Velocity

Overview example

$customer: hold information pertaining to the customer currently logged in

$mudsOnSpecial: is all the types mud on sale at present

$flogger: contains methods that help the promotion

<html>
	<body>
		Hello $customer.Name!
		<table>
			#foreach( $mud in $mudsOnSpecial )
				#if ( $customer.hasPurchased($mud))
					<tr>
						<td>
							$flogger.getPromo( $mud )
						</td>
					</tr>
				#end
			#end
		</table>
	</body>
</html>

Intro for Velocity Template Language (VTL)

References

VTL use references to embed dynamic content in a web site, and a variable is one type of reference

#set( $a = "Velocity")

VTL statements start with #

字符串以双引号包裹时可以允许使用velocity变量

References begin with $ and are used to get something, Directives begin with # and are used to do something

Hello World

<html>
	<body>
		#set($foo = "Velocity")
		Hello $foo World!
	</body>
</html>

Comments

单行注释：

## This is a single line comment.

多行注释：

This is text that is outside the multi-line comment.
Online visitors can see it.

#*
  Thus begins a multi-line comment. Online visitors won't
  see this text because the Velocity Templating Engine will
  ignore it.
*#

Here is text outside the multi-line comment; it is visible.

Index Notation

$foo[0]       ## $foo takes in an Integer lookup
$foo[$i]      ## Using another referebce as the index
$foo["bar"]   ## Passing a string where $foo may be a Map

the bracketed syntax is valid anywhere .get is valid, for example:

#set($foo[0] = 1)
#set($foo.bar[1] = 3)
#set($map["apple"] = "orange")

Formal Reference Notation

${mudSlinger}
${customer.Address}
${purchase.getTotal()}

also, velocity will throw an exception when attempting to call methods or properties that do no exist.

$bar.bogus          ## $bar does not provide property bogus, Exception
$bar.foo.bogus      ## $bar.foo does not provide property bogus, Exception
$bar.retnull.bogus  ## cannot call a property on null, Exception</pre>

Strict rendering mode behavior

$foo                         ## Exception
#set($bar = $foo)            ## Exception
#if($foo == $bar)#end        ## Exception
#foreach($item in $foo)#end  ## Exception

Strict rendering mode is not available in if directives:

#if ($foo)#end                  ## False
#if ( ! $foo)#end               ## True
#if ($foo && $foo.bar)#end      ## False and $foo.bar will not be evaluated
#if ($foo && $foo == "bar")#end ## False and $foo == "bar" wil not be evaluated
#if ($foo1 || $foo2)#end        ## False $foo1 and $foo2 are not defined

to render nothing use $! instead

this is $foo    ## throws an exception because $foo is null
this is $!foo   ## renders to "this is " without an exception
this is $!bogus ## bogus is not in the context so throws an exception

Case Substitution

$foo

$foo.getBar()
## is the same as
$foo.Bar

$data.setUser("jon")
## is the same as
#set( $data.User = "jon" )

$data.getRequest().getServerName()
## is the same as
$data.Request.ServerName
## is the same as
${data.Request.ServerName}

Directives

always begin with a #, the name of directives may be breacketed by a { and a }

eg. to display true enough#elseno way!:

#if($a==1)true enough#elseno way!#end

#if($a==1)true enough#{else}no way!#end

Set

the left of = should be a variable and the right of = should be:

=左边：the Left Hand Side (LHS)

=右边：RHS

Variable reference
String literal
Property reference
Method reference
Number literal
ArrayList
Map

Demostration:

#set( $monkey = $bill ) ## variable reference
#set( $monkey.Friend = "monica" ) ## string literal
#set( $monkey.Blame = $whitehouse.Leak ) ## property reference
#set( $monkey.Plan = $spindoctor.weave($web) ) ## method reference
#set( $monkey.Number = 123 ) ##number literal
#set( $monkey.Say = ["Not", $my, "fault"] ) ## ArrayList
#set( $monkey.Map = {"banana" : "good", "roast beef" : "bad"}) ## Map

The RHS can also be a simple arithmetic expression:

#set( $value = $foo + 1 )
#set( $value = $bar - 1 )
#set( $value = $foo * $bar )
#set( $value = $foo / $bar )

Literals

#set( $directoryRoot = "www" )
#set( $templateName = "index.vm" )
#set( $template = "$directoryRoot/$templateName" )
$template

Output:

www/index.vm

in single quotes the string literal won't be parsed:

#set( $foo = "bar" )
$foo
#set( $blargh = '$foo' )
$blargh

## output: bar
##         $foo

#[[don't parse me]]# syntax allow 大段的代码 unparsed.

#[[
#foreach ($woogie in $boogie)
  nothing will happen to $woogie
#end
]]#

Renders as:

#foreach ($woogie in $boogie)
  nothing will happen to $woogie
#end

Conditionals

#if( $foo )
  <strong>Velocity!</strong>
#end

the conditions that $foo is parsed to be true:

$foo is a boolean (true/false) which has a true value
$foo is a string or a collection which is not null and not empty
$foo is a number which is not equal to zero
$foo is an object (other than a string, a number or a collection) which is not null (and for which the standard public methods for querying the object size, length or boolean/string representations, if they exist, indicate a non-empty, non-zero, non-false object).