How can I use a Factory pattern to instanciate objects read from a configuration?

Question

This is my problem: I'm writing a Java library that allows to process a specific type of message in one of two possible ways (but potentially the client may add other processing modalities). I've created the following classes, according to the strategy pattern:

    interface Processor {
        Message processMessage();
    }

    class ProcessorOne<M extends Message> implements Processor{
        M processMessage(){ //do stuff }
    }

    class ProcessorTwo<M extends Message> implements Processor{
        M processMessage(){  //do other stuff }
    }

    abstract class Message{
    }

    //example
    class MessageTypeOne extends Message{
    }
    //example
    class MessageTypeTwo extends Message{
    }

Now, the Client uses a configuration in such a way that for every concrete type of Message he says: "MessageTypeOne: ProcessorOne, MessageTypeTwo: ProcessorTwo, MessageTypeThree: ProcessorOne", establishing that messages of type MessageTypeOne have to processed by class ProcessorOne and so on.. The configuration could be a xml file or any other type of configuration, this is up to the client.

My question is: How can I create a "link" between what Client will read from configuration (using its own way of parsing, according to the configuration type he has chosen) and the instantiation of the concrete processors using a Factory pattern?

Thanks in advance

Answer 1

I think it's difficult to archive that. You can create processor instance based on their name through Factory pattern, but cannot associate these two class in config file. I think the easist way is use different Factory for your business logic. For example:

    Map<String, String> conf = null; // What parsed from config
    String messageType = // What you have 
    Processor processor = ProcessorFactory.createProcessor(conf.get(messageType));
    Message message = MessageFactory.createMessage(messageType);
    // processing