Java线程join()

join 方法是一个阻塞方法，用来进行线程之间的交流。线程 A 调用 线程 B 的 join 方法，则线程 A 将阻塞，线程 B 执行结束后 线程 A 开始执行。

 was trying to do some analysis on calling start and join simultaneously,

    //Starting and Joining 
    for (Thread thread : threadArray) {
        thread.start();
        thread.join();
    }
compared with start first and then join.

    //Starting Them
    for (Thread thread : threadArray) {
        thread.start();
    }
    //Joining Them
    for (Thread thread : threadArray) {
        thread.join();
    }

第一种方法，实际上是所有threads run sequentially. 因为main thread每一次都要等当前thread结束后才能执行下面的语句；
第二种方法，是可以实现threads run concurrently的.main thread会等待他们全部执行完之后才结束。

另一个例子：

public class ThreadJoinExample {

    public static void main(String[] args) {
        Thread t1 = new Thread(new MyRunnable(), "t1");
        Thread t2 = new Thread(new MyRunnable(), "t2");
        Thread t3 = new Thread(new MyRunnable(), "t3");
        
        t1.start();
        
        //start second thread after waiting for 2 seconds or if it's dead
        try {
            t1.join(2000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        
        t2.start();
        
        //start third thread only when first thread is dead
        try {
            t1.join();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        
        t3.start();
        
        //let all threads finish execution before finishing main thread
        try {
            t1.join();
            t2.join();
            t3.join();
        } catch (InterruptedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
        
        System.out.println("All threads are dead, exiting main thread");
    }

}

class MyRunnable implements Runnable{

    @Override
    public void run() {
        System.out.println("Thread started:::"+Thread.currentThread().getName());
        try {
            Thread.sleep(4000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println("Thread ended:::"+Thread.currentThread().getName());
    }   
}

输出：

Thread started:::t1
Thread ended:::t1
Thread started:::t2
Thread started:::t3
Thread ended:::t2
Thread ended:::t3
All threads are dead, exiting main thread

All threads are dead, exiting main thread