join方法的作用是使所属线程对象正常执行run方法，而对当前线程无限期阻塞，直到所属线程销毁后再执行当前线程的逻辑。 一、先看普通的无join方法 NoJoin.java

public class NoJoin  extends  Thread{    @Override    public void run() {        try {            long count = (long)(Math.random()*100);            System.out.println(count);            Thread.sleep(count);        } catch (InterruptedException e) {            e.printStackTrace();        }    }}

public class NoJoinTest {    public static  void main(String[] args){        NoJoin noJoin = new NoJoin();        noJoin.start();        System.out.println("执行到main....");    }}

输出：

现在要先输出时间，再执行main方法。

只需要添加join方法即可。

/** * Created by Administrator on 2016/1/5 0005. * * join方法的作用是使所属线程对象正常执行run方法，而对当前线程无限期阻塞，直到所属线程销毁后再执行当前线程的逻辑。 */public class JoinTest {    public static  void main(String[] args){        try {            NoJoin noJoin = new NoJoin();            noJoin.start();            noJoin.join();//join            System.out.println("执行到main....");        } catch (InterruptedException e) {            e.printStackTrace();        }    }}

 

查看join源代码：

/**     * Waits at most {
    @code millis} milliseconds for this thread to     * die. A timeout of {
    @code 0} means to wait forever.     *     * 
 This implementation uses a loop of {
    @code this.wait} calls     * conditioned on {
    @code this.isAlive}. As a thread terminates the     * {
    @code this.notifyAll} method is invoked. It is recommended that     * applications not use {
    @code wait}, {
    @code notify}, or     * {
    @code notifyAll} on {
    @code Thread} instances.     *     * @param  millis     *         the time to wait in milliseconds     *     * @throws  IllegalArgumentException     *          if the value of {
    @code millis} is negative     *     * @throws  InterruptedException     *          if any thread has interrupted the current thread. The     *          interrupted status of the current thread is     *          cleared when this exception is thrown.     */    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;            }        }    }

可见，join内部是对象的wait方法，当执行wait(mils)方法时，一定时间会自动释放对象锁。