Java servlet - implementing command pattern

Question

I'm trying to understand how to implement the 'Command design' into my java servlet. Here is an example of the servlet:

protected void processRequest(HttpServletRequest ...) {

    String action = request.getParameter("action"); 
    String destination = null;

    } if (action.equals("Home")) 
        destination = "Home";
    } else if (action.equals("Add")) {
         String var = request...

         try {
            db.add(var);
         } (DatabaseException e) {
            request.setAttribute("errorMessage", e.getMessage());
         }
         ...
    }
 }

What I understand is that every if statement should be converted to a separate class with an execute function that handles for example the adding of an object. What I don't understand is how the requests are handles, are they handles in the execute function and from there returned to the Servlet/JSP (my catched errors are put in a request and used in my JSP files)?

I hope someone can give me a basic structure and show me how to begin implementing it.

Answer 1

how to implement the 'Command design' into my java servlet.

What is Command pattern?

In object-oriented programming, the command pattern is a behavioral design pattern in which an object is used to represent and encapsulate all the information needed to call a method at a later time. This information includes the method name, the object that owns the method and values for the method parameters.

_{Source: wekipedia}

To implement Command Design Pattern in your case servlet's HttpServletRequest action parameter are home & add, I am considering two more like delete, update, etc.

you said,

What I understand is that every if statement should be converted to a separate class with an execute function that handles for example the adding of an object.

ok, create one interface with execute method return type as String i.e. view name. like:

public interface Action {
   public String execute(HttpServletRequest request, HttpServletResponse response) throws Exception
}

create concrete action classes for all action's like:

public class HomeAction implements Action {
    public String execute(HttpServletRequest request, HttpServletResponse response) throws Exception {
         //perform your home related action here..
         return "home";
    }
}

then you said,

my catched errors are put in a request and used in my JSP files.

so implement add action class like:

public class AddAction implements Action {
    public String execute(HttpServletRequest request, HttpServletResponse response) throws Exception {

         //get request parameters here..

         try {
               //perform add action which throws exception..
               return "success";
         }catch(DatabaseException e) {
               request.setAttribute("errorMessage", e.getMessage());// to show exception msg in jsp.
              //log exception to console or file here..
              return "failure";
         }
    }
}

Same implement for update & delete, then in your Servlet keep all action's in a map like:

//Declare one map in your servlet to hold all concrete action objects.
private Map<String,Action> actionMap = new HashMap<String,Action>();

Put all action objects into actionMap in constructor or in Servlet#init(ServletConfig config) like:

actionMap.put("home", new HomeAction());
actionMap.put("add", new AddAction());
actionMap.put("update", new UpdateAction());
actionMap.put("delete", new DeleteAction());

perform actions without if & else condition in Servlet#service(HttpServletRequest request, HttpServletResponse response)

protected void service(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

        String actionKey = request.getParameter("action");
        Action action = actionMap.get(actionKey);
        String view = action.execute(request, response);

        //Here, if view is if failure then, forward to jsp, to available request attributes in jsp. 
        //      if view is success redirect to jsp..

}

Answer 2

You can create views as many as no of commands. for e.g. home.jsp, add.jsp etc.

Create a controller servlet that's sole purpose is to inspect the command and dispatch the corrected view based on command.

Basically I am talking about a very well known MVC [Model-View-Controller] pattern.

For more info have a look at Model–view–controller.