如何为Guice指定默认的Enum实例？

Question

I'd need something like

@DefaultInstance(Level.NORMAL)
enum Level {NORMAL, FANCY, DEBUGGING}

which would make Guice to return Level.NORMAL for the expression

injector.getInstance(Level.class)

There's no such thing like @DefaultInstance . As a workaround I've tried @ProvidedBy with a trivial Provider , but it doesn't work.

Answer 1

Maybe overriding modules could help you. A default level can be configured using AppLevel module:

public class AppModule extends AbstractModule {
    @Override
    public void configure() {
        bind(Level.class).toInstance(Level.NORMAL);
        // other bindings
    }
}

and a specific one can be configured in a small overriding module:

public class FancyLevelModule extends AbstractModule {
    @Override
    public void configure() {
        bind(Level.class).toInstance(Level.FANCY);
    }
}

At the end just create an injector overriding the AppModule with a specific Level config:

public static void main(String[] args) {
    Injector injector = 
        Guice.createInjector(
            Modules.override(new AppModule()).with(new FancyLevelModule())
    );

    System.out.println("level = " + injector.getInstance(Level.class));
}

UPDATE

This problem can be solved in a bit different way. Let's say that Level is used in a class as an injected field:

class Some
{
  @Injected(optional = true)
  private Level level = Level.NORMAL;
}

A default level will be initialized as part of the creation of instances of Some . If some Guice config module declares some other level it will be optionally injected.

Answer 2

A solution, but unfortunatelly not using annotations, would be:

enum Level 
{
    NORMAL, FANCY, DEBUGGING;

    static final Level defaultLevel = FANCY; //put your default here
}

then define module like this:

public class DefaultLevelModule extends AbstractModule 
{
    @Override public void configure() 
    {
        bind(Level.class).toInstance(Level.defaultLevel);
    }
}

Answer 3

It's the issue 295 and looks like a very trivial bug.

I've patched it for myself and maybe one day somebody there will fix this very old issue, too.