join是线程的一个方法，在API中，它的英文解释是Waits for this thread to die.等待此线程死亡。谁等待此线程死亡呢？不难看出,这过程中至少存在两个线程，一个调用线程的线程，一个被调用的线程。我们通过一个简单的示例来理解这句话。
package com.doufu.thread.t01;
class ThreadA extends Thread{
private String name;
public ThreadA(String name){
this.name = name;
}
@Override
public void run() {
try {
Thread.sleep(1000);//让此线程等待1秒
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println(name);
}
}
public class JoinTest01 {
public static void main(String[] args) throws InterruptedException {
ThreadA t = new ThreadA("T1");
t.start();
t.join();
System.out.println("main");
}
}
在上面代码中，调用线程的线程是main即主线程,被调用的线程是t。t.join()的意思即等待t的死亡(运行结束),等待t运行结束的线程就是main。所以上面的代码执行结果应该是先打印T1,再打印main。
通过以上代码，可以理解，在A线程中调用另外一个线程B的join方法，即让线程A进入Block状态，直到线程B运行结束。下面再看一个示例：
只贴出了修改的部分
public class JoinTest01 {
public static void main(String[] args) throws InterruptedException {
ThreadA t = new ThreadA("T1");
t.join();
t.start();
System.out.println("main");
}
}
运行结果：
看这输出结果，是不是觉得不对，不是好说了，在main中调用t的join方法，会让main先挂起，直到t执行结束吗，怎么先打印main了呢？我们来看下Thread类的join源码：
public final void join() throws InterruptedException {
join(0);
}
join()，实际上是调用另外一个join(long millis)的方法，我们再来看下join(long millis)方法
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) {//进入这里
while (isAlive()) {//线程必须是活着的
wait(0);
}
} else {
while (isAlive()) {
long delay = millis - now;
if (delay <= 0) {
break;
}
wait(delay);
now = System.currentTimeMillis() - base;
}
}
}
从源码中看出，如果调用join()方法时，线程不是活着的，则无效。以下是isAlive()的源码，注意看下方法注释
/**
* 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();
我们来看一个面试题：
写道
现在有T1、T2、T3三个线程，你怎样保证T2在T1执行完后执行，T3在T2执行完后执行？
此面试题，我们可以用join来实现
class ThreadA extends Thread{
private String name;
public ThreadA(String name){
this.name = name;
}
@Override
public void run() {
try {
Thread.sleep(1000);//让此线程等待1秒
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println(name);
}
}
class ThreadB extends Thread{
private String name;
private Thread thread;
public ThreadB(String name,Thread thread){
this.name = name;
this.thread = thread;
}
@Override
public void run() {
try {
thread.join();//让此线程进入Block
Thread.sleep(1000);//让此线程等待1秒
} catch (InterruptedException e1) {
e1.printStackTrace();
}
System.out.println(name);
}
}
public class JoinTest01 {
public static void main(String[] args) throws InterruptedException {
ThreadA t1 = new ThreadA("T1");
ThreadB t2 = new ThreadB("T2",t1);
ThreadB t3 = new ThreadB("T3",t2);
t1.start();
t2.start();
t3.start();
}
}
运行结果：
以下更简单的实现方式：
public class JoinTest {
static class JoinThread implements Runnable{
private String name;
public JoinThread(String name){
this.name = name;
}
@Override
public void run() {
System.out.println(name);
try {
Thread.sleep(5000);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println("结束"+name);
}
}
public static void main(String[] args) throws Exception {
for(int i=0;i<3;i++){
Thread t = new Thread(new JoinThread("t"+i));
t.start();
t.join();
}
System.out.println("HAHAHA");
}
}
运行结果：
使用CountDownLatch实现
import java.util.concurrent.CountDownLatch;
public class CoutDownLatchTest {
private static final CountDownLatch c1 = new CountDownLatch(1);
private static final CountDownLatch c2 = new CountDownLatch(2);
public static void main(String[] args) {
Thread t1 = new Thread(){
public void run(){
System.out.println("结束"+1);
c1.countDown();
c2.countDown();
}
};
Thread t2 = new Thread(){
public void run(){
try {
c1.await();
System.out.println("结束"+2);
c2.countDown();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
};
Thread t3 = new Thread(){
public void run(){
try {
c2.await();
System.out.println("结束"+3);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
};
t3.start();
t2.start();
t1.start();
System.out.println("HAHAHA");
}