Processes template source files substituting variables and expressions into placeholders in a template source text to produce the desired output using a streaming approach. This engine has equivalent functionality to the SimpleTemplateEngine but creates the template using writable closures making it potentially more scalable for large templates or in streaming scenarios.

The template engine uses JSP style <% %> script and <%= %> expression syntax or GString style expressions. The variable 'out' is bound to the writer that the template is being written to.

Frequently, the template source will be in a file but here is a simple example providing the template as a string:

def binding = [
    firstname : "Grace",
    lastname  : "Hopper",
    accepted  : true,
    title     : 'Groovy for COBOL programmers'
]
def engine = new groovy.text.GStringTemplateEngine()
def text = '''\
Dear <%= firstname %> $lastname,

We <% if (accepted) print 'are pleased' else print 'regret' %> \
to inform you that your paper entitled
'$title' was ${ accepted ? 'accepted' : 'rejected' }.

The conference committee.
'''
def template = engine.createTemplate(text).make(binding)
println template.toString()

This example uses a mix of the JSP style and GString style placeholders but you can typically use just one style if you wish. Running this example will produce this output:

Dear Grace Hopper,

We are pleased to inform you that your paper entitled
'Groovy for COBOL programmers' was accepted.

The conference committee.

The template engine can also be used as the engine for groovy.servlet.TemplateServlet by placing the following in your web.xml file (plus a corresponding servlet-mapping element):

<servlet>
  <servlet-name>GStringTemplate</servlet-name>
  <servlet-class>groovy.servlet.TemplateServlet</servlet-class>
  <init-param>
    <param-name>template.engine</param-name>
    <param-value>groovy.text.GStringTemplateEngine</param-value>
  </init-param>
</servlet>

In this case, your template source file should be HTML with the appropriate embedded placeholders.

'<%= "test" %> of expr and <% test = 1 %>${test} script.'

'<%= "test" %> of expr and <% test = 1 %>${test} script.'

{ out -> out << "${"test"} of expr and "; test = 1 ; out << "${test} script."}.asWritable()