join方法

join重载方法

1 join()
2 join(long millis)     //参数为毫秒
3 join(long millis,int nanoseconds)    //第一参数为毫秒，第二个参数为纳秒

public class JoinDemo implements Runnable{
    public void run() {
        System.err.println("join thread demo ");
    }

    public static void main(String[] args) throws Exception {
        System.err.println("main thread start... ");
        Runnable r = new JoinDemo();
        Thread t = new Thread(r);
        t.setName("ibli joinTest ...");
        t.start();
//        t.join();
        System.err.println("main thread end... ");
    }
}

public class JoinDemo implements Runnable{
    public void run() {
        System.err.println("join thread demo ");
        try {
            // 线程睡眠4s
            Thread.sleep(4000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        List<String> strings = null;
        System.err.println(strings.get(0));
    }

    public static void main(String[] args) throws Exception {
        System.err.println("main thread start... ");
        Runnable r = new JoinDemo();
        Thread t = new Thread(r);
        t.setName("ibli joinTest ...");
        t.start();
        // 但是主线程join的超时时间是1s
        t.join(1000);
        System.err.println("main thread end... ");
    }
}

main thread start... 
join thread demo 
main thread end... 
Exception in thread "ibli joinTest ..." java.lang.NullPointerException
	at com.ibli.threadTest.api.JoinDemo.run(JoinDemo.java:14)
	at java.lang.Thread.run(Thread.java:748)

public final void join() throws InterruptedException {
        join(0);
}

public final synchronized void join(long millis)
    throws InterruptedException {
        long base = System.currentTimeMillis();
        long now = 0;
        
        // 首先校验参数是否合法
        if (millis < 0) {
            throw new IllegalArgumentException("timeout value is negative");
        }

        // 如果join方法没有参数，则相当于直接调用wait方法
        if (millis == 0) {
            while (isAlive()) {
                wait(0);
            }
        } else {
            while (isAlive()) {
                long delay = millis - now;
                if (delay <= 0) {
                    break;
                }
                wait(delay);
                now = System.currentTimeMillis() - base;
            }
        }
    }

public final native boolean isAlive();

void JavaThread::exit(bool destroy_vm, ExitType exit_type) {
  // Notify waiters on thread object. This has to be done after exit() is called
  // on the thread (if the thread is the last thread in a daemon ThreadGroup the
  // group should have the destroyed bit set before waiters are notified).
  ensure_join(this);
}

static void ensure_join(JavaThread* thread) {
  // We do not need to grap the Threads_lock, since we are operating on ourself.
  Handle threadObj(thread, thread->threadObj());
  assert(threadObj.not_null(), "java thread object must exist");
  ObjectLocker lock(threadObj, thread);
  // Ignore pending exception (ThreadDeath), since we are exiting anyway
  thread->clear_pending_exception();
  // Thread is exiting. So set thread_status field in  java.lang.Thread class to TERMINATED.
  java_lang_Thread::set_thread_status(threadObj(), java_lang_Thread::TERMINATED);
  // Clear the native thread instance - this makes isAlive return false and allows the join()
  // to complete once we've done the notify_all below
  //这里是清除native线程，这个操作会导致isAlive()方法返回false
  java_lang_Thread::set_thread(threadObj(), NULL);
  // 在这里唤醒等待的线程
  lock.notify_all(thread);
  // Ignore pending exception (ThreadDeath), since we are exiting anyway
  thread->clear_pending_exception();
}