The Chain of Responsibility Pattern

The Chain of Responsibility pattern is useful when there is a chain of components that need to accomplish the same operation. Each component may have a default implementation and/or cancel the processing chain.

Let’s say we want to write a class that handles http requests. For each request, we want to log it, verify its authentication, and parse the content. Each operation can be designed as a class implementing the same interface.

public interface HttpRequestHandler {

   HttpRequestHandler next();
   void setNext(HttpRequestHandler next);

   // allows to chain other handlers
   default void add(HttpRequestHandler handler) {
      if (next() != null) {
        next().add(handler);
      } else {
        setNext(handler);
      }
   }; 

   void process(HttpRequest request);
}

public class Logger implements HttpRequestHandler {

   private Log log = ...
   private HttpRequestHandler next;

   // implements getter/setter
   // ...

   public void process(HttpRequest request) {
      log.log(request); // order can changed if we want the operations to start at the end of the chain
      if (next != null) next.process(request);
   }
}

public class Authenticator implements HttpRequestHandler {

    private HttpRequestHandler next;

    // implements getter/setter
    // ...

    public void process(HttpRequest request) {
      if (authenticate(request)) {
         if (next != null) next.process(request);
      }
      // the chain is disrupted if the request does not pass the security check
    }

    private boolean authenticate(HttpRequest request) {
      // ...
    }
}


public class Parser implements HttpRequestHandler {

    private HttpRequestHandler next;

    // implements getter/setter
    // ...

    public void process(HttpRequest request) {
      parse(request);
      if (next != null) next.process(request);
    }

    private void parse(HttpRequest request) {
       // ...
    }
}

The good thing with this design is that we can add more operations without too much effort. Now we can have our client code manipulating the handlers:

public class WebServer {

   public HttpResponse respondTo(HttpRequest request) {
      handlers().handle(request);
   }

   private HttpRequestHandler handlers() {
      Logger logger = new Logger();
      Authenticator auth = new Authenticator();
      Parser parser = new Parser();
      logger.add(auth);
      logger.add(parser);
      return logger;
   }

}