Optional Scoped injection in Guice

Question

I need to inject a field only if it is available in the current scope, and null otherwise. For example:

public class Thinger implements Provider<SomeSuch> {
    public @Inject(optional=true) HttpServletRequest request;        

    public SomeSuch get() {
        return request == null ? new WhosIt() : WhatsIt();
    }
}

However, if HttpServletRequest is bound (which it is) but not in scope, I get a ProvisioningException. I have been able to find an elegant way to do this so I am relegated to do something like.

HttpServletRequest request = null;
try {
    request = injector.getInstance(HttpServletRequest.class);
} catch(ProvisioningException e) {}

Which just feels all manner of wrong. Is there a proper way to do this?

score 3 · Answer 1 · answered Jan 29 '11 at 16:24

What exactly determines your class to be available? The HttpServletRequest in somehow counterintuitive to me, since not having a request within a non request-scoped service sounds like an error to me.

One idea would (in general) be to write a custom Provider for a Holder with just a get/set method. In the provider you can run the checks, whether or not your Thing is available in the current scope, it always returns a Holder of the type you need, but it might be empty/null depending on the thing being available. Since you always return a Holder the Injector should be fine. You just need to check for null in the component you are injecting this into.

Hope this helps.

How can we check if we are "inside" one scope? – Bruno Medeiros May 01 '14 at 16:33 — Bruno Medeiros, May 01 '14 at 16:33

Optional Scoped injection in Guice

1 Answers1