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October 20, 2009

Groovy Goodness: Using the Template Engines

Groovy has a built-in template mechanism to generate text dynamically. We can use the SimpleTemplateEngine, GStringTemplateEngine and XmlTemplateEngine to create a dynamic text from a static source. The XmlTemplateEngine is best used for generating XML, the GStringTemplateEngine is best used for streaming scenarios and otherwise we use the SimpleTemplateEngine.

To use a template engine we must read in the static text from a file, URL or String, bind dynamic data to the template engine and get the generated text. These steps are very easy to follow and that makes using the Groovy template engine so easy.

import groovy.text.*

// SimpleTemplateEngine.
def simple = new SimpleTemplateEngine()
def source = '''Dear $name,
Please respond to this e-mail before ${(now + 7).format("dd-MM-yyyy")}
Kind regards, mrhaki'''
def binding = [now: new Date(109, 11, 1), name: 'Hubert Klein Ikkink']
def output = simple.createTemplate(source).make(binding).toString()

assert "Dear Hubert Klein Ikkink,\nPlease respond to this e-mail before 08-12-2009\nKind regards, mrhaki" == output

// GStringTemplateEngine with out variable.
def gstring = new GStringTemplateEngine()
def gsource = '''Dear <%= name %>,
Text is created for <% if (gstring) out << 'GStringTemplateEngine' else out << 'other template engine'%>.'''
def gbinding = [name: 'mrhaki', gstring: true]
def goutput = gstring.createTemplate(gsource).make(gbinding).toString()

assert "Dear mrhaki,\nText is created for GStringTemplateEngine." == goutput

// XmlTemplateEngine with gsp:scriplet and gsp:expression tags.
def xmlEngine = new XmlTemplateEngine()
def xml = '''<?xml version="1.0"?>
<users xmlns:gsp='http://groovy.codehaus.org/2005/gsp'>
    <gsp:scriptlet>users.each {</gsp:scriptlet>
        <user id="${it.id}"><gsp:expression>it.name</gsp:expression></user>
    <gsp:scriptlet>}</gsp:scriptlet>
</users>'''
def xmlBinding = [users: [
    new Expando(id: 1, name: 'mrhaki'),
    new Expando(id: 2, name: 'Hubert')]
]
def xmlOutput = xmlEngine.createTemplate(xml).make(xmlBinding).toString()

assert '''\
<users>
  <user id='1'>
    mrhaki
  </user>
  <user id='2'>
    Hubert
  </user>
</users>
''' == xmlOutput