Question

Here is my client:

class Client {
    @Inject(optional=true) Service service;
}

Sometimes that Service isn’t needed, and we know that information when the JVM starts (i.e before the binder is run).
How do I make the binding optional? If I don’t specify a binding at all it tries to new the Service (and fails because there is no zero-argument constructor: “Error while injecting at package.Client.service(Service.java:40): Could not find a suitable constructor in package.Service.“), and I can’t do:

binder.bind(Service.class).toInstance(null);

because Guice seems to disallow nulls. Any ideas?

Answer 1

Are you using Guice 2.0? I’ve tried this both with Service being an interface (service field is always null) and with it being a class (null if it can’t create a new instance with a JIT binding, an instance if it can). Both seem like what you’d expect.

In neither case did I use a binding like bind(Service.class).toInstance(null). If you do this, then you need to make sure that all injection points for Service specify that they allow it to be null. This can be done by annotating the injection point with any annotation called @Nullable (you can make your own or use an existing one):

class Client {
    @Inject @Nullable Service service;
}