Making Guice Injector Singleton for Consistency

Question

I was wondering how the injector is typically used. I understand that it's used mainly at start-up time, but how would you use it at run-time to create an object of certain implementation?

For example, I have an interface User with implementation of UserA, UserB and UserC. My idea of using Guice at run-time is to wrap it in a Singleton class so that I can get retrieve it rather than creating a new injector every time I want to create an instance of User object.

//Create in singleton wrapper class
Injector injector = Guice.createInjector(new SecurityModule());

//Retrieve and use at run-time (Manager is the singleton wrapper class)
Injector injector = Manager.getInstance().getInjector();
User user = injector.getInstance(User.class);

Is this how it's typically done? Basically, I want to make sure that I centralize all configuration so that there is less error (e.g. using the wrong module). It's also easier to manage so if I decided to use a different configuration module, I can just change it in a class, rather than in every Java class that is using it.

Thanks, Jack

score 2 · Answer 1 · answered Apr 13 '11 at 17:51

You shouldn't need to use the Injector directly in an application except in rare and specific circumstances, and even then you definitely should not make it a static Singleton. If you want to be able to retrieve new instances of User using Guice somewhere in your application, you should just inject a Provider<User> and call get() on it when you need.

score 1 · Answer 2 · answered Jun 03 '13 at 08:49

I would write:

public class Manager {
    private static Manager instance;
    // getInstance singleton method left out for brevity
    private Injector injector;
    private Manager() {
        injector = Guice.createInjector(...);
    }
    public <T> T get(Class<? extends T> cls) {
        return injector.getInstance(cls);
    }
}

so you can do

Manager.getInstance().get(User.class) and not have to use the Injector from Guice, allowing you to change Dependency Injection framework without changing your application code.

Making Guice Injector Singleton for Consistency

2 Answers2