Implementing communication protocol in Java using state pattern design

Question

Apologies if this is answered elsewhere; couldn\'t find enough information to convince myself of the best way to do this. I also realize this is a lengthy explanation with no co

Answer 1

I have used an enum for this style of parsing with one enum for each state. An example is here http://vanillajava.blogspot.co.uk/2011/06/java-secret-using-enum-as-state-machine.html

interface Context {
    ByteBuffer buffer();
    State state();
    void state(State state);
}
interface State {
    /**
       * @return true to keep processing, false to read more data.
     */
    boolean process(Context context);
}
enum States implements State {
    XML {
        public boolean process(Context context) {
            if (context.buffer().remaining() < 16) return false;
            // read header
            if(headerComplete)
                context.state(States.ROOT);
            return true;
        }
    }, ROOT {
        public boolean process(Context context) {
            if (context.buffer().remaining() < 8) return false;
            // read root tag
            if(rootComplete)
                context.state(States.IN_ROOT);
            return true;
        }
    }
}

public void process(Context context) {
    socket.read(context.buffer());
    while(context.state().process(context));
}

Answer 2

I would first consider the following points :

1. Are your state components (WAITING_FOR_COMMAND, COMMAND_RECEIVED etc )central to your application logic ? That is to say , does it really matter a lot what state the parser is in , and what state it transitions to , for the app to work correctly ? What are the values that each of the state is going to hold ? Do these values differ greatly from one state to another ? If your answers are yes , then probably you have a valid case for State pattern.But I have often seen state machines used in places where it was simply an overkill.

2. Your use-case seems more like in need of Command pattern and an Interpreter pattern ( as it seems you are writing a grammar ).