Why do people use Velocity and/or NVelocity?

Question

Over the past year I have heard alot about Velocity and NVelocity. Reading their documentation and doing searches on the net hasn't given me the answers I was looking for.

In what situation would I use this library in my development? What problem does it solve that didn't already have a solution?

Answer 1

Since the dawn of web apps, people started to think about separation of concerns in many applications, including web applications. The challenge is to separate what is view code from what is business code, or logic code. When jsps first arrived, many people where coding lots of logic in jsps directly (stuff like db access and other), breaking the basic principle of separation of concerns (jsps should be responsible for the presentation, not for the logic).

Velocity, Freemarker and others are templating engines that allows the separation of UI logic and business logic, thus facilitating changes in the presentation while minimizing the changes on the business side. These template engines have facilities for doing common UI tasks such as showing a block of html if some condition holds true, or iterating over a list while maintaining logic code outside of the view. This is fundamental for maintaining a complex application in the long run.

Answer 2

I think it's important to point out that compared to JSP/ASP.NET as a templating mechanisim, Velocity/NVelocity really 'ENFORCE' the seperation of concern.

With <% .. %> of JSP/ASP.NET, any Java/.NET code is allowed. Which is why sometimes you do see business logic code in these files.

With Velocity/NVelocity you can't embed long series of code. Instead you are really forced to pass in computed values, which Velocity/NVelocity picks up and displays them according to the way the template is designed.

Another point would be that they can work outside of a Web Container environment (at least Velocity can AFAIK). Imagine that you had designed a report template with JSP/ASP.NET. It works fine from the web. And then suddenly there is a change request to have it be done from a Desktop application. Rather than embed a Web Container in it, you could initialize Velocity/NVelocity, compute the values, then render the template.