java - 由不同类的线程更新的值不会反射(reflect)在 java 中的另一个类中

Question

我编辑了我的问题并使其变得非常简单。首先，同一个文件中有两个类HashMapClass:创建 ConccurentHashMap 的实例和NewThread:更新hashMap

public class HashMapClass {

public static volatile ConcurrentHashMap serverMap = new ConcurrentHashMap();

public static void main(String args[]) {

    NewThread nt = new NewThread();
    nt.start();
}
}

class NewThread extends Thread {

@Override
public void run() {
    HashMapClass.serverMap.put("Ishan", new Integer(3));

System.out.println("Hash map is empty or not " + HashMapClass.serverMap.isEmpty());
  try {
        Thread.sleep(10000);
    } catch (InterruptedException ex) {
        Logger.getLogger(NewThread.class.getName()).log(Level.SEVERE, null, ex);
    }
    }
}

现在这里显示 hashMap 不为空，但如果我尝试从其他类 New Class 访问它，它显示为空

public class NewClass {

public static void main(String s[]) {
    System.out.println("Hash map is empty or not " + HashMapClass.serverMap.isEmpty());
    Set setMap = HashMapClass.serverMap.entrySet();
    Iterator i = setMap.iterator();
    for (int f = 0; i.hasNext(); ++f) {
        Map.Entry me = (Map.Entry) i.next();
        System.out.println("key is" + me.getKey() + "value is :" + me.getValue());
    }
}

}

此类永远不会用数据更新。我希望现在很容易理解。

Answer 1

HashMap 不是线程安全的，因此如果需要跨线程共享 HashMap，则需要引入某种形式的同步。

或者，更简单的是，使用 ConcurrentHashMap which is thread safe .

另一方面，您可能会从阅读 Java tutorial on concurrency 中受益。 ，尤其是Memory consistency errors上的部分.

编辑

根据您的评论，请参阅一个简单的示例，我认为该示例可以隔离您的问题 - 我的机器上的输出是:

Hash map empty in main (1st attempt)? true
Hash map empty in run? false
Hash map empty in main (2nd attempt)? false

public class Test {

    public static ConcurrentMap serverMap = new ConcurrentHashMap();

    public static void main(String[] args) throws InterruptedException {
        NewThread nt = new NewThread();
        nt.start();
        System.out.println("Hash map empty in main (1st attempt)? " + serverMap.isEmpty());
        Thread.sleep(200);
        System.out.println("Hash map empty in main (2nd attempt)? " + serverMap.isEmpty());
    }

    static class NewThread extends Thread {

        @Override
        public void run() {
            serverMap.put("Ishan", new Integer(3));

            System.out.println("Hash map empty in run? " + serverMap.isEmpty());
        }
    }
}

第二次编辑

您可以从另一个类应用相同的逻辑 - 只需确保调用 NewThread.start 并等待足够的时间(或者直接调用 NewThread.run，以便它在同一个线程中运行，您不必等待) :

public class NewClass {

    public static void main(String s[]) throws InterruptedException {
        new Test.NewThread().start();
        System.out.println("Hash map is empty or not (1): " + Test.serverMap.isEmpty());
        Thread.sleep(100);
        System.out.println("Hash map is empty or not (2): " + Test.serverMap.isEmpty());
        Set setMap = Test.serverMap.entrySet();
        Iterator i = setMap.iterator();
        for (int f = 0; i.hasNext(); ++f) {
            Map.Entry me = (Map.Entry) i.next();
            System.out.println("key is" + me.getKey() + "value is :" + me.getValue());
        }
    }
}