This template engine is a template engine primarily aimed at generating XML-like markup (XML, XHTML, HTML5, …​), but that can be used to generate any text based content. Unlike traditional template engines, this one relies on a DSL that uses the builder syntax. Here is a sample template:

xmlDeclaration()
cars {
   cars.each {
       car(make: it.make, model: it.model)
   }
}

If you feed it with the following model:

model = [cars: [new Car(make: 'Peugeot', model: '508'), new Car(make: 'Toyota', model: 'Prius')]]

It would be rendered as:

<?xml version='1.0'?>
<cars><car make='Peugeot' model='508'/><car make='Toyota' model='Prius'/></cars>

The key features of this template engine are:

  • a markup builder like syntax

  • templates are compiled into bytecode

  • fast rendering

  • optional type checking of the model

  • includes

  • internationalization support

  • fragments/layouts

1. The template format

1.1. Basics

Templates consist of Groovy code. Let’s explore the first example more thoroughly:

xmlDeclaration()                                (1)
cars {                                          (2)
   cars.each {                                  (3)
       car(make: it.make, model: it.model)      (4)
   }                                            (5)
}
1 renders the XML declaration string.
2 opens a cars tag
3 cars is a variable found in the template model, which is a list of Car instances
4 for each item, we create a car tag with the attributes from the Car instance
5 closes the cars tag

As you can see, regular Groovy code can be used in the template. Here, we are calling each on a list (retrieved from the model), allowing us to render one car tag per entry.

In a similar fashion, rendering HTML code is as simple as this:

yieldUnescaped '<!DOCTYPE html>'                                                    (1)
html(lang:'en') {                                                                   (2)
    head {                                                                          (3)
        meta('http-equiv':'"Content-Type" content="text/html; charset=utf-8"')      (4)
        title('My page')                                                            (5)
    }                                                                               (6)
    body {                                                                          (7)
        p('This is an example of HTML contents')                                    (8)
    }                                                                               (9)
}                                                                                   (10)
1 renders the HTML doctype special tag
2 opens the html tag with an attribute
3 opens the head tag
4 renders a meta tag with one http-equiv attribute
5 renders the title tag
6 closes the head tag
7 opens the body tag
8 renders a p tag
9 closes the body tag
10 closes the html tag

The output is straightforward:

<!DOCTYPE html><html lang='en'><head><meta http-equiv='"Content-Type" content="text/html; charset=utf-8"'/><title>My page</title></head><body><p>This is an example of HTML contents</p></body></html>
With some configuration, you can have the output pretty printed, with newlines and indent automatically added.

1.2. Support methods

In the previous example, the doctype declaration was rendered using the yieldUnescaped method. We have also seen the xmlDeclaration method. The template engine provides several support methods that will help you render contents appropriately:

Method Description Example

yield

Renders contents, but escapes it before rendering

Template:

yield 'Some text with <angle brackets>'

Output:

Some text with &lt;angle brackets&gt;

yieldUnescaped

Renders raw contents. The argument is rendered as is, without escaping.

Template:

yieldUnescaped 'Some text with <angle brackets>'

Output:

Some text with <angle brackets>

xmlDeclaration

Renders an XML declaration String. If the encoding is specified in the configuration, it is written in the declaration.

Template:

xmlDeclaration()

Output:

<?xml version='1.0'?>

If TemplateConfiguration#getDeclarationEncoding is not null:

Output:

<?xml version='1.0' encoding='UTF-8'?>

comment

Renders raw contents inside an XML comment

Template:

comment 'This is <a href="foo.html">commented out</a>'

Output:

<!--This is <a href="foo.html">commented out</a>-->

newLine

Renders a new line. See also TemplateConfiguration#setAutoNewLine and TemplateConfiguration#setNewLineString.

Template:

p('text')
newLine()
p('text on new line')

Output:

<p>text</p>
<p>text on new line</p>

pi

Renders an XML processing instruction.

Template:

pi("xml-stylesheet":[href:"mystyle.css", type:"text/css"])

Output:

<?xml-stylesheet href='mystyle.css' type='text/css'?>

tryEscape

Returns an escaped string for an object, if it is a String (or any type derived from CharSequence). Otherwise returns the object itself.

Template:

yieldUnescaped tryEscape('Some text with <angle brackets>')

Output:

Some text with &lt;angle brackets&gt;

1.3. Includes

The MarkupTemplateEngine supports inclusion of contents from another file. Included contents may be:

  • another template

  • raw contents

  • contents to be escaped

Including another template can be done using:

include template: 'other_template.tpl'

Including a file as raw contents, without escaping it, can be done like this:

include unescaped: 'raw.txt'

Eventually, inclusion of text that should be escaped before rendering can be done using:

include escaped: 'to_be_escaped.txt'

Alternatively, you can use the following helper methods instead:

  • includeGroovy(<name>) to include another template

  • includeEscaped(<name>) to include another file with escaping

  • includeUnescaped(<name>) to include another file without escaping

Calling those methods instead of the include xxx: syntax can be useful if the name of the file to be included is dynamic (stored in a variable for example). Files to be included (independently of their type, template or text) are found on classpath. This is one of the reasons why the MarkupTemplateEngine takes an optional ClassLoader as constructor argument (the other reason being that you can include code referencing other classes in a template).

If you don’t want your templates to be on classpath, the MarkupTemplateEngine accepts a convenient constructor that lets you define the directory where templates are to be found.

1.4. Fragments

Fragments are nested templates. They can be used to provide improved composition in a single template. A fragment consists of a string, the inner template, and a model, used to render this template. Consider the following template:

ul {
    pages.each {
        fragment "li(line)", line:it
    }
}

The fragment element creates a nested template, and renders it with a model which is specific to this template. Here, we have the li(line) fragment, where line is bound to it. Since it corresponds to the iteration of pages, we will generate a single li element for each page in our model:

<ul><li>Page 1</li><li>Page 2</li></ul>

Fragments are interesting to factorize template elements. They come at the price of the compilation of a fragment per template, and they cannot be externalized.

1.5. Layouts

Layouts, unlike fragments, refer to other templates. They can be used to compose templates and share common structures. This is often interesting if you have, for example, a common HTML page setup, and that you only want to replace the body. This can be done easily with a layout. First of all, you need to create a layout template:

layout-main.tpl
html {
    head {
        title(title)                (1)
    }
    body {
        bodyContents()              (2)
    }
}
1 the title variable (inside the title tag) is a layout variable
2 the bodyContents call will render the body

Then what you need is a template that includes the layout:

layout 'layout-main.tpl',                                   (1)
    title: 'Layout example',                                (2)
    bodyContents: contents { p('This is the body') }        (3)
1 use the main-layout.tpl layout file
2 set the title variable
3 set the bodyContents

As you can see, bodyContents will be rendered inside the layout, thanks to the bodyContents() call in the layout file. As a result, the template will be rendered as this:

<html><head><title>Layout example</title></head><body><p>This is the body</p></body></html>

The call to the contents method is used to tell the template engine that the block of code is in fact a specification of a template, instead of a helper function to be rendered directly. If you don’t add contents before your specification, then the contents would be rendered, but you would also see a random string generated, corresponding to the result value of the block.

Layouts are a powerful way to share common elements across multiple templates, without having to rewrite everything or use includes.

Layouts use, by default, a model which is independent from the model of the page where they are used. It is however possible to make them inherit from the parent model. Imagine that the model is defined like this:

model = new HashMap<String,Object>();
model.put('title','Title from main model');

and the following template:

layout 'layout-main.tpl', true,                             (1)
    bodyContents: contents { p('This is the body') }
1 note the use of true to enable model inheritance

then it is not necessary to pass the title value to the layout as in the previous example. The result will be:

<html><head><title>Title from main model</title></head><body><p>This is the body</p></body></html>

But it is also possible to override a value from the parent model:

layout 'layout-main.tpl', true,                             (1)
    title: 'overridden title',                               (2)
    bodyContents: contents { p('This is the body') }
1 true means inherit from the parent model
2 but title is overridden

then the output will be:

<html><head><title>overridden title</title></head><body><p>This is the body</p></body></html>

2. Rendering contents

2.1. Creation of a template engine

On the server side, rendering templates require an instance of groovy.text.markup.MarkupTemplateEngine and a groovy.text.markup.TemplateConfiguration:

TemplateConfiguration config = new TemplateConfiguration();         (1)
MarkupTemplateEngine engine = new MarkupTemplateEngine(config);     (2)
Template template = engine.createTemplate("p('test template')");    (3)
Map<String, Object> model = new HashMap<>();                        (4)
Writable output = template.make(model);                             (5)
output.writeTo(writer);                                             (6)
1 creates a template configuration
2 creates a template engine with this configuration
3 creates a template instance from a String
4 creates a model to be used in the template
5 bind the model to the template instance
6 render output

There are several possible options to parse templates:

  • from a String, using createTemplate(String)

  • from a Reader, using createTemplate(Reader)

  • from a URL, using createTemplate(URL)

  • given a template name, using createTemplateByPath(String)

The last version should in general be preferred:

Template template = engine.createTemplateByPath("main.tpl");
Writable output = template.make(model);
output.writeTo(writer);

2.2. Configuration options

The behavior of the engine can be tweaked with several configuration options accessible through the TemplateConfiguration class:

Option Default value Description Example

declarationEncoding

null

Determines the value of the encoding to be written when xmlDeclaration is called. It does not influence the writer you are using as output.

Template:

xmlDeclaration()

Output:

<?xml version='1.0'?>

If TemplateConfiguration#getDeclarationEncoding is not null:

Output:

<?xml version='1.0' encoding='UTF-8'?>

expandEmptyElements

false

If true, empty tags are rendered in their expanded form.

Template:

p()

Output:

<p/>

If expandEmptyElements is true:

Output:

<p></p>

useDoubleQuotes

false

If true, use double quotes for attributes instead of simple quotes

Template:

tag(attr:'value')

Output:

<tag attr='value'/>

If useDoubleQuotes is true:

Output:

<tag attr="value"/>

newLineString

System default (system property line.separator)

Allows to choose what string is used when a new line is rendered

Template:

p('foo')
newLine()
p('baz')

If newLineString='BAR':

Output:

<p>foo</p>BAR<p>baz</p>

autoEscape

false

If true, variables from models are automatically escaped before rendering.

autoIndent

false

If true, performs automatic indentation after new lines

autoIndentString

four (4) spaces

The string to be used as indent.

autoNewLine

false

If true, performs automatic insertion of new lines

baseTemplateClass

groovy.text.markup.BaseTemplate

Sets the super class of compiled templates. This can be used to provide application specific templates.

locale

Default locale

Sets the default locale for templates.
Once the template engine has been created, it is unsafe to change the configuration.

2.3. Automatic formatting

By default, the template engine will render output without any specific formatting. Some configuration options can improve the situation:

  • autoIndent is responsible for auto-indenting after a new line is inserted

  • autoNewLine is responsible for automatically inserting new lines based on the original formatting of the template source

In general, it is recommended to set both autoIndent and autoNewLine to true if you want human-readable, pretty printed, output:

config.setAutoNewLine(true);
config.setAutoIndent(true);

Using the following template:

html {
    head {
        title('Title')
    }
}

The output will now be:

<html>
    <head>
        <title>Title</title>
    </head>
</html>

We can slightly change the template so that the title instruction is found on the same line as the head one:

html {
    head { title('Title')
    }
}

And the output will reflect that:

<html>
    <head><title>Title</title>
    </head>
</html>

New lines are only inserted where curly braces for tags are found, and the insertion corresponds to where the nested content is found. This means that tags in the body of another tag will not trigger new lines unless they use curly braces themselves:

html {
    head {
        meta(attr:'value')          (1)
        title('Title')              (2)
        newLine()                   (3)
        meta(attr:'value2')         (4)
    }
}
1 a new line is inserted because meta is not on the same line as head
2 no new line is inserted, because we’re on the same depth as the previous tag
3 we can force rendering of a new line by explicitly calling newLine
4 and this tag will be rendered on a separate line

This time, the output will be:

<html>
    <head>
        <meta attr='value'/><title>Title</title>
        <meta attr='value2'/>
    </head>
</html>

By default, the renderer uses four(4) spaces as indent, but you can change it by setting the TemplateConfiguration#autoIndentString property.

2.4. Automatic escaping

By default, contents which is read from the model is rendered as is. If this contents comes from user input, it can be sensible, and you might want to escape it by default, for example to avoid XSS injection. For that, the template configuration provides an option which will automatically escape objects from the model, as long as they inherit from CharSequence (typically, `String`s).

Let’s imagine the following setup:

config.setAutoEscape(false);
model = new HashMap<String,Object>();
model.put("unsafeContents", "I am an <html> hacker.");

and the following template:

html {
    body {
        div(unsafeContents)
    }
}

Then you wouldn’t want the HTML from unsafeContents to be rendered as is, because of potential security issues:

<html><body><div>I am an <html> hacker.</div></body></html>

Automatic escaping will fix this:

config.setAutoEscape(true);

And now the output is properly escaped:

<html><body><div>I am an &lt;html&gt; hacker.</div></body></html>

Note that using automatic escaping doesn’t prevent you from including unescaped contents from the model. To do this, your template should then explicitly mention that a model variable should not be escaped by prefixing it with unescaped., like in this example:

Explicit bypass of automatic escaping
html {
    body {
        div(unescaped.unsafeContents)
    }
}

2.5. Common gotchas

2.5.1. Strings containing markup

Say that you want to generate a <p> tag which contains a string containing markup:

p {
    yield "This is a "
    a(href:'target.html', "link")
    yield " to another page"
}

and generates:

<p>This is a <a href='target.html'>link</a> to another page</p>

Can’t this be written shorter? A naive alternative would be:

p {
    yield "This is a ${a(href:'target.html', "link")} to another page"
}

but the result will not look as expected:

<p><a href='target.html'>link</a>This is a  to another page</p>

The reason is that the markup template engine is a streaming engine. In the original version, the first yield call generates a string which is streamed to the output, then the a link is generated and streamed, and then the last yield call is streamed, leading in an execution in order. But with the string version above, the order of execution is different:

  • the yield call requires an argument, a string

  • that arguments needs to be evaluated before the yield call is generated

so evaluating the string leads to an execution of the a(href:…​) call before yield is itself called. This is not what you want to do. Instead, you want to generate a string which contains markup, which is then passed to the yield call. This can be done this way:

p("This is a ${stringOf {a(href:'target.html', "link")}} to another page")

Note the stringOf call, which basically tells the markup template engine that the underlying markup needs to be rendered separately and exported as a string. Note that for simple expressions, stringOf can be replaced by an alternate tag notation that starts with a dollar sign:

p("This is a ${$a(href:'target.html', "link")} to another page")
It is worth noting that using stringOf or the special $tag notation triggers the creation of a distinct string writer which is then used to render the markup. It is slower than using the version with calls to yield which perform direct streaming of the markup instead.

2.6. Internationalization

The template engine has native support for internationalization. For that, when you create the TemplateConfiguration, you can provide a Locale which is the default locale to be used for templates. Each template may have different versions, one for each locale. The name of the template makes the difference:

  • file.tpl: default template file

  • file_fr_FR.tpl: french version of the template

  • file_en_US.tpl: american english version of the template

  • …​

When a template is rendered or included, then:

  • if the template name or include name explicitly sets a locale, the specific version is included, or the default version if not found

  • if the template name doesn’t include a locale, the version for the TemplateConfiguration locale is used, or the default version if not found

For example, imagine the default locale is set to Locale.ENGLISH and that the main template includes:

Use an explicit locale in include
include template: 'locale_include_fr_FR.tpl'

then the template is rendered using the specific template:

Bypass the template configuration
Texte en français

Using an include without specifying a locale will make the template engine look for a template with the configured locale, and if not, fallback to the default, like here:

Don’t use a locale in include
include template: 'locale_include.tpl'
Fallback to the default template
Default text

However, changing the default locale of the template engine to Locale.FRANCE will change the output, because the template engine will now look for a file with the fr_FR locale:

Don’t fallback to the default template because a locale specific template was found
Texte en français

This strategy lets you translate your templates one by one, by relying on default templates, for which no locale is set in the file name.

2.7. Custom template classes

By default, templates created inherit the groovy.text.markup.BaseTemplate class. It may be interesting for an application to provide a different template class, for example to provide additional helper methods which are aware of the application, or customized rendering primitives (for HTML, for example).

The template engine provides this ability by setting an alternative baseTemplateClass in the TemplateConfiguration:

config.setBaseTemplateClass(MyTemplate.class);

The custom base class has to extend BaseClass like in this example:

public abstract class MyTemplate extends BaseTemplate {
    private List<Module> modules
    public MyTemplate(
            final MarkupTemplateEngine templateEngine,
            final Map model,
            final Map<String, String> modelTypes,
            final TemplateConfiguration configuration) {
        super(templateEngine, model, modelTypes, configuration)
    }

    List<Module> getModules() {
        return modules
    }

    void setModules(final List<Module> modules) {
        this.modules = modules
    }

    boolean hasModule(String name) {
        modules?.any { it.name == name }
    }
}

This example shows a class which provides an additional method named hasModule, which can then be used directly in the template:

if (hasModule('foo')) {
    p 'Found module [foo]'
} else {
    p 'Module [foo] not found'
}

3. Type checked templates

3.1. Optional type checking

Even if templates are not type checked, they are statically compiled. This means that once the templates are compiled, performance should be very good. For some applications, it might be good to make sure that templates are valid before they are actually rendered. This means failing template compilation, for example, if a method on a model variable doesn’t exist.

The MarkupTemplateEngine provides such a facility. Templates can be optionally type checked. For that, the developer must provide additional information at template creation time, which is the types of the variables found in the model. Imagine a model exposing a list of pages, where a page is defined as:

Page.groovy
public class Page {

    Long id
    String title
    String body
}

Then a list of pages can be exposed in the model, like this:

Page p = new Page();
p.setTitle("Sample page");
p.setBody("Page body");
List<Page> pages = new LinkedList<>();
pages.add(p);
model = new HashMap<String,Object>();
model.put("pages", pages);

A template can use it easily:

pages.each { page ->                    (1)
    p("Page title: $page.title")        (2)
    p(page.text)                        (3)
}
1 iterate on pages from the model
2 page.title is valid
3 page.text is not (should be page.body)

Without type checking, the compilation of the template succeeds, because the template engine doesn’t know about the model until a page is actually rendered. This means that the problem would only surface at runtime, once the page is rendered:

Runtime error
No such property: text

In some situations, this can be complicated to sort out or even notice. By declaring the type of the pages to the template engine, we’re now capable of failing at compile time:

modelTypes = new HashMap<String,String>();                                          (1)
modelTypes.put("pages", "List<Page>");                                              (2)
Template template = engine.createTypeCheckedModelTemplate("main.tpl", modelTypes)   (3)
1 create a map which will hold the model types
2 declare the type of the pages variables (note the use of a string for the type)
3 use createTypeCheckedModelTemplate instead of createTemplate

This time, when the template is compiled at the last line, an error occurs:

Template compilation time error
[Static type checking] - No such property: text for class: Page

This means that you don’t need to wait for the page to be rendered to see an error. The use of createTypeCheckedModelTemplate is mandatory.

3.2. Alternative declaration of types

Alternatively, if the developer is also the one who writes the templates, it is possible to declare the types of the expected variables directly in the template. In this case, even if you call createTemplate, it will be type checked:

Inline declaration of types
modelTypes = {                          (1)
    List<Page> pages                    (2)
}

pages.each { page ->
    p("Page title: $page.title")
    p(page.text)
}
1 types need to be declared in the modelTypes header
2 declare one variable per object in the model

3.3. Performance of type checked templates

An additional interest of using type checked models is that performance should improve. By telling the type checker what are the expected types, you also let the compiler generate optimized code for that, so if you are looking for the best performance, consider using type checked templates.