join方法的作用是等待執行緒銷燬，當主執行緒中呼叫了其他執行緒的join方法時，主執行緒需要等到該執行緒執行完畢才可以繼續執行，示例如下：

public class MyThread extends Thread
{
    public void run()
    {
        try
        {
            int value = (int)(Math.random() * 10000);
            System.out.println(value);
            Thread.sleep(value);
        } 
        catch
 
 (InterruptedException e)
        {
            e.printStackTrace();
        }
    }
}

public static void main(String[] args) throws Exception
{
    MyThread mt = new MyThread();
    mt.start();
    mt.join();
    System.out.println("主執行緒");
}

另外join方法與wait方法一樣，也是在阻塞時需要釋放鎖，原因是join方法的實現中呼叫了wait方法，原始碼如下：

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;
        }
    }
    }