Unsafe類park,unpark詳解

/**
* 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方法用於Block current thread。在以下情況下返回

1 針對當前線程已經調用過unpark(多次調用unpark的效果和調用一次unpark的效果一樣)

 

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方法馬上返回

2 在當前線程中斷的時候或者調用unpark的時候

  

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 如果是相對時間也就是isAbsolute為false（注意這里后面的單位納秒）到期的時候

public static void main(String[] args) throws Exception {
        
      Unsafe unsafe = UnsafeUtil.getUnsafe();
      //相對時間后面的參數單位是納秒
      unsafe.park(false, 3000000000l);
      
      System.out.println("SUCCESS!!!");
    }

 

　3秒后 park返回

4 如果是絕對時間也就是isAbsolute為true(注意后面的單位是毫秒)到期的時候

public static void main(String[] args) throws Exception {
        
      Unsafe unsafe = UnsafeUtil.getUnsafe();
      
      long time = System.currentTimeMillis()+3000;
      
      //絕對時間后面的參數單位是毫秒
      unsafe.park(true, time);
      
      System.out.println("SUCCESS!!!");
    }

3秒后 park返回

5 無理由的返回

 

unpark方法最好不要在調用park前對當前線程調用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);