java - 并发同步问题

Question

public class Demo {
  public Demo() {
  }

  public static void main(String[] args) {
    Concurrency c = new Concurrency();
    Thread t1 = new Thread(c);
    t1.setName("t1");
    Thread t2 = new Thread(c);
    t2.setName("t2");
    Thread t3 = new Thread(c);
    t3.setName("t3");
    Thread t4 = new Thread(c);
    t4.setName("t4");

    t1.start();
    t2.start();
    t3.start();
    t4.start();
  }
}


class Concurrency implements Runnable {
  private String value= new String("I");
  static Integer s =2;

  @Override
  public void run() {
    function();
  }

  public void function() {
    synchronized(s){

    s = s * 5;
    System.out.println("Current thread is " + Thread.currentThread() + s);
    // s.notify();      
  }
}

我编写了示例程序来测试同步。我得到以下输出:

Current thread is Thread[t2,5,main]50
Current thread is Thread[t3,5,main]1250
Current thread is Thread[t4,5,main]1250
Current thread is Thread[t1,5,main]50

这意味着当多个线程运行时，同步也无法同步，并且当如上所述调用 s.notify() 时，我还会收到 IlleagalMonitorStateException 。

请让我知道它到底在做什么以及为什么同步失败。也欢迎我解决这个问题。

//调用 s.notify() 后，我收到以下错误。

 Exception in thread "t1" Exception in thread "t2" Exception in thread "t3"   
    java.lang.IllegalMonitorStateException
    Current thread is Thread[t1,5,main]50
    Current thread is Thread[t2,5,main]50
    Current thread is Thread[t3,5,main]250
    Current thread is Thread[t4,5,main]1250
        at java.lang.Object.notify(Native Method)
        at Concurrency.function(Demo.java:42)
        at Concurrency.run(Demo.java:32)
        at java.lang.Thread.run(Unknown Source)
    Exception in thread "t4" java.lang.IllegalMonitorStateException
        at java.lang.Object.notify(Native Method)
        at Concurrency.function(Demo.java:42)
        at Concurrency.run(Demo.java:32)
        at java.lang.Thread.run(Unknown Source)
    java.lang.IllegalMonitorStateException
        at java.lang.Object.notify(Native Method)
        at Concurrency.function(Demo.java:42)
        at Concurrency.run(Demo.java:32)
        at java.lang.Thread.run(Unknown Source)
    java.lang.IllegalMonitorStateException
        at java.lang.Object.notify(Native Method)
        at Concurrency.function(Demo.java:42)
        at Concurrency.run(Demo.java:32)
        at java.lang.Thread.run(Unknown Source)

Answer 1

当你这样做

s = s * 5;

您正在将一个新对象分配给s，因此同步实际上适用于不同对象，因此本质上是被破坏的。幕后真正发生的事情类似于

s = new Integer(s.intValue() * 5);

您需要的是一个并发安全的可变整数容器。幸运的是有“AtomicInteger”。

static final AtomicInteger s = new AtomicInteger(2);

...

s.getAndSet(s.get()*5);