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  <code>true</code> if this thread is alive;*          <code>false</code> otherwise.*/public final native boolean isAlive();/* <p>* 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.* <p>...*/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;@Slf4j
public 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);}@Overridepublic  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 start
2019-07-29 20:14:21,551   [t1] INFO  JoinTest  - current thread is : t1 run
2019-07-29 20:14:21,551   [t1] INFO  JoinTest  - current thread is : t1 run
2019-07-29 20:14:21,551   [t1] INFO  JoinTest  - current thread is : t1 run
2019-07-29 20:14:21,551   [t1] INFO  JoinTest  - current thread is : t1 run
2019-07-29 20:14:21,551   [t1] INFO  JoinTest  - current thread is : t1 run
2019-07-29 20:14:21,551   [t1] INFO  JoinTest  - current thread is : t1 run
2019-07-29 20:14:21,551   [t1] INFO  JoinTest  - current thread is : t1 run
2019-07-29 20:14:21,551   [t1] INFO  JoinTest  - current thread is : t1 run
2019-07-29 20:14:21,551   [t1] INFO  JoinTest  - current thread is : t1 run
2019-07-29 20:14:21,551   [t1] INFO  JoinTest  - current thread is : t1 run
2019-07-29 20:14:21,551   [t1] INFO  JoinTest  - current thread is : t1 end
2019-07-29 20:14:21,551   [main] INFO  JoinTest  - current thread is : main run
2019-07-29 20:14:21,551   [main] INFO  JoinTest  - current thread is : main end