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Thread类线程结束会唤醒使用其对象做锁而睡眠的线程

时间:2019-04-18 12:12:18      阅读:223      评论:0      收藏:0      [点我收藏+]

标签:基础   new   知识   openjdk   any   rap   有一个   system   exce   

首先回顾一下我们的基础知识。

 

sleep:

  线程睡眠,不会释放锁

 

wait:

  线程等待。释放锁。

 

notity:

  唤醒随机一个当前对象等待的线程,并不会释放锁

 

notityAll:

  唤醒所有当前对象等待的线程,并不会释放锁

 

遇到问题:

  代码如下:

  

package com.zhen.ten_chapter.question;

/**
 * @author zhen
 * @Date 2019/4/18 10:17
 */
public class ThreadDemo {
    public static void main(String[] args) {
        To10X tos = new To10X(1, 10);
        tos.start();
        int res = tos.getResult();
        System.out.println(res);

    }

   static class To10X extends Thread {
       
        private int point = 1;
        private int end = 10;

        private int result = 0;

        public To10X(int start, int end) {
            this.point = start;
            this.end = end;
            
        }

        @Override
        public  synchronized void run() {
            
                while (point != end + 1) {
                    result += point++;
                    System.out.println(Thread.currentThread().getName() + "::" + result);
                }
                this.notify();
        }

        public  synchronized int getResult() {
             
                while(result == 0) {
                    try {
                        System.out.println("锁定前");
                        this.wait();
                        System.out.println("锁定后");
                    } catch (Exception e) {
                        e.printStackTrace();
                    }
                }

            return result;
        }
    }
}

  程序发现wait总是会被唤醒。

  怀疑是不是总是先wait然后再被notity了,于是将notify注释掉了,但依然被唤醒。

 

什么原因?

  我关注点放在了static关键字上了,依稀记得static 方法的锁锁对象是类,那么static类里面的成员方法的锁对象是不是也是类呢?

    当然是我多想了,但是我依然将static 的内部类改为了一个普通内部类,然后用实例化对象去创建的对应对象与执行方法。可是结果依旧是一样。

  我接下来怀疑是主线程的原因

      因为其他线程都是从主线程上衍生出来的,线程不是很熟练,依稀记得一些所谓守护线程等概念。于是简单写了一个Demo类,主线程wait,然后开一个线程获取到锁执行到结束,然后发现能wait住主线程

那是什么原因呢?

  接下来我觉得一定是有什么东西唤醒了主线程,notity被我注释掉了,notityAll没有编写。查看wait的api,发现它没有自动唤醒的说法,有一个参数是延迟等待的意思。它明确声明了只能靠notity和notifyAll唤醒。百度大法好。我狂百度,发现有人遇到和我一样的问题,参考链接:  https://blog.csdn.net/nmyangym/article/details/7850882#commentBox 。为此,我也去看了一下所谓的jdk的Thread类的join源码,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;
            }
        }
    }

  它确实调用了wait,但是调用了wait能保证唤醒吗?我感觉方向没问题了,继续针对点去百度

参考链接:

  https://segmentfault.com/q/1010000016744022?utm_source=tag-newest

  原来,Thread对象在线程结束的时候,会自动调用一次notifyAll语法,线程结束会执行join方法,join的jdk写法我们看到过,大牛给出的依据是openJDK中的源码:

 

int ret = pthread_create(&tid, &attr, (void* (*)(void*)) java_start, thread);

static void *java_start(Thread *thread) {
  ...
  thread->run();
  return 0;
}

void JavaThread::run() {
  ...
  thread_main_inner();
}

void JavaThread::thread_main_inner() {
  ...
  this->exit(false);
  delete this;
}

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
  java_lang_Thread::set_thread(threadObj(), NULL);
  lock.notify_all(thread);
  // Ignore pending exception (ThreadDeath), since we are exiting anyway
  thread->clear_pending_exception();
}

  

 

结论:

  尽量不要用线程对象做同步锁的钥匙,线程结束的时候它会自动调用this.notifyAll()

 

Thread类线程结束会唤醒使用其对象做锁而睡眠的线程

标签:基础   new   知识   openjdk   any   rap   有一个   system   exce   

原文地址:https://www.cnblogs.com/aigeileshei/p/10728739.html

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