public class JoinDetail extends Thread{
    public JoinDetail(String name) {
        super(name);
    }

    @Override
    public void run() {
        long a = System.currentTimeMillis();
        System.out.println(Thread.currentThread().getName() + "start");
        try {
        Thread.sleep(10000);
    } catch (InterruptedException e) {
        e.printStackTrace();
    }
        a = System.currentTimeMillis() - a;
        System.out.println(Thread.currentThread().getName() + "end" + a);
}

    public static void main(String[] args) {
        long currentTime = System.currentTimeMillis();
        JoinDetail j1 = new JoinDetail("j1");
        HaveThreadLock j2 = new HaveThreadLock("j2",j1);
        j1.start();
        j2.start();
        /*
        try {
            Thread.sleep(10);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        */
        try {
            j1.join(3000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println("主線程執(zhí)行完畢");
        System.out.println("總共耗時(shí)" + (System.currentTimeMillis() - currentTime));
    }
}

執(zhí)行結(jié)果

j2begin
j1start
j2end5000
主線程執(zhí)行完畢
總共耗時(shí)5002
j1end10000

將注釋

        try {
            Thread.sleep(10);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

打開(kāi)
執(zhí)行結(jié)果

j2begin
j1start
j2end5001
主線程執(zhí)行完畢
總共耗時(shí)8004
j1end10001

join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) {
            while (isAlive()) {
                wait(0);
            }
        } else {
            while (isAlive()) {
                long delay = millis - now;
                if (delay <= 0) {
                    break;
                }
                wait(delay);
                now = System.currentTimeMillis() - base;
            }
        }
    }

線程1調(diào)用線程2的join方法部凑，會(huì)把線程2對(duì)象本身當(dāng)作syschronized鎖對(duì)象喉磁，獲取到了鎖然后再判斷線程2的alive狀態(tài)并且wait姿鸿。
分析結(jié)果
在注釋情況下履腋，main線程首先獲取j1對(duì)象鎖，然后給join方法中的base賦值危彩，wait釋放鎖徒欣，使j2線程獲取到鎖逐样，sleep5秒，然后main線程wait時(shí)間超時(shí)重新獲取j1鎖發(fā)現(xiàn)delay<0了打肝，接著main線程結(jié)束脂新。
在打開(kāi)注釋情況下，j2線程先獲取j1對(duì)象鎖粗梭，sleep5秒争便，然后main線程調(diào)用j1.join，wait5秒断医，接著main線程結(jié)束滞乙。