java - 这是哪种单例？

Question

public class ConnectionManager {

    private static Map <String, ConnectionManager> managerInstances = new HashMap<String, ConnectionManager>();

    private String dsKey;
    private ConnectionManager(String dsKey){
        this.dsKey = dsKey;
    }

    public static ConnectionManager getInstance(String dsKey){
        ConnectionManager managerInstance = managerInstances.get(dsKey);

        if (managerInstance == null) {
            synchronized (ConnectionManager.class) {
                managerInstance = managerInstances.get(dsKey);
                if (managerInstance == null) {
                    managerInstance = new ConnectionManager(dsKey);
                    managerInstances.put(dsKey, managerInstance);
                }
            }
        }
        return managerInstance;
    }
}

我最近在某个地方看到了这段代码，其中没有按照 GoF 的书籍定义使用单例模式。 Singleton 正在存储它自己的实例的Map。

这能称为什么样的单例呢？或者这是 Singleton 的有效使用吗？

Answer 1

这不是单例。它的名字叫multiton pattern .

Rather than have a single instance per application, the multiton pattern instead ensures a single instance per key.