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| <a name="overview"></a><div class="section"><h2>Overview</h2> |
| |
| <p> |
| Velocity is a template engine that can be used for many purposes. Some common types of applications |
| which use Velocity are: |
| </p> |
| <ul> |
| <li>Web applications. Web designers create HTML pages with placeholders for dynamic information. |
| The page is processed with <a href="http://velocity.apache.org//tools/devel/view/" class="externalLink">VelocityViewServlet</a> or any of a |
| number of <a href="http://wiki.apache.org/velocity/PoweredByVelocity" class="externalLink">frameworks</a> which support Velocity. |
| This approach to web application development |
| is called Model-View-Controller or MVC and is intended to be a direct replacement |
| for applications developed with Java Server Pages (JSPs) or PHP. |
| </li> |
| |
| <li>Source code generation. Velocity can be used to generate Java source code, SQL, or PostScript |
| based on templates. The <a href="http://wiki.apache.org/velocity/PoweredByVelocity" class="externalLink">PoweredByVelocity</a> |
| page lists a number of open source |
| and commercial development software packages which use Velocity in this manner. |
| </li> |
| |
| <li>Automatic emails. Many applications generate automatic emails for account signup, password reminders |
| or automatically sent reports. Using Velocity, the email template can be stored in a text |
| file rather than directly embedded in your Java code. |
| </li> |
| |
| <li>XML transformation. Velocity provides an ant task called <a href="anakia.html">Anakia</a> which reads |
| an XML file and makes it available to a Velocity template. A common application is to |
| convert documentation stored in a generic "xdoc" format into a styled HTML document. |
| </li> |
| </ul> |
| </div> |
| |
| |
| <a name="how_it_works"></a><div class="section"><h2>How it Works</h2> |
| <p> |
| Velocity allows web page designers and other template writers to include markup statements |
| called <em>references</em> in the page. These references are pulled from a <em>Context</em> |
| object -- essentially a hashtable that provides get and set |
| methods for retrieving and setting objects -- and the corresponding values are inserted |
| directly in a page. Velocity provides basic control statements, that can loop over a collection |
| of values (foreach) or conditionally show a block of text (if/else). The ability to |
| call arbitrary Java methods, include other files, and to create macros that can be repeatedly |
| used make this a powerful yet easy-to-use approach for creating dynamic web page or other |
| text files. |
| </p> |
| |
| <p> |
| Velocity enforces a Model-View-Controller (MVC) style of development |
| by separating Java code from HTML template code. Unlike JSPs, |
| Velocity does not allow Java code to be embedded in pages. Unlike PHP, |
| Velocity does not implement features with other functions. The MVC |
| approach is one of Velocity's great strengths, and allows for more |
| maintainable and better-designed web pages. |
| </p> |
| |
| <p> |
| Although MVC-style development can sometimes lead to longer incubation |
| periods for web sites, particularly if the developers involved are new |
| to MVC, this approach saves time over the long term (believe us, we have |
| been doing this for a long time now). The MVC abstraction prevents web page |
| designers from messing with a software engineer's Java code, and |
| programmers from unduly influencing the look of web sites. Velocity enforces |
| a contract that defines what roles people play in the web site development |
| process. |
| </p> |
| </div> |
| |
| |
| <a name="extending_velocitys_capability"></a><div class="section"><h2>Extending Velocity's Capability</h2> |
| <p>While Velocity is generally useful within an application as is, there are a number of ways |
| its capabilities can be extended. |
| </p> |
| |
| <ul> |
| <li>Special types of objects, generically called "Tools", contain methods but no data. |
| When placed into the Velocity context the template can call these methods to |
| do basic tasks like formatting numbers or escaping HTML entities. |
| </li> |
| <li> |
| Velocity provides a selection of <em>resource loaders</em> that can retrieve |
| templates from text files, the classpath, even a database. But if that's not |
| enough you can write your own resource loader to retrieve in a custom manner. |
| </li> |
| <li> |
| <em>Event Handlers</em> provide hooks to perform custom actions upon certain |
| events, such as the insertion of a reference into text. |
| </li> |
| <li> |
| Advanced users can write a custom <em>Introspector</em> which retrieves |
| reference properties and methods. For example, you might create an introspector |
| that retrieves data from Lucene or other search engine indexes. |
| </li> |
| <li>Finally, for the truly adept, the grammar for Velocity is processed in a parser |
| generated by |
| <a href="https://javacc.dev.java.net/" class="externalLink">JavaCC</a> (Java Compiler |
| Compiler) using the JJTree extension to create an Abstract Syntax Tree. |
| By changing the JavaCC specification file and recompiling, the Velocity syntax |
| itself can be changed. |
| </li> |
| </ul> |
| |
| </div> |
| |
| <a name="acknowledgement"></a><div class="section"><h2>Acknowledgement</h2> |
| <p> |
| Velocity's design concept is borrowed from <a href="http://www.webmacro.org/" class="externalLink">WebMacro</a>. Those involved in the |
| Velocity project acknowledge and appreciate the development and design work |
| that went into the WebMacro project. |
| </p> |
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| 2000-2007 |
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| The Apache Software Foundation |
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| Last Published: 2007-03-06 19:34:54 |
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