notifyAll notify顺序

今天看到一篇问题，提问线程唤醒顺序。

具体代码如下：

 import java.util.LinkedList;
import java.util.List;

public class ThreadRunSort {

    /** 
     * 对象锁 
     */  
    private final Object object = new Object();  
    private List<Integer> sleep = new LinkedList<>();
    private List<Integer> notify = new LinkedList<>();
    /** 
     * 该线程作为一个唤醒线程 
     */  
    public void startThread(int i) {  
        Thread t = new Thread(new Runnable() {  
            @Override  
            public void run() {  
                synchronized (object) {  
                    try {
                        System.out.println(Thread.currentThread().getName()+"进入休眠");
                        sleep.add(i);
                        object.wait();
                        System.out.println(Thread.currentThread().getName()+"线程已经唤醒");
                        notify.add(i);
                    } catch (InterruptedException e) {
                        // TODO Auto-generated catch block
                        e.printStackTrace();
                    }
                }  
            }  
        });  
        t.setName("Thread"+i);
        t.start();  
    }  

    public static void main(String[] args) {  
        ThreadRunSort a = new ThreadRunSort();
        for(int i =1;i<22;i++){
            a.startThread(i);
        }

        try {
            Thread.sleep(1000);
        } catch (InterruptedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
        System.out.println();
        for(int i =1;i<22;i++){
            synchronized (a.object) {
                a.object.notify();
            }
        }

        try {
            Thread.sleep(3000);
        } catch (InterruptedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
        System.out.println("休眠顺序"+a.sleep);
        System.out.println("唤醒顺序"+a.notify);


    }    
}

发现，输入的顺序是不定的，比如：

休眠顺序[1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21]
唤醒顺序[1, 5, 4, 3, 2, 7, 6, 20, 19, 18, 17, 16, 15, 14, 13, 12, 11, 10, 9, 8, 21]

 

 

但是，当修改了其中notify代码：

for(int i =1;i<22;i++){
            try {
                Thread.sleep(10);  // 在这里sleep确保notify会顺序执行
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            synchronized (a.object) {
                a.object.notify();
            }
        }

 

顺序就变成

休眠顺序[1, 3, 4, 2, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21]
唤醒顺序[1, 3, 4, 2, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21]

 

可以理解在wait时候，放入的是一个FIFO的队列？每次notify其实就是出队的形式。（上面顺序有问题是因为线程按顺序出队后，抢占锁的顺序和CPU有关）

 

但是，把代码改成notifyAll后：

休眠顺序[1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21]
唤醒顺序[21, 20, 19, 18, 17, 16, 15, 14, 13, 12, 11, 10, 9, 8, 7, 6, 5, 4, 3, 2, 1]

变成了stack出栈的形式。。

具体的问题还要再研究下。。。。

 

原问题传送门：https://ask.csdn.net/questions/387308#answer_972330