auto scan for guice

Question

I have never used guice before, and I wanted to try it out on an example project with jersey based JAX-RS API backed by a service-bean. I followed this guide: http://randomizedsort.blogspot.de/2011/05/using-guice-ified-jersey-in-embedded.html and was able to bring it to work. My setup is very simple, a JAX-RS resource is invoked via Guice and has a field that is annotated @Inject and injected by Guice:

@Path("configuration")
@Produces(MediaType.APPLICATION_JSON)
@Singleton
public class ConfigurationResource {

    @Inject
    private ConfigurationService configurationService;

So far so good, everything works like it should, besides following: I am using GuiceServletContextListener for setting things up and have to name each component explicitly:

@WebListener
public class GuiceInitializer extends GuiceServletContextListener{
    @Override
    protected Injector getInjector() {
        return Guice.createInjector(new JerseyServletModule() {
            @Override
            protected void configureServlets() {
                //resources
                bind(ConfigurationResource.class);

                //services
                bind(ConfigurationService.class).to(ConfigurationServiceImpl.class);

                // Route all requests through GuiceContainer
                serve("/management/*").with(GuiceContainer.class);
            }
        });
    }
}

I find it pretty inconvenient to explicitly name all dependencies. I have worked with standalone jersey before and it's perfectly capable of auto-scanning for resources in defined packages. Also Spring and CDI are capable of mapping implementation to interfaces without need to explicitly name them.

Now the question part:

• is there any autoscan extension/setting for guice? I found some on the internet, but it's hard to tell which of them are still useable and uptodate.

• is there any other possibility to make configuration of implementations and resources more convenient?

thanks in advance. Leon

Answer 1

I do not think Guice has built in support for someting like the component-scan of Spring framework. However, it is not difficult to simulate this feature in Guice.

You simply need to write a helper module like the following

import com.google.inject.AbstractModule;
import org.reflections.Reflections;

import java.lang.annotation.Annotation;
import java.util.Arrays;
import java.util.HashSet;
import java.util.Set;

/**
 * To use this helper module, call install(new ComponentScanModule("com.foo", Named.class); in the configure method of 
 * another module class.
 */
public final class ComponentScanModule extends AbstractModule {
    private final String packageName;
    private final Set<Class<? extends Annotation>> bindingAnnotations;

    @SafeVarargs
    public ComponentScanModule(String packageName, final Class<? extends Annotation>... bindingAnnotations) {
        this.packageName = packageName;
        this.bindingAnnotations = new HashSet<>(Arrays.asList(bindingAnnotations));
    }

    @Override
    public void configure() {
        Reflections packageReflections = new Reflections(packageName);
        bindingAnnotations.stream()
            .map(packageReflections::getTypesAnnotatedWith)
            .flatMap(Set::stream)
            .forEach(this::bind);
    }
}

To component scan a package like com.foo and sub packages for classes carrying @Singleton, use it in this way:

public class AppModule extends AbstractModule {
   public void configure() {
     install(new ComponentScanModule("com.foo", Singleton.class));
   }
}