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<document>

 <properties>
  <title>Velocity Overview</title>
  <author email="dev@velocity.apache.org">Velocity Documentation Team</author>
 </properties>

 <body>


 <section name="Overview" href="Overview">

 <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/">VelocityViewServlet</a> or any of a
           number of <a href="http://wiki.apache.org/velocity/PoweredByVelocity">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">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>
 </section>


 <section name="How it Works" href="HowitWorks">
 <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>
 </section>


 <section name="Extending Velocity's Capability" href="ExtendingVelocity'sCapability">
    <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/">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>

 </section>

 <section name="Acknowledgement" href="Acknowledgement">
    <p>
        Velocity's design concept is borrowed from <a
        href="http://www.webmacro.org/">WebMacro</a>. Those involved in the
        Velocity project acknowledge and appreciate the development and design work
        that went into the WebMacro project.
     </p>

 </section>

</body>
</document>
