xdocs/docs/overview.xml - velocity-engine - Git at Google

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    http://www.apache.org/licenses/LICENSE-2.0

  Unless required by applicable law or agreed to in writing,
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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>
	<?xml version="1.0" encoding="UTF-8"?>
	<!--
	Licensed to the Apache Software Foundation (ASF) under one
	or more contributor license agreements. See the NOTICE file
	distributed with this work for additional information
	regarding copyright ownership. The ASF licenses this file
	to you under the Apache License, Version 2.0 (the
	"License"); you may not use this file except in compliance
	with the License. You may obtain a copy of the License at

	http://www.apache.org/licenses/LICENSE-2.0

	Unless required by applicable law or agreed to in writing,
	software distributed under the License is distributed on an
	"AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
	KIND, either express or implied. See the License for the
	specific language governing permissions and limitations
	under the License.
	-->

	<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>