Unsafe类park,unpark详解

/**
* Unblock the given thread blocked on <tt>park</tt>, or, if it is
* not blocked, cause the subsequent call to <tt>park</tt> not to
* block. Note: this operation is "unsafe" solely because the
* caller must somehow ensure that the thread has not been
* destroyed. Nothing special is usually required to ensure this
* when called from Java (in which there will ordinarily be a live
* reference to the thread) but this is not nearly-automatically
* so when calling from native code.
* @param thread the thread to unpark.
*
*/
public native void unpark(Object thread);

park方法用于Block current thread。在以下情况下返回

1 针对当前线程已经调用过unpark(多次调用unpark的效果和调用一次unpark的效果一样)

 

public static void main(String[] args) throws Exception {
        
      Unsafe unsafe = UnsafeUtil.getUnsafe();
        
      Thread currThread = Thread.currentThread();
      
      unsafe.unpark(currThread);
      unsafe.unpark(currThread);
      unsafe.unpark(currThread);
      
      unsafe.park(false, 0);
      
      System.out.println("SUCCESS!!!");
    }

 

park方法马上返回

2 在当前线程中断的时候或者调用unpark的时候

  

public static void main(String[] args) throws Exception {
        
      Unsafe unsafe = UnsafeUtil.getUnsafe();
        
      Thread currThread = Thread.currentThread();
      new Thread(()->{
          try {
            Thread.sleep(3000);
            currThread.interrupt();
            //unsafe.unpark(currThread);
        } catch (Exception e) {}
      }).start();
      
      unsafe.park(false, 0);
      
      System.out.println("SUCCESS!!!");
    }

 

　park在3秒后返回

3 如果是相对时间也就是isAbsolute为false（注意这里后面的单位纳秒）到期的时候

public static void main(String[] args) throws Exception {
        
      Unsafe unsafe = UnsafeUtil.getUnsafe();
      //相对时间后面的参数单位是纳秒
      unsafe.park(false, 3000000000l);
      
      System.out.println("SUCCESS!!!");
    }

 

　3秒后 park返回

4 如果是绝对时间也就是isAbsolute为true(注意后面的单位是毫秒)到期的时候

public static void main(String[] args) throws Exception {
        
      Unsafe unsafe = UnsafeUtil.getUnsafe();
      
      long time = System.currentTimeMillis()+3000;
      
      //绝对时间后面的参数单位是毫秒
      unsafe.park(true, time);
      
      System.out.println("SUCCESS!!!");
    }

3秒后 park返回

5 无理由的返回

 

unpark方法最好不要在调用park前对当前线程调用unpark

/**
     * Unblock the given thread blocked on <tt>park</tt>, or, if it is
     * not blocked, cause the subsequent call to <tt>park</tt> not to
     * block.  Note: this operation is "unsafe" solely because the
     * caller must somehow ensure that the thread has not been
     * destroyed. Nothing special is usually required to ensure this
     * when called from Java (in which there will ordinarily be a live
     * reference to the thread) but this is not nearly-automatically
     * so when calling from native code.
     * @param thread the thread to unpark.
     *
     */
    public native void unpark(Object thread);