2006 The Apache Software Foundation Version 1.5 Velocity The Apache Velocity Developers

The Velocity Users 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 examples are equally applicable to other pages and templates. Thanks for choosing Velocity!

This guide has been compiled from the xdoc Version which was included with Velocity up to version 1.4. We explicitly acknowledge the efforts of all the contributors, especially Jason Van Zyl and John Castura in preparing this first version.

This guide is for you if you want to learn how to use the Velocity Templating Language (VTL). It is intended for designers, template writers and developers that are familiar with creating content templates or web pages. If you want to embed the Velocity engine into your own application, you probably want to consult the Velocity Developers Guide available from http://velocity.apache.org/ and use this guide as a reference.

If you encounter any mistakes in this manual or have other feedback related to the Velocity Users Guide, please send email to one of the Velocity mailing lists: user@velocity.apache.org This mailing list is intended for discussion about using Velocity, e.g. writing templates or integrating Velocity into your application. If you need clarification about the content of this guide or have questions about the examples, please send them to this list. dev@velocity.apache.org This mailing list is intended for discussion about developing Velocity itself. If you found a mistake in this guide, want to improve its contents or the formatting framework, please use this mailing list. Please check also the Velocity Homepage located at http://velocity.apache.org/ for up-to-date information, new releases and general Velocity information.

The Apache Velocity templating engine (or short Velocity is a template engine written in 100% pure Java. It permits web page designers to reference methods defined in Java code. Template developers can work in parallel with Java programmers to develop web sites according to the Model-View-Controller (MVC) model, meaning that they can focus solely on creating templates, and programmers can focus solely on writing Java code. Velocity separates Java code from the template pages, making a project more maintainable over the long run. In a web context, Velocity provides a viable alternative to Java Server Pages (JSPs) or PHP. Velocity can be used to generate web pages, Java source code, SQL, PostScript and other output from templates. It can be used either as a standalone utility for generating source code and reports, or as a component integrated into frameworks or larger applications. Velocity provides template services for a number of other projects, e.g. the Turbine web application framework (http://jakarta.apache.org/turbine/). In this context, Velocity offers a template service that allows development of web applications according to a true MVC model.

Suppose you are a page designer for an online store that specializes in selling fruits. Let's call it The Online Fruit Store. Customers place orders for various types and quantities of fruits. They login to your site using their username and password, which allows them to view their orders and buy more fruits. Right now, apples on sale, which are very popular. A minority of your customers regularly buy mangos, which are also on sale, though not as popular and usually relegated to the margin of your web page. Information about each customer is tracked in a database, so one day the question arises: Why not use Velocity to target special deals on fruit to the customers who are most interested in those types of fruits? Velocity makes it easy to customize web pages to your online visitors. As a web site designer at The Online Fruit Store, you will modify the web page that the customer sees 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 and that $fruitSpecial will be all the types of fruit on sale at present. The $promoTool 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. The next example shows how to display promotions using Velocity. <html> <body> Hello $customer.Name! <table> #foreach( $fruit in $fruitSpecial ) #if ( $customer.hasPurchased($fruit) ) <tr> <td> $promoTool.getPromo($fruit) </td> </tr> #end #end </table> </body> </html> The exact details of the #foreach and #if directive will be described in greater depth in the directives chapter; what is important is the impact this short piece of code has on your web site. When a customer with a penchant for mangos logs in, and mangos are on sale, that is what this customer will see, prominently displayed. If another customer with a long history of apple purchases logs in, the notice of an apple sale will be front and center. The flexibility of Velocity is enormous and limited only by your creativity. This manual should help you get started with Velocity and the Velocity Template Language. It has a companion manual, the Velocity Developers Guide, which describes how to integrate Velocity into your own web applications (this is what the software engineers at your company want to read).

The Velocity Template Language (VTL) is meant to provide an easy, simple and clean way to incorporate dynamic content in a web page. Even a web page developer with little or no programming experience will 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. They can refer to something defined in the Java code, or it can get its value from a VTL statement in the web page itself. #set( $a = "Velocity" ) This VTL statement begins with the # character and contains a directive: set. When an online visitor requests your web page, Velocity 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 reference. The reference 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 reference is $a and the value is Velocity. A reference always begins with the $ character. The value on the right side represents a string value. These 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 ot interpolate, such as "Hello $name", where the $name will be replaced by its 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 reference. The reference, $a, can then be used in the template to output Velocity.

Once a value has been assigned to a reference, you can use it anywhere in your HTML document. In the following example, a value is assigned to $name and later referenced. <html><body> #set( $name = "Velocity" ) Hello $name World! </body></html> The result is a web page that reads 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.

Like other formal languages, Velocity consists of a number of elements: Statements and Directives References Comments Each of these elements will be introduced in this chapter.

Velocity directives are language elements than allow the template designer to manipulate the rendering of the template. A directive is part of a Velocity statement: ## Velocity single-line statement #set($a = 10) ## Velocity multi-line statement #foreach($i in [0..10]) Counter is $i #end Begin of Velocity statement (hash symbol) Velocity directive Expression in brackets Multi-line statement body Multi-line statement end Velocity knows about single-line statements and multi-line statements. A single-line statement starts with a single # character, followed by directive itself. Some directives also take parameters in brackets. A single-line statement then ends immediately. #set($name = "apple") #include("header.html") #parse("header.vm") #stop A multi-line statement starts like a single-line statement, but also has a statement body. This body ends with #end Technically speaking, the opening statement and #end can be on the same line. We will still call it a multi-line statement. . #foreach($i in [1..10]) The count is $i #end #literal This is a literal block of text #end #if ($fruit == "apple") This is an apple. #else This is not an apple. #end The #if/#else/#end combo is a special case because it might contain multiple statement bodies between its elements. If you need to explicitly separate a Velocity directive from surrounding text, it is possible to wrap it in curly braces ({ and }): #{set}($name = "apple") #{foreach}($i in [1..10]) The count is $i #{end} All available Velocity directives are listed in the directives chapter. While the hash sign is technically not part of the directive, we will still speak about the #if or #set directive in the latter chapters.

Velocity references are the glue which connect templates to the application. Template designers and application developers must agree on a common set of references used by the Java application and the Velocity templates. Every Velocity reference represents a Java object. A Velocity reference starts with a $ character followed by an identifier name. ## Simple reference The total amout is $total. ## Property reference You currently have $cart.items Items in your Shopping cart. ## Method reference We offer you a discount of $cart.calculateDiscount() for your purchase. ## Creating a new reference from Velocity #set($linesPerPage = 20) If you need to separate a reference identifier name from the surrounding template, you can wrap the reference identifier in curly braces ({ and }): The total amout is ${total}. You currently have ${cart.items} Items in your Shopping cart. Velocity references are discussed in depth in the references chapter.

Comments allow descriptive text to be included in a template 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 code is doing, or any other purpose you find useful. Like the Java programming language, Velocity has single-line and block comments. A single-line comment begins with ## and finishes at the end of the line. ## This is a single-line comment. This is visible in the output ## This is a comment. If you are going to write a few lines of commentary, there is 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 will not see this text because the Velocity templating engine will ignore it. *# Here is text outside the multi-line comment; it is visible. Multi-line comments can start and end in arbitrary columns of the template. 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. It is similar to the Javadoc comment block in the Java programming language and starts with #**. #** This is a VTL comment block and may be used to store such information as the document author and versioning information: @author @version 5 *#

Most of the time, there is no problem, rendering a Velocity template because the rendering engine is quite smart about finding Velocity statements and references and distinguishing them from regular text. ## Renders as normal text, because the dollar sign ## is not followed by a letter. I bought a 4 lb. sack of potatoes for only $2.50! ## Renders as variable I bought a 4 lb. sack of potatoes for only $money. Velocity allows for explicit escaping of references and directives using the \ (backslash) character. If the character following the \ would start a new directive or reference, then this character is output verbatim. This can lead to some unexpected behaviour, especially with directives. \ ## Renders as a single backslash (no # or $ follows) \\ \# \$ ## Renders as \\ \# \$ (no directive or reference follows) \#end ## Renders as #end, backslash escapes an existing directive Unfortunately, the escaping of VTL elements contains a number of quirks that make them actually hard to use It is really mind-boggling how we came up with this. It is tied more to the internal workings of the Velocity engine than any user-friendlyness. This is definitely subject to change in future Velocity versions. Until then, please be kind to our mistakes. . As you have seen, the behaviour of the \ differs whether a directive or just text follows: $a ## $a is a reference but an unbound one (it has no value) ## This renders as $a in the output \$a ## Escaping an unbound reference renders it as \$a #set($a = 10) ## When a value is assigned to the reference... $a ## ... then it renders as 10 ... \$a ## ... and gets escaped and now renders as $a. Escaping VTL statements works in the same way. \#end ## Renders as #end \# end ## Renders as \# end (note the space) \#set ($a = 10) ## Renders as #set ($a = 10) ## If $a has been assigned a value: #set ($a = 10) \#set ($a = 20) ## Renders as #set (10 = 20) ## The backslash does not escape a whole multi-line statement ## This is a syntax error: (#end without a starting statement) \#if ($a == 10) #end Using Velocity Macros, the behaviour of escaping # changes a bit: ...and you probably thought, the worst part was over... #apple ## Renders as #apple, because it is not a VTL ## statement \#apple ## Renders as \#apple #macro(apple) Apple #end #apple ## This is now a syntax error ## (#apple Velocimacro invocation needs brackets) #apple() ## Renders as Apple (invocation of a Velocimacro) \#apple ## renders as #apple, because it is now a VTL ## statement after Velocimacro definition.

There are three types of references in the VTL: variables, properties and methods. They are the glue that connects the business logic of your application written in Java to your templates. The Java code and the templates must use the same set of references, so you as a template designer and the developers writing the application logic must agree on the references available for use on the templates. Every reference that is output when a template is rendered, is converted to a Java string by calling the toString() method on the Java object which represents the reference. If you want a reference rendered in a special way, you can ask your Java developers to implement or override this method.

All references consist 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 and underscore (- and _) Velocity identifiers are, just as Java variable names, case-sensitive.

Variables are the simplest type of references, because each reference is also a variable. Each variable represents a Java object. $foo $bar $foo-bar $foo_bar $fooBar1 Velocity keeps an internal map from identifiers to variables called the context. The is the place where Java code can put objects to be referenced from the template. A variable must be assigned a value before it can be referenced from the template. Assigning a value to a Velocity variable is the same as placing an object in the context from the Java code of your application. ## Puts a String object representing "bar" in the context #set( $foo = "bar" ) Assigning a new value using the #set statement to a variable does change the object present in the context.

Velocity allows you to access properties through a short-hand notation. The objects to look up the properties must be available through a Velocity variable and the notation consists of a leading variable followed by the dot (".") character and another VTL identifier. $customer.address $purchase.total $cart.customerDiscount A property name can represent the following elements depending on the object used for look-up: If the object has a method get<property> where the property name is not modified, this method is invoked else if the object is a Java bean (has methods conforming to the Sun Java Bean specification for accessing bean properties), the bean getter is executed to access the value finally if the object used to look up the property has a get(String) method, invoke this method. Take the first example, $customer.address. It can have multiple meanings: The sequence of these lookups is determined by the Velocity Uberspector. This is merely the default lookup. A Java developer can customize this behaviour by using a custom Uberspector. when the object has a method getaddress(), invoke this method when the object is a Java bean with a property address, invoke its getter, getAddress() when the object has a method get(String), invoke this method, passing address as parameter. when the object has a method isAddress(), invoke this method. When a property name refers to a getter method, $obj.property and $obj.Property will both invoke the same method (either getproperty() or getProperty()). However, if the object represented by $obj has a get(String) method, $obj.property and $obj.Property will pass different values to this get(String) method. This can lead to hard-to-find problems. It is a good practice to standardize the capitalization of property names in your application While lots of the available Velocity literature uses upper-case capitalization, it is actually a good idea to use lower-case capitalization as this is consistent with JSTL and the regular bean property notation. . If you wonder about setting property values, please look up the #set() directive chapter. The setting of properties is discussed there.

Velocity is quite clever when figuring out which method corresponds to a requested property Some people would say that it is entirely too clever. . 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() For upper-case property names like $customer.Address, it is slightly different: getAddress() getaddress() get("Address") isAddress() As regular bean properties tend to be written in lowercase notation, you should talk to your programmers that they don't add both lower-case and upper-case methods to their objects. It would be a bad idea anyway. There are some exotic naming cases, such as a property starting with multiple upper-case letters which are treated specially according to the Java Bean specification. Velocity does not conform to this specification to the last letter, so if you have a method called getURL(), you cannot use $obj.url to invoke the method, you must use $obj.URL or $obj.uRL. This might change in future versions of Velocity.

A method is part of a context object, written in Java code, and is capable of doing something useful, like running a calculation or arriving at a decision. Method invocation is indicated by round parentheses following the method name. These parentheses can optionally contain a parameter list. $cart.calculateTotal() $customer.updatePurchases($cart) $shop.checkInventory() $cart.addTaxes(8, 16) # Property access $page.setTitle( "My Home Page" ) $customer.getAddress() $purchase.getTotal() The last few examples are similar to the examples seen earlier when discussing properties. As stated there, the properties notation is a short-hand notation for the invocation of bean getters and setters By explicitly stating the method name and parameters, the Uberspector is not used. This is useful if you object has both, get(String) and explicit property getter methods. . The main difference between properties and methods is that you can specify a parameter list to a method. $shop.getFruits() $sun.getPlanets() $album.get("logo") $shop.setCustomer($customer) $sun.setPlanetCount(9) $album.put("logo", "newlogo") ## equal property getters and setters $shop.fruits $sun.Planets $album.logo #set ($shop.customer = $customer) #set ($sun.planetCount = 9) #set ($album.logo = "New Logo") The last #set example will not work in Velocity versions before 1.5 unless the $album object implements java.util.Map. This is a bug in older Velocity versions. Not every method invocation can be replaced by short hand notation. Even if a method is called set<property>() or get<property>(), it must still adhere to bean getter and setter method signatures. ## Can't pass a parameter with $sun.planet $sun.getPlanet(3) ## Velocity only allows shorthand notation for getter and setter $shop.orderFruits() ## Can't pass a parameter list $book.setAuthorAndTitle("George Orwell", "Homage to Catalonia")

When writing templates, you might encounter situations in which it is necessary to explicitly separate a reference from the surrounding template text. In the examples above, this was done implicitly through whitespace. Additionally there is a formal notation which wraps the identifiers with curly braces: ${fruit} ${customer.address} ${purchase.getTotal()} Suppose you were building an extension to your fruit shop where juices are sold. $fruit contains the name of the juice which should be sold. Using the shorthand notation would be inadequate for this task. Consider the following example: You have selected $fruitjuice. There is ambiguity here, and Velocity assumes that $fruitjuice, not $fruit, is the identifier that you want to use. Finding no value for $fruitjuice, it will return $fruitjuice. Using formal notation can resolve this problem. You have selected ${fruit}juice. Now Velocity knows that $fruit, not $fruitjuice, is the reference. Formal notation is often useful when references are directly adjacent to text in a template.

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 probably 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 $email. Formal and quiet reference notation can be used together: <input type="text" name="email" value="$!{email}"/> It is very easy to confuse the quiet reference notation with the boolean not-Operator. Using the not-Operator, you use !${foo}, while the quiet reference notation is $!{foo}. And yes, you will end up sometimes with !$!{foo}...

References allow template designers to generate dynamic content for web sites, while directives permit web designers to truly take charge of the appearance and content of the web site. A directive is a script element that can be used to manipulate the rendering of the template. As described above, Velocity directives are part of either single or multi-line statements and are preceded by a hash sign (#). While the hash sign is technically not part of the directive, we will still speak about the #if or #set directive. Velocity knows about the following directives: If is possible to add custom directives through the Velocity configuration. These are merely the directives shipped with the default Velocity configuration. It is even possible to turn some of these off. #set (single-line statement) #literal (multi-line statement) #if / #elseif / #else (multi-line statement) #foreach (multi-line statement) #include (single-line statement) #parse (single-line statement) #stop (single-line statement) #macro (multi-line statement, see Chapter 7)

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. ## Assigning a variable value #set( $fruit = "apple" ) ## Assigning a property value #set( $customer.Favourite = $fruit ) 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 List Map Expression ## variable reference #set( $fruit = $selectedFruit ) ## string literal #set( $fruit.flavor = "sweet" ) ## property reference #set( $fruit.amount = $cart.total ) ## method reference #set( $fruit.color = $colorlist.selectColor($select) ) ## number literal #set( $fruit.value = 123 ) ## List #set( $fruit.sorts = ["Apple", "Pear", "Orange"] ) ## Map #set( $fruit.shapes = {"Apple" : "round", "Pear" : "conical"}) For the List example the elements defined with the [..] operator are accessible using the methods defined in the java.util.List class. So, for example, you could access the first element above using $fruit.sorts.get(0). Similarly, for the Map example, the elements defined within the {..} operator are accessible using the methods defined in the java.util.Map class. You can access the first element above using $fruit.shapes.get("Apple") to return the String round, or even $fruit.shapes.Apple to return the same value. The RHS can also be an arithmetic expression as described in the Math chapter below. #set( $value = $foo + 1 ) #set( $value = $bar - 1 ) #set( $value = $foo * $bar ) #set( $value = $foo / $bar )

In its default configuration, Velocity treats null values on the RHS special. If your developers don't change the default configuration, you must understand that it is not possible to assign a null value through a set directive to a reference. Starting with Velocity 1.5, a runtime property (directive.set.null.allowed) exists that removes this restriction. If you do not want null values to be treated special, tell your application developers that they should look up this property in the Velocity Developers Guide and set it to true. Removing this restriction is one of the new features of Velocity 1.5. If you are maintaining existing template code from older Velocity versions, you probably want to keep the old behaviour. For new developments based on Velocity 1.5 and beyond, we strongly recommend that you don't treat null values on the RHS special (set directive.set.null.allowed to be true). This will be the default behaviour in future Velocity versions. The remainder of this chapter assumes that you kept the default configuration of Velocity. If the RHS is a property or method reference that evaluates to null, it will not be assigned to the LHS and the LHS will not be altered. This is very different from the behaviour in e.g. the Java programming language and needs some discussion. In the following piece of code, two queries are executed using the same set of template references: #set( $result = $query.criteria("fruit") ) The result of the first query is $result #set( $result = $query.criteria("customer") ) The result of the second query is $result If $query.criteria("name") returns the string "apple", and $query.criteria("customer") returns null, the above VTL will render as the follows: The result of the first query is apple The result of the second query is apple In the second set directive, the RHS is null, because $query.criteria("customer") returned null. So the RHS will not be modified (and the old value retained). Unfortunately, this can lead to hard-to-detect errors, for example with #foreach loops that attempt to change a reference using a #set directive, then immediately test that reference with an #if directive: #set( $criteria = ["fruit", "customer"] ) #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). One solution to this would be to reset the value of $result in every loop iteration: #set( $criteria = ["name", "address"] ) #foreach( $criterion in $criteria ) #set( $result = "" ) #set( $result = $query.criteria($criterion) ) #if( $result != "" ) Query was successful #end #end However, the easiest way is to use the quiet reference notation: #set( $criteria = ["name", "address"] ) #foreach( $criterion in $criteria ) #set( $result = $!query.criteria($criterion) ) #if( $result != "" ) Query was successful #end #end If the query fails, $result gets the empty string assigned.

When 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( $fruit = "apple" ) $fruit #set( $veggie = '$fruit' ) $veggie The output will be: apple $fruit By default, rendering unparsed text using single quotes is activated in Velocity. This default can be changed by changing the runtime.interpolate.string.literals property to be false. (See the Velocity Developers Guide for more information).

The #literal directive allows you to easily use large chunks of uninterpreted content in VTL code. This can be especially useful in place of escaping multiple directives. #literal is a multi-line statement and needs a #end directive to close the statement body. #literal() #foreach ($fruit in $basket) nothing will happen to $fruit #end #end The output will be: #foreach ($fruit in $basket) nothing will happen to $fruit #end Even if the $fruit or $basket references have a value assigned, the text will still be output uninterpreted. While #literal suppresses the evaluation of Velocity directives and references at run-time, the body is still parsed. #literal is not a comment block, if you have a syntax error (e.g. a missing #end) inside the block, Velocity will still report a parse error.

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. Consider the following piece of code: #if( $display ) <strong>Velocity!</strong> #end The variable $display is evaluated to determine whether it is true, which will happen under one of two circumstances: $display is a java.lang.Boolean object (True/False) which has a true value. The value is not null. The Velocity context only contains Java objects, so any method that returns a boolean primitive will automatically wrapped into a java.lang.Boolean object. The content between the #if and the #end statements becomes the output if the evaluation is true. In this case, if $display is true, the output will be: Velocity!. Conversely, if $display has a null value, or if it is a boolean false, the statement evaluates as false, and there is no output. An #elseif or #else element can be used with an #if element. Note that Velocity will stop at the first expression that is found to be true: #set ($direction = 15) #set ($wind = 6) #if( $direction < 10 ) <strong>Go North</strong> #elseif( $direction == 10 ) <strong>Go East</strong> #elseif( $wind == 6 ) <strong>Go South</strong> #else <strong>Go West</strong> #end In this example, $direction is greater than 10, so the first two comparisons fail. Next $wind is compared to 6, which is true, so the output is Go South. When you wish to include text immediately following a #else directive you need to use curly brackets surrounding the directive to differentiate it from the following text: #if( $foo == $bar)it is true!#{else}it is not!#end</li>

#foreach iterates over the elements given in a List or Array element. <ul> #foreach( $product in $allProducts ) <li>$product</li> #end </ul> Unlike other languages, the Velocity templating language allows only loops where the number of iterations is predetermined. There is no do..while or repeat..until loop in Velocity. This has been a conscious decision to make sure that the rendering process of Velocity does always terminate It is stil possible to write a template that contains endless loops. It is just harder. . A VTL foreach loop takes two arguments, the first one is the loop variable and the second is an object that can be iterated over. Velocity supports a number of object types in its default configuration The list of objects can be changed and extended, please see the Velocity Developers Guide on how to do this. : Any array type java.lang.Collection - The loop iterates over the Iterator returned by obj.iterator() java.lang.Map - The loop will iterate over the values of the Map using the iterator returned by values().iterator() java.lang.Iterator - Velocity uses this iterator directly java.lang.Enumeration - Velocity wrapping the Enumeration into an Iterator Using an Iterator or Enumeration object directly in a template has the side effect that the loop cannot be evaluated multiple times because the loop consumes the object that gets iterated. Be especially careful if you use such an object inside a Velocimacro or an included template. Velocity provides you with a loop counter reference which contains the number of the current iteration of the loop: <table> #foreach( $customer in $customerList ) <tr><td>$velocityCount</td><td>$customer.Name</td></tr> #end </table> The default name for the loop counter and its starting value is specified through runtime settings. By default it starts at 1 and is called $velocityCount. If you need these values changed, consult your Java developers and the Velocity Developers Guide. # Default name of the loop counter # variable reference. directive.foreach.counter.name = velocityCount # Default starting value of the loop # counter variable reference. directive.foreach.counter.initial.value = 1 The Velocity loop counter variable does support nested loops, so the following example would work as expected: #foreach ($category in $allCategories) Category # $velocityCount is $category #foreach ($item in $allItems) Item # $velocitycount is $item #end #end The loop counter is local to its surrounding loop and any inner loops hides but not resets it. Please note that it is not possible to access the count variable of the outer loop directly inside the inner loop You can assign its value to another variable before entering the inner loop using #set and access it through this variable inside the inner loop. . To avoid deadlocks and denial of service attacks, it is possible to set a maximum allowed number of times that a loop may be executed. By default there is no limit, but it can be set to an arbitrary number in the Velocity configuration. # The maximum allowed number of loops. directive.foreach.maxloops = -1

The #stop directive stops the rendering of a template. The remaining part of the template after this directive is discarded. The most common usage of this directive is debugging templates It is possible to do strange things by using the #stop directive inside #if ... #else ... #end blocks. As #stop only ends the rendering if it is actually encountered, it is possible to skip over the directive. . This part gets rendered in the output. #stop This part does not get rendered. The #stop directive can also be written as #stop(). While #stop ends the rendering process, it does not affect the template parsing. If you have a syntax error in the VTL code after the #stop directive, this error will still be reported.

In Velocity, just as in other rendering techniques like JSP, it is important to build output from multiple input files. For this, you need to be able to pull external resources into the rendering process. For convenience, we will talk about files in the reminder of this chapter. A template needs not to be a file and strictly spoken, a template can be loaded from arbitrary sources including databases. Using files for templates and referring to their names as file pathes with directories is just the most common way. Velocity supports the loading of external templates by the #include and #parse directive, which both load external resources into the rendering process of a template. Velocity does not allow you to include arbitrary templates into your templates for security reasons. All included elements will be loaded with a resource loader set up in the Velocity configuration. Please consult the Developers Guide for information on how to do this.

The #include directive allows the template designer to import a local file. Its contents replace the #include directive in the template. These contents are inserted as-is, they are not rendered through the template engine. Multiple files can be loaded through a single #include directive, their names must be separated by commas. ## Load the file one.txt into this template #include( "one.txt" ) ## Load multiple files into this template #include( "one.txt","two.txt","three.txt" ) 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. #set ($seasonalgreetings = "christmas.txt") #include( "greetings.txt", $seasonalgreetings )

The #parse directive works similar to #include, but the contents of the file will be part of the template rendering process and all VTL elements in the file will be evaluted. The output will replace the #parse directive in the importing template. #parse( "one.vm" ) Like the #include directive, #parse can take a variable rather than a template. Unlike the #include directive, #parse will only take a single argument, you cannot load multiple templates with a single #parse directive. Recursion is permitted; VTL templates can have #parse statements referring to templates that in turn have #parse statements. To avoid infinitive recursion, the number of levels is limited through a configuration property. It is not possible to configure an infinite recursion depth. The default maximum number of recursions is 20 As we said, we wanted to make it hard to write templates that contain endless loops. Allowing unlimited recursion would have been too easy. . # Maximum number of #parse levels directive.parse.max.depth = 10 ## ######################################## ## Template "countdown.vm" ## ######################################## Count down. #set( $count = 8 ) #parse( "counter.vm" ) All done with countdown.vm! ## ######################################## ## ######################################## ## Template "counter.vm ## ######################################## $count #set( $count = $count - 1 ) #if( $count > 0 ) #parse("counter.vm" ) #else All done with counter.vm! #end ## ######################################## The resulting output: Count down. 8 7 6 5 4 3 2 1 0 All done with counter.vm! All done with countdown.vm!

Velocity knows a number of relational and logical operators. They can be used everywhere an expression is evaluated, most prominently in #if and #set directives. Each operator is available in two notations, a short version which is roughly equal to the Java notation and a text representation which can be used to avoid problems e.g. with XML templates and the <, > and && operators. #set ($a = true) #set ($b = false) #if ($a || $b) short version 'or' was true #end #if ($a and $b) text version 'and' was true #end Type of operator short version text version equal Note that the semantics of the equal operator are slightly different than Java where == can only be used to test object equality. In Velocity the equivalent operator can be used to directly compare numbers, strings, or objects. When the objects are of different classes, the string representations are obtained by calling toString() for each object and then compared. == eq not equal != ne greater than > gt greater or equal than >= ge less than < lt less or equal than <= le logical and && and logical or || or logical not ! not Unlike other languages, Velocity does not consider the number 0 (zero) or the empty String ('') to be equivalent to false. Only the boolean value false (primitive oder Boolean.FALSE) and null are evaluated to false. Everything else evaluates to true.This was a conscious decision by the Velocity designers and is not likely to change. Here is a simple example to illustrate how the equivalent operator is used. #set ($foo = "deoxyribonucleic acid") #set ($bar = "ribonucleic acid") #if ($foo == $bar) In this case it is clear they aren't equivalent. So... #else They are not equivalent and this will be the output. #end The text versions of all logical operators can be used to avoid problems with &, <, and > in XML templates.

#if( $this && $that ) <strong>This AND that</strong> #end The #if directive will only evaluate to true if both $this and $that are true. If $this is false, the expression will evaluate to false; $that will not be evaluated. If $this is true, then Velocity will then check the value of $that; if $that is true, then the entire expression is true and This AND that becomes the output. If $that is false, then there will be no output as the entire expression is false.

Logical OR operators work the same way as AND operators, 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( $this || $that ) <strong>This OR That</strong> #end If $this is true, the Velocity templating engine has no need to look at $that; whether $that is true or false, the expression will be true, and This OR That will be output. If $this is false, however, $that must be checked. In this case, if $that is also false, the expression evaluates to false and there is no output. On the other hand, if $that is true, then the entire expression is true, and the output is This OR That

With logical NOT operators, there is only one argument: #if( !$that ) <strong>NOT that</strong> #end Here, the if $that is true, then !$that evaluates to false, and there is no output. If $that is false, then !$that evaluates to true and NOT that will be output. Be careful not to confuse this with the quiet reference $!foo.

The #macro directive allows you to name a section of a VTL template and re-insert it multiple times into the template. A Velocity macro (or short Velocimacro) can be used in many different scenarios. In the following example, a Velocimacro is created to save keystrokes and minimize typographic errors: #macro( d ) <tr><td></td></tr> #end This example defines a Velocimacro called d. It now can be uses as any other VTL directive: #d() When this template is rendered, Velocity replaces #d() with the HTML table row containing a single, empty cell defined above. Using a Velocimacro in a template is called macro invocation. A Velocimacro definition starts with the #macro directive and are ended with #end. The enclosed block is the macro body. Whenever a Velocimacro is invoked, its body is inserted in the template. It is rendered at every invokation through the templating engine, with the invocation arguments replacing the arguments in the Velocimacro definition. A Velocimacro can take any number of arguments including zero, but when it is invoked, it must be called with exactly the same number of arguments that it was defined with. Here is a Velocimacro that takes two arguments, a color and a list of objects: #macro( tablerows $color $values ) #foreach( $value in $values ) <tr><td bgcolor=$color>$value</td></tr> #end #end #set( $greatlakes = ["Superior","Michigan","Huron","Erie","Ontario"] ) #set( $color = "blue" ) <table> #tablerows( $color $greatlakes ) </table> The tablerows Velocimacro takes exactly two arguments. The first argument takes the place of $color, and the second argument takes the place of $values. Anything that can be put into a VTL template can go into the body of a Velocimacro. Notice that $greatlakes takes the place of $values. When this template is rendered, the following output is generated: <table> <tr><td bgcolor="blue">Superior</td></tr> <tr><td bgcolor="blue">Michigan</td></tr> <tr><td bgcolor="blue">Huron</td></tr> <tr><td bgcolor="blue">Erie</td></tr> <tr><td bgcolor="blue">Ontario</td></tr> </table> In the example above, the #tablerows Velocimacro was defined as an inline macro. Another way to declare a macro is in a Velocimacro library, it is then called a global macro. An inline Velocimacro is only visible in the template where it is defined, a global Velocimacro is accessible from all templates. Global definitions must be done in a a Velocimacro template library, which is a template file that contains the macros and they must be referenced explicitly through the Velocity configuration. Please see the Velocity Reference Guide for more information on how to define and load a macro library. If the #tablerows($color $values) Velocimacro is defined a Velocimacro template library, it could be in any template. In the fruit store it could be used to list fruits available: #set( $fruits = ["apple", "pear", "orange", "strawberry"] ) #set($cellbgcol = "white" ) <table> #tablerows( $cellbgcol $fruits ) </table> When rendering this template, Velocity would find the #tablerows Velocimacro in a template library (defined in the Velocity configuration) and generate the following output: <table> <tr><td bgcolor="white">apple</td></tr> <tr><td bgcolor="white">pear</td></tr> <tr><td bgcolor="white">orange</td></tr> <tr><td bgcolor="white">strawberry</td></tr> </table>

Velocimacros can take as arguments any of the following VTL elements: A reference, starting with $ A String literal, enclosed in single or double quotes A Number literal (1, 2, 3 etc.) An IntegerRange ([ 1..2] or [$start .. $end]) An ObjectArray ([ "a", "b", "c"]) true and false for boolean values representing true and false When passing references as arguments to Velocimacros, please note that references are passed by name. This means that their value is evaluated each time it is used inside the Velocimacro. This feature allows you to pass references with method calls and have the method called at each use: #macro( callme $a ) $a $a $a #end #callme( $foo.bar() ) results in the method bar() of the reference $foo being called 3 times. If you are used to the behaviour of method or procedure calls in programming languages like Java or C, please be sure to fully understand the implications of passing by name. Unlike these programming languages, the reference is not evaluated at macro invocation time but at every occurence inside the Velocimacro. Don't treat Velocimacros as procedures or methods! The easiest way to avoid any gotchas is not passing any stateful object into a Velocimacro. This can be done by assigning the result of a method invocation to a new reference: #set( $myval = $foo.bar() ) #callme( $myval )

There are a number of properties in the Velocity configuration that influence the behaviour of the Velocimacros. A complete list of all configuration options is in the Velocity Reference 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 turned off in the resource loaders (see the Velocity Reference Guide on how to do this). This feature is intended for development, not for production.

Must a Velocimacro be defined before it can be used? Yes. Velocimacros must be defined before they are first used in a template. This means that your inline #macro declarations must be found by the engine before usage. This is important to remember if you try to #parse a template containing inline #macro directives. Because the #parse invocation happens at runtime, and the parser looks at the Velocimacro definitions earlier, at parsetime. So loading a set of Velocimacro definitions using #parse does not work. If you need to use a set of Velocimacros on more than one template, declare it as a Velocimacro Library in the Velocity configuration. Can I use a directive or another Velocimacro as an argument to a Velocimacro? Example: #center( #bold("hello") ) No. A directive isn't a valid argument to a directive, and for most practical purposes, a Velocimacro is a directive. However, there are things you can do. One easy solution is to take advantage of the fact that a doublequoted string (") 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 Velocimacro, not at the calling level. In other words, the argument to the Velocimacro is passed in in its entirety and evaluated within the Velocimacro it was passed into. This allows you to do things like: #macro( inner $foo ) inner: $foo #end #macro( outer $foo ) #set($bar = "value") outer: $foo #end #set($bar = 'calltimelala') #outer( "#inner($bar)" ) Where the output is Outer: inner: value because the evaluation of the "#inner($bar)" happens inside #outer, so the $bar value set inside #outer is the one that is used. This is an intentional and jealously guarded feature - arguments are passed by name into Velocimacros, so you can hand Velocimacros 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 the rowColor() method called repeatedly, rather than just once. To avoid that, invoke the method outside of the Velocimacro, and pass the value into the Velocimacro. #set($color = $bar.rowColor()) #foo( $color ) Can I register Velocimacros via #parse? Yes. Velocimacros may be defined in a file that is loaded into a template with #parse. This is especially helpful if you wish to have a library of macros used by multiple templates. As an alternative you may also use the velocimacro.library facility to have Velocity load your Velocimacros at startup. What is Velocimacro Autoreloading? 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 Activating auto-reload imposes a performance penalty and should be turned off in production.

Although VTL in this user guide is often displayed with newlines and whitespaces, the VTL shown below #set( $fruits = ["apple", "pear", "orange", "strawberry"] ) #foreach( $fruit in $fruits ) $fruit #end is equally valid as the following snippet: Send me #set($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.

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Velocity has a number of built-in math operators 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 )

The range operator can be used in conjunction with #set and #foreach statements. It creates an array of java.lang.Integer objects: [n..m] Both n and m must either be or produce integers. If m is greater than n, the range will count down: ## Example 1: #foreach( $cnt in [1..5] ) $cnt #end Result: 1 2 3 4 5 ## Example 2: #foreach( $cnt in [2..-2] ) $cnt #end Result: 2 1 0 -1 -2 ## Example 3: #set( $range = [0..1] ) #foreach( $cnt in $range ) $cnt #end Result: 0 1 ## Example 4: [1..3] Result: [1..3] This is not a range (not in a #set or #foreach directive) As any other operator, the range operator is only evaluated in Velocity directives. This is why example 4 above does not evaluate the range operator.

String concatenation in VTL is a bit tricky, because it depends on the context in which a String should be concatenated. Concatenating multiple references in the template output is done by simply putting them next to each other. The references are evaluated and rendered to the output without any blanks between them. #set( $size = "Big" ) #set( $name = "Ben" ) ## Renders as 'The clock is BigBen.' The clock is $size$name. To create a new String object that contains the evaluation of one or more references, use the #set directive and a doublequoted string to concatenate the elements. #set( $size = "Big" ) #set( $name = "Ben" ) #set($clock = "$size$name" ) ## Renders as 'The clock is BigBen.' The clock is $clock. In this example, $clock is a new String object that contains the evaluation results of $size and $name. These can be simple references as shown above or more complex elements like property or method references. #set ($monthdays = [1..30]) #set ($description = "Days in April:") #set ($result = "$description: $monthdays.size()") ## Renders as 'Days in April: 30' $result