【JAVA 并发】一同步访问共享的可变数据

谁借莪１个温暖的怀抱￠ 2022-07-18 04:10 105阅读 0赞

## **不要使用Thread.stop()** ##

因为unsafe , 要停止一个线程 , 建议的做法一个线程轮询 一个boolean域 .

public class StopThread{
        
      private static boolean stopRequested ; 
      public static void main(String[] args) throws InterruptedException{
        Thread backgroundThread = new Thread(new Runnable(){
          public void run(){
            int i=0;
            while(!stopRequested)
                i++;
          }
        });
    
        backgroundThread.start();
    
        TimeUtil.SECONDS.sleep(1);
        stoprequested = true ;
      }
    }

这个程序不会停止,因为后台程序永远在循环.

因为没有同步,就不能保证后台线程看到主线程对stopRequested的修改.

**下面加上同步**这里的同步不是为了互斥,而是是两个线程通信

public class StopThread1 {
        
        private static boolean stop;
    
        private static  void setStop(){
            stop = true ;
        }
    
        private static synchronized boolean getStop(){
            return stop;
        }
    
        public static void main(String[] args) throws InterruptedException {
            Thread backgroundThread = new Thread(new Runnable() {
                public void run() {
                    int i = 0;
                    while (!getStop())
                        i++;
                }
            });
            backgroundThread.start();
            TimeUnit.SECONDS.sleep(1);
            setStop();
        }
    }

## **volatile 通信 , 不能实现互斥** ##

public class StopThread2 {
        
        private static volatile boolean stop;
    
        public static void main(String[] args) throws InterruptedException {
            Thread backgroundThread = new Thread(new Runnable() {
                public void run() {
                    int i = 0;
                    while (!stop)
                        i++;
                }
            });
            backgroundThread.start();
            TimeUnit.SECONDS.sleep(1);
            stop = true;
    
        }
    }

## **++ / – 不是原子操作** ##

private static volatile int nextSerizlNumber =  0;
    public static int generateSerizlNumber(){
      return nextSerialNumber ++ ; 
    }

首先读取值,写回新的值,如果第二个线程在第一个线程读取旧值时,写如了新的值,这将是安全性失败 safety failure .

修改

private static final AtomicLong nextSerialNum = new AtomicLong();
    public static long generateSerialNumber(){
      return nextSerialNum.getAndIncrement();
    }

**当多个线程共享可变数据的时候,每个读或写数据的线程都必须执行同步.**