Velocity - Developer's Guide

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Contents
Introduction and Getting Started Resources How Velocity Works The Fundamental Pattern To Singleton Or Not To Singleton... Singleton Model Separate Instance The Context The Basics Support for Iterative Objects for #foreach() Context Chaining Objects Created by the Template Other Context Issues Using Velocity in Servlets Servlet Programming Deployment Using Velocity in General Applications The Velocity Helper Class Exceptions Miscellaneous Details Application Attributes EventCartridge and Event Handlers Velocity Configuration Keys and Values Configuring the Log System Using Log4j With Existing Category Simple Example of a Custom Logger Configuring the Resource Loaders (template loaders) Resource Loaders Configuration Examples Pluggable Resource Manager and Resource Cache Template Encoding for Internationalization Velocity and XML FAQ (Frequently Asked Questions) Summary Appendix 1 : Deploying the Example Servlet Jakarta Tomcat Caucho Technology's Resin BEA WebLogic
Introduction
Velocity is a Java-based template engine, a simple and powerful development tool that allows you to easily create and render documents that format and present your data. In this guide, we hope to give an overview of the basics of development using Velocity, focusing on the two main areas for Velocity usage : servlet-based WWW development general application use You will see that there is no real difference between these, other than we make servlet development with Velocity very easy if you use our provided class VelocityServlet as a base class for your servlet, and offer a utility class to help with application development. Getting Started While this information is found elsewhere on the Velocity site and in the documentation, it is included here for completeness. Getting Velocity running on your computer is very easy. Note that all directory references are relative the root of the Velocity distribution tree. Get the Velocity distribution. This is available as a release, nightly snapshot or directly from the CVS code repository. Any are fine, although for the latest features, the nightly snapshot is most likely the best way. For more information, go here. If you don't have Jakarta Ant, the Java build tool already installed, please do so. It is required for building Velocity, although not required for using Velocity. Go to the `build` directory in the distribution. Type `ant <build target>` where <build target> is one of: `jar` builds the complete Velocity jar in the `bin` directory. This jar will be called 'velocity-X.jar', where 'X' is the current version number. This jar does not include necessary dependencies for Velocity. If you use this target, you must get the Collections component jar from Jakarta Commons and add to your CLASSPATH (or WEB-INF/lib). If you wish to use the built-in logging or template conversion, you must include the appropriate jars in your CLASSPATH or webapp's WEB-INF/lib. For convenience, you can use the `jar-dep` target to build a jar with ORO, Logkit and Commons Collections included. `jar-dep` builds the complete Velocity jar in the `bin` directory, including necessary support for logging from the Jakarta Avalon Logkit package, critical configuration support from the Jakarta Commons and the necesary support for WebMacro template conversion using the Jakarta ORO package. `jar-core` builds a slimmer Velocity jar in the `bin` directory, called 'velocity-core-X.jar'. This jar contains the core Velocity functionality, and doesn't include example and utility things like Anakia, Texen or the VelocityServlet support baseclass. It has the same external dependency requirements as the regular `jar` target. `jar-util` builds a utility Velocity jar in the `bin` directory, called 'velocity-util-X.jar'. This jar contains utility code, specifically Anakia, Texen, and the WebMacro template conversion utility. It has the same external dependency requirements as the regular `jar` target. `jar-servlet` builds a utility Velocity jar in the `bin` directory, called 'velocity-servlet-X.jar'. This jar contains utility code for servlet programmers. It has the same external dependency requirements as the regular `jar` target. `jar-J2EE` builds a complete jar, like the 'jar' target, that includes any components that require J2EE support. Currently, this includes only org.apache.velocity.runtime.resource.loader.DataSourceResourceLoader. As usual, it is placed in the `bin` directory, called 'velocity-j2ee-X.jar'. NOTE : if you wish to use this build target, you must place (or link) a copy of j2ee.jar into the build/lib directory. We do not provide it as part of the distribution. A good source is http://java.sun.com/. It has the same external dependency requirements as the regular `jar` target. `jar-J2EE-dep` build a complete jar with J2EE support and includes logging support from the Jakarta Avalon Logkit and regexp support fromt the Jakarta ORO package. See the notes on the `jar-dep` target, above. `examples` builds the example code in the example programs found in the `examples` directory. This build target will also build the forumdemo example project. `forumdemo` builds the example webapplication in the `examples/forumdemo` directory. `docs` builds these docs in the `docs` directory using Velocity's Anakia XML transformation tool. Allowing you to use Velocity templates in place of stylesheets - give it a try! Note: This target requires that the jakarta-site2 project is located as a peer directory to the jakarta-velocity distribution directory. Please see the note in the build.xml file for this target for further information. `jar-src` bundles all the Velocity source code into a single jar, placed in the `bin` directory. `javadocs` builds the Javadoc class documentation in the `docs/api` directory `test` (after jar) will test Velocity against it's testbed suite of test routines `help` lists the build targets that are available. While not required, testing the build is a good idea. Use the `test` target mentioned above. That's it! Velocity is ready to be used. Put the jar into your classpath, or into other appropriate places (such as the lib directory of your webapp if using with servlets) If you want to play with the examples, which is highly recommended when getting started, use build the examples via `ant examples`. Dependencies Velocity uses elements of the Java 2 API such as collections, and therefore building requires the Java 2 Standard Edition SDK (Software Development Kit). To run Velocity, the Java 2 Standard Edition RTE (Run Time Environment) is required (or you can use the SDK, of course). Velocity also is dependent upon a few packages for general functionality. They are included in the `build/lib` directory for convenience, but the default build target (see above) does not include them. If you use the default build target, you must add the dependencies to your classpath. Jakarta Commons Collections - required. Jakarta Avalon Logkit - optional, but very common. Needed if using the default file-based logging in Velocity. Jakarta ORO - optional. Needed when using the `org.apache.velocity.convert.WebMacro` template conversion utility.
Resources
There are quite a few resources and examples available to the programmer, and we recommend that you look at our examples, documentation and even the source code. Some great sources are : The user and developer community : join us via the mail-lists. Mail-list archives : http://www.mail-archive.com is a good one. Type 'velocity' into the search box to see both our -user and -dev archives. source code : `src/java/...` : all the source code to the Velocity project application example 1 : `examples/app_example1` : a simple example showing how to use Velocity in an application program. application example 2 : `examples/app_example2` : a simple example showing how to use Velocity in an application program using the Velocity application utility class. servlet example : `examples/servlet_example1` : a simple example showing how to use Velocity in a servlet. logger example : `examples/logger_example` : a simple example showing how to create a custom logging class and register it with Velocity to receive all log messages. XML example : `examples/xmlapp_example` : a simple example showing how to use JDOM to read and access XML document data from within a Velocity template. It also includes a demonstration of a recursive Velocimacro that walks the document tree. event example : `examples/event_example` : An example that demonstrates the use of the event handling API in Velocity 1.1 Anakia application : `examples/anakia` : example application showing how to use Velocity for creating stylesheet renderings of xml data Forumdemo web app : `examples/forumdemo` : working example of a simple servlet-based forum application documentation : `docs` : all the generated documentation for the Velocity project in html API documentation : `docs/api` : the generated Javadoc documentation for the Velocity project templates : `test/templates` : a large collection of template examples in our testbed directory, these are a great source of useage examples of VTL, the Velocity Template Language context example : `examples/context_example` : two examples showing how the Velocity context can be extended. For advanced users. All directory references above are relative to the distribution root directory.
How Velocity Works
> 'The Fundamental Pattern' When using Velocity in an application program or in a servlet (or anywhere, actually), you will generally do the following : Initialize Velocity. This applies to both usage patterns for Velocity, the Singleton as well as the 'separate runtime instance' (see more on this below), and you only do this once. Create a Context object (more on what that is later). Add your data objects to the Context. Choose a template. 'Merge' the template and your data to produce the ouput. In code, using the singleton pattern via the `org.apache.velocity.app.Velocity` class, this looks like
>     import java.io.StringWriter;
>     import org.apache.velocity.VelocityContext;
>     import org.apache.velocity.Template;
>     import org.apache.velocity.app.Velocity;
>     import org.apache.velocity.exception.ResourceNotFoundException;
>     import org.apache.velocity.exception.ParseErrorException;
>     import org.apache.velocity.exception.MethodInvocationException;

>     Velocity.init();

>     VelocityContext context = new VelocityContext();

>     context.put( "name", new String("Velocity") );

>     Template template = null;

>     try
>     {
>        template = Velocity.getTemplate("mytemplate.vm");
>     }
>     catch( ResourceNotFoundException rnfe )
>     {
>        // couldn't find the template
>     }
>     catch( ParseErrorException pee )
>     {
>       // syntax error : problem parsing the template
>     }
>     catch( MethodInvocationException mie )
>     {
>       // something invoked in the template
>       // threw an exception
>     }
>     catch( Exception e )
>     {}

>     StringWriter sw = new StringWriter();

>     template.merge( context, sw );
That's the basic pattern. It is very simple, isn't it? This is generally what happens when you use Velocity to render a template. You probably won't be writing code exactly like this - we provide a few tools to help make it even easier than this for both servlet and application programmers. Later on in this guide, we will talk about using Velocity in both servlets as well as general applications, and we discuss the tools we provide to make things easier. In each case, though, the above sequence is what is happening either explicitly, or behind the scenes.
To Singleton Or Not To Singleton...
> As of Velocity 1.2 and later, developers now have two options for using the Velocity engine, the singleton model and the separate instance model. The same core Velocity code is used for both approaches, which are provided to make Velocity easier to integrate into your Java application. Singleton Model This is the legacy pattern, where there is only one instance of the Velocity engine in the JVM (or web application, depending) that is shared by all. This is very convenient as it allows localized configuration and sharing of resources. For example, this is a very appropriate model for use in a Servlet 2.2+ compliant web application as each web application can have it's own instance of Velocity, allowing that web application's servlet to share resources like templates, a logger, etc. The singleton is accessable via the `org.apache.velocity.app.Velocity` class, and and example of use :
>     import org.apache.velocity.app.Velocity;
>     import org.apache.velocity.Template;

>     ...

>     /*
>      *  Configure the engine - as an example, we are using
>      *  ourselves as the logger - see logging examples
>      */

>     Velocity.setProperty( Velocity.RUNTIME_LOG_LOGSYSTEM, this);

>     /*
>      *  now initialize the engine
>      */

>     Velocity.init();

>     ...

>     Template t = Velocity.getTemplate("foo.vm");
> Please note that the Singleton model is used in the `org.apache.velocity.servlet.VelocityServlet` base class, a utility class provided with the distribution to make writing servlets easier. While extending this class is the most common and convenient way to write servlets using Velocity, you are free to not use this class if you needs require something different. Separate Instance New in version 1.2, the separate instance allows you to create, configure and use as many instances of Velocity as you wish in the same JVM (or web application.) This is useful when you wish to support separate configurations, such as template directories, loggers, etc in the same application. To use separate instances, use the `org.apache.velocity.app.VelocityEngine` class. An example, which parallels the above singleton example, looks like :
>     import org.apache.velocity.app.VelocityEngine;
>     import org.apache.velocity.Template;

>     ...

>     /*
>      *  create a new instance of the engine
>      */

>     VelocityEngine ve = new VelocityEngine();

>     /*
>      *  configure the engine.  In this case, we are using
>      *  ourselves as a logger (see logging examples..)
>      */

>     ve.setProperty( VelocityEngine.RUNTIME_LOG_LOGSYSTEM, this);

>     /*
>      *  initialize the engine
>      */

>     ve.init();

>     ...

>     Template t = ve.getTemplate("foo.vm");
As you can see, this is very simple and straightforward. Except for some simple syntax changes, using Velocity as a singleton or as separate instances requires no changes to the high-level structure of your application or templates. As a programmer, the classes you should use to interact with the Velocity internals are the `org.apache.velocity.app.Velocity` class if using the singleton model, or `org.apache.velocity.app.VelocityEngine` if using the non-singleton model ('separate instance'). At no time should an application use the internal `Runtime, RuntimeConstants, RuntimeSingleton or RuntimeInstance` classes in the `org.apache.velocity.runtime` package, as these are intended for internal use only and may change over time. As mentioned above, the classes you should use are located in the `org.apache.velocity.app` package, and are the `Velocity` and `VelocityEngine` classes. If anything is missing or needed from those classes, do not hesitate to suggest changes - these classes are intended for the application developer.
The Context
> The Basics The concept of the 'context' is central to Velocity, and is a common technique for moving a container of data around between parts of a system. The idea is that the context is a 'carrier' of data between the Java layer (or you the programmer) and the template layer ( or the designer ). You as the programmer will gather objects of various types, whatever your application calls for, and place them in the context. To the designer, these objects, and their methods and properties, will become accessable via template elements called references. Generally, you will work with the designer to determine the data needs for the application. In a sense, this will become an 'API' as you produce a data set for the designer to access in the template. Therefore, in this phase of the development process it is worth devoting some time and careful analysis. While Velocity allows you to create your own context classes to support special needs and techniques (like a context that accesses an LDAP server directly, for example), a good basic implementation class called VelocityContext is provided for you as part of the distribution. VelocityContext is suitable for all general purpose needs, and we strongly recommended that you use it. Only in exceptional and advanced cases will you need to extend or create your own context implementation. Using VelocityContext is as simple as using a normal Java Hashtable class. While the interface contains other useful methods, the two main methods you will use are
>     public Object put(String key, Object value);
>     public Object get(String key);
> Please note that like a Hashtable, the value must be derived from java.lang.Object, and must not be null. Fundamental types like int or float must be wrapped in the appropriate wrapper classes. That's really all there is to basic context operations. For more information, see the API documentation included in the distribution. Support for Iterative Objects for #foreach() As a programmer, you have great freedom in the objects that you put into the context. But as with most freedoms, this one comes with a little bit of responsibility, so understand what Velocity supports, and any issues that may arise. Velocity supports serveral types of collection types suitable for use in the VTL `#foreach()` directive. `Object []` Regular object array, not much needs to be said here. Velocity will internally wrap your array in a class that provides an Iterator interface, but that shouldn't concern you as the programmer, or the template author. `java.util.Collection` Velocity will use the `iterator()` method to get an Iterator to use in the loop, so if you are implementing a Collection interface on your object, please ensure that `iterator()` returns a working Iterator. `java.util.Map` Here, Velocity depends upon the `values()` method of the interface to get a `Collection` interface, on which `iterator()` is called to retrieve an Iterator for the loop. `java.util.Iterator` USE WITH CAUTION : This is currently supported only provisionally - the issue of concern is the 'non-resettablity' of the Iterator. If a 'naked' Iterator is placed into the context, and used in more than one #foreach(), subsequent #foreach() blocks after the first will fail, as the Iterator doesn't reset. `java.util.Enumeration` USE WITH CAUTION : Like `java.util.Iterator`, this is currently supported only provisionally - the issue of concern is the 'non-resettablity' of the Enumeration. If a 'naked' Enumeration is placed into the context, and used in more than one #foreach(), subsequent #foreach() blocks after the first will fail, as the Enumeration doesn't reset. In the case of the `Iterator` and `Enumeration`, it is recommended that they are placed in the context only when it cannot be avoided, and you should let Velocity find the appropriate reusable iterative interface when that is sufficient and possible. There are good reasons to use the `java.util.Iterator` interface directly (large data sets via JDBC, for example), but if it can be avoided, it might be better to use something else. By 'directly' , we meant doing something like:
>     Vector v = new Vector();
>     v.addElement("Hello");
>     v.addElement("There");

>     context.put("words", v.iterator() );
> where the Iterator itself is placed into the context. Instead, if you simply did:
>     context.put("words", v );
> then all would be fine: Velocity would figure out that Vector implement Collection (via List), and therefore will find the `iterator()` method, and use that to get a 'fresh' Iterator for its use each time it needs to. With just a plain Iterator (the first snippet above...), once velocity has used it in a `#foreach()`, Velocity has no way of getting a new one to use for the next `#foreach()` it is used in. The result is no output from any subsequent `#foreach()` blocks using that reference. This above isn't meant to give the impression that iterating over collections in Velocity is something that requires great care and thought. Rather, the opposite is true, in general. Just be careful when you place an Iterator into the context. Context Chaining An innovative feature of Velocity's context design is the concept of context chaining. Also sometimes referred to as context wrapping, this advanced feature allows you to connect separate contexts together in a manner that makes it appear as one 'contiguous' context to the template. This is best illustrated by an example :
>     VelocityContext context1 = new VelocityContext();

>     context1.put("name","Velocity");
>     context1.put("project", "Jakarta");
>     context1.put("duplicate", "I am in context1");

>     VelocityContext context2 = new VelocityContext( context1 );

>     context2.put("lang", "Java" );
>     context2.put("duplicate", "I am in context2");

>     template.merge( context2, writer );
> In the code above, we have set up context2 such that it chains context1. This means that in the template, you can access any of the items that were put into either of the two VelocityContext objects, as long as there is no duplication of the keys used to add objects. If that is the case, as it is above for the key 'duplicate', the object stored in the nearest context in the chain will be available. In this example above, the object returned would be the string "I am in context2". Note that this duplication, or 'covering', of a context item does not in any way harm or alter the covered object. So in the example above, the string "I am in context1" is alive and well, still accessable via context1.get("duplicate"). But in the example above, the value of the reference '$duplicate' in the template would be 'I am in context2', and the template has no access to the covered string 'I am in context1'. Note also that you have to be careful when you are relying on the template to add information to a context that you will examine later after the rendering. The changes to the context via `#set()` statements in a template will affect only the outer context. So make sure that you don't discard the outer context, expecting the data from the template to have been placed onto the inner one. This feature has many uses, the most common so far is providing layered data access and toolsets. As mentioned before, the Velocity context mechanism is also extendable, but beyond the current scope of this guide. If you are interested, please see the classes in the package `org.apache.velocity.context` to see how the provided contexts are put together. Futher, there are a few examples in the `examples/context_example` directory in the distribution which show alternate implementations, including [a goofy] one that uses a database as the backing storage. Please note that these examples are unsupported and are there for demonstration/educational purposes only. Objects Created in the Template There are two common situations where the Java code must deal with objects created at runtime in the template : When a template author calls a method of an object placed into the context by Java code.
>     #set($myarr = ["a","b","c"] )
>     $foo.bar( $myarr )
> When a template adds objects to the context, the Java code can access those objects after the merge process is complete.
>     #set($myarr = ["a","b","c"] )
>     #set( $foo = 1 )
>     #set( $bar = "bar")
Dealing with these cases if very straighforward, as there are just a few things to know: The VTL RangeOperator [ 1..10 ] and ObjectArray ["a","b"] are `java.util.ArrayList` objects when placed in the context or passed to methods. Therefore, your methods that are designed to accept arrays created in the template should be written with this in mind. Numbers will be Integers in the context, and strings will be, of course, Strings. Velocity will properly 'narrow' args to method calls, so calling `setFoo( int i )` with an int placed into the context via `#set()` will work fine. Other Context Issues One of the features provided by the VelocityContext (or any Context derived from AbstractContext) is node specific introspection caching. Generally, you as a the developer don't need to worry about this when using the VelocityContext as your context. However, there is currently one known usage pattern where you must be aware of this feature. The VelocityContext will accumulate intropection information about the syntax nodes in a template as it visits those nodes. So, in the following situation: You are iterating over the same template using the same VelocityContext object. Template caching is off. You request the Template from getTemplate() on each iteration. It is possible that your VelocityContext will appear to 'leak' memory (it is really just gathering more introspection information.) What happens is that it accumulates template node introspection information for each template it visits, and as template caching is off, it appears to the VelocityContext that it is visiting a new template each time. Hence it gathers more introspection information and grows. It is highly recommended that you do one or more of the following : Create a new VelocityContext for each excursion down through the template render process. This will prevent the accumulation of introspection cache data. For the case where you want to reuse the VelocityContext because it's populated with data or objects, you can simply wrap the populated VelocityContext in another, and the 'outer' one will accumulate the introspection information, which you will just discard. Ex. `VelocityContext useThis = new VelocityContext( populatedVC );` This works because the outer context will store the introspection cache data, and get any requested data from the inner context (as it is empty.) Be careful though - if your template places data into the context and it's expected that it will be used in the subsequent iterations, you will need to do one of the other fixes, as any template #set() statements will be stored in the outermost context. See the discussion in Context chaining for more information. Turn on template caching. This will prevent the template from being re-parsed on each iteration, resulting the the VelocityContext being able to not only avoid adding to the introspection cache information, but be able to use it resulting in a performance improvement. Reuse the Template object for the duration of the loop iterations. Then you won't be forcing Velocity, if the cache is turned off, to reread and reparse the same template over and over, so the VelocityContext won't gather new introspection information each time.
Using Velocity In Servlets
> Servlet Programming The most common use of Velocity is in the area of Java Servlet programming for the WWW. There are many reasons why Velocity is well suited for this task, one of the primary ones is Velocity's enforcement of the separation of the presentation (or view) layer from the code layer. There are many resources on this subject, including this. The basic technique of using Velocity in a servlet environment is very simple. In a nutshell, all you must do is extend the provided VelocityServlet base class and implement a single method, `handleRequest()`. That's really all that is required to use Velocity in your servlet development. As of Velocity 1.1, there are two `handleRequest()` methods : `public Template handleRequest( Context )` This is the older of the two methods. This method requires that you return a valid Template object. If not valid, or `null`, this is considered an error condition, and will result in the `error()` error handling method being called. You may override the `error()` if you wish. If returning a `null` is something you expect to do (for example, you will want to redirect requests) it is recommended that you use the newer method, listed next. `public Template handleRequest( HttpServletRequest, HttpServletResponse, Context )` This is the newer of the two `handleRequest()` methods, implemented in version 1.1. The difference with this method is that the `HttpServletRequest` and `HttpServletResponse` objects are passed to you as arguments to the method, as well as in the `Context`. The other difference is that this method can return `null` to indicate that all processing has been handled by the method, and that Velocity should do nothing further than call `requestCleanup()`. This is extremely useful is you wish to redirect the request, for example. As always, please refer to the Javadoc API documentation for the definitive and latest notes. The following code is similar to the SampleServlet.java class included in the distribution in the examples directory.
>     public class SampleServlet extends VelocityServlet
>     {
>         public Template handleRequest( HttpServletRequest request,
>                                        HttpServletResponse response,
>                                        Context context )
>         {

>             String p1 = "Jakarta";
>             String p2 = "Velocity";

>             Vector vec = new Vector();
>             vec.addElement( p1 );
>             vec.addElement( p2 );

>             context.put("list", vec );

>             Template template = null;

>             try
>             {
>                 template =  getTemplate("sample.vm");
>             }
>             catch( ResourceNotFoundException rnfe )
>             {
>               // couldn't find the template
>             }
>             catch( ParseErrorException pee )
>             {
>               // syntax error : problem parsing the template
>             }
>             catch( Exception e )
>             {}

>             return template;
>         }
>     }
> Look familiar? With the exception of creating the context object, which is done for you by the VelocityServlet base class, and the merge() step which is also done for you by the VelocityServlet base class, it's identical to the basic code pattern we mentioned at the beginning of this guide. We take the context, add our application data, and return a template. The default `Context` object that is passed into the `handleRequest()` methods contains both the current `HttpServletRequest` and `HttpServletResponse` objects. They are placed in the context using the the constants `VelocityServlet.REQUEST` (value = 'req') and `VelocityServlet.RESPONSE` (value = 'res') respectively. To access and use these objects in your Java code :
>     public Template handleRequest(  Context context )
>     {
>         HttpServletRequest request =  (HttpServletRequest) context.get( REQUEST );
>         HttpServletResponse response =  (HttpServletResponse) context.get( RESPONSE );

>        ...
> and in your templates:
>     #set($name = $req.getParameter('name') )
> For more advanced uses, the VelocityServlet base class allows you to override parts of the handling of the request processing. The following methods may be overridden : `Properties loadConfiguration( ServletConfig )` Allows you to override the normal configuration mechanism and add or alter the configuation properties. This is useful for overriding or augmenting template and log paths, to set the absolute path into the webapp root at runtime. `Context createContext(HttpServletRequest, HttpServletResponse )` Allows you to create the Context object yourself. This allows more advanced techniques, such as chaining or pre-loading with tools or data. The default implementation simply returns a VelocityContext object with the request and response objects placed inside. The request and response objects are wrapped in simple wrapper classes to avoid introspection problems that may occurr in some servlet container implementations. You can use the request and repsponse objects normally, accessing methods of either from the template. Just note that they aren't specifically javax.servlet.XXXX classes, if that is important to you. `void setContentType( HttpServletRequest,HttpServletResponse )` Allows you to examine the request and set the content type yourself, depending on the request or client. The default implementation sets the content type to be that either specified in the velocity.properties, if any, or the default, "text/html" if not specified in the properties. `void mergeTemplate( Template, Context, HttpServletResponse )` Allows you to produce the output stream. The VelocityServlet uses a pool of very efficient Writer classes, so this would usually be overridden in special situations. `void requestCleanup( HttpServletRequest, HttpServletResponse , Context )` Allows you to do any cleanup or resource reclamation at the end of the request processing. The default does nothing. `protected void error( HttpServletRequest, HttpServletResponse, Exception )` Error handler that is called an exception occurrs in request processing. Default implementation will send a simple HTML message with stacktrace and exception information back to the user. Override for custom client messages and more advanced problem handling. For further information, please see the Javadoc API documentation. Deployment When you deploy your Velocity-based servlets, you will certainly want to ensure that your properties file is used to configure the Velocity runtime. Under Tomcat, one way to accomplish this is by placing your velocity.properties file into the root directory of your web app (webapps/appname ) and then add the following to your WEB-INF/web.xml file :
>     <servlet>
>       <servlet-name>MyServlet</servlet-name>
>       <servlet-class>com.foo.bar.MyServlet</servlet-class>
>       <init-param>
>           <param-name>properties</param-name>
>           <param-value>/velocity.properties</param-value>
>       </init-param>
>     </servlet>
Assuming all is right, this will ensure that when MyServlet is loaded, it will use the velocity.properties file to initialize itself rather than relying on it's internal defaults. Note that Velocity uses a singleton model for it's central core Runtime class, so it is a very good idea to put the velocity-XX.jar into the WEB-INF/lib directory in all web applications that use Velocity to ensure that the web app classloader is managing your Runtime instance, rather than putting it in the CLASSPATH or the top level lib directory of the servlet runner. This deployment method will ensure that different web applications will not be subject to Velocity configuration conflicts.
Using Velocity In General Applications
> As Velocity was designed to be a general-use tool, it is just as useful in general application programs as it is servlets. In general, you can use the same programming pattern discussed at the beginning of this guide, but there are a few utility methods provided for application use, just like we provide the VelocityServlet base class for ease of use in servlet programming. The only new responsibility you have as the application programmer is to initialize the Velocity runtime engine, but that is easy. The Velocity Helper Class Velocity contains an application utility class called Velocity ( `org.apache.velocity.app.Velocity` ). The purpose of this class is to provide the necessary methods required to initialize Velocity, as well as useful utility routines to make life easier in using Velocity. This class is documented in the project's javadoc, so please look there for definitive details. This documentation is intended to be of a tutorial nature; therefore for compete API information, the Javadoc is the definitive source. The Velocity runtime engine is a singleton instance that provides resource, logging and other services to all Velocity users running in the same JVM. Therefore, the runtime engine is initialized only once. You can attempt to initialize Velocity more than once, but only the first initialization will apply. The rest of the attempts will be ignored. The Velocity utility class currently provides five methods used in configuration of the runtime engine. The five configuration methods are : `setProperty( String key, Object o )` Sets the property `key` with the value `o`. The value is typically a String, but in special cases can also be a comma-separated list of values (in a single String, ex."foo, bar, woogie") as well as other things that will arise. `Object getProperty( String key )` Returns the value of the property key. Note that you must be aware of the type of the return value, as they can be things other than Strings. `init()` Initializes the runtime with the default properties provided in the distribution.(These are listed below in the section pertaining to properties.) `init( Properties p )` Initialize the runtime with the properties contained in the `java.util.Properties` object passed as an argument. `init( String filename )` initilizes the runtime using the properties found in the properties file filename Note that in each case, the default properties will be used as a base configuration, and any additional properties specified by the application will replace individual defaults. Any default properties not overwritten will remain in effect. This has the benefit that only the properties you are interested in changing need to be specified, rather than a complete set. Another thing to note is that the `init()` calls may be called more than once without harm in an application. However, the first call to any of the `init()` functions will configure the engine with the configuration properties set at that point, and any further configuration changes or `init()` calls will be ignored. The most common approaches to initializing Velocity will be something like : Setup the configuration values you wish to set in a file in the same format as org/apache/velocity/runtime/defaults/velocity.properties (the default set), or in a `java.util.Properties`, and then call either `init( filename )` or `init( Properties )` Set the configuration values individually using `setProperty()` and then call `init()`. This method is generally used by more advanced applications that already have their own configuration management system - this allows the application so configure Velocity based upon values it generates at runtime, for example. Once the runtime is initialized, you can do with it what you wish.. This mostly revolves around rendering templates into an output stream, and the Velocity utility class allows you to do this easily. Currently, here are the methods and a brief description of what they do : `evaluate( Context context, Writer out, String logTag, String instring )` `evaluate( Context context, Writer writer, String logTag, InputStream instream )` These methods will render the input, in either the form of String or InputStream to an output Writer, using a Context that you provide. This is a very convenienient method to use for token replacement of strings, or if you keep 'templates' of VTL-containing content in a place like a database or other non-file storage, or simply generate such dynamically. `invokeVelocimacro( String vmName, String namespace, String params[], Context context, Writer writer )` Allows direct access to Velocimacros. This can also be accomplished via the `evaluate()` method above if you wish. Here you simply name the vm you wish to be called, create an array of args to the VM, a Context of data, and Writer for the output. Note that the VM args must be the 'keys' of the data objects in the Context, rather than literal data to be used as the arg. This will probably change. `mergeTemplate( String templateName, Context context, Writer writer )` Convenient access to the normal template handling and rendering services of Velocity. This method will take care of getting and rendering the template. It will take advantage of loading the template according to the properties setting for the file resource loader, and therefore provides the advantage of file and parsed template caching that Velocity offers. This is the most efficient way to access templates, and is recommended unless you have special needs. `boolean templateExists( String name )` Determines if a template `name` is able to be found by the currently configured resource loaders. Once we know about these basic helpers, it is easy to write Java program that uses Velocity. Here it is:
>     import java.io.StringWriter;
>     import org.apache.velocity.app.Velocity;
>     import org.apache.velocity.VelocityContext;

>     public class Example2
>     {
>         public static void main( String args[] )
>         {
>             /* first, we init the runtime engine.  Defaults are fine. */

>             Velocity.init();

>             /* lets make a Context and put data into it */

>             VelocityContext context = new VelocityContext();

>             context.put("name", "Velocity");
>             context.put("project", "Jakarta");

>             /* lets render a template */

>             StringWriter w = new StringWriter();

>             Velocity.mergeTemplate("testtemplate.vm", context, w );
>             System.out.println(" template : " + w );

>             /* lets make our own string to render */

>             String s = "We are using $project $name to render this.";
>             w = new StringWriter();
>             Velocity.evaluate( context, w, "mystring", s );
>             System.out.println(" string : " + w );
>         }
>     }
> When we run this program, and have the template `testtemplate.vm` in the same directory as our program (because we used the default configuration properties, and the defaul place to load templates from is the current directory...), our output should be :
>     template : Hi!  This Velocity from the Jakarta project.

>     string : We are using Jakarta Velocity to render this.
> where the template we used, testtemplate.vm, is
>     Hi!  This $name from the $project project.
> That's all there is to it! Note that we didn't have to use both `mergeTemplate()` and `evaluate()` in our program. They are both included there for demonstration purposes. You will probably use only one of the methods, but depending on you application requirements, you are free to do what you wish. This appears to be a little different from the 'fundamental pattern' that was mentioned at the beginning of this guide, but it really is the same thing. First, you are making a context and filling it with the data needed. Where this examples differs is that in the part of the above example where `mergeTemplate()` is used, `mergeTemplate()` is doing the work of getting the template and merging it for you, using the lower-level calls in the Runtime class. In the second example, you are making your template dynamically via the String, so that is analgous to the 'choose template' part of the process, and the `evaluate()` method does the merging for you using lower level calls. So the example above sticks to the same simply pattern of using the Velocity template engine, but the utility functions do some of the repeated drudge work, or allow you other options for your template content other than template files. Exceptions There are three exceptions that Velocity will throw during the parse / merge cycle. This are additional to the exceptions that will come from IO problems, etc. They are found in the package `org.apache.velocity.exception` and are: `ResourceNotFoundException` Thrown when the resource managment system cannot find a resource (template) that was requested. `ParseErrorException` Thrown when a VTL syntax error is found when parsing a resource (template). `MethodInvocationException` Thrown when a method of object in the context thrown an exception during render time. This exception wraps the thrown exception and propogates it to the application. This allows you to handle problems in your own objects at runtime. In each case, a message is put into the runtime log. For more information, see the Javadoc API documentation. Miscellaneous Details While the above example used the default properties, setting your own properties is very simple. All you have to do is make a properties file somewhere and pass the name of that file to the `init(String)` method of the Velocity utility class, or make a `java.util.Properties` object, add the desired properties and values, and pass that to the `init(Properties)` method. The latter method is convenient, because you can either fill it directly from a separate properties file via the `load()` method, or even better, you can fill it dynamically from your own application / framework's property set at runtime. This gives you the freedom to combine all of the properties for your app into one properties file. If we wanted to use a different directory than the current directory to load our template from, we could do something like this :
>      ...

>     import java.util.Properties;
>      ...

>     public static void main( String args[] )
>     {
>         /* first, we init the runtime engine.  */

>         Properties p = new Properties();
>         p.setProperty("file.resource.loader.path", "/opt/templates");
>         Velocity.init( p );

>         /* lets make a Context and put data into it */

>      ...
And, assuming you have a directory `/opt/templates` and the template `testtemplate.vm` is in there, then things would work just fine. If you try this and have a problem, be sure to look at the velocity.log for information - the error messages are pretty good for figuring out what is wrong.
Application Attributes
> Application Attributes are name-value pairs that can be associated with a RuntimeInstance (either via the `VelocityEngine` or the `Velocity` singleton) and accessed from any part of the Velocity engine that has access to the RuntimeInstance. This feature was designed for applications that need to communicate between the application layer and custom parts of the Velocity engine, such as loggers, resource loaders, resource managers, etc. The Application Attribute API is very simple. From the application layer, there is a method of the `VelocityEngine` and the `Velocity` classes :
>         public void setApplicationAttribute( Object key, Object value );
> through which an application can store on Object under an application (or internal component) specified key. There are no restrictions on the key or the value. The value for a key may be set at any time - it is not required that this be set before init() is called. Internal components can access the key-value pairs if they have access to the object via the `RuntimeServices` interface, using the method
>         public Object getApplicationAttribute( Object key );
Note that internal components cannot set the value of the key, just get it. if the internal component must communicate information to the application layer, it must do so via the Object passed as the value.
EventCartridge and Event Handlers
> > Starting in version 1.1, a fine-grain event handling system was added to Velocity. The EventCartridge is a class in which you register your event handlers, and then the EventCartridge acts as the delivery agent from which the Velocity engine will access the event handlers at merge time if needed. Currently, there are 3 events that can be handled, and all are found in the `org.apache.velocity.app.event` package. `org.apache.velocity.app.event.NullSetEventHandler` When a #set() results in a null assignment, this is normally logged. The `NullSetEventHandler` allows you to 'veto' the logging of this condition.
> >     public interface NullSetEventHandler extends EventHandler
> >     {
> >         public boolean shouldLogOnNullSet( String lhs, String rhs );
> >     }
> > `org.apache.velocity.app.event.ReferenceInsertionEventHandler` A `ReferenceInsertionEventHandler` allows the developer to intercept each write of a reference ($foo) value to the output stream and modify that output.
> >     public interface  ReferenceInsertionEventHandler extends EventHandler
> >     {
> >         public Object referenceInsert( String reference, Object value  );
> >     }
> > `org.apache.velocity.app.event.MethodExceptionEventHandler` When a user-supplied method throws an exception, the `MethodExceptionEventHandler` is invoked with the Class, method name and thrown Exception. The handler can either return a valid Object to be used as the return value of the method call, or throw the passed-in or new Exception, which will be wrapped and propogated to the user as a `MethodInvocationException`
> >     public interface MethodExceptionEventHandler extends EventHandler
> >     {
> >         public Object methodException( Class claz, String method, Exception e )
> >              throws Exception;
> >     }
> Using the EventCartridge Using the EventCartridge is fairly straightforward. The following abbreviated example was taken from `org.apache.velocity.test.misc.Test`.
>      ...

>     import org.apache.velocity.app.event.EventCartridge;
>     import org.apache.velocity.app.event.ReferenceInsertionEventHandler;
>     import org.apache.velocity.app.event.MethodExceptionEventHandler;
>     import org.apache.velocity.app.event.NullSetEventHandler;

>      ...

>     public class Test implements ReferenceInsertionEventHandler,
>                                  NullSetEventHandler,
>                                  MethodExceptionEventHandler
>     {
>         public void myTest()
>         {
>             ....

>             /*
>              *  now, it's assumed that Test implements the correct methods to
>              *  support the event handler interfaces.  So to use them, first
>              *  make a new cartridge
>              */
>             EventCartridge ec = new EventCartridge();

>             /*
>              * then register this class as it contains the handlers
>              */
>             ec.addEventHandler(this);

>             /*
>              * and then finally let it attach itself to the context
>              */
>             ec.attachToContext( context );

>             /*
>              * now merge your template with the context as you normally
>              * do
>              */

>             ....
>         }

>         /*
>          *  and now the implementations of the event handlers
>          */
>         public Object referenceInsert( String reference, Object value  )
>         {
>             /*  do something with it */
>             return value;
>         }

>         public boolean shouldLogOnNullSet( String lhs, String rhs )
>         {
>             if ( /* whatever rule */ )
>                 return false;

>             return true;
>         }

>         public Object methodException( Class claz, String method, Exception e )
>              throws Exception
>         {
>             if ( /* whatever rule */ )
>                return "I should have thrown";

>             throw e;
>         }
>     }
Velocity Configuration Keys and Values
> Velocity's runtime configuration is controlled by a set of configuration keys listed below. Generally, these keys will have values that consist of either a String, or a comma-separated list of Strings, referred to as a CSV for comma-separated values. There is a set of default values contained in Velocity's jar, found in /src/java/org/apache/velocity/runtime/defaults/velocity.defaults, that Velocity uses as it's configuration baseline. This ensures that Velocity will always have a 'correct' value for it's configuration keys at startup, although it may not be what you want. Any values specified before init() time will replace the default values. Therefore, you only have toconfigure velocity with the values for the keys that you need to change, and not worry about the rest. Further, as we add more features and configuration capability, you don't have to change your configuration files to suit - the Velocity engine will always have default values. Please sees the section above Using Velocity In General Applications for discussion on the configuration API. Below are listed the configuration keys that control Velocity's behavior. Organized by category, each key is listed with it's current default value to the right of the '=' sign. Runtime Log `runtime.log = velocity.log` Full path and name of log file for error, warning, and informational messages. The location, if not absolute, is relative to the 'current directory'. `runtime.log.logsystem` This property has no default value. It is used to give Velocity an instantiated instance of a logging class that supports the interface `org.apache.velocity.runtime.log.LogSystem.`, which allows the combination of Velocity log messages with your other application log messages. Please see the section Configuring the Log System for more information. `runtime.log.logsystem.class = org.apache.velocity.runtime.log.AvalonLogSystem` Class to be used for the Velocity-instantiated log system. `runtime.log.error.stacktrace = false` `runtime.log.warn.stacktrace = false` `runtime.log.info.stacktrace = false` Turns on stacktracing for the three error categories. These produce a large amount of log output. `runtime.log.invalid.references = true` Property to turn off the log output when a reference isn't valid. Good thing to turn of in production, but very valuable for debugging. `runtime.log.logsystem.avalon.logger = name` Allows user to specify an existing logger name in the Avalon hierarchy without having to wrap with a LogSystem interface. Note: You must also specify `runtime.log.logsystem.class = org.apache.velocity.runtime.log.AvalonLogSystem` as the default logsystem may change. There is no guarantee that the Avalon log system will remain the default log system. Character Encoding `input.encoding = ISO-8859-1` Character encoding for input (templates). Using this, you can use alternative encoding for your templates, such as UTF-8. `output.encoding = ISO-8859-1` Character encoding for output streams from the VelocityServlet and Anakia. #foreach() Directive `directive.foreach.counter.name = velocityCount` Used in the #foreach() directive, defines the string to be used as the context key for the loop count. A template would access the loop count as $velocityCount. `directive.foreach.counter.initial.value = 1` Default starting value for the loop counter reference in a #foreach() loop. #include() and #parse() Directive `directive.include.output.errormsg.start =` `directive.include.output.errormsg.end =` Defines the beginning and ending tags for an in-stream error message in the case of a problem with the #include() directive. If both the .start and .end tags are defined, an error message will be output to the stream, of the form '.start msg .end' where .start and .end refer to the property values. Output to the render stream will only occur if both the .start and .end (next) tag are defined. `directive.parse.maxdepth = 10` Defines the allowable parse depth for a template. A template may #parse() another template which itself may have a #parse() directive. This value prevents runaway #parse() recursion. Resource Management `resource.manager.logwhenfound = true` Switch to control logging of 'found' messages from resource manager. When a resource is found for the first time, the resource name and classname of the loader that found it will be noted in the runtime log. `resource.loader = <name> (default = File)` Multi-valued key. Will accept CSV for value. Pulic name of a resource loader to be used. This public name will then be used in the specification of the specific properties for that resource loader. Note that as a multi-valued key, it's possible to pass a value like "file, class" (sans quotes), indicating that following will be configuration values for two loaders. `<name>.loader.description = Velocity File Resource Loader` Description string for the given loader. `<name>.resource.loader.class = org.apache.velocity.runtime.resource.loader.FileResourceLoader` Name of implementation class for the loader. The default loader is the file loader. `<name>.resource.loader.path = .` Multi-valued key. Will accept CSV for value. Root(s) from which the loader loads templates. Templates may live in subdirectories of this root. ex. homesite/index.vm This configuration key applies currently to the FileResourceLoader and JarResourceLoader. `<name>.resource.loader.cache = false` Controls caching of the templates in the loader. Default is false, to make life easy for development and debugging. This should be set to true for production deployment. When 'true', the `modificationCheckInterval` property applies. This allows for the efficiency of caching, with the convenience of controlled reloads - useful in a hosted or ISP environment where templates can be modifed frequently and bouncing the application or servlet engine is not desired or permitted. `<name>.resource.loader.modificationCheckInterval = 2` This is the number of seconds between modification checks when caching is turned on. When this is an integer > 0, this represents the number of seconds between checks to see if the template was modified. If the template has been modified since last check, then it is reloaded and reparsed. Otherwise nothing is done. When <= 0, no modification checks will take place, and assuming that the property `cache` (above) is true, once a template is loaded and parsed the first time it is used, it will not be checked or reloaded after that until the application or servlet engine is restarted. To illustrate, here is an example taken right from the default Velocity properties, showing how setting up the FileResourceLoader is managed
>     resource.loader = file

>     file.resource.loader.description = Velocity File Resource Loader
>     file.resource.loader.class = org.apache.velocity.runtime.resource.loader.FileResourceLoader
>     file.resource.loader.path = .
>     file.resource.loader.cache = false
>     file.resource.loader.modificationCheckInterval = 2
Velocimacro `velocimacro.library = VM_global_library.vm` Multi-valued key. Will accept CSV for value. Filename(s) of Velocimacro library to be loaded when the Velocity Runtime engine starts. These Velocimacros are accessable to all templates. The file is assumed to be relative to the root of the file loader resource path. `velocimacro.permissions.allow.inline = true` Determines of the definition of new Velocimacros via the #macro() directive in templates is allowed. The default value is true, meaning any template can define and use new Velocimacros. Note that depending on other properties, those #macro() statements can replace global definitions. `velocimacro.permissions.allow.inline.to.replace.global = false` Controls if a Velocimacro defind 'inline' in a template can replace a Velocimacro defined in a library loaded at startup. `velocimacro.permissions.allow.inline.local.scope = false` Controls 'private' templates namespaces for Velocimacros. When true, a #macro() directive in a template creates a Velocimacro that is accessable only from the defining template. This means that Velocimacros cannot be shared unless they are in the global or local library loaded at startup. (See above.) It also means that templates cannot interfere with each other. This property also allows a technique where there is a 'default' Velocimacro definition in the global or local library, and a template can 'override' the implementation for use within that template. This occurrs because when this property is true, the template's namespace is searched for a Velocimacro before the global namespace, therefore allowing the override mechanism. `velocimacro.context.localscope = false` Controls whether reference access (set/get) within a Velocimacro will change the context, or be of local scope in that Velocimacro. `velocimacro.library.autoreload = false` Controls Velocimacro library autoloading. When set to `true` the source Velocimacro library for an invoked Velocimacro will be checked for changes, and reloaded if necessary. This allows you to change and test Velocimacro libraries without having to restart your application or servlet container, just like you can with regular templates. This mode only works when caching is off in the resource loaders (e.g. `file.resource.loader.cache = false` ). This feature is intended for development, not for production. String Interpolation `runtime.interpolate.string.literals = true` Controls interpolation mechanism of VTL String Literals. Note that a VTL StringLiteral is specifically a string using double quotes that is used in a #set() statement, a method call of a reference, a parameter to a VM, or as an argument to a VTL directive in general. See the VTL reference for further information. Runtime Configuration `parser.pool.size = 20` This property sets the number of parsers that Velocity will create at startup and keep in a pool. The default of 20 parsers should be more than enough for most uses. In the event that Velocity does run out of parsers, it will indicate so in the log, and dynamically create them as needed. Note that they will not be added to the pool. This is a slow operation compared to the normal parser pooling, but this is considered an exceptional condition. If you see a log message, please increment this property. Pluggable Introspection `runtime.introspector.uberspect = org.apache.velocity.util.introspection.UberspectImpl` This property sets the 'Uberspector', the introspection package that handles all introspection strategies for Velocity. The default works just fine, so only replace if you have something really interesting and special to do.
Configuring the Log System
> Velocity has a few nice logging features to allow both simplicity and flexibility. Without any extra configuration, Velocity will setup a file-based logger that will output all logging messages to a file called `velocity.log` in the 'current directory' where Velocity was initialized. For more advanced users, you may integrate your current logging facilities with Velocity to have all log messages sent to your logger. Starting with version 1.3, Velocity will automatically use either the Jakarta Avalon Logkit logger, or the Jakarta Log4j logger. It will do so by using whatever it finds in the current classpath, starting first with Logkit. If Logkit isn't found, it tries Log4j. To utilize this feature, simply use the 'non-dependency' Velocity jar (because Logkit is baked into the jar with dependencies) and place either the logkit or log4j jar in your classpath. In general, you have the following logging options : Default Configuration By default, Velocity will create a file-based logger in the current directory. See the note above regarding automatic detection of Logkit or Log4j to use as the default logging system. Existing Log4j Category Starting with version 1.3, Velocity will log it's output to an existing Log4j Category setup elsewhere in the application. To use this feature you must Make sure that the Log4j jar is in your classpath. (You would do this anyway since you are using Log4j in the application using Velocity.) Configure Velocity to use the `SimpleLog4JLogSystem` class. Specify the name of the existing Category to use via the 'runtime.log.logsystem.log4j.category' property. This approach replaces and deprecates the older `Log4JLogSystem` class. To see how this is done in code, see the example below. Custom Standalone Logger You can create a custom logging class - you simply must implement the interface `org.apache.velocity.runtime.log.LogSystem` and then simply set the configuration property `runtime.log.logsystem.class` with the classname, and Velocity will create an instance of that class at init time. You may specify the classname as you specify any other properties. See the information on the Velocity helper class as well as the configuration keys and values. Please note that through oversight, the interface to `org.apache.velocity.runtime.log.LogSystem` was changed in v1.2 to support the separable instances of the Velocity runtime. If you have an exisiting pre v1.2 custom logger that is going to be instantiated by the Velocity LogManager, you must add the `init( RuntimeServices )` method. Integrated Logging You can integrate Velocity's logging capabilities with your applications existing logging system, simply by implementing the `org.apache.velocity.runtime.log.LogSystem` interface. Then, pass an instance of your logging class to Velocity via the `runtime.log.logsystem` configuration key before initializing the Velocity engine, and Velocity will log messages to your applications logger. See the information on the Velocity helper class as well as the configuration keys and values. Using Log4j With Existing Category Here is an example of how to configure Velocity to log to an existing Log4j Category.
>     import org.apache.velocity.app.VelocityEngine;
>     import org.apache.velocity.runtime.RuntimeConstants;

>     import org.apache.log4j.Category;
>     import org.apache.log4j.BasicConfigurator;

>     public class Log4jCategoryExample
>     {
>         public static String CATEGORY_NAME = "velexample";

>         public static void main( String args[] )
>             throws Exception
>         {
>             /*
>              *  configure log4j to log to console
>              */

>             BasicConfigurator.configure();

>             Category log = Category.getInstance( CATEGORY_NAME );

>             log.info("Hello from Log4jCategoryExample - ready to start velocity");

>             /*
>              *  now create a new VelocityEngine instance, and
>              *  configure it to use the category
>              */

>             VelocityEngine ve = new VelocityEngine();

>             ve.setProperty( RuntimeConstants.RUNTIME_LOG_LOGSYSTEM_CLASS,
>                 "org.apache.velocity.runtime.log.SimpleLog4JLogSystem" );

>             ve.setProperty("runtime.log.logsystem.log4j.category", CATEGORY_NAME);

>             ve.init();

>             log.info("this should follow the initialization output from velocity");
>         }
>     }
> Note that the above example can be found in `examples/logger_example`. Simple Example of a Custom Logger Here is an example of how to use an instantiation of your class that implements Velocity's logging system as the logger. Note that we are not passing the name of the class to use, but rather a living, existing instantiation of the class to be used. All that is required is that it support the `LogSystem` interface.
>     import org.apache.velocity.runtime.log.LogSystem;
>     import org.apache.velocity.runtime.RuntimeServices;
>     ...

>     public class MyClass implements LogSystem
>     {

>     ...

>         public MyClass()
>         {
>             ...

>             try
>             {
>                 /*
>                  *  register this class as a logger
>                  */
>                 Velocity.setProperty(Velocity.RUNTIME_LOG_LOGSYSTEM, this );
>                 Velocity.init();
>             }
>             catch (Exception e)
>             {
>                 /*
>                  *  do something
>                  */
>             }
>         }

>         /**
>          *  This init() will be invoked once by the LogManager
>          *  to give you current RuntimeServices intance
>          */
>         public void init( RuntimeServices rsvc )
>         {
>             // do nothing
>         }

>         /**
>          *  This is the method that you implement for Velocity to call
>          *  with log messages.
>          */
>         public void logVelocityMessage(int level, String message)
>         {
>             /*  do something useful */
>         }
>     ...
>     }
Configuring Resource Loaders
> Resource Loaders One of the fundamental and important parts about Velocity is the resource management system and the resource loaders. They are referred to as 'resources' here rather than 'templates' because the resource management system will also handle non-template reasources, specifically things that are loaded via the #include() directive. The resource loader system if very flexible, allowing one or more resource loaders to be in operation at the same time. This allows tremendous flexibility in configuration and resource managment, and futher allows you to write your own resource loaders for your special needs. There are currently four kinds of resource loaders that are included with Velocity, each described below. Note that in the example configuration properties given, a common name for the loader is shown (ex.'file' in `file.resource.loader.path`). This 'common name' may not work for your configuration. Please read the section on resource configuration properties to understand how this system works. Also, each of these loaders is located in the package `org.apache.velocity.runtime.resource.loader`. FileResourceLoader : This loader gets resources from the filesystem. It's configuration properties include : `file.resource.loader.path` = <path to root of templates> `file.resource.loader.cache` = true/false `file.resource.loader.modificationCheckInterval` = <seconds between checks> This is the default loader, and is configured, by default to get templates from the 'current directory'. In the case of using Velocity with servlets, this can be a problem as you don't want to have to keep your templates in the directory from which you start your servlet engine. Please see the section on developing servlets with Velocity for more information. JarResourceLoader : This loader gets resource from specific jar files. It is very similar to the FileResourceLoader, except that you have the convenience of bundling your templates into jars. The properties are identical, except for `jar.resource.loader.path`, where you provide the full location of the jar(s) you wish to load resources from. To specify a jar for the loader.path you use the standard JAR URL syntax of `java.net.JarURLConnection`. ClasspathResourceLoader : This loader gets resources from the classloader. In general, this means that the ClasspathResourceLoader will load templates placed in the classpath (in jars, for example) While the classpath is a source of great pain and suffering in general, it is a very useful mechanism when working on a Servlet Spec 2.2 (or newer) compliant servlet runner. Tomcat is an example of such. To use this loader effectively, all you must do is jar your templates, and put that jar into the WEB-INF/lib directory of your webapp. There are no configuration options to worry about, nor is the absolute vs. relative path an issue, as it is with Jar and File resource loaders. Again, please note that the ClasspathResourceLoader is not only for use with a servlet container, but can be used in any application context. DataSourceResourceLoader : This loader will load resources from a DataSource such as a database. This loader is not built as part of the standard build as it requires J2EE support. To build this loader, please download the J2EE distribution, move the j2ee.jar into the `build/lib` directory, and then build the new velocity jar with the `jar-j2ee` build target. For more information on this loader, please see the javadoc for the class `org.apache.velocity.runtime.resource.loader.DataSourceResourceLoader`. Configuration Examples Configuring the resource loaders for Velocity is straightforward. The properties that control the are listed in the resource configuration section, for further reference. The first step in configuring one or more resource loaders is do 'declare' them by name to Velocity. Use the property `resource.loader` and list one or more loader names. You can use anything you want - these names are used to associate configuration properties with a given loader.
>     resource.loader = file
> That entry declares that we will have a resource loader known as 'file'. The next thing to do is to set the important properties. The most critical is to declare the class to use as the loader :
>     file.resource.loader.class = org.apache.velocity.runtime.resource.loader.FileResourceLoader
> In this case, we are telling velocity that we are setting up a resource loadercalled 'file', and are using the class `org.apache.velocity.runtime.resource.loader.FileResourceLoader` to be the class to use. The next thing we do is set the properties important to this loader.
>     file.resource.loader.path = /opt/templates
>     file.resource.loader.cache = true
>     file.resource.loader.modificationCheckInterval = 2
> Here, we set a few things. First, we set the path to find the templates to be `/opt/templates`. Second, we turned caching on, so that after a template or static file is read in, it is cached in memory. And finally, we set the modification check interval to 2 seconds, allowing Velocity to check for new templates. Those are the basics. What follows are a few examples of different configuraitons. Do-nothing Default Configuration : As the name says, there is nothing you have to do or configure to get the default configuration. This configuration uses the FileResourceLoader with the current directory as the default resource path, and caching is off. As a properties set, this is expressed as :
>     resource.loader = file

>     file.resource.loader.description = Velocity File Resource Loader
>     file.resource.loader.class = org.apache.velocity.runtime.resource.loader.FileResourceLoader
>     file.resource.loader.path = .
>     file.resource.loader.cache = false
>     file.resource.loader.modificationCheckInterval = 0
> Multiple Template Path Configuration : This configuration uses the FileResourceLoader with several directories as 'nodes' on the template search path. We also want to use caching, and have the templates checked for changes in 10 second intervals. As a properties set, this is expressed as :
>     resource.loader = file

>     file.resource.loader.description = Velocity File Resource Loader
>     file.resource.loader.class = org.apache.velocity.runtime.resource.loader.FileResourceLoader
>     file.resource.loader.path = /opt/directory1, /opt/directory2
>     file.resource.loader.cache = true
>     file.resource.loader.modificationCheckInterval = 10
> Multiple Loader Configuration : This configuration sets up three loaders at the same time, the FileResourceLoader, the ClasspathResourceLoader, and the JarResourceLoader. The loaders are set-up such that the FileResourceLoader is consulted first, then the ClasspathResourceLoader, and finally the JarResourceLoader. This would allow you to qickly drop a template into the file template area to replace on of the templates found in the classpath (usually via a jar) without having to rebuild the jar.
>     #
>     # specify three resource loaders to use
>     #
>     resource.loader = file, class, jar

>     #
>     # for the loader we call 'file', set the FileResourceLoader as the
>     # class to use, turn off caching, and use 3 directories for templates
>     #
>     file.resource.loader.description = Velocity File Resource Loader
>     file.resource.loader.class = org.apache.velocity.runtime.resource.loader.FileResourceLoader
>     file.resource.loader.path = templatedirectory1, anotherdirectory, foo/bar
>     file.resource.loader.cache = false
>     file.resource.loader.modificationCheckInterval = 0

>     #
>     #  for the loader we call 'class', use the ClasspathResourceLoader
>     #
>     class.resource.loader.description = Velocity Classpath Resource Loader
>     class.resource.loader.class = org.apache.velocity.runtime.resource.loader.ClasspathResourceLoader

>     #
>     # and finally, for the loader we call 'jar', use the JarResourceLoader
>     # and specify two jars to load from
>     #
>     jar.resource.loader.description = Velocity Jar  Resource Loader
>     jar.resource.loader.class = org.apache.velocity.runtime.resource.loader.JarResourceLoader
>     jar.resource.loader.path = jar:file:/myjarplace/myjar.jar, jar:file:/myjarplace/myjar2.jar
> Node that the three names 'file', 'class', and 'jar' are merely for your convenience and sanity. They can be anything you want - they are just used to associate a set of properties together. However, it is recommended that you use names that give some hint of the function. Note that while all three require very little configuration information for proper operation, the ClasspathResourceLoader is the simplest. Pluggable Resource Manager and Resource Cache The Resource Manager is the main part of the resource (template and static content) management system, and is responsible for taking application requests for templates, finding them in the available resource loaders, and then optionally caching the parsed template. The Resource Cache is the mechanism that the Resource Manager uses to cache templates for quick reuse. While the default versions of these two facilities are suitable for most applications, for advanced users it now is possible to replace the default resource manager and resource cache with custom implementations. A resource manager implementation must implement the `org.apache.velocity.runtime.resource.ResourceManager` interface. A description of the requirements of a resource manager is out of scope for this document. Implementors are encouraged to review the default implementation. To configure Velocity to load the replacement implementation, use the configuration key :
>     resource.manager.class
> This key is also defined as a contstant `RuntimeConstants.RESOURCE_MANAGER_CLASS` A resource cache implementation must implement the `org.apache.velocity.runtime.resource.ResourceCache` interface As with the resource manager, a description of the requirements of a resource manager is out of scope for this document. Implementors are encouraged to review the default implementation. To configure Velocity to load the replacement implementation, use the configuration key :
>     resource.manager.cache.class
This key is also defined as a contstant `RuntimeConstants.RESOURCE_MANAGER_CACHE_CLASS`
Template Encoding for Internationalization
As of version 1.1, Velocity allows you to specify the character encoding of your template resources on a template by template basis. The normal resource API's have been extended to take the encoding as an argument : `org.apache.velocity.servlet.VelocityServlet` : `public Template getTemplate( String template, String encoding )` `org.apache.velocity.app.Velocity` : `public static Template getTemplate(String name, String encoding)` `public static boolean mergeTemplate( String templateName, String encoding, Context context, Writer writer )` `` The value for the encoding argument is the conventional encoding specification supported by your JVM, for example "UTF-8" or "ISO-8859-1". For the official names for character sets, see here. Note that this applies only to the encoding of the template itself - the output encoding is an application specific issue.
Velocity and XML
> Velocity's flexibility and simple template language makes it an ideal environment for working with XML data. Anakia is an example of how Velocity is used to replace XSL for rendering output from XML. The Velocity site, including this documentation, is generated from XML source using Anakia. The Jakarta site is also rendered using Anakia. Generally, the pattern for dealing with XML in Velocity is to use something like JDOM to process your XML into a data structure with convenient Java access. Then, you produce templates that access data directly out of the XML document - directly though the JDOM tree. For example, start with an XML document such as :
>     <?xml version="1.0"?>

>     <document>
>      <properties>
>       <title>Developer's Guide</title>
>       <author email="geirm@apache.org">Velocity Documentation Team</author>
>      </properties>
>     </document>
> Now make a little Java program that includes code similar to:
>     ...

>     SAXBuilder builder;
>     Document root = null;

>     try
>     {
>         builder = new SAXBuilder(  "org.apache.xerces.parsers.SAXParser" );
>         root = builder.build("test.xml");
>     }
>     catch( Exception ee)
>     {}

>     VelocityContext vc = new VelocityContext();
>     vc.put("root", root );

>     ...
> (See the Anakia source for details on how to do this, or the Anakia example in the `examples` directory in the distribution.) Now, make a regular Velocity template :
>     <html>
>       <body>
>         The document title is
>         $root.getChild("document").getChild("properties").getChild("title").getText()
>       </body>
>     </html>
> and render that template as you normally would, using the Context containing the JDOM tree. Of course, this isn't the prettiest of examples, but it shows the basics - that you can easily access XML data directly from a Velocity template. One real advantage of styling XML data in Velocity is that you have access to any other object or data that the application provides. You aren't limited to just using the data present in the XML document. You may add anything you want to the context to provide additional information for your output, or provide tools to help make working with the XML data easier. Bob McWhirter's Werken Xpath is one such useful tool - an example of how it is used in Anakia can be found in `org.apache.velocity.anakia.XPathTool`. One issue that arises with XML and Velocity is how to deal with XML entities. One technique is to combine the use of Velocimacros when you need to render an entity into the output stream :
>     ## first, define the Velocimacro somewhere

>     #macro( xenc $sometext )$tools.escapeEntities($sometext)#end

>     ## and use it as

>     #set( $sometext = " < " )
>     <text>#xenc($sometext)</text>
> where the escapeEntities() is a method that does the escaping for you. Another trick would be to create an encoding utility that takes the context as a constructor parameter and only implements a method:
>     public String get(String key)
>     {
>         Object obj = context.get(key)
>         return (obj != null) ? Escape.getText( obj.toString() ) : "";
>     }
> Put it into the context as "xenc". Then you can use it as :
>     <text>$xenc.sometext</text>
> This takes advantage of Velocity's introspection process - it will try to call get("sometext") on the $xenc object in the Context - then the xenc object can then get the value from the Context, encode it, and return it. Alternatively, since Velocity makes it easy to implement custom Context objects, you could implement your own context which always applies the encoding to any string returned. Be careful to avoid rendering the output of method calls directly, as they could return objects or strings (which might need encoding). Place them first into the context with a #set() directive and the use that, for example :
>     #set( $sometext = $jdomElement.getText() )
>     <text>$sometext</text>
The previous suggestions for dealing with XML entities came from Christoph Reck, an active participant in the Velocity community. We are very grateful for his [unknowing] contribution to this document, and hope his ideas weren't mangled too badly :)
FAQ (Frequently Asked Questions)
> In no apparent order, here are questions and answers that repeatedly arise by developers using Velocity. As we get more, we will move this out to a separate document. Why Can't I Access Class Members and Constants from VTL? The short answer is because we don't introspect for fields. We don't do that because we wish to promote the idea that you hide your raw data in your data objects. There are two solutions to this issue. The first is that you should be writing accessors for data elements you wish to be publicly exposed. That of course won't work for instances where you don't have the source, or simply are too lazy. There is a class for the latter group, `org.apache.velocity.app.FieldMethodizer` which introspects your class, and exposes the public static fields in a way that allows easy access from a template. Suppose you have a class :
>         public class Foo
>         {
>             public static String PATH_ROOT = "/foo/bar";

>             ....
>         }
> Then, in your code, you would put a `Foo` into the context like this
>           context.put("myfoo", new FieldMethodizer( new Foo() ) );
> Then to access this field in your template, you would simply access in a manner similar to Java :
>            $myfoo.PATH_ROOT
If you have a crushing need to access public non-static members (or even private if you are so driven), then you would have to exend / modify the `FieldMethodizer` yourself. (And we still recommend you provide accessors....) Where does Velocity look for Templates? By default, without any configuration on your part, Velocity will look for templates in files, and look for them in the current directory (or relative to the current directory, if you prepend a path to your template, like 'foo/bar.vm'). Velocity does this to make it as easy as possible to use out of the box. It has been argued that Velocity should do it from the 'root' directory, but it's never clear what that is on filesystems where there are multiple roots (like - "C:\", "D:\", etc). For more information, see the section on resource loaders as well as the section on configuration keys, values and defaults.
Summary
We hope this brief guide was a helpful introduction to using Velocity in your Java projects, and thank you for you interest in Velocity. We welcome any and all comments you may have about this documentation and the Velocity template engine itself. Please submit all detailed, thoughtful and constructive feedback through our mail lists.
Appendix 1 : Deploying the Example Servlet
A continuing source of frustration for beginning servlet users is getting all the pieces into place and working. Using a servlet engine like Tomcat or Resin is far from obvious for the first time user (and even for more experienced users...). The following are the basic instructions, to the best of our knowledge, for getting the SampleServlet example working. Note that the sample template `sample.vm` and the servlet code itself, `SampleServlet.java` are found in the `examples/servlet_example` directory. Although some servlet engines (Resin, for example) will compile the servlet for you, it would be best if you compiled the examples first. To do this, see the section on building Velocity, and use the `examples` target. Jakarta Tomcat Setting up under Jakarta Tomcat is fairly straightforward. The 'webapp' directory is where Tomcat automatically looks for it's 'web applications', so this is where we will set things up. First, make a new 'webapp' by creating a directory called velexample in Tomcat's webapps directory, and make a new directory structure as follows : velexample velexample/WEB-INF velexample/WEB-INF/lib velexample/WEB-INF/classes Put the Velocity jar into the velexample/WEB-INF/lib directory. Note that with v1.2 and newer, you either have to either use the jar from the distribution (or that you build yourself) that contains all the dependencies ( ex. velocity-dep-1.2.jar) or you must add the dependency jars yourself to the WEB-INF/lib directory. Please see the section "Getting Started" and "Dependencies", above. Put the SampleServlet.class into the velexample/WEB-INF/classes directory Put the sample.vm template into the velexample directory. The SampleServlet is written to use the root of the webapp as the source of the templates, so no configuration is needed. At this point, you should be able to start (or restart) Tomcat and access the servlet. To access the servlet, point your web browser at : http://localhost:8080/velexample/servlet/SampleServlet or if that doesn't work : http://<your computer's ip address>:8080/velexample/servlet/SampleServlet You should see the sample output. Caucho Technology's Resin Setting up the example servlet under Caucho Technology's Resin servlet engine is also very simple. The following instructions were tested with the version 1.2.5 release. The following assume that you have unzip-ed or untar-ed the distribution, know how to start the servlet engine (something like bin/httpd.sh under unix...), and know where the doc directory is (in the root of the distribution). Copy the SampleServlet.class file into the doc/WEB-INF/classes directory. Copy the sample.vm template file into the doc/ directory Copy the Velocity jar (and any required dependencies - see note above in Tomcat setup section) into the doc/WEB-INF/lib directory. Start resin. To access the servlet, point your web browser at : http://localhost:8080/servlet/SampleServlet or if that doesn't work : http://<your computer's ip address>:8080/servlet/SampleServlet and you should see the output. Note that this appeared to be a simpler configuration - with the Tomcat example, you set up a complete, new, separate webapp, whereas the Resin instructions don't, although both should provide a sufficient setup to let you play with Velocity. Note that while we wish we could, we can't answer questions about the servlet engines. Please use the resources provided by the servlet engine provider. BEA WebLogic Paw Dybdahl <pdy@csg.csc.dk> contributed this description of using Velocity with WebLogic, as well as some good general suggestions and examples for working with servlets.
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