How do I implement a front controller in Java?

Question

I'm writing a very simple web framework using Java servlets for learning purposes. I've done this before in PHP, and it worked by consulting the request URI, then instantiating the appropriate class and method.

This worked fine in PHP, as one can do something like $c = new $x; $x->$y;. I'm unsure however of how to translate this to Java, or even if this is an appropriate way to go about it.

So far, I've tried:

Router router = new Router(request.getPathInfo());
String className = router.route(); //returns com.example.controller.Foo

Class c = Class.forName(className);
Object x = c.newInstance();

Foo y = (Foo) x;
y.doSomething();

This seems fine for a couple of routes, but doesn't seem like it would scale well, nor would it allow for sourcing routes from a configuration file.

How should I make it work?

Community · Accepted Answer · 2017-05-23 12:08:42Z

2

Get hold of actions in a Map<String, Action> where the String key represents less or more a combination of request method and request pathinfo. I've posted similar answer before here: Java Front Controller

You can fill such a map either statically (hardcoding all actions) or dynamically (convention over configuration, looking up classes in a certain package, or scanning the entire classpath for classes with a certain annotation or implementing a certain interface).

And just stick to Servlet. The Filter isn't there for. At highest use it to forward the request to the controller Servlet. In the Servlet, just implement HttpServlet#service().

edited May 23, 2017 at 12:08

CommunityBot

11 silver badge

answered Feb 10, 2010 at 11:40

BalusC

1.1m377 gold badges3.7k silver badges3.6k bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

deamon Over a year ago

The benefit of a filter is, that you can use it together with servlets that are not mapped by the front controller. Wicket uses this approach.

BalusC Over a year ago

That would be a MVC "patch" to existing servlet based apps, not a fullfledged MVC framework.

deamon · Accepted Answer · 2010-02-10 14:56:09Z

2

I would use a Servlet Filter as Front Controller. The router would connect paths with request dispatchers. In the doFilter method you would convert ServletRequest to HttpServletRequest, extract the request path and match it against the registered mappings. The result of this mapping is a request dispatcher you would dispatch the request with.

In pseudo code:

doFilter(ServletRequest request, ServletResponse response) {
  httpServletRequest = (HttpServletRequest) request;
  path = httpServletRequest.getRequestURI();
  dispatcher = router.getTarget(path);
  dispatcher.dispatch(request, response);
}

Depending on your need the default routing mechanism of the Servlet API could be sufficient.

edited Feb 10, 2010 at 14:56

answered Feb 10, 2010 at 9:12

deamon

93.1k117 gold badges332 silver badges461 bronze badges

6 Comments

BalusC Over a year ago

Pathinfo isn't available in filters.

deamon Over a year ago

pathinfo is available after casting ServletRequest to HttpServletRequest.

BalusC Over a year ago

I didn't mean that :) It will always return null in filters.

deamon Over a year ago

Maybe I remember something wrong with casting here, but I've written a filter that dispatches requests based on the path.

BalusC Over a year ago

For that normally request URI/URL is been used (and yes, from the downcasted HttpServletRequest).

|

Tom Duckering · Accepted Answer · 2010-02-10 08:59:38Z

0

Not quite sure what you're after but you might want to take a look at Java servlets. Granted many web frameworks are abstracted above plain servlets, but it's a jolly good place to start learning about Java web apps if you ask me (which indirectly you did ;) )

Download the Java servlet specification here: Java Servlet Spec - it's quite interesting.

answered Feb 10, 2010 at 8:59

Tom Duckering

2,7372 gold badges23 silver badges27 bronze badges

1 Comment

etheros Over a year ago

Oops, I forgot to mention that my code so far is implemented as a servlet.

user7094 · Accepted Answer · 2010-02-10 09:03:28Z

How should you make it work? However you want it to. If you're just doing it for learning purposes, whatever you do will be fine.

I would suggest having all your actions implement the same interface though (maybe extend Servlet) so that you don't have to compile in all different classes.

Then you can essentially do what you're doing, except that your cast to Foo becomes a cast to Servlet and then you don't have to have a special case for all your different classes.

You can then also load up the routes from configuration (maybe an XML file).

Essentially what you're doing is implemented by the Struts 1 framework so it might be worthwhile reading up on that (it's open-source so you can also look at the source if you want).

Collectives™ on Stack Overflow

How do I implement a front controller in Java?

4 Answers 4

2 Comments

6 Comments

1 Comment

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

4 Answers 4

2 Comments

6 Comments

1 Comment

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related