Interfaces - register class inheriting interface to create objects based on the class

Question

How can I "register" a class which implements an interface to create objects based on the "registered" class? Let me explain what I want to do:

I an coding a request processor for my thread pooled server and I want the request processor can be "configured" on the server so I can choose which request processor the server should use for it's requests. Here is how I think it should have worked:

// yes I know the interface can't extend the abstract Thread class ...
// see in the Server class what I want to do with it
public interface RequestProcessor extends Thread {

public final Socker socket;

RequestProcessor(final Socket paramSocket) {
    socket = paramSocket;
}

// abstract so each request processor implementing this interface
// has to handle the request in his specific way
abstract void run();

}

public class RequestProcessorA implements RequestProcessor {

RequestProcessorA(final Socket paramSocket) {
    super(paramSocket);
}

@Override
public void run() {
    // do something with the request
}

}

public class RequestProcessorB implements RequestProcessor {

RequestProcessorB(final Socket paramSocket) {
    super(paramSocket);
}

@Override
public void run() {
    // do something different with the request
}

}

So having this as interface (or as abstract class to be able to extend the Thread class) I want to create a server and tell him to use wheater RequestProcessorA or RequestProcessorB like:

public class Server extends Thread {

private final RequestProcessor processor;
private final ServerSocker server;

Server(final int paramPort, final RequestProcessor paramProcessor) {
    processor = paramProcessor;
    server = new ServerSocket(paramPort);
}

@Override
public void run() {
    while (true) {
        Socket socket = server.accept();
        // how can I create a new RequestProcessorA object if "processor" is type of RequestProcessorA???
        // how can I create a new RequestProcessorB object if "processor" is type of RequestProcessorB???
        // how can I create a new CustomProcessor object if "processor" is type of CustomProcessor???
        RequestProcessor rp = ???;
        // add rp to the thread pool
    }
}

}

So how to design such requirements? I wan't to make this an extendable library so others can simply use the server and just have to code their own request processors by extendimg/implementing my interface/abstract class and simply "register" their own request processor?

I hope you guys understand what I mean. And sorry if this is a dup but I dont really know the technical terms for this :/

@EJP so this isn't a simple interface/abstract class question? — Pali
– Pali, Commented Dec 10, 2015 at 21:32
@Pali This is about creation of instances, so it's more complicated than that. — Aasmund Eldhuset
– Aasmund Eldhuset, Commented Dec 10, 2015 at 21:35
@Pali No, it is an interface/abstract class/factory/registration question, and that is the answer that Java provides. — user207421
– user207421, Commented Dec 10, 2015 at 21:39

Aasmund Eldhuset · Accepted Answer · 2015-12-10 21:33:07Z

3

You will want to use the Abstract Factory pattern. Create one factory interface, and one implementation for each class you might want to produce:

public interface IRequestProcessorFactory {
    RequestProcessor create(Socket paramSocket);
}

and implement it like this:

public class RequestProcessorAFactory implements IRequestProcessorFactory {
    public RequestProcessor create(Socket paramSocket) {
        return new RequestProcessorA(paramSocket);
    }
}

and pass an instance of it to Server. Then, call

RequestProcessor rp = factory.create(socket);

to obtain what will either be a RequestProcessorA or RequestProcessorB, depending on which factory was passed.

answered Dec 10, 2015 at 21:33

Aasmund Eldhuset

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3 Comments

John Over a year ago

Would you not consider this Factory method pattern rather than Abstract Factory ?

Pali Over a year ago

Working like a charm! Thank you very much random citizen!

Aasmund Eldhuset Over a year ago

Great! @John: I believe that this is Abstract Factory, since we have multiple factories, one per class that we might want to produce. I don't think it matters that the parent type is an interface rather than an abstract class. Look at the class diagram in the Wikipedia article.

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