java - 创建一个返回单例的工厂

Question

我有公共(public)代码(我称之为 Controller 的多个类)需要由项目中的多个包共享。 我正在考虑创建一个工厂，返回这些 Controller 。 因此，工厂将有一个 HashMap ，它可以返回请求的 Controller 或创建一个新 Controller (如果未创建)。 Controller 有共同的代码，因为我不需要创建这些 Controller 的多个实例，我认为它们应该是单例的。

这看起来是个好方法吗？

Answer 1

我想你需要的是 Multiton pattern .

The multiton pattern is a design pattern similar to the singleton, which allows only one instance of a class to be created. The multiton pattern expands on the singleton concept to manage a map of named instances as key-value pairs. Rather than have a single instance per application (e.g. the java.lang.Runtime object in the Java programming language) the multiton pattern instead ensures a single instance per key.

public class FooMultiton {
    private static final Map<Object, FooMultiton> instances = new HashMap<Object, FooMultiton>();

    private FooMultiton() {
        // no explicit implementation
    }

    public static synchronized FooMultiton getInstance(Object key) {

        // Our "per key" singleton
        FooMultiton instance = instances.get(key);

        if (instance == null) {
            // Lazily create instance
            instance = new FooMultiton();

            // Add it to map   
            instances.put(key, instance);
        }

        return instance;
    }

    // other fields and methods ...
}

The controllers have common code and since I don't need to create multiple instances of these controller, I think they should be singletons.

您需要单个实例不一定意味着您需要一个单例模式。您可以拥有一个实例并将其传递给后续调用。不必使用 private 构造函数强制执行单例。

另请阅读 Evil Singletons有关 Singletons 缺点的更多信息。读完之后，如果您仍然觉得需要 Singleton，那就继续吧。