具有循环依赖的HK2注射

Question

I am using HK2 as dependency injection framework. I am instantiating a State Machine inside my application. So, A <-> B is quite possible because depending on the input, one can transition back to A from B.

@Singleton 
class StateManager extends SomeInterface{

    @Inject
    private B b;

    @Inject
    private A a;
}

@Singleton 
class A extends State{

    @Inject
    private B b;

    @Inject
    private StateManager sm;

}

@Singleton 
class B extends State{

    @Inject
    private A a;

    @Inject
    private StateManager sm;
}

How can I achieve something like this? Above example is just for illustration. I have more states than this. I tried to see if it is a duplicate but couldn't find one that deals with. When I try to ServiceLocator.getService(StateManager.class) to get this graph, I get circular dependency exception. What is the better of doing such things?

Answer 1

You should be able to to inject javax.inject.Provider<B> . From the Javadoc

Provides instances of T . Typically implemented by an injector. For any type T that can be injected, you can also inject Provider<T> . Compared to injecting T directly, injecting Provider<T> enables:

• retrieving multiple instances.
• lazy or optional retrieval of an instance.
• breaking circular dependencies.
• abstracting scope so you can look up an instance in a smaller scope from an instance in a containing scope.

Then just call get() when you need it

B b = bProvider.get();