Velocity is often used to generate web pages in applications, usually as a direct replacement for JSP. Some of the benefits of using Velocity to generate web pages are:
This document provides some basic info on getting started with Velocity in a web application.
The primary purpose of the core Velocity engine is simply to generate text based on a template. Consequently, Velocity does not contain any web-related functionality in and of itself. To make a web application, you will need a framework to respond to HTTP requests, handle user authentication, make business logic calls, and generate a response. There are several strong contenders.
As a side note, you may also come across references to VelocityServlet, which is a deprecated servlet that was included in the Velocity Core up to version 1.4. Since VelocityServlet is no longer being maintained we strongly recommend you use VelocityViewServlet in Velocity Tools instead.
There are a few issues with Velocity that are specific to web applications. Here is a brief discussion of the most commonly encountered issues.
Velocity provides the ability to call any method of an object acting as a reference. This can be useful when displaying information into the page but is dangerous when object or application state is modified.
For example, the following code safely calls the size() method of a list and displays the result.
An example of an unsafe operation concerns a financial web page with an object in the context that calculates data year by year. The method calculateNextYear() calculates data for the next year and advances an internal counter:
The problem with this approach is that the code cannot be repeated in multiple parts of the page. You may not intend to do so, but it's easy to forget this when cutting and pasting or writing control statements (such as #if or #foreach). This becomes more of an issue when you are dealing with application or session-level state.
The (strongly) recommended practice is to only use Velocity for inserting information into text. Method calls can be useful to retrieve information. However, it's generally a bad idea to use a method call to change object state, and it's always a bad idea to change application state.
If you find yourself needing to change object state (as in the previous example) try precalculating all the possible values in the controller and putting them in a List or Map. Any changes to application state should always be done by the controller.
On a related note, you should always put a List or Set into the context instead of an Iterator or Enumeration. This allows the collection to be used more than once in the page with no change in behavior.
Any user-entered text that contains special HTML or XML entities (such as <, >, or &) needs to be escaped before included in the web page. This is required, both to ensure the text is visible, and also to prevent dangerous cross-site scripting. Unlike, for example, JSTL (the Java Standard Tag Language found in Java Server Pages), Velocity does not contain any HTML-specific escaping functionality. However, you have three options:
Note that other kinds of escaping are sometimes required. For example, in style sheets the @ character needs to be escaped, and in Javascript strings the single apostrophe ' needs to be escaped.
Since a web application is running on a central server, that typically has multiple users and confidential resources, care must be taken to make certain that the web application is secure. Most standard web security principles apply to a web application built with Velocity. A few specific issues (such as system configuration, more on cross-site scripting, and method introspection) are written up in this article on Building Secure Applications with Velocity.
A minor point is that Velocity, in the absence of any log-related configuration, creates a log file in the current directory. When Velocity is used in a web application the "current directory" is usually the current directory from which the application server is started. If you start seeing the file "velocity.log" files in random places on your server filesystem, check the Velocity log configuration. Typically this occurs when Velocity is used within a web application outside of web page generation (e.g. for sending email).
What follows is a brief tutorial on building a simple web app with VelocityViewServlet. For more information, consult the Velocity Tools documentation.