NVelocity：语法
User Guide - Contents
- About this Guide
- 什么是NVelocity?
- NVelocity能为我们做什么?
- Velocity Template Language (VTL): An Introduction
- Hello Velocity World!
- 注释
- References
- Formal Reference Notation
- Quiet Reference Notation
- Getting literal
- Case Substitution
- 指令
- Escaping VTL Directives
- VTL: Formatting Issues
- Other Features and Miscellany
- Feedback
About this Guide
主要解释Velocity Template Language (VTL)的语法. 大部分示例主要演示NVelocity在B/S系统的应用(比如CMS), 当然也可以应用到C/S程序.
本文章内容主要摘自Apache 官方velocity的语法讲解
What is NVelocity?
NVelocity是Velocity(Java)的.net移植. 使用该模板引擎可以生产aspx、jsp、html、SQL……在我的项目里面主要用于静态生成。
NVelocity能为我们做什么?
The Mud Store Example
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?
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.
You meet with software engineers at your company, and everyone has agreed that $customer will hold information pertaining to the customer currently logged in, that $mudsOnSpecial will be all the types mud on sale at present. The $flogger 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.
You could embed the following VTL statement in the web page:
<HTML>
<BODY>
Hello $customer.Name!
<table>
#foreach( $mud in $mudsOnSpecial )
#if ( $customer.hasPurchased($mud) )
<tr>
<td>
$flogger.getPromo( $mud )
</td>
</tr>
#end
#end
</table>
The exact details of the foreach 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.
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 presence. As you get more familiar with these elements, you will begin to unleash the power of Velocity.
Velocity Template Language (VTL): An Introduction
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.
VTL uses references 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 statement in the web page itself. Here is an example of a VTL statement that could be embedded in an HTML document:
#set( $a = "Velocity" )
VTL语句以#开始后接指令: set. 当用户请求页面, Velocity模板引擎会查找所有的#, 来判定哪些是需要解析的VTL语句.
The # character is followed by a directive, set. The set directive uses an expression (enclosed in brackets) -- an equation that assigns a value to a variable. The variable is listed on the left hand side and its value on the right hand side; the two are separated by an = character.
In the example above, the variable is $a and the value is Velocity. This variable, like all references, begins with the $ 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 $name will be replaced by the current value before that string literal is assigned to the left hand side of the =
The following rule of thumb may be useful to better understand how Velocity works: References begin with $ and are used to get something. Directives begin with # and are used to do something.
In the example above, #set is used to assign a value to a variable. The variable, $a, can then be used in the template to output "Velocity".
Hello Velocity World!
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 $foo and later referenced.
<html>
<body>
#set( $foo = "Velocity" )
Hello $foo World!
</body>
<html>
The result is a web page that prints "Hello Velocity World!".
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 set directive will be revisited in greater detail later on.
Comments
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.
## This is a single line comment.
A single line comment begins with ## 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 #* 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.
Here are a few examples to clarify how single line and multi-line comments work:
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.
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:
#**
This is a VTL comment block and
may be used to store such information
as the document author and versioning
information:
@author
@version 5
*#
References
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.
Everything coming to and from a reference is treated as a String object. If there is an object that represents $foo (such as an Integer object), then Velocity will call its .toString() method to resolve the object into a String.
Variables
The shorthand notation of a variable consists of a leading "$" character followed by a VTL Identifier. 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:
- alphabetic (a .. z, A .. Z)
- numeric (0 .. 9)
- hyphen ("-")
- underscore ("_")
Here are some examples of valid variable references in the VTL:
$foo
$mudSlinger
$mud-slinger
$mud_slinger
$mudSlinger1
When VTL references a variable, such as $foo, the variable can get its value from either a set directive in the template, or from the Java code. For example, if the Java variable $foo has the value bar at the time the template is requested, bar replaces all instances of $foo on the web page. Alternatively, if I include the statement
#set( $foo = "bar" )
The output will be the same for all instances of $foo that follow this directive.
Properties
The second flavor of VTL references are properties, and 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. These are examples of valid property references in the VTL:
$customer.Address
$purchase.Total
Take the first example, $customer.Address. It can have two meanings. It can mean, Look in the hashtable identified as customer and return the value associated with the key Address. But $customer.Address can also be referring to a method (references that refer to methods will be discussed in the next section); $customer.Address could be an abbreviated way of writing $customer.getAddress(). When your page is requested, Velocity will determine which of these two possibilities makes sense, and then return the appropriate value.
Methods
A method is defined in the Java code and is capable of doing something useful, like running a calculation or arriving at a decision. Methods are references that consist of a leading "$" character followed a VTL Identifier, followed by a VTL 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 (")"). These are examples of valid method references in the VTL:
$customer.getAddress()
$purchase.getTotal()
$page.setTitle( "My Home Page" )
$person.setAttributes( ["Strange", "Weird", "Excited"] )
The first two examples -- $customer.getAddress() and $purchase.getTotal() -- may look similar to those used in the Properties section above, $customer.Address and $purchase.Total. If you guessed that these examples must be related some in some fashion, you are correct!
VTL Properties can be used as a shorthand notation for VTL Methods. 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.
The shorthand notation can be used for the following Methods
$sun.getPlanets()
$annelid.getDirt()
$album.getPhoto()
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.
$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
Property Lookup Rules
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 $customer.address, the sequence is
- getaddress()
- getAddress()
- get("address")
- isAddress()
- getAddress()
- getaddress()
- get("Address")
- isAddress()
Formal Reference Notation
Shorthand notation for references was used for the examples listed above, but there is also a formal notation for references, which is demonstrated below:
${mudSlinger}
${customer.Address}
${purchase.getTotal()}
In almost all cases you will use the shorthand notation for references, but in some cases the formal notation is required for correct processing.
Suppose you were constructing a sentence on the fly where $vice 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:
Jack is a $vicemaniac.
There is ambiguity here, and Velocity assumes that $vicemaniac, not $vice, is the Identifier that you mean to use. Finding no value for $vicemaniac, it will return $vicemaniac. Using formal notation can resolve this problem.
Jack is a ${vice}maniac.
Now Velocity knows that $vice, not $vicemaniac, is the reference. Formal notation is often useful when references are directly adjacent to text in a template.
Quiet Reference Notation
When Velocity encounters an undefined reference, its normal behavior is to output the image of the reference. For example, suppose the following reference appears as part of a VTL template.
<input type="text" name="email" value="$email"/>
When the form initially loads, the variable reference $email has no value, but you prefer a blank text field to one with a value of "$email". Using the quiet reference notation circumvents Velocity's normal behavior; instead of using $email in the VTL you would use $!email. So the above example would look like the following:
<input type="text" name="email" value="$!email"/>
Now when the form is initially loaded and $email still has no value, an empty string will be output instead of "$email".
Formal and quiet reference notation can be used together, as demonstrated below.
<input type="text" name="email" value="$!{email}"/>
Getting literal
VTL uses special characters, such as $ and #, to do its work, so some added care should be taken where using these characters in your templates. This section deals with escaping the $ character.
Currency
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.
Escaping Valid VTL References
Cases may arise where there is the potential for Velocity to get confused. Escaping special characters is the best way to handle VTL's special characters in your templates, and this can be done using the backslash ( \ ) character.
#set( $email = "foo" )
$email
If Velocity encounters a reference in your VTL template to $email, it will search the Context for a corresponding value. Here the output will be foo, because $email is defined. If $email is not defined, the output will be $email.
Suppose that $email is defined (for example, if it has the value foo), and that you want to output $email. There are a few ways of doing this, but the simplest is to use the escape character.
## The following line defines $email in this template:
#set( $email = "foo" )
$email
\$email
\\$email
\\\$email
renders as
foo
$email
\foo
\$email
Note that the \ character bind to the $ from the left. The bind-from-left rule causes \\\$email to render as \\$email. Compare these examples to those in which $email is not defined.
$email
\$email
\\$email
\\\$email
renders as
$email
\$email
\\$email
\\\$email
Notice Velocity handles references that are defined differently from those that have not been defined. Here is a set directive that gives $foo the value gibbous.
#set( $foo = "gibbous" )
$moon = $foo
The output will be: $moon = gibbous -- where $moon is output as a literal because it is undefined and gibbous is output in place of $foo.
It is also possible to escape VTL directives; this is described in more detail in the Directives section.
Case Substitution
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:
$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}
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.
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 getFoo() is referred to in a template by $bar.foo, Velocity will first try $getfoo. If this fails, it will then try $getFoo. Similarly, when a template refers to $bar.Foo, Velocity will try $getFoo() first and then try getfoo().
Note: References to instance variables in a template are not resolved. Only references to the attribute equivalents of JavaBean getter/setter methods are resolved (i.e. $foo.Name does resolve to the class Foo's getName() instance method, but not to a public Name instance variable of Foo).
Directives
References allow template designers to generate dynamic content for web sites, while directives -- easy to use script elements that can be used to creatively manipulate the output of Java code -- permit web designers to truly take charge of the appearance and content of the web site.
Directives always begin with a #. Like references, the name of the directive may be bracketed by a { and a } symbol. This is useful with directives that are immediately followed by text. For example the following produces an error:
#if($a==1)true enough#elseno way!#end
In such a case, use the brackets to separate #else from the rest of the line.
#if($a==1)true enough#{else}no way!#end
#set
The #set 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:
#set( $primate = "monkey" )
#set( $customer.Behavior = $primate )
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:
- Variable reference
- String literal
- Property reference
- Method reference
- Number literal
- ArrayList
- Map
These examples demonstrate each of the aforementioned types:
#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
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).
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.
The RHS can also be a simple arithmetic expression:
#set( $value = $foo + 1 )
#set( $value = $bar - 1 )
#set( $value = $foo * $bar )
#set( $value = $foo / $bar )
If the RHS is a property or method reference that evaluates to null, it will not 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:
#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
If $query.criteria("name") returns the string "bill", and $query.criteria("address") returns null, the above VTL will render as the following:
The result of the first query is bill
The result of the second query is bill
This tends to confuse newcomers who construct #foreach loops that attempt to #set a reference via a property or method reference, then immediately test that reference with an #if directive. For example:
#set( $criteria = ["name", "address"] )
#foreach( $criterion in $criteria )
#set( $result = $query.criteria($criterion) )
#if( $result )
Query was successful
#end
#end
In the above example, it would not be wise to rely on the evaluation of $result to determine if a query was successful. After $result has been #set (added to the context), it cannot be set back to null (removed from the context). The details of the #if and #foreach directives are covered later in this document.
One solution to this would be to pre-set $result to false. Then if the $query.criteria() call fails, you can check.
#set( $criteria = ["name", "address"] )
#foreach( $criterion in $criteria )
#set( $result = false )
#set( $result = $query.criteria($criterion) )
#if( $result )
Query was successful
#end
#end
Unlike some of the other Velocity directives, the #set directive does not have an #end statement.
LiteralsWhen using the #set directive, string literals that are enclosed in double quote characters will be parsed and rendered, as shown:
#set( $directoryRoot = "www" )
#set( $templateName = "index.vm" )
#set( $template = "$directoryRoot/$templateName" )
$template
The output will be
www/index.vm
However, when the string literal is enclosed in single quote characters, it will not be parsed:
#set( $foo = "bar" )
$foo
#set( $blargh = '$foo' )
$blargh
bar
$foo
By default, this feature of using single quotes to render unparsed text is available in Velocity. This default can be changed by editing velocity.properties such that stringliterals.interpolate=false.
Alternately, the #literal script element allows the template designer to easily use large chunks of uninterpreted content in VTL code. This can be especially useful in place of escaping multiple directives.
#literal()
#foreach ($woogie in $boogie)
nothing will happen to $woogie
#end
#end
Renders as:
#foreach ($woogie in $boogie)
nothing will happen to $woogie
#end
Conditionals
If / ElseIf / ElseThe #if 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:
#if( $foo )
<strong>Velocity!</strong>
#end
The variable $foo is evaluated to determine whether it is true, which will happen under one of two circumstances: (i) $foo 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 boolean - the introspection infrastructure will return a Boolean of the same logical value.
The content between the #if and the #end statements become the output if the evaluation is true. In this case, if $foo is true, the output will be: "Velocity!". Conversely, if $foo has a null value, or if it is a boolean false, the statement evaluates as false, and there is no output.
An #elseif or #else element can be used with an #if 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 $foo has a value of 15 and $bar has a value of 6.
#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
In this example, $foo is greater than 10, so the first two comparisons fail. Next $bar is compared to 6, which is true, so the output is Go South.
Relational and Logical Operators
Velocity uses the equivalent operator to determine the relationships between variables. Here is a simple example to illustrate how the equivalent operator is used.
#set ($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
Note that the semantics of == are slightly different than Java where == 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 toString() for each object and then compared.
Velocity has logical AND, OR and NOT operators as well. For further information, please see the VTL Reference Guide Below are examples demonstrating the use of the logical AND, OR and NOT operators.
## logical AND
#if( $foo && $bar )
<strong> This AND that</strong>
#end
The #if() directive will only evaluate to true if both $foo and $bar are true. If $foo is false, the expression will evaluate to false; $bar will not be evaluated. If $foo is true, the Velocity Templating Engine will then check the value of $bar; if $bar is true, then the entire expression is true and This AND that becomes the output. If $bar is false, then there will be no output as the entire expression is false.
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.
## logical OR
#if( $foo || $bar )
<strong>This OR That</strong>
#end
If $foo is true, the Velocity Templating Engine has no need to look at $bar; whether $bar is true or false, the expression will be true, and This OR That will be output. If $foo is false, however, $bar must be checked. In this case, if $bar is also false, the expression evaluates to false and there is no output. On the other hand, if $bar is true, then the entire expression is true, and the output is This OR That
With logical NOT operators, there is only one argument :
##logical NOT
#if( !$foo )
<strong>NOT that</strong>
#end
Here, the if $foo is true, then !$foo evaluates to false, and there is no output. If $foo is false, then !$foo evaluates to true and NOT that will be output. Be careful not to confuse this with the quiet reference $!foo which is something altogether different.
There are text versions of all logical operators, including eq, ne, and, or, not, gt, ge, lt, and le.
One more useful note. When you wish to include text immediately following a #else 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 #else).
#if( $foo == $bar)it's true!#{else}it's not!#end</li>
Loops
Foreach LoopThe #foreach element allows for looping. For example:
<ul>
#foreach( $product in $allProducts )
<li>$product</li>
#end
</ul>
This #foreach loop causes the $allProducts list (the object) to be looped over for all of the products (targets) in the list. Each time through the loop, the value from $allProducts is placed into the $product variable.
The contents of the $allProducts variable is a Vector, a Hashtable or an Array. The value assigned to the $product variable is a Java Object and can be referenced from a variable as such. For example, if $product was really a Product class in Java, its name could be retrieved by referencing the $product.Name method (ie: $Product.getName()).
Lets say that $allProducts 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:
<ul>
#foreach( $key in $allProducts.keySet() )
<li>Key: $key -> Value: $allProducts.get($key)</li>
#end
</ul>
Velocity provides an easy way to get the loop counter so that you can do something like the following:
<table>
#foreach( $customer in $customerList )
<tr><td>$velocityCount</td><td>$customer.Name</td></tr>
#end
</table>
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 velocity.properties file. Here's what the loop counter properties section of the velocity.properties file appears:
# 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
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 velocity.properties file. This is useful as a fail-safe.
# The maximum allowed number of loops.
directive.foreach.maxloops = -1
Include
The #include script element allows the template designer to import a local file, which is then inserted into the location where the #include 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.
#include( "one.txt" )
The file to which the #include directive refers is enclosed in quotes. If more than one file will be included, they should be separated by commas.
#include( "one.gif","two.txt","three.htm" )
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.
#include( "greetings.txt", $seasonalstock )
Parse
The #parse script element allows the template designer to import a local file that contains VTL. Velocity will parse the VTL and render the template specified.
#parse( "me.vm" )
Like the #include directive, #parse can take a variable rather than a template. Any templates to which #parse refers must be included under TEMPLATE_ROOT. Unlike the #include directive, #parse will only take a single argument.
VTL templates can have #parse statements referring to templates that in turn have #parse statements. By default set to 10, the directive.parse.max.depth line of the velocity.properties allows users to customize maximum number of #parse referrals that can occur from a single template. (Note: If the directive.parse.max.depth property is absent from the velocity.properties file, Velocity will set this default to 10.) Recursion is permitted, for example, if the template dofoo.vm contains the following lines:
Count down.
#set( $count = 8 )
#parse( "parsefoo.vm" )
All done with dofoo.vm!
It would reference the template parsefoo.vm, which might contain the following VTL:
$count
#set( $count = $count - 1 )
#if( $count > 0 )
#parse( "parsefoo.vm" )
#else
All done with parsefoo.vm!
#end
After "Count down." is displayed, Velocity passes through parsefoo.vm, 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 dofoo.vm and output the "All done with dofoo.vm!" message.
Stop
The #stop script element allows the template designer to stop the execution of the template engine and return. This is useful for debugging purposes.
#stop
Velocimacros
The #macro 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.
#macro( d )
<tr><td></td></tr>
#end
The Velocimacro being defined in this example is d, and it can be called in a manner analogous to any other VTL directive:
#d()
When this template is called, Velocity would replace #d() with a row containing a single, empty data cell.
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.
#macro( tablerows $color $somelist )
#foreach( $something in $somelist )
<tr><td bgcolor=$color>$something</td></tr>
#end
#end
The Velocimacro being defined in this example, tablerows, takes two arguments. The first argument takes the place of $color, and the second argument takes the place of $somelist.
Anything that can be put into a VTL template can go into the body of a Velocimacro. The tablerows Velocimacro is a foreach statement. There are two #end statements in the definition of the #tablerows Velocimacro; the first belongs to the #foreach, the second ends the Velocimacro definition.
#set( $greatlakes = ["Superior","Michigan","Huron","Erie","Ontario"] )
#set( $color = "blue" )
<table>
#tablerows( $color $greatlakes )
</table>
Notice that $greatlakes takes the place of $somelist. When the #tablerows Velocimacro is called in this situation, the following output is generated:
<table>
<tr><td bgcolor="blue">Superior</td></tr>
<tr><td bgcolor="blue">Michigan</td></tr>
<tr><td bgcolor="blue">Huron</td></tr>
<tr><td bgcolor="blue">Erie</td></tr>
<tr><td bgcolor="blue">Ontario</td></tr>
</table>
Velocimacros can be defined inline in a Velocity template, meaning that it is unavailable to other Velocity templates on the same web site. Defining a Velocimacro such that it can be shared by all templates has obvious advantages: it reduces the need to redefine the Velocimacro on numerous templates, saving work and reducing the chance of error, and ensures that a single change to a macro available to more than one template.
Were the #tablerows($color $list) 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 mushroom.vm devoted to all things fungi, the #tablerows Velocimacro could be invoked to list the parts of a typical mushroom:
#set( $parts = ["volva","stipe","annulus","gills","pileus"] )
#set( $cellbgcol = "#CC00FF" )
<table>
#tablerows( $cellbgcol $parts )
</table>
When fulfilling a request for mushroom.vm, Velocity would find the #tablerows Velocimacro in the template library (defined in the velocity.properties file) and generate the following output:
<table>
<tr><td bgcolor="#CC00FF">volva</td></tr>
<tr><td bgcolor="#CC00FF">stipe</td></tr>
<tr><td bgcolor="#CC00FF">annulus</td></tr>
<tr><td bgcolor="#CC00FF">gills</td></tr>
<tr><td bgcolor="#CC00FF">pileus</td></tr>
</table>
Velocimacros can take as arguments any of the following VTL elements :
- Reference : anything that starts with '$'
- String literal : something like "$foo" or 'hello'
- Number literal : 1, 2 etc
- IntegerRange : [ 1..2] or [$foo .. $bar]
- ObjectArray : [ "a", "b", "c"]
- boolean value true
- boolean value false
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
#macro( callme $a )
$a $a $a
#end
#callme( $foo.bar() )
results in the method bar() of the reference $foo being called 3 times.
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.
If you need to circumvent this feature, you can always just get the value from the method as a new reference and pass that :
#set( $myval = $foo.bar() )
#callme( $myval )
Several lines in the velocity.properties file allow for flexible implementation of Velocimacros. Note that these are also documented in the Developer Guide.
velocimacro.library - A comma-separated list of all Velocimacro template libraries. By default, Velocity looks for a single library: VM_global_library.vm. The configured template path is used to find the Velocimacro libraries.
velocimacro.permissions.allow.inline - 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.
velocimacro.permissions.allow.inline.to.replace.global - 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 velocimacro.library property. The default, false, prevents Velocimacros defined inline in a template from replacing those defined in the template libraries loaded at startup.
velocimacro.permissions.allow.inline.local.scope - 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.
velocimacro.context.localscope - 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.
velocimacro.library.autoreload - This property controls Velocimacro library autoloading. The default value is false. 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.
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.
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 velocimacro.library facility to have Velocity load your VMs at startup.
Escaping VTL Directives
VTL directives can be escaped with the backslash character ("\") in a manner similar to valid VTL references.
## #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" )
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:
#if( $jazz )
Vyacheslav Ganelin
#end
If $jazz is true, the output is
Vyacheslav Ganelin
If $jazz is false, there is no output. Escaping script elements alters the output. Consider the following case:
\#if( $jazz )
Vyacheslav Ganelin
\#end
Whether $jazz is true or false, the output will be
#if($ jazz )
Vyacheslav Ganelin
#end
In fact, because all script elements are escaped, $jazz is never evaluated for its boolean value. Suppose backslashes precede script elements that are legitimately escaped:
\\#if( $jazz )
Vyacheslav Ganelin
\\#end
In this case, if $jazz is true, the output is
\ Vyacheslav Ganelin
\
To understand this, note that the #if( arg ) when ended by a newline (return) will omit the newline from the output. Therefore, the body of the #if() block follows the first '\', rendered from the '\\' preceding the #if(). The last \ is on a different line than the text because there is a newline after 'Ganelin', so the final \\, preceding the #end is part of the body of the block.
If $jazz is false, the output is
\
Note that things start to break if script elements are not properly escaped.
\\\#if( $jazz )
Vyacheslave Ganelin
\\#end
Here the #if is escaped, but there is an #end remaining; having too many endings will cause a parsing error.
VTL: Formatting Issues
Although VTL in this user guide is often displayed with newlines and whitespaces, the VTL shown below
#set( $imperial = ["Munetaka","Koreyasu","Hisakira","Morikune"] )
#foreach( $shogun in $imperial )
$shogun
#end
is equally valid as the following snippet that Geir Magnusson Jr. posted to the Velocity user mailing list to illustrate a completely unrelated point:
Send #set($foo=["$10 and ","a pie"])#foreach($a in $foo)$a#end please.
Velocity's behaviour is to gobble up excess whitespace. The preceding directive can be written as:
Send me
#set( $foo = ["$10 and ","a pie"] )
#foreach( $a in $foo )
$a
#end
please.
or as
Send me
#set($foo = ["$10 and ","a pie"])
#foreach ($a in $foo )$a
#end please.
In each case the output will be the same.
Other Features and Miscellany
Math
Velocity has a handful of built-in mathematical functions that can be used in templates with the set directive. The following equations are examples of addition, subtraction, multiplication and division, respectively:
#set( $foo = $bar + 3 )
#set( $foo = $bar - 4 )
#set( $foo = $bar * 6 )
#set( $foo = $bar / 2 )
When a division operation is performed between two integers, the result will be an integer. Any remainder can be obtained by using the modulus (%) operator.
#set( $foo = $bar % 5 )
Range Operator
The range operator can be used in conjunction with #set and #foreach statements. Useful for its ability to produce an object array containing integers, the range operator has the following construction:
[n..m]
Both n and m must either be or produce integers. Whether m is greater than or less than n will not matter; in this case the range will simply count down. Examples showing the use of the range operator as provided below:
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]
Produces the following output:
First example:
1 2 3 4 5
Second example:
2 1 0 -1 -2
Third example:
0 1
Fourth example:
[1..3]
Note that the range operator only produces the array when used in conjunction with #set and #foreach directives, as demonstrated in the fourth example.
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.
Advanced Issues: Escaping and !
When a reference is silenced with the ! character and the ! character preceded by an \ escape character, the reference is handled in a special way. Note the differences between regular escaping, and the special case where \ precedes ! follows it:
#set( $foo = "bar" )
$\!foo
$\!{foo}
$\\!foo
$\\\!foo
This renders as:
$!foo
$!{foo}
$\!foo
$\\!foo
Contrast this with regular escaping, where \ precedes $:
\$foo
\$!foo
\$!{foo}
\\$!{foo}
This renders as:
$foo
$!foo
$!{foo}
\bar
Velocimacro Miscellany
This section is a mini-FAQ on topics relating to Velocimacros. This section will change over time, so it's worth checking for new information from time to time.
Note : Throughout this section, 'Velocimacro' will commonly be abbreviated as 'VM'.
Can I use a directive or another VM as an argument to a VM?Example : #center( #bold("hello") )
No. A directive isn't a valid argument to a directive, and for most practical purposes, a VM is a directive.
However..., there are things you can do. One easy solution is to take advantage of the fact that 'doublequote' (") renders its contents. So you could do something like
#set($stuff = "#bold('hello')" )
#center( $stuff )
You can save a step...
#center( "#bold( 'hello' )" )
Please note that in the latter example the arg is evaluated inside 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 :
#macro( inner $foo )
inner : $foo
#end
#macro( outer $foo )
#set($bar = "outerlala")
outer : $foo
#end
#set($bar = 'calltimelala')
#outer( "#inner($bar)" )
Where the output is
Outer : inner : outerlala
because the evaluation of the "#inner($bar)" happens inside #outer(), so the $bar value set inside #outer() is the one that's used.
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
#macro( foo $color )
<tr bgcolor=$color><td>Hi</td></tr>
<tr bgcolor=$color><td>There</td></tr>
#end
#foo( $bar.rowColor() )
And have rowColor() called repeatedly, rather than just once. To avoid that, invoke the method outside of the VM, and pass the value into the VM.
#set($color = $bar.rowColor())
#foo( $color )
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.
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 velocimacro.library facility to have Velocity load your VMs at startup.
There is a property, meant to be used in development, not production :
velocimacro.library.autoreload
which defaults to false. When set to true along with
<type>.resource.loader.cache = false
(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.
Here is what a simple set of configuration properties would look like.
file.resource.loader.path = templates
file.resource.loader.cache = false
velocimacro.library.autoreload = true
Don't keep this on in production.
String Concatenation
A common question that developers ask is How do I do String concatenation? Is there any analogue to the '+' operator in Java?.
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.
In the regular 'schmoo' of a template (when you are mixing it in with regular content) :
#set( $size = "Big" )
#set( $name = "Ben" )
The clock is $size$name.
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
#set( $size = "Big" )
#set( $name = "Ben" )
#set($clock = "$size$name" )
The clock is $clock.
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' :
#set( $size = "Big" )
#set( $name = "Ben" )
#set($clock = "${size}Tall$name" )
The clock is $clock.
Now the output is 'The clock is BigTallBen'. The formal notation is needed so the parser knows you mean to use the reference '$size' versus '$sizeTall' which it would if the '{}' weren't there.
Feedback
If you encounter any mistakes in this manual or have other feedback related to the Velocity User Guide, please email the Velocity developers list. Thanks!