java - Java中的等待和通知死锁情况

Question

1. 线程 t1 在 wait() 被命中后将进入死锁。虽然t2 中有 notify()。代码陷入僵局。未收到打印语句 - “等待释放后::::”

2. 我可以看到两个线程在 counterAdd() 中竞争获取监视器。因此，我认为通知会起作用。

   package com.java.thread.practice;
   
     public class WaitAndNotify4 {
   
           int counter = 0;
   
           /*
              CounterAdd() is to be accessed by both t1 and t2.
              If not synchronized not giving consistent output. 
           */ 
           synchronized int counterAdd(){
               return counter++;
           }
   
           public static void main(String[] args) throws InterruptedException{
   
               // Creating method to call the threads.
               WaitAndNotify4 andNotify4 = new WaitAndNotify4();
               andNotify4.testRaceCondition();
   
           }
   
           private  void testRaceCondition() throws InterruptedException {
   
               // Thread t1 created and launched.
   
               Thread t1 = new Thread(new Runnable(){
                   @Override
                   public void run() {
   
                       for(int i=0; i<5; i++){
                           synchronized(this){
                           if(i== 1){
                               System.out.println("Calling wait after count 1");
                               try {
                                // Assuming that this wait will be resumed by notify in t2.
   
                                   wait();
                                   System.out.println("After wait is released :::: ");
                               } catch (InterruptedException e) {
                                   // TODO Auto-generated catch block
                                   e.printStackTrace();
                               }
   
                           }
                           }
                           counterAdd();
                       }
                   }
               });
   
   
               Thread t2 = new Thread(new Runnable(){
                   @Override
                   public void run() {
                       // TODO Auto-generated method stub
   
                       for(int i=0; i<5; i++){
                           if(i==2){
   
                               synchronized(this){
                                   System.out.println("Before releasing the counter :::::");
                                   notify();
                                   System.out.println("After releasing the counter :::::");
                               }
   
                           }
                           counterAdd();
                       }
   
                   }
               });
   
   
   
               t1.start();
               t2.start();
   
               t1.join();
               t2.join();
   
               System.out.println(" Sum value is found as ::::: "+counter);
   
           }
       }
   

Answer 1

您正在不同的对象上进行同步。第一种情况是在对象 t1 上，第二种情况是在 t2 上，在 andNotify4 上的方法 counterAdd 中。为了始终锁定 andNotify4，您需要执行类似的操作。

public class Main {
    private int counter = 0;

    synchronized int counterAdd() {
        return counter++;
    }

    public static void main(String[] args) throws InterruptedException {
        Main andNotify4 = new Main();
        andNotify4.testRaceCondition();
    }

    private void testRaceCondition() throws InterruptedException {
        Thread t1 = new Thread(new Runnable() {
            @Override
            public void run() {
                for (int i = 0; i < 5; i++) {
                    synchronized (Main.this) {
                        if (i == 1) {
                            System.out.println("Calling wait after count 1");
                            try {
                                Main.this.wait();
                                System.out.println("After wait is released :::: ");
                            } catch (InterruptedException e) {
                                e.printStackTrace();
                            }
                        }
                    }
                    counterAdd();
                }
            }
        });

        Thread t2 = new Thread(new Runnable() {
            @Override
            public void run() {
                for (int i = 0; i < 5; i++) {
                    if (i == 2) {
                        synchronized (Main.this) {
                            System.out.println("Before releasing the counter :::::");
                            Main.this.notify();
                            System.out.println("After releasing the counter :::::");
                        }
                    }
                    counterAdd();
                }
            }
        });

        t1.start();
        t2.start();

        t1.join();
        t2.join();

        System.out.println(" Sum value is found as ::::: " + counter);
    }
}