如何使用泛型实现ServiceLocator？

Question

Having this interface:

public interface ServiceLocator {
    <T> void setService(Class<T> klass, Factory<T> factory)

    <T> void setConstant(Class<T> klass, T value)

    <T> T getObject(Class<T> klass)

}

How do I implement it? I mean, how I declare the structure data? Is this right?

private Map<Class, Factory> services = new HashMap<>();
private Map<Class, Object> constants = new HashMap<>();

Answer 1

private Map<Class, Factory> services = new HashMap<>();
private Map<Class, Object> constants = new HashMap<>();

If you wanna use this structures you can declare it like that:

private Map<Class<?>, Factory<T>> services = new HashMap<>();
private Map<Class<?>, Object> constants = new HashMap<>();

Remember to put Factory<T> to catch the type. Using Class provides you to use AnyClass.class as Key

Btw you can put all in one Map like private Map<Class<?>, Object> and at the get function cast it if it's a Factory or not ;)

Answer 2

Solution:

private Map<Class<?>, Factory<?>> services = new HashMap<>();
private Map<Class<?>, Object> constants = new HashMap<>();

Adding -Xlint:unchecked directive to the compiler options. And @SuppressWarnings("unchecked") on the methods where you need it. In my case is on getObject method.

Answer 3

Guava has an interface intended for a similar purpose: ClassToInstanceMap<B> . The API is:

<T extends B> T getInstance(Class<T> type);
<T extends B> T putInstance(Class<T> type, T value);

Looking at the implementation in MutableClassToInstanceMap.java , we see that it uses a HashMap<Class<? extends B>, B> HashMap<Class<? extends B>, B> wrapped in a ConstrainedMap . Its MapConstraint uses java.lang.Class.cast() to ensure that all the values in the map are legal instances of their corresponding keys.

Since B is often just Object , the implementation's HashMap ends up just being a HashMap<Class<? extends Object>, Object> HashMap<Class<? extends Object>, Object> .