Groovy - Template Engines

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Groovy’s template engine operates like a mail merge (the automatic addition of names and addresses from a database to letters and envelopes in order to facilitate sending mail, especially advertising, to many addresses) but it is much more general.

Simple Templating in Strings

If you take the simple example below, we are first defining a name variable to hold the string “Groovy”. In the println statement, we are using $ symbol to define a parameter or template where a value can be inserted.

def name = "Groovy" 
println "This Tutorial is about ${name}"

If the above code is executed in groovy, the following output will be shown. The output clearly shows that the $name was replaced by the value which was assigned by the def statement.

Simple Template Engine

Following is an example of the SimpleTemplateEngine that allows you to use JSP-like scriptlets and EL expressions in your template in order to generate parametrized text. The templating engine allows you to bind a list of parameters and their values so that they can be replaced in the string which has the defined placeholders.

def text ='This Tutorial focuses on $TutorialName. In this tutorial you will learn 

about $Topic'  

def binding = ["TutorialName":"Groovy", "Topic":"Templates"]  
def engine = new groovy.text.SimpleTemplateEngine() 
def template = engine.createTemplate(text).make(binding) 

println template

If the above code is executed in groovy, the following output will be shown.

Let’s now use the templating feature for an XML file. As a first step let’s add the following code to a file called Student.template. In the following file you will notice that we have not added the actual values for the elements, but placeholders. So $name,$is and $subject are all put as placeholders which will need to replaced at runtime.

<Student> 
   <name>${name}</name> 
   <ID>${id}</ID> 
   <subject>${subject}</subject> 
</Student>

Now let’s add our Groovy script code to add the functionality which can be used to replace the above template with actual values. The following things should be noted about the following code.

  • The mapping of the place-holders to actual values is done through a binding and a SimpleTemplateEngine. The binding is a Map with the place-holders as keys and the replacements as the values.

import groovy.text.* 
import java.io.* 

def file = new File("D:/Student.template") 
def binding = ['name' : 'Joe', 'id' : 1, 'subject' : 'Physics']
				  
def engine = new SimpleTemplateEngine() 
def template = engine.createTemplate(file) 
def writable = template.make(binding) 

println writable

If the above code is executed in groovy, the following output will be shown. From the output it can be seen that the values are successfully replaced in the relevant placeholders.

<Student> 
   <name>Joe</name> 
   <ID>1</ID> 
   <subject>Physics</subject> 
</Student>

StreamingTemplateEngine

The StreamingTemplateEngine engine is another templating engine available in Groovy. This is kind of equivalent to the SimpleTemplateEngine, but creates the template using writeable closures making it more scalable for large templates. Specifically this template engine can handle strings larger than 64k.

Following is an example of how StreamingTemplateEngine are used −

def text = '''This Tutorial is <% out.print TutorialName %> The Topic name 

is ${TopicName}''' 
def template = new groovy.text.StreamingTemplateEngine().createTemplate(text)
  
def binding = [TutorialName : "Groovy", TopicName  : "Templates",]
String response = template.make(binding) 
println(response)

If the above code is executed in groovy, the following output will be shown.

This Tutorial is Groovy The Topic name is Templates

XMLTemplateEngine

The XmlTemplateEngine is used in templating scenarios where both the template source and the expected output are intended to be XML. Templates use the normal ${expression} and $variable notations to insert an arbitrary expression into the template.

Following is an example of how XMLTemplateEngine is used.

Live Demo 
def binding = [StudentName: 'Joe', id: 1, subject: 'Physics'] 
def engine = new groovy.text.XmlTemplateEngine() 

def text = '''\
   <document xmlns:gsp='http://groovy.codehaus.org/2005/gsp'>
      <Student>
         <name>${StudentName}</name>
         <ID>${id}</ID>
         <subject>${subject}</subject>
      </Student>
   </document> 
''' 

def template = engine.createTemplate(text).make(binding) 
println template.toString()

If the above code is executed in groovy, the following output will be shown

   Joe
    
    
   1
    
    
   Physics
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