线程的join()方法

官网描述

join

public final void join()
                throws InterruptedException

Waits for this thread to die.

An invocation of this method behaves in exactly the same way as the invocation

join(0)

Throws:

InterruptedException - if any thread has interrupted the current thread. The interrupted status of the current thread is cleared when this exception is thrown.

等该线程执行完之后再执行后面的业务逻辑

join(long millis) 最多等该线程多少毫秒就执行后面逻辑

join(long millis, int nanos)最多等该线程多少纳秒就执行后面的逻辑

代码演示

package com.dwz.concurrency.chapter5;

import java.util.Optional;
import java.util.stream.IntStream;
/**
 *     在t1线程执行100毫秒，10纳秒之后执行 main主线程相关业务
 */
public class ThreadJoin2 {
    public static void main(String[] args) throws InterruptedException {
        Thread t1 = new Thread(() -> {
            try {
                System.out.println("t1 is running");
                Thread.sleep(10_000);
                System.out.println("t1 is done");
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        });
        
        t1.start();
//        t1.join();--等t1执行完
        t1.join(100, 10);---等t1执行100毫秒，10纳秒之后
       
        Optional.of("All of tasks finish done.").ifPresent(System.out::println);
        IntStream.range(1, 1000).forEach(i -> System.out.println(Thread.currentThread().getName() + "->" + i));        
    }
}

join()在多线程中的使用

package com.dwz.concurrency.chapter5;

import java.util.Optional;
import java.util.stream.IntStream;

/**
 * join()是在start()之后调用，保证t1、t2执行完毕后再执行main线程业务逻辑
 * join()是相对于main线程而言的，t1、t2是同级并发交叉执行的
 */
public class ThreadJoin {
    public static void main(String[] args) throws InterruptedException {
        Thread t1 = new Thread(() -> {
            IntStream.range(1, 1000).forEach(i -> System.out.println(Thread.currentThread().getName() + "->" + i));
        });
        
        Thread t2 = new Thread(() -> {
            IntStream.range(1, 1000).forEach(i -> System.out.println(Thread.currentThread().getName() + "->" + i));
        });
        
        t1.start();
        t2.start();
        t1.join();
        t2.join();
        
        Optional.of("All of tasks finish done.").ifPresent(System.out::println);
        IntStream.range(1, 1000).forEach(i -> System.out.println(Thread.currentThread().getName() + "->" + i));
    }
}

如果采用如下执行顺序：

t1.start();
t1.join();
t2.start();                
t2.join();

结果是t1执行完才会执行t2

main线程的join()

//模拟一个死锁(主线程等待自己死亡之后再死亡，一直会有个等待死亡的线程导致程序进程不能结束)
//start httpServer和JettyHttpServer.start()是两个守护线程，会随着主线程死亡而死亡
//要保持其长连接，可以使用主线程的join()方法，保证主线程一直存活，从而使得守护线程也存活
Thread.currentThread().join();

这个方法使main线程一直存活