join

join

join是Thread方法，它的作用是A线程中子线程B在运行之后调用了B.join()，A线程会阻塞直至B线程执行结束

join源码(只有继承Thread类才能使用)

基于openjdk1.8的源码

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;            }        }    }       /**     * Tests if this thread is alive. A thread is alive if it has     * been started and has not yet died.     *     * @return  true if this thread is alive;     *          false otherwise.     */    public final native boolean isAlive();     /* 
     * Note that the {@code wait} method, as it places the current thread     * into the wait set for this object, unlocks only this object; any     * other objects on which the current thread may be synchronized remain     * locked while the thread waits.     * 
     ...     */    public final native void wait(long timeout) throws InterruptedException;

源码分析

A线程调用了B.join(),获取了B的锁，当B alive，B.wait(0)会让当前线程A阻塞，执行join方法等同于，A线程进入了下列

的语句

syncronized(B){...B.wait...}

代码测试

package com.java.javabase.thread.base;import lombok.extern.slf4j.Slf4j;@Slf4jpublic class JoinTest {    public static void main(String[] args) {        Thread t1 =new ThreadOne("t1");        t1.start();        log.info("current thread is : {} run",Thread.currentThread().getName());        try {            t1.join();        } catch (InterruptedException e) {            log.info("InterruptedException",e);            e.printStackTrace();        }        log.info("current thread is : {} end",Thread.currentThread().getName());    }    static class  ThreadOne extends Thread{        public ThreadOne(String name){            super(name);        }        @Override        public  void run(){            log.info("current thread is : {} start",Thread.currentThread().getName());            for(int i =0;i<10;i++)            {                log.info("current thread is : {} run",Thread.currentThread().getName());            }            log.info("current thread is : {} end",Thread.currentThread().getName());        }    }}

说明

主线程调用t1.join之后，主线程只有t1的锁进入阻塞状态

运行结果

2019-07-29 20:14:21,551   [t1] INFO  JoinTest  - current thread is : t1 start2019-07-29 20:14:21,551   [t1] INFO  JoinTest  - current thread is : t1 run2019-07-29 20:14:21,551   [t1] INFO  JoinTest  - current thread is : t1 run2019-07-29 20:14:21,551   [t1] INFO  JoinTest  - current thread is : t1 run2019-07-29 20:14:21,551   [t1] INFO  JoinTest  - current thread is : t1 run2019-07-29 20:14:21,551   [t1] INFO  JoinTest  - current thread is : t1 run2019-07-29 20:14:21,551   [t1] INFO  JoinTest  - current thread is : t1 run2019-07-29 20:14:21,551   [t1] INFO  JoinTest  - current thread is : t1 run2019-07-29 20:14:21,551   [t1] INFO  JoinTest  - current thread is : t1 run2019-07-29 20:14:21,551   [t1] INFO  JoinTest  - current thread is : t1 run2019-07-29 20:14:21,551   [t1] INFO  JoinTest  - current thread is : t1 run2019-07-29 20:14:21,551   [t1] INFO  JoinTest  - current thread is : t1 end2019-07-29 20:14:21,551   [main] INFO  JoinTest  - current thread is : main run2019-07-29 20:14:21,551   [main] INFO  JoinTest  - current thread is : main end