);
                Thread.sleep(1000);
            }
            System.out.println(threadName + " end.");
        } catch (Exception e) {
            System.out.println("Exception from " + threadName + ".run");
        }
    }
}

public class TestDemo {
    public static void main(String[] args) {
        String threadName = Thread.currentThread().getName();
        System.out.println(threadName + " start.");
        BThread bt = new BThread();
        try {
            bt.join();
            bt.start();
            Thread.sleep(2000);
        } catch (Exception e) {
            System.out.println("Exception from main");
        }
        System.out.println(threadName + " end!");
    }
}