Optional injection in Guice

Question

Is it possible to tell Guice that it's not necessary to inject all constructor arguments? For example, I have a constructor Foo that takes two args of types Bar and Baz. All of them are optional in my system: they may

1. both present
2. both missing
3. only Bar present
4. only Baz present.

That said, it depends on other modules providing these bindings. I want to get something like this:

class Foo {
  private final Bar bar;
  private final Baz baz;

  @Inject(optional = true)
  public Foo(@Nullable Bar bar, @Nullable Baz baz) {
    this.bar = bar;
    this.baz = baz;
  }
}

But I can't really use optional with constructors. Is there a way to do that?

Answer 1

I think the preferred Guice pattern for this is:

public class HolderPatter {

  static class Bar {
    @Inject Bar(BarDependency dependency) {}
  }
  static class Baz {
    @Inject Baz(BazDependency dependency) {}
  }

  static class BarHolder {
    @Inject(optional=true) Bar value = null;
  }

  static class BazHolder {
    @Inject(optional=true) Baz value = null;
  }

  static class Foo {
    private final Bar bar;
    private final Baz baz;

    @Inject
    public Foo(BarHolder bar, BazHolder baz) {
      this.bar = bar.value;
      this.baz = baz.value;
    }
  }
}

Note that this will also allow you to specify sane default values...

Answer 2

The latest version of Guice recently added OptionalBinder which works better than the @Inject(optional=true) method and also adds several advanced features.

See also the thread where OptionalBinder was announced.