Start up the Velocity Template Engine

Generate Web content with Java-based, open source Velocity

By Geir Magnusson Jr., JavaWorld.com, 12/28/01

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One of Velocity's strengths is that it strongly enforces a separation of functional responsibility within the application. It does this by limiting template access to objects that the application code specifically makes available. This means that designers can focus exclusively on the data presentation (the view), and the application programmer can focus on the application control (the controller) and the business logic and data management (the model) in Model-View-Controller (MVC) development. MVC is a well-accepted development pattern that simplifies both development and ongoing maintenance of sophisticated applications.

Where do I use it?

Velocity is successfully used in:

• Servlet-based Web applications
• Java and SQL code generation
• XML processing and transformation
• Text processing, such as RTF file generation

Velocity is most commonly used as a rendering engine for Java servlet-based Web application development, in place of or in conjunction with JSPs and other rendering technologies. Besides the easy, maintainable template syntax, Velocity is used in Web development because its template language can manipulate and present the data, not create data. This discourages programming within the templates. This is a good thing; it keeps your Java code's business and application logic where they belong.

Velocity is well suited to J2EE (Java 2 Platform, Enterprise Edition) Web development because the platform accommodates output technologies other than JSP. While JSP is included in the J2EE specification, J2EE doesn't require its use.

How does it work?

You use the same general process to create a Velocity-based application as you would any application. Let's consider a more interesting example than the Hello World application above. Suppose you operate a pet store and wish to generate an email blast to announce a sale. First, you must design the email and then develop the template and code based on that design.

Design-time considerations

You need to consider three elements for your design:

• Which data to include in the email
• What form the data elements should take (for example, as List, Map, or String)
• What to call those data elements

For this example, let's suppose you decide on three pets for sale, each with a different advertised price. You decide to use a map to associate each pet name and its price, and then store all three maps in a list. You call this list petList, the pet name name, and the price as price in the map. Now that you have identified the relevant data, its representation, and naming criteria, you can write the code and the template's design.

Write the code and template design

Once you agree on data specifics, Velocity lets you write the code and design the template in parallel. The designer integrates the data into the nondata presentation content (like images, text, and so on) in the template. In this case, we simply write in the email body:

  $petList.size() Pets on Sale!
  We are proud to offer these fine pets
  at these amazing prices.  This month only,
  choose from:
  #foreach( $pet in $petList )
    $pet.name for only $pet.price
  #end
   Call Today!

As the programmer, you must: