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<a name="UserGuide-Contents"></a>
<h1>User Guide - Contents</h1>
<div class="subsection">
<ol>
<li>
<a href="#AboutthisGuide">About this Guide</a>
</li>
<li>
<a href="#WhatisVelocity?">What is Velocity?</a>
</li>
<li>
<a href="#WhatcanVelocitydoforme?">What can Velocity do for me?</a>
<ol>
<li>
<a href="#TheMudStoreExample">The Mud Store example</a>
</li>
</ol>
</li>
<li>
<a href="#VelocityTemplateLanguage(VTL):AnIntroduction">Velocity Template Language (VTL): An Introduction</a>
</li>
<li>
<a href="#HelloVelocityWorld!">Hello Velocity World!</a>
</li>
<li>
<a href="#Comments">Comments</a>
</li>
<li>
<a href="#References">References</a>
<ol>
<li>
<a href="#Variables">Variables</a>
</li>
<li>
<a href="#Properties">Properties</a>
</li>
<li>
<a href="#Methods">Methods</a>
</li>
<li>
<a href="#PropertyLookupRules">Property Lookup Rules</a>
</li>
</ol>
</li>
<li>
<a href="#FormalReferenceNotation">Formal Reference Notation</a>
</li>
<li>
<a href="#QuietReferenceNotation">Quiet Reference Notation</a>
</li>
<li>
<a href="#Gettingliteral">Getting literal</a>
<ol>
<li>
<a href="#Currency">Currency</a>
</li>
<li>
<a href="#EscapingValidVTLReferences">Escaping Valid VTL References</a>
</li>
</ol>
</li>
<li>
<a href="#CaseSubstitution">Case Substitution</a>
</li>
<li>
<a href="#Directives">Directives</a>
<ol>
<li>
<a href="#Set">Set</a>
</li>
<li>
<a href="#StringLiterals">Literals</a>
</li>
<li>
<a href="#Conditionals">If-Else Statements</a>
<ol>
<li>
<a href="#RelationalandLogicalOperators">Relational and Logical
Operators</a>
</li>
</ol>
</li>
<li>
<a href="#Loops">Foreach Loops</a>
</li>
<li>
<a href="#Include">Include</a>
</li>
<li>
<a href="#Parse">Parse</a>
</li>
<li>
<a href="#Stop">Stop</a>
</li>
<li>
<a href="#Velocimacros">Velocimacros</a>
</li>
</ol>
</li>
<li>
<a href="#EscapingVTLDirectives">Escaping VTL Directives</a>
</li>
<li>
<a href="#VTL:FormattingIssues">VTL: Formatting Issues</a>
</li>
<li>
<a href="#OtherFeaturesandMiscellany">Other Features and Miscellany</a>
<ol>
<li>
<a href="#Math">Math</a>
</li>
<li>
<a href="#RangeOperator">Range Operator</a>
</li>
<li>
<a href="#AdvancedIssues:Escapingand!">Advanced Issues: Escaping and
!</a>
</li>
<li>
<a href="#VelocimacroMiscellany">Velocimacro Miscellany</a>
</li>
<li>
<a href="#StringConcatenation">String Concatenation</a>
</li>
</ol>
</li>
<li>
<a href="#Feedback">Feedback</a>
</li>
</ol>
</div>
<a name="AboutthisGuide"></a>
<h1>About this Guide</h1>
<div class="subsection">
<p>
The Velocity User Guide is intended to help page designers and
content providers get acquainted with Velocity and the syntax of its
simple yet powerful scripting language, the Velocity Template
Language (VTL). Many of the examples in this guide deal with using
Velocity to embed dynamic content in web sites, but all VTL examples
are equally applicable to other pages and templates.
</p>
<p>
Thanks for choosing Velocity!
</p>
</div>
<a name="WhatisVelocity?"></a>
<h1>What is Velocity?</h1>
<div class="subsection">
<p>
Velocity is a Java-based template engine. It permits web page
designers to reference methods defined in Java code. Web designers
can work in parallel with Java programmers to develop web sites
according to the Model-View-Controller (MVC) model, meaning that web
page designers can focus solely on creating a well-designed site,
and programmers can focus solely on writing top-notch code. Velocity
separates Java code from the web pages, making the web site more
maintainable over the long run and providing a viable alternative to
<a href="http://java.sun.com/products/jsp/">Java Server Pages</a>
(JSPs) or <a href="http://www.php.net/">PHP</a>.
</p>
<p>
Velocity can be used to generate web pages, SQL, PostScript and
other output from templates. It can be used either as a standalone
utility for generating source code and reports, or as an integrated
component of other systems. When complete, Velocity will provide
template services for the <a href="http://java.apache.org/turbine/">Turbine</a> web application
framework. Velocity+Turbine will provide a template service that
will allow web applications to be developed according to a true MVC
model.
</p>
</div>
<a name="WhatcanVelocitydoforme?"></a>
<h1>What can Velocity do for me?</h1>
<div class="subsection">
<a name="TheMudStoreExample"></a>
<h2>The Mud Store Example</h2>
<div class="subsection">
<p>
Suppose you are a page designer for an online store that specializes
in selling mud. Let's call it "The Online Mud Store". Business is
thriving. Customers place orders for various types and quantities of
mud. They login to your site using their username and password,
which allows them to view their orders and buy more mud. Right now,
Terracotta Mud is on sale, which is very popular. A minority of your
customers regularly buys Bright Red Mud, which is also on sale,
though not as popular and usually relegated to the margin of your
web page. Information about each customer is tracked in your
database, so one day the question arises, Why not use Velocity to
target special deals on mud to the customers who are most interested
in those types of mud?
</p>
<p>
Velocity makes it easy to customize web pages to your online
visitors. As a web site designer at The Mud Room, you want to make
the web page that the customer will see after logging into your
site.
</p>
<p>
You meet with software engineers at your company, and everyone has
agreed that <em>$customer</em> will hold information pertaining to
the customer currently logged in, that <em>$mudsOnSpecial</em> will
be all the types mud on sale at present. The <em>$flogger</em>
object contains methods that help with promotion. For the task at
hand, let's concern ourselves only with these three references.
Remember, you don't need to worry about how the software engineers
extract the necessary information from the database, you just need
to know that it works. This lets you get on with your job, and lets
the software engineers get on with theirs.
</p>
<p>
You could embed the following VTL statement in the web page:
</p>
<div class="source">
<HTML>
<BODY>
Hello $customer.Name!
<table>
#foreach( $mud in $mudsOnSpecial )
#if ( $customer.hasPurchased($mud) )
<tr>
<td>
$flogger.getPromo( $mud )
</td>
</tr>
#end
#end
</table>
</pre></div>
<p>
The exact details of the <em>foreach</em> statement will be
described in greater depth shortly; what's important is the impact
this short script can have on your web site. When a customer with a
penchant for Bright Red Mud logs in, and Bright Red Mud is on sale,
that is what this customer will see, prominently displayed. If
another customer with a long history of Terracotta Mud purchases
logs in, the notice of a Terracotta Mud sale will be front and
center. The flexibility of Velocity is enormous and limited only by
your creativity.
</p>
<p>
Documented in the VTL Reference are the many other Velocity
elements, which collectively give you the power and flexibility you
need to make your web site a web <em>presence</em>. As you get more
familiar with these elements, you will begin to unleash the power of
Velocity.
</p>
</div>
</div>
<a name="VelocityTemplateLanguage(VTL):AnIntroduction"></a>
<h1>Velocity Template Language (VTL): An Introduction</h1>
<div class="subsection">
<p>
The Velocity Template Language (VTL) is meant to provide the
easiest, simplest, and cleanest way to incorporate dynamic content
in a web page. Even a web page developer with little or no
programming experience should soon be capable of using VTL to
incorporate dynamic content in a web site.
</p>
<p>
VTL uses <em>references</em> to embed dynamic content in a web site,
and a variable is one type of reference. Variables are one type of
reference that can refer to something defined in the Java code, or
it can get its value from a VTL <em>statement</em> in the web page
itself. Here is an example of a VTL statement that could be embedded
in an HTML document:
</p>
<div class="source">
#set( $a = "Velocity" )
</pre></div>
<p>
This VTL statement, like all VTL statements, begins with the
<em>#</em> character and contains a directive: <em>set</em>. When an
online visitor requests your web page, the Velocity Templating
Engine will search through your web page to find all <em>#</em>
characters, then determine which mark the beginning of VTL
statements, and which of the <em>#</em> characters that have nothing
to do with VTL.
</p>
<p>
The <em>#</em> character is followed by a directive, <em>set</em>.
The <em>set</em> directive uses an expression (enclosed in brackets)
-- an equation that assigns a <em>value</em> to a <em>variable</em>.
The variable is listed on the left hand side and its value on the
right hand side; the two are separated by an <em>=</em> character.
</p>
<p>
In the example above, the variable is <em>$a</em> and the value is
<em>Velocity</em>. This variable, like all references, begins with
the <em>$</em> character. 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, such as "Hello $name",
where the <em>$name</em> will be replaced by the current value before
that string literal is assigned to the left hand side of the <em>=</em>
</p>
<p>
The following rule of thumb may be useful to better understand how
Velocity works: <strong>References begin with <em>$</em> and are
used to get something. Directives begin with <em>#</em> and are used
to do something.</strong>
</p>
<p>
In the example above, <em>#set</em> is used to assign a value to a
variable. The variable, <em>$a</em>, can then be used in the
template to output "Velocity".
</p>
</div>
<a name="HelloVelocityWorld!"></a>
<h1>Hello Velocity World!</h1>
<div class="subsection">
<p>
Once a value has been assigned to a variable, you can reference the
variable anywhere in your HTML document. In the following example, a
value is assigned to <em>$foo</em> and later referenced.
</p>
<div class="source">
<html>
<body>
#set( $foo = "Velocity" )
Hello $foo World!
</body>
<html>
</pre></div>
<p>
The result is a web page that prints "Hello Velocity World!".
</p>
<p>
To make statements containing VTL directives more readable, we
encourage you to start each VTL statement on a new line, although
you are not required to do so. The <em>set</em> directive will be
revisited in greater detail later on.
</p>
</div>
<a name="Comments"></a>
<h1>Comments</h1>
<div class="subsection">
<p>
Comments allows descriptive text to be included that is not placed
into the output of the template engine. Comments are a useful way of
reminding yourself and explaining to others what your VTL statements
are doing, or any other purpose you find useful. Below is an example
of a comment in VTL.
</p>
<div class="source">
## This is a single line comment.
</pre></div>
<p>
A single line comment begins with <em>##</em> and finishes at the
end of the line. If you're going to write a few lines of commentary,
there's no need to have numerous single line comments. Multi-line
comments, which begin with <em>#*</em> and end with <em>*#</em>, are
available to handle this scenario.
</p>
<div class="source">
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.
</pre></div>
<p>
Here are a few examples to clarify how single line and multi-line
comments work:
</p>
<div class="source">
This text is visible. ## This text is not.
This text is visible.
This text is visible. #* This text, as part of a multi-line
comment, is not visible. This text is not visible; it is also
part of the multi-line comment. This text still not
visible. *# This text is outside the comment, so it is visible.
## This text is not visible.
</pre></div>
<p>
There is a third type of comment, the VTL comment block, which may
be used to store such information as the document author and
versioning information:
</p>
<div class="source">
#**
This is a VTL comment block and
may be used to store such information
as the document author and versioning
information:
@author
@version 5
*#
</pre></div>
</div>
<a name="References"></a>
<h1>References</h1>
<div class="subsection">
<p>
There are three types of references in the VTL: variables,
properties and methods. As a designer using the VTL, you and your
engineers must come to an agreement on the specific names of
references so you can use them correctly in your templates.
</p>
<p>
Everything coming to and from a reference is treated as a String
object. If there is an object that represents <em>$foo</em> (such as
an Integer object), then Velocity will call its
<code>.toString()</code> method to resolve the object into a String.
</p>
<p>
<a name="Variables"><strong>Variables</strong></a>
<br />
The shorthand notation of a variable consists of a leading "$"
character followed by a VTL <em>Identifier</em>. A VTL Identifier
must start with an alphabetic character (a .. z or A .. Z). The rest
of the characters are limited to the following types of characters:
</p>
<p>
<ul>
<li>alphabetic (a .. z, A .. Z)</li>
<li>numeric (0 .. 9)</li>
<li>hyphen ("-")</li>
<li>underscore ("_")</li>
</ul>
</p>
<p>
Here are some examples of valid variable references in the VTL:
</p>
<div class="source">
$foo
$mudSlinger
$mud-slinger
$mud_slinger
$mudSlinger1
</pre></div>
<p>
When VTL references a variable, such as <em>$foo</em>, the variable
can get its value from either a <em>set</em> directive in the
template, or from the Java code. For example, if the Java variable
<em>$foo</em> has the value <em>bar</em> at the time the template is
requested, <em>bar</em> replaces all instances of <em>$foo</em> on
the web page. Alternatively, if I include the statement
</p>
<div class="source">
#set( $foo = "bar" )
</pre></div>
<p>
The output will be the same for all instances of <em>$foo</em> that
follow this directive.
</p>
<p>
<a name="Properties"><strong>Properties</strong></a>
<br />
The second flavor of VTL references are properties, and properties
have a distinctive format. The shorthand notation consists of a
leading <em>$</em> character followed a VTL Identifier, followed by
a dot character (".") and another VTL Identifier. These are examples
of valid property references in the VTL:
</p>
<div class="source">
$customer.Address
$purchase.Total
</pre></div>
<p>
Take the first example, <em>$customer.Address</em>. It can have two
meanings. It can mean, Look in the hashtable identified as
<em>customer</em> and return the value associated with the key
<em>Address</em>. But <em>$customer.Address</em> can also be
referring to a method (references that refer to methods will be
discussed in the next section); <em>$customer.Address</em> could be
an abbreviated way of writing <em>$customer.getAddress()</em>. When
your page is requested, Velocity will determine which of these two
possibilities makes sense, and then return the appropriate value.
</p>
<p>
<a name="Methods"><strong>Methods</strong></a>
<br />
A method is defined in the Java code and is capable of doing
something useful, like running a calculation or arriving at a
decision. Methods are references that consist of a leading "$"
character followed a VTL Identifier, followed by a VTL <em>Method
Body</em>. 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 (")"). These
are examples of valid method references in the VTL:
</p>
<div class="source">
$customer.getAddress()
$purchase.getTotal()
$page.setTitle( "My Home Page" )
$person.setAttributes( ["Strange", "Weird", "Excited"] )
</pre></div>
<p>
The first two examples -- <em>$customer.getAddress()</em> and
<em>$purchase.getTotal()</em> -- may look similar to those used in
the Properties section above, <em>$customer.Address</em> and
<em>$purchase.Total</em>. If you guessed that these examples must be
related some in some fashion, you are correct!
</p>
<p>
VTL Properties can be used as a shorthand notation for VTL Methods.
The Property <em>$customer.Address</em> has the exact same effect as
using the Method <em>$customer.getAddress()</em>. 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.
</p>
<p>
The shorthand notation can be used for the following Methods
</p>
<div class="source">
$sun.getPlanets()
$annelid.getDirt()
$album.getPhoto()
</pre></div>
<p>
We might expect these methods to return the names of planets
belonging to the sun, feed our earthworm, or get a photograph from
an album. Only the long notation works for the following Methods.
</p>
<div class="source">
$sun.getPlanet( ["Earth", "Mars", "Neptune"] )
## Can't pass a parameter list with $sun.Planets
$sisyphus.pushRock()
## Velocity assumes I mean $sisyphus.getRock()
$book.setTitle( "Homage to Catalonia" )
## Can't pass a parameter list
</pre></div>
<p>
<a name="PropertyLookupRules"><strong>Property Lookup Rules</strong></a>
<br />
As was mentioned earlier, properties often refer to methods of the
parent object. Velocity is quite clever when figuring out which method
corresponds to a requested property. It tries out different alternatives
based on several established naming conventions. The exact lookup
sequence depends on whether or not the property name starts with
an upper-case letter. For lower-case names, such as
<em>$customer.address</em>, the sequence is
<ol>
<li>getaddress()</li>
<li>getAddress()</li>
<li>get("address")</li>
<li>isAddress()</li>
</ol>
For upper-case property names like <em>$customer.Address</em>, it is
slightly different:
<ol>
<li>getAddress()</li>
<li>getaddress()</li>
<li>get("Address")</li>
<li>isAddress()</li>
</ol>
</p>
<p>
<a name="FormalReferenceNotation"><strong>Formal Reference Notation</strong></a>
<br />
Shorthand notation for references was used for the examples listed
above, but there is also a formal notation for references, which is
demonstrated below:
</p>
<div class="source">
${mudSlinger}
${customer.Address}
${purchase.getTotal()}
</pre></div>
<p>
In almost all cases you will use the shorthand notation for
references, but in some cases the formal notation is required for
correct processing.
</p>
<p>
Suppose you were constructing a sentence on the fly where
<em>$vice</em> was to be used as the base word in the noun of a
sentence. The goal is to allow someone to choose the base word and
produce one of the two following results: "Jack is a pyromaniac." or
"Jack is a kleptomaniac.". Using the shorthand notation would be
inadequate for this task. Consider the following example:
</p>
<div class="source">
Jack is a $vicemaniac.
</pre></div>
<p>
There is ambiguity here, and Velocity assumes that
<em>$vicemaniac</em>, not <em>$vice</em>, is the Identifier that you
mean to use. Finding no value for <em>$vicemaniac</em>, it will
return <em>$vicemaniac</em>. Using formal notation can resolve this
problem.
</p>
<div class="source">
Jack is a ${vice}maniac.
</pre></div>
<p>
Now Velocity knows that <em>$vice</em>, not <em>$vicemaniac</em>, is
the reference. Formal notation is often useful when references are
directly adjacent to text in a template.
</p>
<p>
<a name="QuietReferenceNotation"><strong>Quiet Reference Notation</strong></a>
<br />
When Velocity encounters an undefined reference, its normal behavior
is to output the image of the reference. For example, suppose the
following reference appears as part of a VTL template.
</p>
<div class="source">
<input type="text" name="email" value="$email"/>
</pre></div>
<p>
When the form initially loads, the variable reference
<em>$email</em> has no value, but you prefer a blank text field to
one with a value of "$email". Using the quiet reference notation
circumvents Velocity's normal behavior; instead of using
<em>$email</em> in the VTL you would use <em>$!email</em>. So the
above example would look like the following:
</p>
<div class="source">
<input type="text" name="email" value="$!email"/>
</pre></div>
<p>
Now when the form is initially loaded and <em>$email</em> still has
no value, an empty string will be output instead of "$email".
</p>
<p>
Formal and quiet reference notation can be used together, as
demonstrated below.
</p>
<div class="source">
<input type="text" name="email" value="$!{email}"/>
</pre></div>
</div>
<a name="Gettingliteral"></a>
<h1>Getting literal</h1>
<div class="subsection">
<p>
VTL uses special characters, such as <em>$</em> and <em>#</em>, to
do its work, so some added care should be taken where using these
characters in your templates. This section deals with escaping the
<em>$</em> character.
</p>
<p>
<a name="Currency"><strong>Currency</strong></a>
<br />
There is no problem writing "I bought a 4 lb. sack of potatoes at
the farmer's market for only $2.50!" As mentioned, a VTL identifier
always begins with an upper- or lowercase letter, so $2.50 would not
be mistaken for a reference.
</p>
<p>
<a name="EscapingValidVTLReferences"><strong>Escaping Valid VTL References</strong></a>
<br />
Cases may arise where there is the potential for Velocity to get
confused. <em>Escaping</em> special characters is the best way to
handle VTL's special characters in your templates, and this can be
done using the backslash ( <em>\</em> ) character.
</p>
<div class="source">
#set( $email = "foo" )
$email
</pre></div>
<p>
If Velocity encounters a reference in your VTL template to
<em>$email</em>, it will search the Context for a corresponding
value. Here the output will be <em>foo</em>, because <em>$email</em> is
defined. If <em>$email</em> is not defined, the output will be
<em>$email</em>.
</p>
<p>
Suppose that <em>$email</em> is defined (for example, if it has the
value <em>foo</em>), and that you want to output <em>$email</em>. There are a few
ways of doing this, but the simplest is to use the escape character.
</p>
<div class="source">
## The following line defines $email in this template:
#set( $email = "foo" )
$email
\$email
\\$email
\\\$email
</pre></div>
<p>
renders as
</p>
<div class="source">
foo
$email
\foo
\$email
</pre></div>
<p>
Note that the <em>\</em> character bind to the <em>$</em>
from the left. The bind-from-left rule causes <em>\\\$email</em> to
render as <em>\\$email</em>. Compare these examples to those in
which <em>$email</em> is not defined.
</p>
<div class="source">
$email
\$email
\\$email
\\\$email
</pre></div>
<p>
renders as
</p>
<div class="source">
$email
\$email
\\$email
\\\$email
</pre></div>
<p>
Notice Velocity handles references that are defined differently
from those that have not been defined. Here is a set directive that
gives <em>$foo</em> the value <em>gibbous</em>.
</p>
<div class="source">
#set( $foo = "gibbous" )
$moon = $foo
</pre></div>
<p>
The output will be: <em>$moon = gibbous</em> -- where <em>$moon</em>
is output as a literal because it is undefined and <em>gibbous</em>
is output in place of <em>$foo</em>.
</p>
<p>
It is also possible to escape VTL directives; this is described in
more detail in the Directives section.
</p>
</div>
<a name="CaseSubstitution"></a>
<h1>Case Substitution</h1>
<div class="subsection">
<p>
Now that you are familiar with references, you can begin to apply
them effectively in your templates. Velocity references take
advantage of some Java principles that template designers will find
easy to use. For example:
</p>
<div class="source">
$foo
$foo.getBar()
## is the same as
$foo.Bar
$data.setUser("jon")
## is the same as
#set( $data.User = "jon" )
$data.getRequest().getServerName()
## is the same as
$data.Request.ServerName
## is the same as
${data.Request.ServerName}
</pre></div>
<p>
These examples illustrate alternative uses for the same references.
Velocity takes advantage of Java's introspection and bean features
to resolve the reference names to both objects in the Context as
well as the objects methods. It is possible to embed and evaluate
references almost anywhere in your template.
</p>
<p>
Velocity, which is modelled on the Bean specifications defined by
Sun Microsystems, is case sensitive; however, its developers have
strove to catch and correct user errors wherever possible.
When the method <em>getFoo()</em> is referred to in a template
by <code>$bar.foo</code>, Velocity will first try <code>$getfoo</code>.
If this fails, it will then try <code>$getFoo</code>.
Similarly, when a template refers to <code>$bar.Foo</code>, Velocity
will try <em>$getFoo()</em> first and then try <em>getfoo()</em>.
</p>
<p>
Note: <em>References to instance variables in a template are not
resolved.</em> Only references to the attribute equivalents of
JavaBean getter/setter methods are resolved
(i.e. <code>$foo.Name</code> does resolve to the class Foo's
<code>getName()</code> instance method, but not to a public
<code>Name</code> instance variable of Foo).
</p>
</div>
<a name="Directives"></a>
<h1>Directives</h1>
<div class="subsection">
<p>
References allow template designers to generate dynamic content for
web sites, while <em>directives</em> -- easy to use script elements
that can be used to creatively manipulate the output of Java code --
permit web designers to truly take charge of the appearance and
content of the web site.
</p>
<p>
Directives always begin with a <code>#</code>. Like references, the name of the directive may be
bracketed by a <code>{</code> and a <code>}</code> symbol. This is useful with directives
that are immediately followed by text. For example the following produces an error:
</p>
<div class="source">
#if($a==1)true enough#elseno way!#end
</pre></div>
<p>
In such a case, use the brackets to separate <code>#else</code> from the rest of the line.
</p>
<div class="source">
#if($a==1)true enough#{else}no way!#end
</pre></div>
<p> </p>
<a name="Set"><strong>#set</strong></a>
<p>
The <em>#set</em> directive is used for setting the value of a
reference. A value can be assigned to either a variable reference or
a property reference, and this occurs in brackets, as demonstrated:
</p>
<div class="source">
#set( $primate = "monkey" )
#set( $customer.Behavior = $primate )
</pre></div>
<p>
The left hand side (LHS) of the assignment must be a variable
reference or a property reference. The right hand side (RHS) can be
one of the following types:
</p>
<p>
<ul>
<li>Variable reference</li>
<li>String literal</li>
<li>Property reference</li>
<li>Method reference</li>
<li>Number literal</li>
<li>ArrayList</li>
<li>Map</li>
</ul>
</p>
<p>
These examples demonstrate each of the aforementioned types:
</p>
<div class="source">
#set( $monkey = $bill ) ## variable reference
#set( $monkey.Friend = "monica" ) ## string literal
#set( $monkey.Blame = $whitehouse.Leak ) ## property reference
#set( $monkey.Plan = $spindoctor.weave($web) ) ## method reference
#set( $monkey.Number = 123 ) ##number literal
#set( $monkey.Say = ["Not", $my, "fault"] ) ## ArrayList
#set( $monkey.Map = {"banana" : "good", "roast beef" : "bad"}) ## Map
</pre></div>
<p>
NOTE: For the ArrayList example the elements defined with the
[..] operator are accessible using the methods defined
in the ArrayList class. So, for example, you could access
the first element above using $monkey.Say.get(0).
</p>
<p>
Similarly, for the Map example, the elements defined within the
{ } operator are accessible using the methods defined
in the Map class. So, for example, you could access
the first element above using $monkey.Map.get("bannana") to return
a String 'good', or even $monkey.Map.banana to return the
same value.
</p>
<p>
The RHS can also be a simple arithmetic expression:
</p>
<div class="source">
#set( $value = $foo + 1 )
#set( $value = $bar - 1 )
#set( $value = $foo * $bar )
#set( $value = $foo / $bar )
</pre></div>
<p>
If the RHS is a property or method reference that evaluates to
<em>null</em>, it will <b>not</b> be assigned to the LHS.
Depending on how Velocity is configured, it is usually not
possible to remove an existing reference from the context via
this mechanism.
(Note that this can be permitted by changing one of the Velocity configuration properties).
This can be confusing for
newcomers to Velocity. For example:
</p>
<div class="source">
#set( $result = $query.criteria("name") )
The result of the first query is $result
#set( $result = $query.criteria("address") )
The result of the second query is $result
</pre></div>
<p>
If <em>$query.criteria("name")</em> returns the string
"bill", and <em>$query.criteria("address")</em> returns
<em>null</em>, the above VTL will render as the following:
</p>
<div class="source">
The result of the first query is bill
The result of the second query is bill
</pre></div>
<p>
This tends to confuse newcomers who construct <em>#foreach</em>
loops that attempt to <em>#set</em> a reference via a property or
method reference, then immediately test that reference with an
<em>#if</em> directive. For example:
</p>
<div class="source">
#set( $criteria = ["name", "address"] )
#foreach( $criterion in $criteria )
#set( $result = $query.criteria($criterion) )
#if( $result )
Query was successful
#end
#end
</pre></div>
<p>
In the above example, it would not be wise to rely on the
evaluation of <em>$result</em> to determine if a query was
successful. After <em>$result</em> has been <em>#set</em> (added to
the context), it cannot be set back to <em>null</em> (removed from
the context). The details of the <em>#if</em> and <em>#foreach</em>
directives are covered later in this document.
</p>
<p>
One solution to this would be to pre-set <em>$result</em>
to <em>false</em>. Then if the <em>$query.criteria()</em>
call fails, you can check.
</p>
<div class="source">
#set( $criteria = ["name", "address"] )
#foreach( $criterion in $criteria )
#set( $result = false )
#set( $result = $query.criteria($criterion) )
#if( $result )
Query was successful
#end
#end
</pre></div>
<p>
Unlike some of the other Velocity directives, the <em>#set</em>
directive does not have an <em>#end</em> statement.
</p>
<a name="StringLiterals"><strong>Literals</strong></a>
<p>
When using the <em>#set</em> directive, string literals that are
enclosed in double quote characters will be parsed and rendered, as
shown:
</p>
<div class="source">
#set( $directoryRoot = "www" )
#set( $templateName = "index.vm" )
#set( $template = "$directoryRoot/$templateName" )
$template
</pre></div>
<p>
The output will be
</p>
<div class="source">
www/index.vm
</pre></div>
<p>
However, when the string literal is enclosed in single quote
characters, it will not be parsed:
</p>
<div class="source">
#set( $foo = "bar" )
$foo
#set( $blargh = '$foo' )
$blargh
</pre></div>
<div class="source">
bar
$foo
</pre></div>
<p>
By default, this feature of using single quotes to render unparsed
text is available in Velocity. This default can be changed by
editing <code>velocity.properties</code> such that
<code>stringliterals.interpolate=false</code>.
</p>
<p>
Alternately, the <em>#literal</em> script element allows the
template designer to easily use large chunks of uninterpreted
content in VTL code. This can be especially useful in place of <a href="#EscapingVTLDirectives">escaping</a> multiple directives.
</p>
<div class="source">
#literal()
#foreach ($woogie in $boogie)
nothing will happen to $woogie
#end
#end
</pre></div>
<p>
Renders as:
</p>
<div class="source">
#foreach ($woogie in $boogie)
nothing will happen to $woogie
#end
</pre></div>
</div>
<a name="Conditionals"></a>
<h1>Conditionals</h1>
<div class="subsection">
<strong>If / ElseIf / Else</strong>
<p>
The <em>#if</em> directive in Velocity allows for text to be
included when the web page is generated, on the conditional that
the if statement is true. For example:
</p>
<div class="source">
#if( $foo )
<strong>Velocity!</strong>
#end
</pre></div>
<p>
The variable <em>$foo</em> is evaluated to determine whether it is
true, which will happen under one of two circumstances: (i)
<em>$foo</em> is a boolean (true/false) which has a true value, or
(ii) the value is not null. Remember that the Velocity context only
contains Objects, so when we say 'boolean', it will be represented
as a Boolean (the class). This is true even for methods that return
<code>boolean</code> - the introspection infrastructure will return
a <code>Boolean</code> of the same logical value.
</p>
<p>
The content between the <em>#if</em>
and the <em>#end</em> statements become the output if the
evaluation is true. In this case, if <em>$foo</em> is true, the
output will be: "Velocity!". Conversely, if <em>$foo</em> has a
null value, or if it is a boolean false, the statement evaluates
as false, and there is no output.
</p>
<p>
An <em>#elseif</em> or <em>#else</em> element can be used with an
<em>#if</em> element. Note that the Velocity Templating Engine
will stop at the first expression that is found to be true. In the
following example, suppose that <em>$foo</em> has a value of 15
and <em>$bar</em> has a value of 6.
</p>
<div class="source">
#if( $foo < 10 )
<strong>Go North</strong>
#elseif( $foo == 10 )
<strong>Go East</strong>
#elseif( $bar == 6 )
<strong>Go South</strong>
#else
<strong>Go West</strong>
#end
</pre></div>
<p>In this example, <em>$foo</em> is greater than 10, so the first
two comparisons fail. Next <em>$bar</em> is compared to 6, which is
true, so the output is <strong>Go South</strong>.
</p>
<p>
<a name="RelationalandLogicalOperators"><strong>Relational and Logical Operators</strong></a>
</p>
<p>
Velocity uses the equivalent operator to determine the relationships between variables.
Here is a simple example to illustrate how the equivalent operator is used.
</p>
<div class="source">
#set ($foo = "deoxyribonucleic acid")
#set ($bar = "ribonucleic acid")
#if ($foo == $bar)
In this case it's clear they aren't equivalent. So...
#else
They are not equivalent and this will be the output.
#end
</pre></div>
<p>Note that the semantics of <em>==</em> are slightly different than Java where <em>==</em>
can only be used to test object equality. In Velocity the equivalent operator
can be used to directly compare numbers, strings, or objects. When the objects
are of different classes, the string representations are obtained by calling
<code>toString()</code> for each object and then compared.
</p>
<p>
Velocity has logical AND, OR and NOT operators as well.
For further information, please see the
<a href="vtl-reference-guide.html">VTL Reference Guide</a>
Below are examples demonstrating the use of the
logical AND, OR and NOT operators.
</p>
<div class="source">
## logical AND
#if( $foo && $bar )
<strong> This AND that</strong>
#end
</pre></div>
<p>
The <em>#if()</em> directive will only evaluate to true
if both <em>$foo</em>
and <em>$bar</em> are true. If <em>$foo</em> is false, the
expression will evaluate to false; <em>$bar</em> will not be
evaluated. If <em>$foo</em> is true, the Velocity Templating
Engine will then check the value of <em>$bar</em>; if
<em>$bar</em> is true, then the entire expression is true and
<strong>This AND that</strong> becomes the output. If
<em>$bar</em> is false, then there will be no output as the entire
expression is false.
</p>
<p>
Logical OR operators work the same way, except only one of the
references need evaluate to true in order for the entire
expression to be considered true. Consider the following example.
</p>
<div class="source">
## logical OR
#if( $foo || $bar )
<strong>This OR That</strong>
#end
</pre></div>
<p>
If <em>$foo</em> is true, the Velocity Templating Engine has no
need to look at <em>$bar</em>; whether <em>$bar</em> is true or
false, the expression will be true, and <strong>This OR That</strong>
will be output. If <em>$foo</em> is false,
however, <em>$bar</em> must be checked. In this case, if
<em>$bar</em> is also false, the expression evaluates to false and
there is no output. On the other hand, if <em>$bar</em> is true,
then the entire expression is true, and the output is
<strong>This OR That</strong>
</p>
<p>
With logical NOT operators, there is only one argument :
</p>
<div class="source">
##logical NOT
#if( !$foo )
<strong>NOT that</strong>
#end
</pre></div>
<p>
Here, the if <em>$foo</em> is true, then <em>!$foo</em> evaluates to
false, and there is no output. If <em>$foo</em> is false, then
<em>!$foo</em> evaluates to true and <strong>NOT that</strong> will be
output. Be careful not to confuse this with the <em>quiet reference $!foo</em>
which is something altogether different.
</p>
<p>There are text versions of all logical operators, including
<em>eq</em>, <em>ne</em>,
<em>and</em>, <em>or</em>, <em>not</em>, <em>gt</em>, <em>ge</em>, <em>lt</em>,
and <em>le</em>.
</p>
<p>One more useful note. When you wish to include text immediately following
a <em>#else</em> directive you will need to use curly brackets immediately surrounding
the directive to differentiate it from the following text.
(Any directive can be delimited by
curly brackets, although this is most useful for <em>#else</em>).
</p>
<div class="source">
#if( $foo == $bar)it's true!#{else}it's not!#end</li>
</pre></div>
</div>
<a name="Loops"></a>
<h1>Loops</h1>
<div class="subsection">
<strong>Foreach Loop</strong>
<p>
The <em>#foreach</em> element allows for looping. For example:
</p>
<div class="source">
<ul>
#foreach( $product in $allProducts )
<li>$product</li>
#end
</ul>
</pre></div>
<p>
This <em>#foreach</em> loop causes the <em>$allProducts</em> list
(the object) to be looped over for all of the products (targets) in
the list. Each time through the loop, the value from
<em>$allProducts</em> is placed into the <em>$product</em> variable.
</p>
<p>
The contents of the <em>$allProducts</em> variable is a Vector, a
Hashtable or an Array. The value assigned to the <em>$product</em>
variable is a Java Object and can be referenced from a variable as
such. For example, if <em>$product</em> was really a Product class
in Java, its name could be retrieved by referencing the
<em>$product.Name</em> method (ie: <em>$Product.getName()</em>).
</p>
<p>
Lets say that <em>$allProducts</em> is a Hashtable. If you wanted to
retrieve the key values for the Hashtable as well as the objects
within the Hashtable, you can use code like this:
</p>
<div class="source">
<ul>
#foreach( $key in $allProducts.keySet() )
<li>Key: $key -> Value: $allProducts.get($key)</li>
#end
</ul>
</pre></div>
<p>
Velocity provides an easy way to get the loop counter so that you
can do something like the following:
</p>
<div class="source">
<table>
#foreach( $customer in $customerList )
<tr><td>$velocityCount</td><td>$customer.Name</td></tr>
#end
</table>
</pre></div>
<p>
The default name for the loop counter variable reference, which is
specified in the velocity.properties file, is $velocityCount. By
default the counter starts at 1, but this can be set to either 0 or
1 in the <code>velocity.properties</code> file. Here's what the loop
counter properties section of the <code>velocity.properties</code>
file appears:
</p>
<div class="source">
# Default name of the loop counter
# variable reference.
directive.foreach.counter.name = velocityCount
# Default starting value of the loop
# counter variable reference.
directive.foreach.counter.initial.value = 1
</pre></div>
<p>
It's possible to set a maximum allowed number of times that a loop
may be executed. By default there is no max (indicated by a value of 0
or less), but this can be set to an arbitrary number in the
<code>velocity.properties</code> file. This is useful as a fail-safe.
</p>
<div class="source">
# The maximum allowed number of loops.
directive.foreach.maxloops = -1
</pre></div>
</div>
<a name="Include"></a>
<h1>Include</h1>
<div class="subsection">
<p>
The <em>#include</em> script element allows the template designer to
import a local file, which is then inserted into the location where
the <em>#include</em> directive is defined. The contents of the file
are not rendered through the template engine. For security reasons,
the file to be included may only be under TEMPLATE_ROOT.
</p>
<div class="source">
#include( "one.txt" )
</pre></div>
<p>
The file to which the <em>#include</em> directive refers is enclosed
in quotes. If more than one file will be included, they should be
separated by commas.
</p>
<div class="source">
#include( "one.gif","two.txt","three.htm" )
</pre></div>
<p>
The file being included need not be referenced by name; in fact, it
is often preferable to use a variable instead of a filename. This
could be useful for targeting output according to criteria
determined when the page request is submitted. Here is an example
showing both a filename and a variable.
</p>
<div class="source">
#include( "greetings.txt", $seasonalstock )
</pre></div>
</div>
<a name="Parse"></a>
<h1>Parse</h1>
<div class="subsection">
<p>
The <em>#parse</em> script element allows the template designer to
import a local file that contains VTL. Velocity will parse the VTL
and render the template specified.
</p>
<div class="source">
#parse( "me.vm" )
</pre></div>
<p>
Like the <em>#include</em> directive, <em>#parse</em> can take a
variable rather than a template. Any templates to which
<em>#parse</em> refers must be included under TEMPLATE_ROOT. Unlike
the <em>#include</em> directive, <em>#parse</em> will only take a
single argument.
</p>
<p>
VTL templates can have <em>#parse</em> statements referring to
templates that in turn have <em>#parse</em> statements. By default
set to 10, the <em>directive.parse.max.depth</em> line of the
<code>velocity.properties</code> allows users to customize maximum
number of <em>#parse</em> referrals that can occur from a single
template. (Note: If the <em>directive.parse.max.depth</em> property
is absent from the <code>velocity.properties</code> file, Velocity
will set this default to 10.) Recursion is permitted, for example,
if the template <code>dofoo.vm</code> contains the following lines:
</p>
<div class="source">
Count down.
#set( $count = 8 )
#parse( "parsefoo.vm" )
All done with dofoo.vm!
</pre></div>
<p>
It would reference the template <code>parsefoo.vm</code>, which
might contain the following VTL:
</p>
<div class="source">
$count
#set( $count = $count - 1 )
#if( $count > 0 )
#parse( "parsefoo.vm" )
#else
All done with parsefoo.vm!
#end
</pre></div>
<p>
After "Count down." is displayed, Velocity passes through
<code>parsefoo.vm</code>, counting down from 8. When the count
reaches 0, it will display the "All done with parsefoo.vm!" message.
At this point, Velocity will return to <code>dofoo.vm</code> and
output the "All done with dofoo.vm!" message.
</p>
</div>
<a name="Stop"></a>
<h1>Stop</h1>
<div class="subsection">
<p>
The <em>#stop</em> script element allows the template designer to
stop the execution of the template engine and return. This is useful
for debugging purposes.
</p>
<div class="source">
#stop
</pre></div>
</div>
<a name="Velocimacros"></a>
<h1>Velocimacros</h1>
<div class="subsection">
<p>
The <em>#macro</em> script element allows template designers to
define a repeated segment of a VTL template. Velocimacros are very
useful in a wide range of scenarios both simple and complex. This
Velocimacro, created for the sole purpose of saving keystrokes and
minimizing typographic errors, provides an introduction to the
concept of Velocimacros.
</p>
<div class="source">
#macro( d )
<tr><td></td></tr>
#end
</pre></div>
<p>
The Velocimacro being defined in this example is <em>d</em>, and it
can be called in a manner analogous to any other VTL directive:
</p>
<div class="source">
#d()
</pre></div>
<p>
When this template is called, Velocity would replace <em>#d()</em>
with a row containing a single, empty data cell.
</p>
<p>
A Velocimacro could take any number of arguments -- even zero
arguments, as demonstrated in the first example, is an option -- but
when the Velocimacro is invoked, it must be called with the same
number of arguments with which it was defined. Many Velocimacros are
more involved than the one defined above. Here is a Velocimacro that
takes two arguments, a color and an array.
</p>
<div class="source">
#macro( tablerows $color $somelist )
#foreach( $something in $somelist )
<tr><td bgcolor=$color>$something</td></tr>
#end
#end
</pre></div>
<p>
The Velocimacro being defined in this example, <em>tablerows</em>,
takes two arguments. The first argument takes the place of
<em>$color</em>, and the second argument takes the place of
<em>$somelist</em>.
</p>
<p>
Anything that can be put into a VTL template can go into the body of
a Velocimacro. The <em>tablerows</em> Velocimacro is a
<em>foreach</em> statement. There are two <em>#end</em> statements
in the definition of the <em>#tablerows</em> Velocimacro; the first
belongs to the <em>#foreach</em>, the second ends the Velocimacro
definition.
</p>
<div class="source">
#set( $greatlakes = ["Superior","Michigan","Huron","Erie","Ontario"] )
#set( $color = "blue" )
<table>
#tablerows( $color $greatlakes )
</table>
</pre></div>
<p>
Notice that <em>$greatlakes</em> takes the place of
<em>$somelist</em>. When the <em>#tablerows</em> Velocimacro is
called in this situation, the following output is generated:
</p>
<div class="source">
<table>
<tr><td bgcolor="blue">Superior</td></tr>
<tr><td bgcolor="blue">Michigan</td></tr>
<tr><td bgcolor="blue">Huron</td></tr>
<tr><td bgcolor="blue">Erie</td></tr>
<tr><td bgcolor="blue">Ontario</td></tr>
</table>
</pre></div>
<p>
Velocimacros can be defined <em>inline</em> in a Velocity template,
meaning that it is unavailable to other Velocity templates on the
same web site. Defining a Velocimacro such that it can be shared by
all templates has obvious advantages: it reduces the need to
redefine the Velocimacro on numerous templates, saving work and
reducing the chance of error, and ensures that a single change to a
macro available to more than one template.
</p>
<p>
Were the <em>#tablerows($color $list)</em> Velocimacro defined in a
Velocimacros template library, this macro could be used on any of
the regular templates. It could be used many times and for many
different purposes. In the template <code>mushroom.vm</code> devoted
to all things fungi, the <em>#tablerows</em> Velocimacro could be
invoked to list the parts of a typical mushroom:
</p>
<div class="source">
#set( $parts = ["volva","stipe","annulus","gills","pileus"] )
#set( $cellbgcol = "#CC00FF" )
<table>
#tablerows( $cellbgcol $parts )
</table>
</pre></div>
<p>
When fulfilling a request for <code>mushroom.vm</code>, Velocity
would find the <em>#tablerows</em> Velocimacro in the template
library (defined in the <code>velocity.properties</code> file) and
generate the following output:
</p>
<div class="source">
<table>
<tr><td bgcolor="#CC00FF">volva</td></tr>
<tr><td bgcolor="#CC00FF">stipe</td></tr>
<tr><td bgcolor="#CC00FF">annulus</td></tr>
<tr><td bgcolor="#CC00FF">gills</td></tr>
<tr><td bgcolor="#CC00FF">pileus</td></tr>
</table>
</pre></div>
<strong>Velocimacro Arguments</strong>
<p>
Velocimacros can take as arguments any of the following
VTL elements :
</p>
<ul>
<li>
Reference : anything that starts with '$'
</li>
<li>
String literal : something like "$foo" or 'hello'
</li>
<li>
Number literal : 1, 2 etc
</li>
<li>
IntegerRange : [ 1..2] or [$foo .. $bar]
</li>
<li>
ObjectArray : [ "a", "b", "c"]
</li>
<li>
boolean value true
</li>
<li>
boolean value false
</li>
</ul>
<p>
When passing references as arguments to Velocimacros,
please note that references are passed 'by name'.
This means that their value is 'generated' at each
use inside the Velocimacro. This feature allows you
to pass references with method calls and have the
method called at each use. For example, when calling
the following Velocimacro as shown
</p>
<div class="source">
#macro( callme $a )
$a $a $a
#end
#callme( $foo.bar() )
</pre></div>
<p>
results in the method bar() of the reference $foo
being called 3 times.
</p>
<p>
At first glance, this feature appears surprising, but
when you take into consideration the original motivation
behind Velocimacros -- to eliminate cut'n'paste duplication
of commonly used VTL -- it makes sense. It allows you to
do things like pass stateful objects, such as an object
that generates colors in a repeating sequence for
coloring table rows, into the Velocimacro.
</p>
<p>
If you need to circumvent this feature, you can always
just get the value from the method as a new reference
and pass that :
</p>
<div class="source">
#set( $myval = $foo.bar() )
#callme( $myval )
</pre></div>
<strong>Velocimacro Properties</strong>
<p>
Several lines in the <code>velocity.properties</code> file allow for
flexible implementation of Velocimacros. Note that these are also
documented in the <a href="developer-guide.html">Developer Guide</a>.
</p>
<p>
<code>velocimacro.library</code> - A comma-separated list of all
Velocimacro template libraries. By default, Velocity looks for
a single library: <em>VM_global_library.vm</em>. The configured template path
is used to find the Velocimacro libraries.
</p>
<p>
<code>velocimacro.permissions.allow.inline</code> - This property,
which has possible values of true or false, determines whether
Velocimacros can be defined in regular templates. The default,
true, allows template designers to define Velocimacros in the
templates themselves.
</p>
<p>
<code>velocimacro.permissions.allow.inline.to.replace.global</code> -
With possible values of true or false,
this property allows the user to specify if a Velocimacro defined
inline in a template can replace a globally defined template, one
that was loaded on startup via the <code>velocimacro.library</code>
property. The default, <code>false</code>, prevents
Velocimacros defined inline in a template from replacing those
defined in the template libraries loaded at startup.
</p>
<p>
<code>velocimacro.permissions.allow.inline.local.scope</code> - This
property, with possible values of true or false, defaulting to false,
controls if Velocimacros defined inline are 'visible' only to the
defining template. In other words, with this property set to true,
a template can define inline VMs that are usable only by the defining
template. You can use this for fancy VM tricks - if a global VM calls
another global VM, with inline scope, a template can define a
private implementation of the second VM that will be called by the
first VM when invoked by that template. All other templates
are unaffected.
</p>
<p>
<code>velocimacro.context.localscope</code> - This property has the
possible values true or false, and the default is false. When true,
any modifications to the context via #set() within a Velocimacro
are considered 'local' to the Velocimacro, and will not
permanently affect the context.
</p>
<p>
<code>velocimacro.library.autoreload</code> - This property
controls Velocimacro library autoloading. The default value
is <code>false</code>. When set to <code>true</code>
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 <i>off</i>
in the resource loaders (e.g. <code>file.resource.loader.cache = false</code> ).
This feature is intended for development, not for production.
</p>
<strong>Velocimacro Trivia</strong>
<p>
Currently, Velocimacros must be defined before they are first
used in a template. This means that your #macro() declarations
should come before using the Velocimacros.
</p>
<p>
This is important to remember if you try to #parse()
a template containing inline #macro() directives. Because
the #parse() happens at runtime, and the parser decides if
a VM-looking element in the template is a VM at parsetime,
#parse()-ing a set of VM declarations won't work as expected.
To get around this, simply use the <code>velocimacro.library</code>
facility to have Velocity load your VMs at startup.
</p>
</div>
<a name="EscapingVTLDirectives"></a>
<h1>Escaping VTL Directives</h1>
<div class="subsection">
<p>
VTL directives can be escaped with the backslash character ("\") in
a manner similar to valid VTL references.
</p>
<div class="source">
## #include( "a.txt" ) renders as <contents of a.txt>
#include( "a.txt" )
## \#include( "a.txt" ) renders as #include( "a.txt" )
\#include( "a.txt" )
## \\#include ( "a.txt" ) renders as \<contents of a.txt>
\\#include ( "a.txt" )
</pre></div>
<p>
Extra care should be taken when escaping VTL directives that contain
multiple script elements in a single directive (such as in an
if-else-end statements). Here is a typical VTL if-statement:
</p>
<div class="source">
#if( $jazz )
Vyacheslav Ganelin
#end
</pre></div>
<p>
If <em>$jazz</em> is true, the output is
</p>
<div class="source">
Vyacheslav Ganelin
</pre></div>
<p>
If <em>$jazz</em> is false, there is no output. Escaping script elements
alters the output. Consider the following case:
</p>
<div class="source">
\#if( $jazz )
Vyacheslav Ganelin
\#end
</pre></div>
<p>
Whether <em>$jazz</em> is true or false, the output will be
</p>
<div class="source">
#if($ jazz )
Vyacheslav Ganelin
#end
</pre></div>
<p>
In fact, because all script elements
are escaped, <em>$jazz</em> is never evaluated for its boolean value.
Suppose
backslashes precede script elements that are legitimately escaped:
</p>
<div class="source">
\\#if( $jazz )
Vyacheslav Ganelin
\\#end
</pre></div>
<p>
In this case, if <em>$jazz</em> is true, the output is
</p>
<div class="source">
\ Vyacheslav Ganelin
\
</pre></div>
<p>
To understand this, note that the <code>#if( arg ) </code> when
ended by a newline (return) will omit the newline from the output.
Therefore, the body of the <code>#if()</code>
block follows the first '\', rendered
from the '\\' preceding the <code>#if()</code>.
The last \ is on a different
line than the text because there is a newline after 'Ganelin', so
the final \\, preceding the <code>#end</code> is part of the
body of the block.
</p>
<p>
If <em>$jazz</em> is false, the output is
</p>
<div class="source">
\
</pre></div>
<p>
Note that
things start to break if script elements are not properly escaped.
</p>
<div class="source">
\\\#if( $jazz )
Vyacheslave Ganelin
\\#end
</pre></div>
<p>
Here the <em>#if</em> is escaped, but there is an <em>#end</em>
remaining; having too many endings will cause a parsing error.
</p>
</div>
<a name="VTL:FormattingIssues"></a>
<h1>VTL: Formatting Issues</h1>
<div class="subsection">
<p>
Although VTL in this user guide is often displayed with newlines and
whitespaces, the VTL shown below
</p>
<div class="source">
#set( $imperial = ["Munetaka","Koreyasu","Hisakira","Morikune"] )
#foreach( $shogun in $imperial )
$shogun
#end
</pre></div>
<p>
is equally valid as the following snippet that Geir Magnusson Jr.
posted to the Velocity user mailing list to illustrate a completely
unrelated point:
</p>
<div class="source">
Send #set($foo=["$10 and ","a pie"])#foreach($a in $foo)$a#end please.
</pre></div>
<p>
Velocity's behaviour is to gobble up excess whitespace. The
preceding directive can be written as:
</p>
<div class="source">
Send me
#set( $foo = ["$10 and ","a pie"] )
#foreach( $a in $foo )
$a
#end
please.
</pre></div>
<p>
or as
</p>
<div class="source">
Send me
#set($foo = ["$10 and ","a pie"])
#foreach ($a in $foo )$a
#end please.
</pre></div>
<p>
In each case the output will be the same.
</p>
</div>
<a name="OtherFeaturesandMiscellany"></a>
<h1>Other Features and Miscellany</h1>
<div class="subsection">
<a name="Math"></a>
<h2>Math</h2>
<div class="subsection">
<p>
Velocity has a handful of built-in mathematical functions that can
be used in templates with the <em>set</em> directive. The following
equations are examples of addition, subtraction, multiplication and
division, respectively:
</p>
<div class="source">
#set( $foo = $bar + 3 )
#set( $foo = $bar - 4 )
#set( $foo = $bar * 6 )
#set( $foo = $bar / 2 )
</pre></div>
<p>
When a division operation is performed between two integers, the result will be an
integer. Any remainder can be obtained by using the modulus
(<em>%</em>) operator.
</p>
<div class="source">
#set( $foo = $bar % 5 )
</pre></div>
</div>
<a name="RangeOperator"></a>
<h2>Range Operator</h2>
<div class="subsection">
<p>
The range operator can be used in conjunction with <em>#set</em> and
<em>#foreach</em> statements. Useful for its ability to produce an
object array containing integers, the range operator has the
following construction:
</p>
<div class="source">
[n..m]
</pre></div>
<p>
Both <em>n</em> and <em>m</em> must either be or produce integers.
Whether <em>m</em> is greater than or less than <em>n</em> will not
matter; in this case the range will simply count down. Examples
showing the use of the range operator as provided below:
</p>
<div class="source">
First example:
#foreach( $foo in [1..5] )
$foo
#end
Second example:
#foreach( $bar in [2..-2] )
$bar
#end
Third example:
#set( $arr = [0..1] )
#foreach( $i in $arr )
$i
#end
Fourth example:
[1..3]
</pre></div>
<p>
Produces the following output:
</p>
<div class="source">
First example:
1 2 3 4 5
Second example:
2 1 0 -1 -2
Third example:
0 1
Fourth example:
[1..3]
</pre></div>
<p>
Note that the range operator only produces the array when used in
conjunction with <em>#set</em> and <em>#foreach</em> directives, as
demonstrated in the fourth example.
</p>
<p>
Web page designers concerned with making tables a standard size, but
where some will not have enough data to fill the table, will find
the range operator particularly useful.
</p>
</div>
<a name="AdvancedIssues:Escapingand!"></a>
<h2>Advanced Issues: Escaping and !</h2>
<div class="subsection">
<p>
When a reference is silenced with the <em>!</em> character and the
<em>!</em> character preceded by an <em>\</em> escape character, the
reference is handled in a special way. Note the differences between
regular escaping, and the special case where <em>\</em> precedes
<em>!</em> follows it:
</p>
<div class="source">
#set( $foo = "bar" )
$\!foo
$\!{foo}
$\\!foo
$\\\!foo
</pre></div>
<p>
This renders as:
</p>
<div class="source">
$!foo
$!{foo}
$\!foo
$\\!foo
</pre></div>
<p>
Contrast this with regular escaping, where <em>\</em> precedes
<em>$</em>:
</p>
<div class="source">
\$foo
\$!foo
\$!{foo}
\\$!{foo}
</pre></div>
<p>
This renders as:
</p>
<div class="source">
$foo
$!foo
$!{foo}
\bar
</pre></div>
</div>
<a name="VelocimacroMiscellany"></a>
<h2>Velocimacro Miscellany</h2>
<div class="subsection">
<p>
This section is a mini-FAQ on topics relating to Velocimacros. This
section will change over time, so it's worth checking for new information
from time to time.
</p>
<p>
Note : Throughout this section, 'Velocimacro' will commonly be abbreviated
as 'VM'.
</p>
<strong>Can I use a directive or another VM as an argument to a VM?</strong>
<p>
Example : <code>#center( #bold("hello") )</code>
</p>
<p>
No. A directive isn't a valid argument to a directive, and for most practical
purposes, a VM is a directive.
</p>
<p>
<i>However...</i>, there are things you can do. One easy solution is to take
advantage of the fact that 'doublequote' (") renders its contents. So you
could do something like
</p>
<div class="source">
#set($stuff = "#bold('hello')" )
#center( $stuff )
</pre></div>
<p>
You can save a step...
</p>
<div class="source">
#center( "#bold( 'hello' )" )
</pre></div>
<p>
Please note that in the latter example the arg
is evaluated <i>inside</i> the VM, not at the
calling level. In other words, the argument to
the VM is passed in in its entirety and evaluated within the VM
it was passed into. This allows you to do things like :
</p>
<div class="source">
#macro( inner $foo )
inner : $foo
#end
#macro( outer $foo )
#set($bar = "outerlala")
outer : $foo
#end
#set($bar = 'calltimelala')
#outer( "#inner($bar)" )
</pre></div>
<p>
Where the output is
</p>
<div class="source">
Outer : inner : outerlala
</pre></div>
<p>
because the evaluation of the "#inner($bar)" happens inside #outer(), so the
$bar value set inside #outer() is the one that's used.
</p>
<p>
This is an intentional and jealously guarded feature - args are passed 'by
name' into VMs, so you can hand VMs things like stateful references such as
</p>
<div class="source">
#macro( foo $color )
<tr bgcolor=$color><td>Hi</td></tr>
<tr bgcolor=$color><td>There</td></tr>
#end
#foo( $bar.rowColor() )
</pre></div>
<p>
And have rowColor() called repeatedly, rather than just once. To avoid that,
invoke the method outside of the VM, and pass the value into the VM.
</p>
<div class="source">
#set($color = $bar.rowColor())
#foo( $color )
</pre></div>
<strong>Can I register Velocimacros via #parse() ? </strong>
<p>
Currently, Velocimacros must be defined before they are first
used in a template. This means that your #macro() declarations
should come before using the Velocimacros.
</p>
<p>
This is important to remember if you try to #parse()
a template containing inline #macro() directives. Because
the #parse() happens at runtime, and the parser decides if
a VM-looking element in the template is a VM at parsetime,
#parse()-ing a set of VM declarations won't work as expected.
To get around this, simply use the <code>velocimacro.library</code>
facility to have Velocity load your VMs at startup.
</p>
<strong>What is Velocimacro Autoreloading?</strong>
<p>
There is a property, meant to be used in <i>development</i>,
not production :
</p>
<p>
<code>velocimacro.library.autoreload</code>
</p>
<p>
which defaults to false. When set to true <em>along with</em>
</p>
<p>
<code><type>.resource.loader.cache = false</code>
</p>
<p>
(where <type> is the name of the resource loader that you
are using, such as 'file') then the Velocity engine will automatically
reload changes to your Velocimacro library files when you make them,
so you do not have to dump the servlet engine (or application) or do
other tricks to have your Velocimacros reloaded.
</p>
<p>
Here is what a simple set of configuration properties would look like.
</p>
<div class="source">
file.resource.loader.path = templates
file.resource.loader.cache = false
velocimacro.library.autoreload = true
</pre></div>
<p>
Don't keep this on in production.
</p>
</div>
<a name="StringConcatenation"></a>
<h2>String Concatenation</h2>
<div class="subsection">
<p>
A common question that developers ask is
<i>How do I do String concatenation? Is there any
analogue to the '+' operator in Java?</i>.
</p>
<p>
To do concatenation of references in VTL, you
just have to 'put them together'. The context of
where you want to put them together does matter, so
we will illustrate with some examples.
</p>
<p>
In the regular 'schmoo' of a template (when you are mixing
it in with regular content) :
</p>
<div class="source">
#set( $size = "Big" )
#set( $name = "Ben" )
The clock is $size$name.
</pre></div>
<p>
and the output will render as 'The clock is BigBen'.
For more interesting cases, such as when you want to concatenate
strings to pass to a method, or to set a new reference, just
do
</p>
<div class="source">
#set( $size = "Big" )
#set( $name = "Ben" )
#set($clock = "$size$name" )
The clock is $clock.
</pre></div>
<p>
Which will result in the same output. As a final example, when
you want to mix in 'static' strings with your references, you may
need to use 'formal references' :
</p>
<div class="source">
#set( $size = "Big" )
#set( $name = "Ben" )
#set($clock = "${size}Tall$name" )
The clock is $clock.
</pre></div>
<p>
Now the output is 'The clock is BigTallBen'. The
formal notation is needed so the parser knows you
mean to use the reference '$size' versus '$sizeTall'
which it would if the '{}' weren't there.
</p>
</div>
</div>
<a name="Feedback"></a>
<h1>Feedback</h1>
<div class="subsection">
<p>
If you encounter any mistakes in this manual or have
other feedback related to the Velocity User Guide, please
email the
<a href="mailto:dev@velocity.apache.org">Velocity developers list</a>.
Thanks!
</p>
</div>
</div>
<div id="footer">
Copyright © 1999-2007, <a href="http://www.apache.org/">The Apache Software Foundation</a>.
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