# Template engines

version 4.0.0

## 1. Introduction

Groovy supports multiple ways to generate text dynamically including `GStrings`, `printf` and MarkupBuilder just to name a few. In addition to these, there is a dedicated template framework which is well-suited to applications where the text to be generated follows the form of a static template.

## 2. Template framework

The template framework in Groovy consists of a `TemplateEngine` abstract base class that engines must implement and a `Template` interface that the resulting templates they generate must implement.

Included with Groovy are several template engines:

- `SimpleTemplateEngine` - for basic templates

- `StreamingTemplateEngine` - functionally equivalent to `SimpleTemplateEngine`, but can handle strings larger than 64k

- `GStringTemplateEngine` - stores the template as writeable closures (useful for streaming scenarios)

- `XmlTemplateEngine` - works well when the template and output are valid XML

- `MarkupTemplateEngine` - a very complete, optimized, template engine

## 3. SimpleTemplateEngine

Shown here is the `SimpleTemplateEngine` that allows you to use JSP-like scriptlets (see example below), script, and EL expressions in your template in order to generate parametrized text. Here is an example of using the system:

    def text = 'Dear "$firstname $lastname",\nSo nice to meet you in <% print city %>.\nSee you in ${month},\n${signed}'

    def binding = ["firstname":"Sam", "lastname":"Pullara", "city":"San Francisco", "month":"December", "signed":"Groovy-Dev"]

    def engine = new groovy.text.SimpleTemplateEngine()
    def template = engine.createTemplate(text).make(binding)

    def result = 'Dear "Sam Pullara",\nSo nice to meet you in San Francisco.\nSee you in December,\nGroovy-Dev'

    assert result == template.toString()

While it is generally not deemed good practice to mix processing logic in your template (or view), sometimes very simple logic can be useful. E.g. in the example above, we could change this:

    $firstname

to this (assuming we have set up a static import for capitalize inside the template):

    ${firstname.capitalize()}

or this:

    <% print city %>

to this:

    <% print city == "New York" ? "The Big Apple" : city %>

### 3.1. Advanced Usage Note

If you happen to be embedding your template directly in your script (as we did above) you have to be careful about backslash escaping. Because the template string itself will be parsed by Groovy before it is passed to the templating framework, you have to escape any backslashes inside GString expressions or scriptlet 'code' that are entered as part of a Groovy program. E.g. if we wanted quotes around The Big Apple above, we would use:

    <% print city == "New York" ? "\\"The Big Apple\\"" : city %>

Similarly, if we wanted a newline, we would use:

    \\n

in any GString expression or scriptlet 'code' that appears inside a Groovy script. A normal “\n” is fine within the static template text itself or if the entire template itself is in an external template file. Similarly, to represent an actual backslash in your text you would need

    \\\\

in an external file or

    \\\\

in any GString expression or scriptlet 'code'. (Note: the necessity to have this extra slash may go away in a future version of Groovy if we can find an easy way to support such a change.)

## 4. StreamingTemplateEngine

The `StreamingTemplateEngine` engine is functionally equivalent to the `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.

It 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 a file but here we show a simple example providing the template as a string:

    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)

    def binding = [
        firstname : "Grace",
        lastname  : "Hopper",
        accepted  : true,
        title     : 'Groovy for COBOL programmers'
    ]

    String response = template.make(binding)

    assert response == '''Dear Grace Hopper,

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

    The conference committee.'''

## 5. GStringTemplateEngine

As an example of using the `GStringTemplateEngine`, here is the example above done again (with a few changes to show some other options). First we will store the template in a file this time:

test.template

    Dear "$firstname $lastname",
    So nice to meet you in <% out << (city == "New York" ? "\\"The Big Apple\\"" : city) %>.
    See you in ${month},
    ${signed}

Note that we used `out` instead of `print` to support the streaming nature of `GStringTemplateEngine`. Because we have the template in a separate file, there is no need to escape the backslashes. Here is how we call it:

    def f = new File('test.template')
    def engine = new groovy.text.GStringTemplateEngine()
    def template = engine.createTemplate(f).make(binding)
    println template.toString()

and here is the output:

    Dear "Sam Pullara",
    So nice to meet you in "The Big Apple".
    See you in December,
    Groovy-Dev

## 6. XmlTemplateEngine

`XmlTemplateEngine` for use in templating scenarios where both the template source and the expected output are intended to be XML. Templates may use the normal `${expression}` and `$variable` notations to insert an arbitrary expression into the template. In addition, support is also provided for special tags: `<gsp:scriptlet>` (for inserting code fragments) and `<gsp:expression>` (for code fragments which produce output).

Comments and processing instructions will be removed as part of processing and special XML characters such as `<`, `>`, `"` and `'` will be escaped using the respective XML notation. The output will also be indented using standard XML pretty printing.

The xmlns namespace definition for gsp: tags will be removed but other namespace definitions will be preserved (but may change to an equivalent position within the XML tree).

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

    def binding = [firstname: 'Jochen', lastname: 'Theodorou', nickname: 'blackdrag', salutation: 'Dear']
    def engine = new groovy.text.XmlTemplateEngine()
    def text = '''\
        <document xmlns:gsp='http://groovy.codehaus.org/2005/gsp' xmlns:foo='baz' type='letter'>
            <gsp:scriptlet>def greeting = "${salutation}est"</gsp:scriptlet>
            <gsp:expression>greeting</gsp:expression>
            <foo:to>$firstname "$nickname" $lastname</foo:to>
            How are you today?
        </document>
    '''
    def template = engine.createTemplate(text).make(binding)
    println template.toString()

This example will produce this output:

    <document type='letter'>
      Dearest
      <foo:to xmlns:foo='baz'>
        Jochen &quot;blackdrag&quot; Theodorou
      </foo:to>
      How are you today?
    </document>

## 7. The MarkupTemplateEngine

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

### 7.1. The template format

#### 7.1.1. 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.

#### 7.1.2. 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;

#### 7.1.3. 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.

#### 7.1.4. 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.

#### 7.1.5. 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>

### 7.2. Rendering contents

#### 7.2.1. 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);

#### 7.2.2. 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. | See the auto escape section
autoIndent | false | If true, performs automatic indentation after new lines | See the auto formatting section
autoIndentString | four (4) spaces | The string to be used as indent. | See the auto formatting section
autoNewLine | false | If true, performs automatic insertion of new lines | See the auto formatting section
baseTemplateClass | `groovy.text.markup.BaseTemplate` | Sets the super class of compiled templates. This can be used to provide application specific templates. | See the custom templates section
locale | Default locale | Sets the default locale for templates. | See the internationalization section

| Once the template engine has been created, it is unsafe to change the configuration.

#### 7.2.3. 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.

#### 7.2.4. 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)
        }
    }

#### 7.2.5. Common gotchas

##### 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 need 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.

#### 7.2.6. 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 fall back 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.

#### 7.2.7. 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'
    }

### 7.3. Type checked templates

#### 7.3.1. 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.

#### 7.3.2. 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

#### 7.3.3. 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.

## 8. Other solutions

Also, there are other templating solutions that can be used along with Groovy, such as FreeMarker, Velocity, StringTemplate and others.