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.
Building Web Applications with Velocity
Velocity is often used for building web applications In order to use Velocity in a web app you'll need a servlet or servlet-based framework. The easiest way to get started is with VelocityViewServlet in the Velocity Tools subproject. You can also use any of a number of third party frameworks or build your own servlet using the techniques described in this document.We suggest you read this article on getting started with web applications for more detail on the various options.
Downloading Velocity
You can download the latest release version of Velocity or Velocity Tools from the main Apache Velocity download site. For Velocity itself, source is included with the binary download.
If you want to download the latest source, you can do so via the Subversion (svn) source control system, or download a complete nightly snapshot.
Instructions for building Velocity from source can be found in the Build document.
DependenciesVelocity 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.
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:
All directory references above are relative to the distribution root directory.
When using Velocity in an application program or in a servlet (or anywhere, actually), you will generally do the following:
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. However, no matter how to use Velocity the above sequence is what is happening either explicitly, or behind the scenes.
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.
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 its 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");
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 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 the value must be derived from java.lang.Object. 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.
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.
Support for "Static Classes"Not all classes are instantiable. Classes like java.lang.Math do not provide any public constructor, and yet may contain useful static methods. In order to access these static methods from a template, you can simply add the class itself to the context:
context.put("Math", Math.class);
This will allow you to call any public static method in java.lang.Math on the $Math reference in the template.
Context ChainingAn innovative feature of Velocity's context design is the concept of context chaining. Also sometimes referred to as context wrapping, this advanced feature allows you to connect separate contexts together in a manner that makes it appear as one 'contiguous' context to the template.
This is best illustrated by an example:
VelocityContext context1 = new VelocityContext();
context1.put("name","Velocity");
context1.put("project", "Jakarta");
context1.put("duplicate", "I am in context1");
VelocityContext context2 = new VelocityContext( context1 );
context2.put("lang", "Java" );
context2.put("duplicate", "I am in context2");
template.merge( context2, writer );
In the code above, we have set up context2 such that it chains context1. This means that in the template, you can access any of the items that were put into either of the two VelocityContext objects, as long as there is no duplication of the keys used to add objects. If that is the case, as it is above for the key 'duplicate', the object stored in the nearest context in the chain will be available. In this example above, the object returned would be the string "I am in context2".
Note that this duplication, or 'covering', of a context item does not in any way harm or alter the covered object. So in the example above, the string "I am in context1" is alive and well, still accessable via context1.get("duplicate"). But in the example above, the value of the reference '$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 TemplateThere 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:
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:
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:
If you are using VelocityViewServlet or other web frameworks, you may never call Velocity directly. However, if you use Velocity for non-web purposes, or create your own web framework you will need to directly call the Velocity Engine similar to the fundamental pattern shown earlier. One important additional thing to remember is to initialize the Velocity Engine before using it to merge templates.
The Velocity Helper ClassVelocity 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:
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:
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:
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.
ExceptionsVelocity may throw one of several exceptions during the parse / merge cycle. These exceptions extend RuntimeException and do not need to explicitly caught, although each includes specific properties that may help in presenting useful error messages to the end user. The exceptions are found in the package org.apache.velocity.exception and are:
In each case, a message is put into the runtime log. For more information, see the Javadoc API documentation.
Miscellaneous DetailsWhile 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.
If you need to place objects into the Velocity properties then you cannot use the Velocity.init(Properties p) method. Instead you should create a new instance of the org.apache.commons.collections.ExtendedProperties class, copy all properties from an existing Properties object into the ExtendedProperties and then add new properties with your objects to the ExtendedProperties object.
...
VelocityEngine velocityEngine = new VelocityEngine();
ExtendedProperties eprops = null;
if (props==null) {
eprops = new ExtendedProperties();
} else {
eprops = ExtendedProperties.convertProperties(props);
}
// Now set the property with your object instance
eprops.setProperty("name", object);
...
velocityEngine.setExtendedProperties(eprops);
velocityEngine.init();
...
You may want to also consider using the Application Attributes feature described in the following section.
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.
Velocity contains a fine-grained event handling system that allows you to customize the operation of the engine. For example, you may change the text of references that are inserted into a page, modify which templates are actually included with #include or #parse, or capture all invalid references.
All event handler interfaces available in Velocity are in the package org.apache.velocity.app.event. You may create your own implementation or use one of the sample implementations in the package org.apache.velocity.app.event.implement. (See the javadocs for more details on the provided implementations).
org.apache.velocity.app.event.IncludeEventHandler
The IncludeEventHandler can be used to modify the template that is included in a page with #include or #parse. For example, this may be used to make all includes relative to the current directory or to prevent access to unauthorized resources. Multiple IncludeEventHandler's may be chained, with the return value of the final call used as the name of the template to retrieve.org.apache.velocity.app.event.InvalidReferenceEventHandlerpublic IncludeEventHandler extends EventHandler { public String includeEvent( String includeResourcePath, String currentResourcePath, String directiveName ); }
Available implementations include:
org.apache.velocity.app.event.implement.IncludeNotFound org.apache.velocity.app.event.implement.IncludeRelativePath
Normally, when a template contains a bad reference an error message is logged and (unless it is part of a #set or #if), the reference is included verbatim in a page. With the InvalidReferenceEventHandler this behavior can be changed. Substitute values can be inserted, invalid references may be logged, or an exception can be thrown. Multiple InvalidReferenceEventHandler's may be chained. The exact manner in which chained method calls behave will differ per method. (See the javadoc for the details).org.apache.velocity.app.event.MethodExceptionEventHandlerpublic InvalidReferenceEventHandler extends EventHandler { public Object invalidGetMethod( Context context, String reference, Object object, String property, Info info); public boolean invalidSetMethod( Context context, String leftreference, String rightreference, Info info); public Object invalidMethod( Context context, String reference, Object object, String method, Info info); }
Available implementations include:
org.apache.velocity.app.event.implement.ReportInvalidReferences
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. While MethodExceptionEventHandler's can be chained only the first handler is actually called -- all others are ignored.org.apache.velocity.app.event.ReferenceInsertionEventHandlerpublic interface MethodExceptionEventHandler extends EventHandler { public Object methodException( Class claz, String method, Exception e ) throws Exception; }
Available implementations include:
org.apache.velocity.app.event.implement.PrintExceptions
A ReferenceInsertionEventHandler allows the developer to intercept each write of a reference ($foo) value to the output stream and modify that output. Multiple ReferenceInsertionEventHandler's may be chained with each step potentially altering the inserted reference.public interface ReferenceInsertionEventHandler extends EventHandler { public Object referenceInsert( String reference, Object value ); }
Available implementations include:
org.apache.velocity.app.event.implement.EscapeHtmlReference org.apache.velocity.app.event.implement.EscapeJavascriptReference org.apache.velocity.app.event.implement.EscapeSqlReference org.apache.velocity.app.event.implement.EscapeXmlReference
Registering Event Handlers
You may register event handlers in either of two manners. The easiest way to register event handlers is to specify them in velocity.properties. (Event handlers configured in this manner are referred to as "global" event handlers). For example, the following property will escape HTML entities in any inserted reference.
eventhandler.referenceinsertion.class = org.apache.velocity.app.event.implement.EscapeHtmlReference
Most event handler interfaces will also permit event handlers to be chained together. Such a chain may be in a comma separated list or as additional lines with a property/value pair. For example, the following event handler properties install two ReferenceInsertionEventHandler's. The first will apply to references starting with "msg" (for example $msgText) and will escape HTML entities (e.g. turning & into &). The second will escape all references starting with "sql" (for example $sqlText) according to SQL escaping rules. (note that in these examples, the first two properties given relate to the event handler configuration while the second two properties are used by the specific event handler implementation).
eventhandler.referenceinsertion.class = org.apache.velocity.app.event.implement.EscapeHtmlReference eventhandler.referenceinsertion.class = org.apache.velocity.app.event.implement.EscapeSqlReference eventhandler.escape.html.match = /msg.*/ eventhandler.escape.sql.match = /sql.*/
Event handlers may also be attached to a context via an EventCartridge. This allows event handlers to be tied more closely to a specific template merge or group of merges. The event handler will automatically be injected with the current context if it implements the ContextAware interface. (Due to thread-safety reasons this is not possible with global event handlers).
The following code shows how to register an event handler with an EventCartridge and a context.
...
import org.apache.velocity.app.event.EventCartridge;
import org.apache.velocity.app.event.ReferenceInsertionEventHandler;
import org.apache.velocity.app.event.implement.EscapeHtmlReference;
import org.apache.velocity.app.event.implement.EscapeSqlReference;
...
public class Test
{
public void myTest()
{
....
/**
* Make a cartridge to hold the event handlers
*/
EventCartridge ec = new EventCartridge();
/*
* then register and chain two escape-related handlers
*/
ec.addEventHandler(new EscapeHtmlReference());
ec.addEventHandler(new EscapeSqlReference());
/*
* and then finally let it attach itself to the context
*/
ec.attachToContext( context );
/*
* now merge your template with the context as you normally
* do
*/
....
}
}
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 its configuration baseline. This ensures that Velocity will always have a 'correct' value for its 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 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 its 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.LogChute, which allows
the combination of Velocity log messages with your other application
log messages. Please see the section
Configuring Logging
for more information.
runtime.log.logsystem.class =
org.apache.velocity.runtime.log.AvalonLogChute
Class to be used for the Velocity-instantiated log system.
runtime.log.invalid.references = true
Property to turn off the log output when a reference isn't valid.
Good thing to turn off 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 LogChute interface. Note:
You must also specify
runtime.log.logsystem.class =
org.apache.velocity.runtime.log.AvalonLogChute 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.
#define() Directive
define.provide.scope.control = false
Used to turn on the automatic provision of the $define scope control
during #define() calls. The default is false.
Set it to true if you want a local, managed namespace
you can put references in when within a #define block or if you want it for
more advanced #break usage.
#evaluate() Directive
evaluate.provide.scope.control = false
Used to turn on the automatic provision of the $evaluate scope
during #evaluate() or Velocity[Engine].evaluate(...) calls. The default
is false. Set it to true if you want a local, managed namespace
you can put references in during an evaluation or if you want it for
more advanced #break usage.
#foreach() Directive
foreach.provide.scope.control = true
Used to control the automatic provision of the $foreach scope
during #foreach calls. This gives access to the foreach status information
(e.g. $foreach.index or $foreach.hasNext). The default is true.
Set it to false if unused and you want a tiny performance boost.
directive.foreach.maxloops = -1
Maximum allowed number of loops for a #foreach() statement.
directive.foreach.skip.invalid = true
Tells #foreach to simply skip rendering when the object it is
iterating over is not or cannot produce a valid Iterator.
#include() and #parse() Directive
directive.include.output.errormsg.start
= <!-- include error :
directive.include.output.errormsg.end
= see error log -->
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.max.depth = 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.
template.provide.scope.control = false
Used to turn on the automatic provision of the $template scope control
during #parse calls and template.merge(...) calls. The default is false.
Set it to true if you want a secure namespace for your template variables
or more advanced #break control.
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.manager.cache.class
Declares the class to be used
for resource caching. The current default is
org.apache.velocity.runtime.resource.ResourceCacheImpl.
When resource.manager.defaultcache.size is set to 0, then
the default implementation is the standard java ConcurrentHashMap.
Otherwise, a non-zero cache size uses an LRU Map. The default cache size
is 89. Note that the ConcurrentHashMap may be better at thread concurrency.
resource.manager.defaultcache.size
Sets the size of the default implementation of the resource
manager cache size. The default is 89.
resource.loader = <name> (default = file)
Multi-valued key. Will accept CSV for value. Public 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.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.
velocimacro.arguments.strict = false
When set to true, will throw a ParseErrorException when
parsing a template containing a macro with an invalid number of arguments.
Is set to false by default to maintain backwards compatibility with
templates written before this feature became available.
velocimacro.body.reference = false
Defines name of the reference that can be used to get the body content
(an AST block) given for a block macro call (e.g. #@myMacro() has a body #end).
The default reference name is "bodyContent" (e.g. $bodyContent). This block
macro feature was introduced in Velocity 1.7.
macro.provide.scope.control = false
Used to turn on the automatic provision of the $macro scope control
during #macro calls. The default is false.
Set it to true if you need a local namespace in macros or more
advanced #break controls.
<somebodymacro>.provide.scope.control = false
Used to turn on the automatic provision of the $<nameofthemacro> scope control
during a call to a macro with a body (e.g. #@foo #set($foo.a=$b) ... $foo.a #end).
The default is false. Set it to true if you happen to need a namespace just
for your macro's body content or more advanced #break controls.
Strict Reference Setting
runtime.references.strict = false
New in Velocity 1.6, when set to true Velocity will throw a
MethodInvocationException for references that are not
defined in the context, or have not been defined with a #set
directive. This setting will also throw an exception if an attempt is
made to call a non-existing property on an object or if the object is
null. When this property is true then
'directive.foreach.skip.invalid' defaults to true,
but explicitly setting 'directive.foreach.skip.invalid' will
override this default. For a complete discussion see Strict References Setting.
runtime.references.strict.escape = false Changes escape behavior such that putting a forward slash before a reference or macro always escapes the reference or macro and absorbs the forward slash regardless if the reference or macro is defined. For example "\$foo" always renders as "$foo", or "\#foo()" is always rendered as "#foo()". This escape behavior is of use in strict mode since unintended strings of characters that look like references or macros will throw an exception. This provides an easy way to escape these references. However, even in non-strict mode the developer may find this a more consistent and reliable method for escaping.
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.
Math
runtime.strict.math = false
Affects all math operations in VTL. If changed to true, this will cause
Velocity to throw a MathException whenever one side of a math operation
has a null value (e.g. #set( $foo = $null * 5 )) or when
trying to divide by zero. If this value is left false,
then rendering will continue and that math operation will be ignored.
Parser Configuration
parser.pool.class = org.apache.velocity.runtime.ParserPoolImpl
This property selects the implementation for the parser pool. This class
must implement ParserPool. Generally there is no reason to change this though
if you are building a high volume web application you might consider including
an alternate implementation that automatically adjusts the size of the pool.
parser.pool.size = 20
This property is used by the default pooling implementation to
set the number of parser instances 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 overflow instances as needed. Note that these
extra parsers will not be added to the pool, and will be discarded
after use. This will result in very slow operation compared to the
normal usage of pooled parsers, but this is considered an exceptional
condition. A web application using Velocity as its view engine might
exhibit this behavior under extremely high concurrency (such as when
getting Slashdotted). If you see a corresponding message referencing
the parser.pool.size property in your log files, please
increment this property immediately to avoid performance degradation.
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. You can specify a
comma-separated list of Uberspector classes, in which case all Uberspectors are
chained. The default chaining behaviour is to return the first non-null value
for each introspection call among all provided uberspectors. You can modify
this behaviour (for instance to restrict access to some methods) by subclassing
org.apache.velocity.util.introspection.AbstractChainableUberspector (or
implementing directly
org.apache.velocity.util.introspection.ChainableUberspector). This
allows you to create more interesting rules or patterns for Uberspection,
rather than just returning the first non-null value.
Some alternate Uberspectors are provided. You would for instance use runtime.introspector.uberspect = org.apache.velocity.util.introspection.SecureUberspector to avoid template authors to instanciate new classes or to use reflection, or use runtime.introspector.uberspect = org.apache.velocity.util.introspection.UberspectImpl, org.apache.velocity.util.introspection.UberspectPublicFields to expose Java public fields in your templates.
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:
Here is an example of how to configure Velocity to log to an existing Log4j Logger.
import org.apache.velocity.app.VelocityEngine;
import org.apache.velocity.runtime.RuntimeConstants;
import org.apache.log4j.Logger;
import org.apache.log4j.BasicConfigurator;
public class Log4jLoggerExample
{
public static String LOGGER_NAME = "velexample";
public static void main( String args[] )
throws Exception
{
/*
* configure log4j to log to console
*/
BasicConfigurator.configure();
Logger log = Logger.getLogger( LOGGER_NAME );
log.info("Log4jLoggerExample: 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.Log4JLogChute" );
ve.setProperty("runtime.log.logsystem.log4j.logger",
LOGGER_NAME);
ve.init();
log.info("follows initialization output from velocity");
}
}
Note that the above example can be found in examples/logger_example.
Simple Example of a Custom LoggerHere 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 LogChute interface.
import org.apache.velocity.runtime.log.LogChute;
import org.apache.velocity.runtime.RuntimeServices;
...
public class MyClass implements LogChute
{
...
public MyClass()
{
...
try
{
/*
* register this class as a logger with the Velocity singleton
* (NOTE: this would not work for the non-singleton method.)
*/
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 the 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 log(int level, String message)
{
/* do something useful */
}
/**
* This is the method that you implement for Velocity to
* call with log messages.
*/
public void log(int level, String message, Throwable t)
{
/* do something useful */
}
/**
* This is the method that you implement for Velocity to
* check whether a specified log level is enabled.
*/
public boolean isLevelEnabled(int level)
{
/* do something useful */
return someBooleanValue;
}
...
}
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.
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 CacheThe 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
A resource cache implementation may want to limit the cache size (rather than providing an unbounded cache which could consume all available memory). To configure Velocity to set the size for your cache, use the configuration key:
resource.manager.cache.size
This key is also defined as a contstant RuntimeConstants.RESOURCE_MANAGER_CACHE_SIZE
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.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'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:
<document> <properties> <title>Developer's Guide</title> <author email="geirm@apache.org">Velocity Doc 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.getRootElement());
...
(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 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 mailing lists.