來源：http://jakarta.apache.org/velocity/docs/vtl-reference-guide.html

About this Guide

This guide is the reference for the Velocity Template Language (VTL). For more information, please also refer to the Velocity User Guide.

References

Variables

Notation:

$ [ ! ][ { ][ a..z, A..Z ][ a..z, A..Z, 0..9, -, _ ][ } ]

Examples:

• Normal notation: $mud-Slinger_9
• Silent notation: $!mud-Slinger_9
• Formal notation: ${mud-Slinger_9}

Properties

Notation:

$ [ { ][ a..z, A..Z ][ a..z, A..Z, 0..9, -, _ ]* .[a..z, A..Z ][ a..z, A-Z, 0..9, -, _ ]* [ } ]

Examples:

• Regular Notation: $customer.Address
• Formal Notation: ${purchase.Total}

Methods

Notation:

$ [ { ][ a..z, A..Z ][ a..z, A..Z, 0..9, -, _ ]* .[ a..z, A..Z ][ a..z, A..Z, 0..9, -, _ ]*( [ opional parameter list... ] ) [ } ]

Examples:

• Regular Notation: $customer.getAddress()
• Formal Notation: ${purchase.getTotal()}
• Regular Notation with Parameter List: $page.setTitle( "My Home Page" )

VTL Properties can be used as a shorthand notation for VTL Methods that take get and set. Either $object.getMethod() or $object.setMethod() can be abbreviated as $object.Method. 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.

Directives

#set - Establishes the value of a reference

Format:

#set( $ref = [ ", ' ]arg[ ", ' ] )

Usage:

• $ref - The LHS of the assignment must be a variable reference or a property reference.
• arg - The RHS of the assignment, arg is parsed if enclosed in double quotes, and not parsed if enclosed in single quotes. If the RHS evaluates to null , it is not assigned to the LHS.

Examples:

• Variable reference: #set( $monkey = "bill" )
• String literal: #set( $monkey.Friend = "monica" )
• Property reference: #set( $monkey.Blame = $whitehouse.Leak )
• Method reference: #set( $monkey.Plan = $spindoctor.weave($web) )
• Number literal: #set( $monkey.Number = 123 )
• Range operator: #set( $monkey.Numbers = [1..3] )
• Object array: #set( $monkey.Say = ["Not", $my, "fault"] )

The RHS can also be a simple arithmetic expression, such as:

• Addition: #set( $value = $foo + 1 )
• Subtraction: #set( $value = $bar - 1 )
• Multiplication: #set( $value = $foo * $bar )
• Division: #set( $value = $foo / $bar )
• Remainder: #set( $value = $foo % $bar )

#if / #elseif / #else - output conditional on truth of statements

Format:

#if( [condition] ) [output] [ #elseif( [condition] ) [output] ]* [ #else [output] ] #end

Usage:

• condition - If a boolean, considered true if it has a true false; if not a boolean, considered true if not null.
• output - May contain VTL.

Examples:

• Equivalent Operator: #if( $foo == $bar )
• Greater Than: #if( $foo > 42 )
• Less Than: #if( $foo < 42 )
• Greater Than or Equal To: #if( $foo >= 42 )
• Less Than or Equal To: #if( $foo <= 42 )
• Equals Number: #if( $foo == 42 )
• Equals String: #if( $foo == "bar" )
• Boolean NOT: #if( !$foo )

#foreach - Loops through a list of objects

Format:

#foreach( $ref in arg ) statement #end

Usage:

• $ref - The first variable reference is the item.
• arg - May be one of the following: a reference to a list (i.e. object array, collection, or map), an array list, or the range operator.
• statement - What is output each time Velocity finds a valid item in the list denoted above as arg . This output is any valid VTL and is rendered each iteration of the loop.

Examples of the #foreach(), omitting the statement block :

• Reference: #foreach ( $item in $items )
• Array list: #foreach ( $item in ["Not", $my, "fault"] )
• Range operator: #foreach ( $item in [1..3] )

Velocity provides an easy way to get the loop counter so that you can do something like the following:

<table> #foreach( $customer in $customerList ) <tr><td>$velocityCount</td><td>$customer.Name</td></tr> #end </table>

The default name for the loop counter variable reference, which is specified in the velocity.properties file, is $velocityCount. By default the counter starts at 1, but this can be set to either 0 or 1 in the velocity.properties file. Here's what the loop counter properties section of the velocity.properties file appears:

# Default name of the loop counter # variable refernce. counter.name = velocityCount # Default starting value of the loop # counter variable reference. counter.initial.value = 1

#include - Renders local file(s) that are not parsed by Velocity

Format:

#include( arg[, arg2, ... argn] )

• arg - Refers to a valid file under TEMPLATE_ROOT.

Examples:

• String: #include( "disclaimer.txt", "opinion.txt" )
• Variable: #include( $foo, $bar )

#parse - Renders a local template that is parsed by Velocity

Format:

#parse( arg )

• arg - Refers to a template under TEMPLATE_ROOT.

Examples:

• String: #parse( "lecorbusier.vm" )
• Variable: #parse( $foo )

Recursion permitted. See parse_directive.maxdepth in velocity.properties to change from parse depth. (The default parse depth is 10.)

#stop - Stops the template engine

Format:

#stop

Usage:

This will stop execution of the current template. This is good for debugging a template.

#macro - Allows users to define a Velocimacro (VM), a repeated segment of a VTL template, as required

Format:

#macro( vmname $arg1 [ $arg2 $arg3 ... $argn ] ) [ VM VTL code... ] #end

• vmname - Name used to call the VM ( #vmname )
• $arg1 $arg2 [ ... ] - Arguments to the VM. There can be any number of arguments, but the number used at invocation must match the number specified in the definition.
• [ VM VTL code... ] - Any valid VTL code, anything you can put into a template, can be put into a VM.

Once defined, the VM is used like any other VTL directive in a template.

#vmname( $arg1 $arg2 )

VMs can be defined in one of two places:

Template library: can be either VMs pre-packaged with Velocity or custom-made, user-defined, site-specific VMs; available from any template

Inline: found in regular templates, only usable when velocimacro.permissions.allowInline=true in velocity.properties .

Comments

Comments are not rendered at runtime.

Single Line

Example:

## This is a comment.

Multi Line

Example:

#*
This is a multiline comment.
This is the second line
*#

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