Guice assisted inject with several factory methods and null parameters

Question

I have this interface and simple implementation:

public interface Data {    
}

import java.nio.file.Path;
import javax.annotation.Nullable;
import javax.inject.Inject;
import com.google.inject.assistedinject.Assisted;

public class SimpleData implements Data {
    @Inject
    public SimpleData(@Assisted @Nullable Path path) {
    }
}

I want to generate a Factory with different methods using guice.

import java.nio.file.Path;

import javax.annotation.Nullable;

public interface Factory {
    Data create();
    Data load(@Nullable Path path);
}

But the following module configuration:

import com.google.inject.Guice;
import com.google.inject.Injector;
import com.google.inject.assistedinject.FactoryModuleBuilder;

public class Main {
    public static void main(String[] args) {
        Injector injector = Guice.createInjector(
                binder -> binder.install(
                            new FactoryModuleBuilder().implement(Data.class, SimpleData.class)
                                                      .build(Factory.class)));
        Data data = injector.getInstance(Factory.class).create();
    }
}

fails:

Exception in thread "main" com.google.inject.CreationException: Guice creation errors:

    1) No implementation for java.nio.file.Path annotated with @com.google.inject.assistedinject.Assisted(value=) was bound.
      while locating java.nio.file.Path annotated with @com.google.inject.assistedinject.Assisted(value=)
        for parameter 0 at SimpleData.<init>(SimpleData.java:10)
      at Factory.create(Factory.java:1)
      at com.google.inject.assistedinject.FactoryProvider2.initialize(FactoryProvider2.java:539)
      at com.google.inject.assistedinject.FactoryModuleBuilder$1.configure(FactoryModuleBuilder.java:335)

    1 error
        at com.google.inject.internal.Errors.throwCreationExceptionIfErrorsExist(Errors.java:435)
        at com.google.inject.internal.InternalInjectorCreator.injectDynamically(InternalInjectorCreator.java:175)
        at com.google.inject.internal.InternalInjectorCreator.build(InternalInjectorCreator.java:109)
        at com.google.inject.Guice.createInjector(Guice.java:95)
        at com.google.inject.Guice.createInjector(Guice.java:72)
        at com.google.inject.Guice.createInjector(Guice.java:62)
        at Main.main(Main.java:9)

Answer 1

I solved my problem using the annotation @AssistedInject. Quote from the javadoc:

When used in tandem with FactoryModuleBuilder, constructors annotated with @AssistedInject indicate that multiple constructors can be injected, each with different parameters.

So i add the annotation and a constructor to the SimpleData class:

public class SimpleData implements Data {
    @AssistedInject
    public SimpleData(@Assisted Path path) {

    }
    @AssistedInject
    public SimpleData() {

    }
}

i removed the @Nullable annotation from the factory:

import java.nio.file.Path;

public interface Factory {
    Data create();
    Data load(Path path);
}

Answer 2

@Nullable does not mean that if you don't have a binding, then null will be injected. It only allows writing bindings to null. If you don't have a binding and there is no applicable JIT-binding, then injection will fail.

Your factory's create() method requires Guice to find an @Assisted Path binding, but it obviously can't find it since you've never created one, so it fails.

Honestly, I'm not sure if there is a clean way to implement such defaulting. Ideally you should mark Path with some binding annotation and add a default binding to null for it, but @Assisted already is a binding annotation, and it is not possible to have multiple binding annotations on a single injection point. You can try creating a binding for @Assisted Path:

binder.bind(Path.class).annotatedWith(Assisted.class).toInstance(null);

However, I'm not sure if it would work because Assisted can be special to Guice. And even if it will work, it is not very clean - there may be conflicts with other assisted factories accepting Paths.