java双重检查锁定延迟初始化我需要为映射使用volatile关键字吗

Question

从Usage_in_Java & Effective Java 2开始，我知道如果惰性初始化目标是单个变量，我需要volatile关键字

但是，我要执行多个延迟初始化，并将它们存储在ConcurrentHashMap中，怎么样？这个Map也需要易变吗

下面是代码示例

public class Utils {
    private static final Map<Integer, RemoteViews> remoteViewsMap = new ConcurrentHashMap<Integer, RemoteViews>();

    public static RemoteViews getRemoteViews(int resourceId) {
        RemoteViews remoteViews = remoteViewsMap.get(resourceId);
        if (remoteViews == null) {
            synchronized(remoteViewsMap){
                remoteViews = remoteViewsMap.get(resourceId);
                if (remoteViews == null) {
                    remoteViews = new RemoteViews("org.yccheok.gui", resourceId);
                    remoteViewsMap.put(resourceId, remoteViews);
                }
            }
        }
        return remoteViews;
    }
}

上述代码正确且线程安全吗

Answer 1

不需要volatile关键字，因为ConcurrentHashMap作为ConcurrentMap的实现提供了以下内存一致性效果：

actions in a thread prior to placing an object into a ConcurrentMap as a key or value happen-before actions subsequent to the access or removal of that object from the ConcurrentMap in another thread

然而，这不是您通常希望使用并发映射的方式。一般情况如下：

Object existing = concurrentMap.get(key);
// check if this key is already present
if (existing == null) {
  Object newObject = new Object();
  existing = concurrentMap.putIfAbsent(key, newObject); // atomic operation
  // another thread might have already stored mapping for key
  if (existing == null) {
    return newObject;
  }
}
return existing;

注意，它不能防止两个线程同时调用new Object()（如果创建新对象的成本很高，这可能是一个问题），但它允许您一起避免显式同步

更新：对于双重检查锁定，在您的情况下，应该如下所示：

Object value = concurrentMap.get(key);
if (value == null) {
  synchronized (lock) {
    value = concurrentMap.get(key);
    if (value == null) {
      value = new Object();
      concurrentMap.put(key, value);
    }
  }
}
return value;