1. Обработчик запросов

public class Request {
    public String parameter;
    public int argument;
}

public abstract class Worker{
    protected final int argument;

    public Worker(int argument) {
        this.argument = argument;
    }

    public abstract void doWork();
}

public class Worker1 extends Worker {
    private ServiceA serviceA;
    private ServiceB serviceB;

    public Worker1(ServiceA serviceA, ServiceB serviceB, int argument) {
        super(argument);
        this.serviceA = serviceA;
        this.serviceB = serviceB;
    }

    @Override
    public void doWork() {
        System.out.println(String.format("Worker1 starts work with argument %d services %s and %s",  argument, serviceA, serviceB));
    }
}

public class Worker2 extends Worker {
    private ServiceB serviceB;
    private ServiceC serviceC;

    public Worker2(ServiceB serviceB, ServiceC serviceC, int argument) {
        super(argument);
        this.serviceB = serviceB;
        this.serviceC = serviceC;
    }

    @Override
    public void doWork() {
        System.out.println(String.format("Worker2 starts work with argument %d services %s and %s",  argument, serviceB, serviceC));
    }
}

Простая реализация обработчика

public class RequestHandler {
    private final ServiceA serviceA;
    private final ServiceB serviceB;
    private final ServiceC serviceC;

    public RequestHandler(ServiceA serviceA,
                             ServiceB serviceB,
                             ServiceC serviceC) {
        this.serviceA = serviceA;
        this.serviceB = serviceB;
        this.serviceC = serviceC;
    }

    public void handleRequest(Request request) {
        Worker worker = null;
        if (request.parameter.equals("case1")) {
            worker = new Worker1(serviceA, serviceB, request.argument);
        } else if (request.parameter.equals("case2")) {
            worker = new Worker2(serviceB, serviceC, request.argument);
        }
        //Здесь и далее не буду добавлять проверки на корректность входного
        //параметра, чтобы не загромождать код
        worker.setServiceA(serviceA);
        worker.setServiceB(serviceB);
        worker.setServiceC(serviceC);

        worker.doWork();
    }
}

• сразу бросается в глаза внедрение зависимостей через set*-методы. Пришлось добавить эти методы в абстрактный класс;
• значит, и все три зависимости на ServiceA, ServiceB и ServiceC переехали «наверх» в абстрактный класс. Но ему совершенно необязательно знать о сервисах, это дело его наследников. К примеру, Worker1 абсолютно не нуждается в ServiceC;
• if — else if-структура. Она будет очень страшно выглядеть при увеличении количества наследников в иерархии Worker;
• передача зависимостей в конструктор обработчика. Сам обработчик запросов об этих зависимостях не должен знать — они ему ни к чему. Нужны они только самим экземплярам Worker непосредственно,
• Слишком много ответственностей для класса RequestHandler: он умеет маппить запрос на нужный Worker, создавать его и проставлять зависимости. В идеале (согласно принципам SOLID) хочется следовать парадигме «Один класс — одна ответственность».

2. Подключаем Guice

<dependency>
      <groupId>com.google.inject</groupId>
      <artifactId>guice</artifactId>
      <version>${guice.version}</version>
</dependency>

public abstract class Worker {   
    public abstract void doWork();
}

public class Worker1 extends Worker{
    private ServiceA serviceA;
    private ServiceB serviceB;

    @Inject
    public Worker1(ServiceA serviceA, ServiceB serviceB) {
        this.serviceA = serviceA;
        this.serviceB = serviceB;
    }

    @Override
    public void doWork() {
        System.out.println(String.format("Worker1 starts work with %s and %s", serviceA, serviceB));
    }
}

@Singleton
public class RequestHandler {
    private Provider<Worker1> worker1Provider;
    private Provider<Worker2> worker2Provider;

    @Inject
    public RequestHandler(Provider<Worker1> worker1Provider, 
                         Provider<Worker2> worker2Provider) {
        this.worker1Provider = worker1Provider;
        this.worker2Provider = worker2Provider;
    }

    public void handleRequest(Request request) {
        Worker worker = null;
        if (request.parameter.equals("case1")) {
            worker = worker1Provider.get();
        } else if (request.parameter.equals("case2")) {
            worker = worker2Provider.get();
        }
        worker.doWork();
    }
}

Provider<T> — an object capable of providing instances of type T

 public static void main( String[] args ) {
        Request request = new Request();
        request.parameter = "case1";
        request.argument = 5;

        Injector injector = Guice.createInjector();
        RequestHandler requestHandler = injector.getInstance(RequestHandler.class);

        requestHandler.handleRequest(request);

        request.parameter = "case2";
        requestHandler.handleRequest(request);
    }

3. Прокидываем аргументы

@Inject
public Worker1(ServiceA serviceA, ServiceB serviceB, int argument) {

@Singleton
public class WorkerFactory {
    private ServiceA serviceA;
    private ServiceB serviceB;
    private ServiceC serviceC;

    @Inject
    public WorkerFactory(ServiceA serviceA,
                         ServiceB serviceB, 
                         ServiceC serviceC) {
        this.serviceA = serviceA;
        this.serviceB = serviceB;
        this.serviceC = serviceC;
    }

    public Worker1 createWorker1 (int argument) {
        return new Worker1(serviceA, serviceB, argument);
    }

    public Worker2 createWorker2 (int argument) {
        return new Worker2(serviceB, serviceC, argument);
    }
}

Guice AssistedInject

<dependency>
      <groupId>com.google.inject.extensions</groupId>
      <artifactId>guice-assistedinject</artifactId>
      <version>${guice.version}</version>
</dependency>

public interface WorkerFactory {
    Worker1 createWorker1 (int argument);

    Worker2 createWorker2 (int argument);
}

public class Module extends AbstractModule {
    @Override
    protected void configure() {
        install(new FactoryModuleBuilder().implement(Worker1.class, Worker1.class)
                .implement(Worker2.class, Worker2.class)
                .build(WorkerFactory.class));
    }
}

FactoryModuleBuilder — provides a factory that combines the caller's arguments with injector-supplied values to construct objects.

• метод build(WorkerFactory.class) сообщает Guice, что нужно предоставить реализацию фабрики WorkerFactory;
• метод implement принимает два аргумента:

  implement (Class<T> source, Class<? extends T> target)

  source — интерфейс возвращаемого значения, target — его реализация, которую наша фабрика будет предоставлять;
• В данном случае пришлось явным образом задавать создание Worker1 и Worker2, хотя у них есть общий интерфейс Worker. Терпение, на следующем шаге мы это поправим;
• install(new FactoryModuleBuilder()) — завершаем конфигурацию, добавляя новый модуль к нашему.

@AssistedInject
public Worker1(ServiceA serviceA, ServiceB serviceB, @Assisted int argument) 

@Singleton
public class RequestHandler {
    private WorkerFactory workerFactory;

    @Inject
    public RequestHandler(WorkerFactory workerFactory) {
        this.workerFactory = workerFactory;
    }

    public void handleRequest(Request request) {
        Worker worker = null;
        if (request.parameter.equals("case1")) {
            worker = workerFactory.createWorker1(request.argument);
        } else if (request.parameter.equals("case2")) {
            worker = workerFactory.createWorker2(request.argument);
        }

        worker.doWork();
    }
}

    Injector injector = Guice.createInjector(new Module());

4. Параметризуем Factory

public interface WorkerFactory<T extends Worker> {
    T createWorker (int argument);
}

public class Module extends AbstractModule{
    @Override
    protected void configure() {
        install(new FactoryModuleBuilder().implement(Worker.class, Worker1.class)
                .build(new TypeLiteral<WorkerFactory<Worker1>>() {}));

        install(new FactoryModuleBuilder().implement(Worker.class, Worker2.class)
                .build(new TypeLiteral<WorkerFactory<Worker2>>() {}));
    }
}

TypeLiteral<T> — represents a generic type T. Java doesn't yet provide a way to represent generic types, so this class does

@Singleton
public class RequestHandler {
    private WorkerFactory<Worker1> worker1Factory;
    private WorkerFactory<Worker2> worker2Factory;

    @Inject
    public RequestHandler(WorkerFactory<Worker1> worker1Factory,
                          WorkerFactory<Worker2> worker2Factory) {
        this.worker1Factory = worker1Factory;
        this.worker2Factory = worker2Factory;
    }

    public void handleRequest(Request request) {
        Worker worker = null;
        if (request.parameter.equals("case1")) {
            worker = worker1Factory.createWorker(request.argument);
        } else if (request.parameter.equals("case2")) {
            worker = worker2Factory.createWorker(request.argument);
        }

        worker.doWork();
    }
}

5. Multibindings

MapBinder — an API to bind multiple map entries separately, only to later inject them as a complete map.

<dependency>
      <groupId>com.google.inject.extensions</groupId>
      <artifactId>guice-multibindings</artifactId>
      <version>4.2.0</version>
</dependency>

MapBinder<String, WorkerFactory> binder = MapBinder.newMapBinder(binder(), String.class, WorkerFactory.class);

 new TypeLiteral<WorkerFactory<Worker1>>(){}

public class Module extends AbstractModule{
    @Override
    protected void configure() {
        install(new FactoryModuleBuilder().implement(Worker.class, Worker1.class)
                .build(new TypeLiteral<WorkerFactory<Worker1>>() {}));

        install(new FactoryModuleBuilder().implement(Worker.class, Worker2.class)
                .build(new TypeLiteral<WorkerFactory<Worker2>>() {}));

        MapBinder<String, WorkerFactory> binder = MapBinder.newMapBinder(binder(), String.class, WorkerFactory.class);
        binder.addBinding("case1").to(new TypeLiteral<WorkerFactory<Worker1>>(){});
        binder.addBinding("case2").to(new TypeLiteral<WorkerFactory<Worker2>>(){});
    }
}

@Singleton
public class RequestHandler {
    private Map<String, WorkerFactory> workerFactoryMap;

    @Inject
    public RequestHandler(Map<String, WorkerFactory> workerFactoryMap) {
        this.workerFactoryMap = workerFactoryMap;
    }

    public void handleRequest(Request request) {
        Worker worker = workerFactoryMap.get(request.parameter)
                .createWorker(request.argument);
        worker.doWork();
    }
}

Небольшое заключение