There’s now a newer post about the same topic at Self-Contained Systems and ROCA: A complete example using Spring Boot, Thymeleaf and Bootstrap.
What is the best way to build a web application?
On the other hand there’s one word in the term ‘Single Page App’ that makes me afraid: Single. Everything in one page? Really? I mean, it could be really a lot. Think of a medium or big sized web application with a lot of developers working on it. Initial loading times are a small problem compared to organizing the work: client software design, namespaces, merging, testing. Even if that’s just new to us old Java developers, we still have one application for everything. It’s not easy to exchange certain parts, it’s not easy to integrate with other applications, and it’s not easy to integrate other applications into it.
ROCA – Resource-oriented Client Architecture
So what do you do if you don’t want to build a single page application? While working at a customer I came across ROCA (Resource-oriented Client Architecture) which is a set of rules for a web application architecture. It’s a short list, so before I repeat them here, please read them there.
So now you know the rules, but that doesn’t mean you can instantly imagine how such an application would look like. At least I couldn’t. I learned that there are two important aspects:
RESTful communication is stateless, so we have no session state. We have meaningful bookmarkable URIs for each resource and sub-resource, and a resource ideally represents an object from our domain, or a list of objects from our domain. I say ideally, because that’s not a must. In a lot of use cases, a resource made for a web frontend cannot be mapped 1-on-1 to domain objects, but if it does, our life gets easier. To interact with those resources we use the four HTTP methods GET, POST, PUT and DELETE. So if our domain happens to be a movie database, usage could be:
- GET on /movies for displaying all movies
- POST on /movies for adding a movie
- GET on /movies/42 for displaying the movie with id 42
- PUT on /movies/42 for updating the movie with id 42
- DELETE on /movies/42 for deleting the movie with id 42
A GET returns HTML markup (possibly through a template engine), PUT and DELETE are tunneled through a POST, and POST, PUT and DELETE return a redirect URI to follow the POST/REDIRECT/GET pattern.
The movie database – an example application
You learn the most if you try it out – so I built a web application following the ROCA rules (with a little help and inspiration of some people that know more about it than I do). It’s a movie database, and you can find it here on Github. I used Bootstrap, jQuery, Thymeleaf, Spring HATEOAS and Spring MVC for building it, and that’s why:
I really didn’t have much to do with CSS and web design in general before I did this project, so Bootstrap came as a rescue. Everybody can do good-looking user interfaces with Bootstrap. And in a real world project there would probably be someone professional doing the web design.
Spring HATEOAS is a library for dealing with the hypermedia part in RESTful applications. I guess it was intended to use in RESTful web services, but there is no reason for not using it here. We have our domain objects delivered by our services. They have relations to other domain objects. In the browser, those relations are represented by links to another resource or sub-resource, for example the resource /movies/42 has a relation to its list of comments, which can be found following the URI /movies/42/comments. To convert our domain object into a resource, we need to create those links. Spring HATEOAS brings structure into this process by providing a Link and a Resource class, where the latter may contain a domain object and a list of Link objects. Furthermore it contains a ResourceAssembler interface which can be implemented to build special domain-resource-converters for domain classes, doing the conversion in exactly one place. This way it becomes a three-stepped process: getting domain data from a service, converting it into resources and inserting it into the template.
I needed a request/response style web framework, and Spring MVC is one of them. I wanted to check if it fits well. And also I wanted to write a web application without a line of XML, and since that’s possible with Servlet 3.0 and Spring 3.1, I did it here. Note that you need a container capable of Servlet 3.0 to run the application (Tomcat 7 will do).
I encourage you to have a look at the code and let it run. Does it feel good? Or is an SPA maybe a better solution? Let me know what you think.