Two-Stage Guice Provider

by Jean Tessier

This document shows how to pass an object between Guice providers so each one can initialize it in turn.

The finalized code makes up the rest of this repo.

All examples use Guice 7.0. They were tested with Spock 2.4 using Java 21.

Introduction

I got the idea for this pattern as I was trying to reuse a framework. This framework had been written for one specific application but its authors felt it was generic enough to write other applications. I was to be the first one to try and use it outside of its original application.

The framework uses Guice for its dependency injection. Early on, I encountered a problem where the original application did a lot of custom settings up in a provider method of its Module. I needed to have a similar module that would duplicate a lot of the setup, minus the bits specific to the original application, plus some new bits specific to my application. I wanted to extract the common parts to one generic module and trick Guice into chaining providers on the same object to complete its initialization.

This is their story.

Generic Service

Imagine the following service interface:

package service;

public interface Service {
    void setupClient1();
    void setupClient2();
    String getState();
}

It has different setup methods for different clients. This example is to illustrate that different clients might setup the service differently. In real life, there would be no dependency from the service upon its clients. For a more concrete example, the service might be a web server class and each client might configure a different set of URLs.

For the purposes of this discussion, here is a quick specification of what we are looking for:

package service

import spock.lang.Specification

class ServiceImplTest extends Specification {

    def sut = new ServiceImpl()

    def "has default state"() {
        expect:
        sut.state == "generic"
    }

    def "setupClient1 sets the state to client1"() {
        when:
        sut.setupClient1()

        then:
        sut.state == "client1"
    }

    def "setupClient2 sets the state to client2"() {
        when:
        sut.setupClient2()

        then:
        sut.state == "client2"
    }

}

And here is a simple implementation that satisfied it:

package service;

public class ServiceImpl implements Service {
    private String state = "generic";

    public void setupClient1() {
        state = "client1";
    }

    public void setupClient2() {
        state = "client2";
    }

    public String getState() {
        return state;
    }
}

Generic Guice Module

To begin with, we need a base Module that creates a generic Service implementation. Here is a test for it:

package generic

import spock.lang.Specification

class GenericModuleTest extends Specification {

    def sut = new GenericModule()

    def "provides a generic service"() {
        when:
        def actualService = sut.provideService()

        then:
        actualService != null
        actualService.state == "generic"
    }

}

And here is a generic Guice module with a provider method to create a plain ServiceImpl whenever a Service instance is required.

package generic;

import com.google.inject.*;
import service.*;

public class GenericModule extends AbstractModule {
    @Provides
    public Service provideService() {
        return new ServiceImpl();
    }
}

You can use it like this:

Injector injector = Guice.createInjector(new GenericModule());
Service service = injector.getInstance(Service.class);

The variable service now references a plain ServiceImpl.

Specialized Guice Modules

Different clients who require a customized Service instance need to use a their own module instead of the previous GenericModule.

A hypothetical client No. 1 could need a module that follows the following specification, that it calls setupClient1() on an existing Service instance:

package client1

import service.Service
import spock.lang.Specification

class Client1ModuleTest extends Specification {

    def sut = new Client1Module()

    def "configures a generic service for client1"() {
        given:
        def mockService = Mock(Service)

        when:
        def actualService = sut.provideService(mockService)

        then:
        actualService == mockService

        and:
        1 * mockService.setupClient1()
    }

}

This hypothetical client No. 1 could use this module:

package client1;

import com.google.inject.*;
import service.*;

public class Client1Module extends AbstractModule {
    @Provides
    public Service provideService() {
        Service service = new ServiceImpl();
        service.setupClient1();
        return service;
    }
}

A hypothetical client No. 2 could require this specification instead:

package client2

import service.Service
import spock.lang.Specification

class Client2ModuleTest extends Specification {

    def sut = new Client2Module()

    def "configures a generic service for client2"() {
        given:
        def mockService = Mock(Service)

        when:
        def actualService = sut.provideService(mockService)

        then:
        actualService == mockService

        and:
        1 * mockService.setupClient2()
    }

}

Which this implementation satisfies:

package client2;

import com.google.inject.*;
import service.*;

public class Client2Module extends AbstractModule {
    @Provides
    public Service provideService() {
        Service service = new ServiceImpl();
        service.setupClient2();
        return service;
    }
}

These two modules have some problems.

1. There is code duplication between both modules when it comes to instantiating ServiceImpl.
2. Each module is tied to using the ServiceImpl implementation of Service.

It would be better if we could somehow inject a Service implementation and have the module do its custom configuration.

A Solution That Does Not Work

Ideally, I would like to separate the choice of Service implementation, as provided by GenericModule, from the application-specific configuration, as provided by Client1Module and Client2Module.

I could rewrite Client1Module as:

    package client1;

    import com.google.inject.*;
    import service.*;

    public class Client1Module extends AbstractModule {
        @Provides
        public Service provideService(Service service) {
            service.setupClient1();
            return service;
        }
    }

And use it like this:

Injector injector = Guice.createInjector(new GenericModule(), new Client1Module());
Service service = injector.getInstance(Service.class);

I would hope that Guice picks to call Client1Module.provideService() to satisfy my request and injects a Service into this call by calling GenericModule.provideService(). But that's not the case. Because I now have two providers for Service, Guice gets confused and throws an error with the following message:

A binding to service.Service was already configured at generic.GenericModule.provideService().

I need to differentiate between the two providers, somehow.

A Solution That Works

I can use annotations to distinguish between the kinds of Service each provider is responsible for.

First, I qualify the Service provided by GenericModule as "generic".

package generic;

import com.google.inject.*;
import com.google.inject.name.*;
import service.*;

public class GenericModule extends AbstractModule {
    @Provides
    @Named("generic")
    public Service provideService() {
        return new ServiceImpl();
    }
}

I will also qualify the Service provided by client No. 1's module. This way, it is explicit in the calling code which Service I am interested in.

package client1;

import com.google.inject.*;
import com.google.inject.name.*;
import service.*;

public class Client1Module extends AbstractModule {
    @Provides
    @Named("client1")
    public Service provideService(@Named("generic") Service service) {
        service.setupClient1();
        return service;
    }
}

Here, it is made explicit that client No. 1 requires a "client1" flavor of Service, which will be provided by Client1Module.provideService(). The latter requires a "generic" flavor of Service, which will be provided by GenericModule.provideService(). The client simply asks for what it needs and lets Guice take care of the rest.

Injector injector = Guice.createInjector(new GenericModule(), new Client1Module());
Service service = injector.getInstance(Key.get(Service.class, Names.named("client1")));

Client No. 2 would use the following module to obtain a "client2" flavor of Service.

package client2;

import com.google.inject.*;
import com.google.inject.name.*;
import service.*;

public class Client2Module extends AbstractModule {
    @Provides
    @Named("client2")
    public Service provideService(@Named("generic") Service service) {
        service.setupClient2();
        return service;
    }
}

And use it like this:

Injector injector = Guice.createInjector(new GenericModule(), new Client2Module());
Service service = injector.getInstance(Key.get(Service.class, Names.named("client2")));

More Realistic

More realistically, I would declare a Client1 class that requires a "client1" kind of Service:

package client1;

import com.google.inject.*;
import com.google.inject.name.*;
import generic.*;
import service.*;

public class Client1 extends Client {
    @Inject
    public Client1(@Named("client1") Service service) {
        super(service);
    }
}

And a Client2 class that requires a "client2" kind of Service:

package client2;

import com.google.inject.*;
import com.google.inject.name.*;
import generic.*;
import service.*;

public class Client2 extends Client {
    @Inject
    public Client2(@Named("client2") Service service) {
        super(service);
    }
}

Here is a simple, generic Client superclass:

package generic;

import service.*;

public class Client {
    private final Service service;

    public Client(Service service) {
        this.service = service;
    }

    public Service getService() {
        return service;
    }
}

The nice thing is that Client does not need to know about the difference between clients No. 1 and No. 2. It deals exclusively in terms of the generic Service interface. Guice uses the configuration information to pick the right implementation and customize it appropriately.

Injector injector = Guice.createInjector(new GenericModule(), new Client1Module());
Client client = injector.getInstance(Client1.class);

Conclusion

This pattern for using Guice might prove useful.

Another possibility is that the way Service gets customized is all wrong. I could conceive of a Customizer object that would get injected into GenericModule.provideService(). Client1Module and Client2Module would provide bindings to specific types of Customizer, and that would be the end of it. This design might be more robust.

But if you cannot change GenericModule, then maybe the pattern I just covered can be of assistance.

Further Developments

After I sent this document to a few people for review, someone brought up the upcoming @New annotation for Guice. It seems to address some of the same concerns as the pattern I describe on this page. One difference is that my pattern can be chained for as long as necessary, whereas you can use the @New annotation only once.

---

Date	Edit
2009-01-28	First draft.
2009-01-29	Last substantial edit.
2020-04-28	Making sure it all still works with the latest version of Guice, convert tests to Spock
2020-05-03	Remove empty configure() methods that are no longer needed in Guice 4.2.3.