GWTP bind enum in GIN

Question

I'm trying to put a enumeration into the ginjector with these lines of code:

ClientGinjector.java

MyEnum getMyEnum();

ClientModule.java

bind(MyEnum.class).in(Singleton.class);

But when I'm trying to compile I get the following error:

[ERROR] Error injecting bla.blup.MyEnum: Unable to create or inherit binding: Binding requested for constant key 'bla.blup.MyEnum' but no explicit binding was found

Can anyone please help me?

Answer 1

An enum class cannot be constructed, its only valid instances are its enum values. That menas you have to bind a specific enum value that will be injected into whatever field or parameter of that enum type.

Guice/GIN encourages you to use binding annotations for constants, so you can possibly inject different constant values depending on context; e.g.

@Named("foo") @Inject MyEnum myEnum;

–

bindConstant().annotatedWith(Names.named("foo")).to(MyEnum.FOO);

If you don't want to use a binding annotation (because you know you'll only ever want a single enum value throughout your app), you cannot use bindConstant(), but you can use toInstance:

@Inject MyEnum myEnum;

…

bind(MyEnum.class).toInstance(MyEnum.FOO);

This will only work in Guice though, not in GIN, which doesn't have toInstance. In GIN, you have to use a Provider class or a @Provides method:

class MyEnumProvider implements Provider<MyEnum> {
  @Override
  public MyEnum get() {
    return MyEnum.FOO;
  }
}
…
bind(MyEnum.class).toProvider(MyEnumProvider.class);

or

@Provides
MyEnum provideMyEnum() {
  return MyEnum.FOO;
}

Both approaches above will also work with Guice.