java - HashMap 更新未反射(reflect)在线程之间

Question

我有一个包含 hahsmap 的单例类，hashmap 被初始化为类变量。该 map 已正确更新，因为当我添加和打印它的大小时，它发生了变化，但是，当我从不同的线程调用它时， map 始终为空。发生这种情况是否有特殊原因？

如果这有任何区别，我正在使用 ConccurentHashMap。

谢谢

Singleton decleration:

    public class ClientRegistryDetailsSingleton {

        private static ClientRegistryDetailsSingleton instance = null;
        private ConcurrentHashMap<String, Integer> tickerToNumberRegistered = new ConcurrentHashMap<String,Integer>();


        protected ClientRegistryDetailsSingleton() {
             // Exists only to defeat instantiation.
        }

        public static ClientRegistryDetailsSingleton getInstance() {
            if(instance == null) {
               instance = new ClientRegistryDetailsSingleton();
            }
            return instance;
        }

        public void setTickerToNumberRegistered(ConcurrentHashMap<String, Integer> tickerToNumberRegistered) {
            this.tickerToNumberRegistered = tickerToNumberRegistered;
        }

        public ConcurrentHashMap<String, Integer> getTickerToNumberRegistered() {
            return tickerToNumberRegistered;
        }


    public void addToClienets(String ticker){}

    public void removeFromClients(String ticker){}
}

从另一个线程调用它:

String[] splitForTicker = message.split(",");

        ConcurrentHashMap<String, Integer> map = ClientRegistryDetailsSingleton.getInstance().getTickerToNumberRegistered();
        System.out.println("The number of items in the map from senders persepctive" + map.size());

输出:

The number of items in the map from senders persepctive 0
2012-11-12 14:29:12,495 [Process messages received] INFO  com.feed.feedReceive.ProcessFeedStreamLine - Successfully received a message from the feed
The number of items in the map from senders persepctive 0
1 :the size of the map now someone has added
2012-11-12 14:29:15,495 [Process messages received] INFO  com.feed.feedReceive.ProcessFeedStreamLine - Successfully received a

message from the feed The number of items in the map from senders persepctive 0

Singleton 的新代码

public class ClientRegistryDetailsSingleton {

    private static ClientRegistryDetailsSingleton instance = new ClientRegistryDetailsSingleton();
    private volatile ConcurrentHashMap<String, Integer> tickerToNumberRegistered = new ConcurrentHashMap<String,Integer>();

    protected ClientRegistryDetailsSingleton() {
         // Exists only to defeat instantiation.
    }

    public static synchronized ClientRegistryDetailsSingleton getInstance() {
        return instance;
    }

    public synchronized ConcurrentHashMap<String, Integer> getTickerToNumberRegistered() {
        return tickerToNumberRegistered;
    }

    public void addToClienets(String ticker){}

    public void removeFromClients(String ticker){}
}

Answer 1

发布的代码中存在竞争条件，如果两个线程调用 getInstance() 并且单例尚未构建，则可能导致构建多个单例实例:

public static ClientRegistryDetailsSingleton getInstance() {
    if(instance == null) {                                   // Line 1
        instance = new ClientRegistryDetailsSingleton();     // Line 2
    }
}

可能执行两个线程，T1 和 T2:

• T1 在第 1 行执行检查并进入 if 分支。
• T1 已挂起，instance 仍为 null。
• T2 在第 1 行执行检查并进入 if 分支。
• T2 构造类并分配给实例。
• T2 返回实例给调用者。
• T2 暂停。
• T1 再次启动并构造另一个实例并分配给instance。

单实例的构造必须是线程安全的。可能的解决方案是:

• 使 getInstance() 方法同步。

• 不要使用惰性初始化(如果可能):

  private static final ClientRegistryDetailsSingleton instance =
      new ClientRegistryDetailsSingleton();