Apache Velocity User Guide, with basic information about Apache Velocity's Template Language(**VTL**) information.">

Apache Velocity Language Basics | Wendy's Corner

Apache Velocity Language Basics

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Syntax Basics

All VTL statements, begins with the # character. For rendering process: the Velocity Templating Engine will:

  1. search through your web page to find all # characters

  2. Then determine which mark the beginning of VTL statements, and which of the # characters that have nothing to do with VTL.

It is recommended to start each VTL statement on a new line.

References

There are three types of references in the VTL: variables, properties and methods.

In VTL, References begin with $ and are used to get something. Directives begin with # and are used to do something.

Variables

All variables/references in VTL begins with the $ character,followed by a VTL Identifier.

  • A VTL Identifier must start with an alphabetic character (i.e. a to z case insensitive). The rest of the characters are limited to the following types of characters: alphabetic, numeric, - or _.

In VTL, the variable can get its value from either a set directive in the template, or from the Java code. Example: for #set( $foo = "bar" ), "bar" will replace all $foo variables in VTL.

Once a value has been assigned to a variable, you can reference the variable anywhere in your HTML document. Example:

<html>
<body>
    #set( $foo = "Velocity" )
    Hello $foo World!
</body>
<html>
String

String values are always enclosed in quotes, either single or double quotes:

  • Single quotes will ensure that the quoted value will be assigned to the reference as is.

  • Double quotes allow you to use velocity references and directives to interpolate

  • Example: for "Hello $name", the $name will be replaced by the referenced value.

Properties

Properties have a distinctive format.

The shorthand notation consists of a leading $ character followed a VTL Identifier, followed by a dot character . and another VTL Identifier

Example: $customer.Address has two meanings:

  1. VTL should look in the hashtable named as customer, then return the value associated with the key Address.

  2. VTL should find a referring to a method called $customer.getAddress().

Methods

Methods are references that consist of a leading $ character followed a VTL Identifier, followed by a VTL Method Body.

A VTL Method Body consists of a VTL Identifier followed by an left parenthesis character (, followed by an optional parameter list, followed by right parenthesis character ).

Example:

$customer.getAddress()
$purchase.getTotal()
$page.setTitle( "My Home Page" )
$person.setAttributes( ["Strange", "Weird", "Excited"] )
a shorthand notation for VTL Methods

In the example above, The property $customer.Address has the exact same effect as using the method $customer.getAddress().

It is generally preferable to use a Property when available.

The main difference between Properties and Methods is that you can specify a parameter list to a Method.

Array as Lists

As of Velocity 1.6, all array references are treated as fixed-length lists. User can call java.util.List methods on array references. Example:

$myarray.isEmpty()
$myarray.size()
$myarray.get(2)
$myarray.set(1, 'test')
vararg methods

From Velocity 1.6, setter methods can have different parameter list:

$sun.setPlanets('Earth', 'Mars', 'Neptune')
$sun.setPlanets('Mercury')
$sun.setPlanets()
##  Will just pass in an empty, zero-length array

Other Syntax Information

Comments

In VTL, A single line comment begins with ## and finishes at the end of the line. Example: ## This is a single line comment.

Multi-line comments, which begin with #* and end with *#, are available to handle this scenario.

This is text that is outside the multi-line comment.
Online visitors can see it.
*#
 Thus begins a multi-line comment. Online visitors won't
 see this text because the Velocity Templating Engine will
 ignore it.
*#
Here is text outside the multi-line comment; it is visible.

User can also set javadoc-style annotation in VTL comment block:

#**
This is a VTL comment block and
may be used to store such information
as the document author and versioning
information:
@author
@version 5
*#
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