Velocity Template Reference
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Many areas of NexPort Campus support the use of system or context properties that allow you to include text that dynamically changes for each user. For instance you can create group pages that display a logged in user's name or the current date. In assignment descriptions you can include information about the assignment including the current score, status, due date or many other properties. This is all possible because these text areas are actually templates written using the Velocity Templating Language (VTL).
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 NexPort Campus, but all VTL examples are equally applicable to other pages and templates.
Many areas of NexPort Campus support the use of system or context properties that allow you to include text that dynamically changes for each user. For instance you can create group pages that display a logged in user's name or the current date. In assignment descriptions you can include information about the assignment including the current score, status, due date or many other properties. This is all possible because these text areas are actually templates written using the Velocity Templating Language (VTL).
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 NexPort Campus, but all VTL examples are equally applicable to other pages and templates.
Velocity is a template engine. It permits editors and content designers to reference proterties defined by NexPort. This allows curriculum designers to create robust content that dynamically changes.
Velocity is a template engine. It permits editors and content designers to reference proterties defined by NexPort. This allows curriculum designers to create robust content that dynamically changes.
Velocity is a template engine. It permits editors and content designers to reference proterties defined by NexPort. This allows curriculum designers to create robust content that dynamically changes.
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 in their campus and curriculum.
VTL uses references to embed dynamic content in the text displayed in various areas of NexPort Campus, and a variable is one type of reference. Variables are one type of reference that can refer to something defined in NexPort Campus, 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:
This VTL statement, like all VTL statements, begins with the # character and contains a directive: set. When an online visitor requests your web page, the Velocity Templating Engine will search through your web page to find all # characters, then determine which mark the beginning of VTL statements, and which of the # characters that have nothing to do with VTL.
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. Values are always enclosed in quotes; with Velocity there is no confusion about data types, as only strings (text-based information) may be passed to variables.
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".
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 in their campus and curriculum.
VTL uses references to embed dynamic content in the text displayed in various areas of NexPort Campus, and a variable is one type of reference. Variables are one type of reference that can refer to something defined in NexPort Campus, 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:
This VTL statement, like all VTL statements, begins with the # character and contains a directive: set. When an online visitor requests your web page, the Velocity Templating Engine will search through your web page to find all # characters, then determine which mark the beginning of VTL statements, and which of the # characters that have nothing to do with VTL.
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. Values are always enclosed in quotes; with Velocity there is no confusion about data types, as only strings (text-based information) may be passed to variables.
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".
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.
The result is a paragraph (<p>)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.
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.
The result is a paragraph (<p>)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 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.
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.
Here are a few examples to clarify how single line and multi-line comments work:
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.
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.
Here are a few examples to clarify how single line and multi-line comments work:
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.
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.
Here are a few examples to clarify how single line and multi-line comments work:
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:
When VTL references a variable, such as $foo, the variable can get its value from either a set directive in the template. 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
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:
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 $customer.getAddress(). When your page is requested, Velocity will determine which of these two possibilities makes sense, and then return the appropriate value.
Methods A method is defined in NexPort Campus and is capable of doing something useful, like running a calculation or arriving at a decision. Methods are references that consist of a leading "$" 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:
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
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.
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:
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:
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.
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.
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:
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.
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.
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.
renders as
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.
renders as
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.
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.
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:
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.Namedoes resolve to the class Foo'sgetName()instance method, but not to a publicNameinstance variable of Foo).
References allow template designers to generate dynamic content for their campus, while directives -- easy to use script elements that can be used to creatively manipulate the output of the template-- permit web designers to truly take charge of the appearance and content of your campus.
#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:
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
These examples demonstrate each of the aforementioned types:
NOTE: In the last 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).
The RHS can also be a simple arithmetic expression:
If the RHS is a property or method reference that evaluates to null, it will not be assigned to the LHS. It is not possible to remove an existing reference from the context via this mechanism. This can be confusing for newcomers to Velocity. For example:
If $query.criteria("name") returns the string "bill", and $query.criteria("address") returns null, the above VTL will render as the following:
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:
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.
Unlike some of the other Velocity directives, the #set directive does not have an #end statement.
String Literals
When using the #set directive, string literals that are enclosed in double quote characters will be parsed and rendered, as shown:
The output will be
However, when the string literal is enclosed in single quote characters, it will not be parsed:
By default, this feature of using single quotes to render unparsed text is available in Velocity. This default can be changed by editing
velocity.propertiessuch thatstringliterals.interpolate=false.
If / ElseIf / Else
The #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:
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 aBooleanof 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.
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.
Please note that currently, Velocity's numeric comparisons are constrained to Integers - anything else will evaluate to false. The only exception to this is equality '==', where Velocity requires that the objects on each side of the '==' is of the same class.
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.
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.
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 :
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.
Foreach Loop
The #foreach element allows for looping. For example:
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:
Velocity provides an easy way to get the loop counter so that you can do something like the following:
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.propertiesfile. Here's what the loop counter properties section of thevelocity.propertiesfile appears:
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.
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.
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.
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.
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 parse_directive.maxdepth line of the
velocity.propertiesallows users to customize maximum number of #parse referrals that can occur from a single template. (Note: If the parse_directive.maxdepth property is absent from thevelocity.propertiesfile, Velocity will set this default to 10.) Recursion is permitted, for example, if the templatedofoo.vmcontains the following lines:It would reference the template
parsefoo.vm, which might contain the following VTL: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 todofoo.vmand output the "All done with dofoo.vm!" message.
The #stop script element allows the template designer to stop the execution of the template engine and return. This is useful for debugging purposes.
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.
The Velocimacro being defined in this example is d, and it can be called in a manner analogous to any other VTL directive:
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.
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.
Notice that $greatlakes takes the place of $somelist. When the #tablerows Velocimacro is called in this situation, the following output is generated:
Velocimacros can be defined inline in a Velocity template, meaning that it is unavailable to other Velocity templates in the same campus. Defining a Velocimacro such that it can be shared by all templates has obvious advantages: it reduces the need to redefine the Velocimacro on numerous templates, saving work and reducing the chance of error, and ensures that a single change to a macro available to more than one template.
Were the #tablerows($color $list) Velocimacro defined in a Velocimacros template library, this macro could be used on any of the regular templates. It could be used many times and for many different purposes. In the template
mushroom.vmdevoted to all things fungi, the #tablerows Velocimacro could be invoked to list the parts of a typical mushroom:When fulfilling a request for
mushroom.vm, Velocity would find the #tablerows Velocimacro in the template library (defined in thevelocity.propertiesfile) and generate the following output:Velocimacros can take as arguments any of the following VTL elements :
Reference : anything that starts with '$'
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
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 :
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 thevelocimacro.libraryproperty. 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 isfalse. When set totruethe source Velocimacro library for an invoked Velocimacro will be checked for changes, and reloaded if necessary. This allows you to change and test Velocimacro libraries without having to restart your application or servlet container, just like you can with regular templates. This mode only works when caching is off in the resource loaders (e.g.file.resource.loader.cache = false). This feature is intended for development, not for production.Velocimacro Trivia
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.libraryfacility to have Velocity load your VMs at startup.
VTL directives can be escaped with the backslash character ("\") in a manner similar to valid VTL references.
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 is true, the output is
If $jazz is false, there is no output. Escaping script elements alters the output. Consider the following case:
Whether $jazz is true or false, the output will be
In fact, because all script elements are escaped, $jazz is never evaluated for it's boolean value. Suppose backslashes precede script elements that are legitimately escaped:
In this case, if $jazz is true, the output is
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#endis part of the body of the block.If $jazz is false, there is no output. Note that things start to break if script elements are not properly escaped.
Here the #if is escaped, but there is an #end remaining; having too many endings will cause a parsing error.
Although VTL in this user guide is often displayed with newlines and whitespaces, the VTL shown below
is equally valid as the following snippet that Geir Magnusson Jr. posted to the Velocity user mailing list to illustrate a completely unrelated point:
Velocity's behaviour is to gobble up excess whitespace. The preceding directive can be written as:
or as
In each case the output will be the same.
MathVelocity 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:
When a division operation is performed, the result will be an integer. Any remainder can be obtained by using the modulus (%) operator.
Only integers (...-2, -1, 0, 1, 2...) are permissible when performing mathematical equations in Velocity; when a non-integer is used, it is logged and a null will be returned as the output.
Range OperatorThe 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:
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:
Produces the following output:
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:
This renders as:
Contrast this with regular escaping, where __ precedes $:
This renders as:
Velocimacro MiscellanyThis section is a mini-FAQ on topics relating to Velocimacros. Thissection will change over time, so it's worth checking for new informationfrom time to time.
Note : Throughout this section, 'Velocimacro' will commonly be abbreviatedas 'VM'.
Can I use a directive or another VM as an argument to a VM?
Example :
#center( #bold("hello") )No. A directive isn't a valid argument to a directive, and for most practicalpurposes, a VM is a directive.
However..., there are things you can do. One easy solution is to takeadvantage of the fact that 'doublequote' (") renders it's contents. So youcould do something like
You can save a step...
Please note that in the latter example the argis evaluated inside the VM, not at thecalling level. In other words, the argument tothe VM is passed in in its entirety and evaluated within the VMit was passed into. This allows you to do things like :
Where the output is
because the evaluation of the "#inner($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 'byname' into VMs, so you can hand VMs things like stateful references such as
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.
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.libraryfacility to have Velocity load your VMs at startup.What is Velocimacro Autoreloading?
There is a property, meant to be used in development, not production :
velocimacro.library.autoreloadwhich 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.
Don't keep this on in production.
String ConcatenationA 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) :
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
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' :
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.
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Velocity has logical AND, OR and NOT operators as well. For further information, please see the Below are examples demonstrating the use of the logical AND, OR and NOT operators.
Velocimacro Arguments
Velocimacro Properties
Several lines in the velocity.properties file allow for flexible implementation of Velocimacros. Note that these are also documented in the .
Can I register Velocimacros via #parse() ?
If you encounter any mistakes in this manual or have other feedback related to the Velocity User Guide, please email the . Thanks!
