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

Specifically this template engine can handle strings larger than 64k which still causes problems for the other groovy template engines.

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 text = '''\
Dear <% out.print firstname %> ${lastname},

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

The conference committee.
'''

def template = new groovy.text.StreamingTemplateEngine().createTemplate(text)

print template.make(binding)

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.

StreamingTemplateEngine as a servlet engine

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

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

Debugging Template Code

The template engine makes an effort to throw descriptive exceptions with context lines, ie:

 groovy.text.TemplateExecutionException: Template parse error at line 4:
          3: We <% if (accepted) out.print 'are pleased' else out.print 'regret' %> to inform you that your paper entitled
      --> 4: '$txitle' was ${ accepted ? 'accepted' : 'rejected' }.
          5:
at test.run(test.groovy:18)

Caused by: groovy.lang.MissingPropertyException: No such property: txitle for class: groovy.tmp.templates.StreamingTemplateScript1
... 1 more

and sanitize the exceptions to make things readable.

When the exceptions are not enough, it might sometimes be useful to view the actual script source generated by the template engine. This would conceptually be equivalent to viewing the .java file generated for a jsp page. The source is not currently very readable and until we get a built in groovy code pretty printer, we will probably continue to opt for compactness rather than readability.

With that being said, viewing the source might still have some value. For this reason the script source is accessible via the template.scriptSource property, i.e.:

    println template.scriptSource

In the above example.

  { parentClass, stringSectionList, binding, out ->
     //code generated by parsing the template data
  }

  { binding, out ->
     //code generated by parsing the template data
  }

  public Writable make(final Map map) {
    final Closure template = this.template.curry(new Object[]{map});
    return (Writable) template;
  }

Alice why is a ${bird} like a writing desk

Alice why is a $bird like a writing desk

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

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

Alice why is a ${bird} like a writing desk

out << "Alice why is a ";
out << """${bird}""";
out << " like a writing desk";

Alice why is a $bird like a writing desk

out << "Alice why is a ";
out << bird;
out << " like a writing desk";

Alice why is a $bird like a writing desk