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The Joy of Mustache: Server Side Templates for the JVM

I don’t do much server-side templating, but when I do…​ well frankly, I tend to forget things. Every template language has its strengths and weaknesses, and they all have syntax to remember, and more frequently to forget. Recently I completed some work on the old Spring Petclinic, converting it to use Thymeleaf in the view layer, and re-organizing the code to be a bit more "modern". I enjoyed working with Thymeleaf 3, and found it a pleasant experience, but had to spend a lot of time scanning documentation and samples. Then I had another little project that needed some templates, and I remembered my fondness for Mustache, which we added to Spring Boot back in version 1.2, and which plays an important role in the excellent Spring REST Docs tool. I added spring-boot-starter-mustache to my new project, and was up and running within seconds.

I want to show you what a neat little tool JMustache is for server side rendering of HTML (or anything else in plain text come to that). I always liked Mustache because of its simplicity - it’s "just enough" templating - and you really couldn’t wish for a cleaner, leaner, more lightweight library than this one, if you have to render templates in the JVM. There is one jar file with no dependencies, and it adds 78kb to your classpath, which isn’t going to hurt anyone, and will put a smile on many faces. It has very few features, which is excellent for people who can’t remember syntax, and the manual is short, comprehensive, readable, and useful.

If you carry on reading, as we build up a sample application, you will see how to build HTML pages with Mustache, rendering static and dynamic content, building forms and menus, and abstracting the layout of the pages into separate components. The simplicity of Mustache shines through, and guides you to put logic in Java, keeping the templates as clean as possible. As a sidebar you will see how to secure an application with a custom login form in a slightly unusual but interesting way.

Sample Code

There is some sample code following the text in GitHub. It is a tiny Spring MVC application, also using Spring Security. If you want to see it develop in stages along with the text, you can use some tags:

  • "base" is a starting point with a working application

  • "includes" creates a re-usable layout using a header and a footer

  • "layout" is a slightly more advanced implementation using a Mustache lambda

  • "menus" adds some more UI elements using more Spring Boot and Mustache features

At every stage you can checkout the tag and run the app. There is a Maven wrapper in the root of the project, so you can build and run it from the command line, e.g.

$ git clone https://github.com/dsyer/mustache-sample
$ cd mustache-sample
$ git checkout base
$ ./mvnw spring-boot:run

Instead of running from the command line you can import the project into your favourite IDE and run the main method in the DemoApplication.

The app runs on http://localhost:8080, and you can authenticate with any username and password (even empty!). There are no real features in the sample app, but it does have login and logout and a home page, to provide some hooks to show the templating features.

Getting Started

Spring Boot has autoconfiguration support for JMustache, so it is easy to get up and running with a Spring MVC application. You could generate a project from the Spring Initializr, and ask it for spring-boot-starter-mustache.

Spring Boot automatically configures a ViewResolver for JMustache, so you can implement a home page by providing a controller that returns a view name, e.g.

HomeController.java
@Controller
class HomeController {
  @GetMapping("/")
  String home() {
    return "index";
  }
}

With this controller, when the user visits the home page ("/") Spring will render a template at classpath:/templates/index.html, which means in the directory src/main/resources/templates in your project. For example you could drop this in and confirm that it works:

index.html
<!doctype html>
<html lang="en">
  <body>
    <h1>Demo</h1>
    <div>Hello World</div>
  </body>
</html>

Make the Application Secure

There’s no dynamic (templated) content there yet. Let’s make the app secure and add a login form, at which point you will need the dynamic content. So add spring-cloud-starter-security to your dependencies and the home page will be automatically protected. Suppose you want to have a login form at "/login", so you’ll need the controller:

LoginController.java
@Controller
@RequestMapping("/login")
class LoginController {

	@GetMapping
	public String form() {
		return "login";
	}

}

You will also need some basic security configuration, which you can add as a method in the main application, if you extend a base class from Spring Security:

DemoApplication.java
@SpringBootApplication
public class DemoApplication extends WebSecurityConfigurerAdapter {

  @Override
  protected void configure(HttpSecurity http) throws Exception {
    http.authorizeRequests()
          .antMatchers("/login", "/error").permitAll()
          .antMatchers("/**").authenticated()
        .and().exceptionHandling()
          .authenticationEntryPoint(new LoginUrlAuthenticationEntryPoint("/login"));
  }

}
Note
this is a slightly unconventional configuration for form login, because it isn’t using the built-in formLogin(), which would automatically add the authentication entry point. If that’s distracting then skip the next section and just add .formLogin() to your configuration instead of the exceptionHandling() above (everything after the .and())

Custom Authentication Processing with Spring MVC

Spring Security implements form login (and everything else really) using a Filter, so that’s what you would get with the built in formLogin() configuration. Just to make things interesting, you are going to do the authentication in a Spring MVC handler, enabling you to add some custom logic, and MVC is easier to work with than filters.

So let’s extend the LoginController with a method to handle username/password authentication (it is easy to extend to more complicated logic). The main thing it needs to do is validate the input and if it is a real user, populate the SecurityContext:

LoginController.java
@PostMapping
public void authenticate(@RequestParam Map<String, String> map) throws Exception {
  Authentication result = new UsernamePasswordAuthenticationToken(
      map.get("username"), "N/A",
      AuthorityUtils.commaSeparatedStringToAuthorityList("ROLE_USER"));
  SecurityContextHolder.getContext().setAuthentication(result);
}
Note
In this simple example there is only a "happy path" - all users are authenticated. Obviously this is not a very secure authentication process, and you would want to throw an AuthenticationException, e.g. BadCredentialsException, in a real controller. The exception would be handled by Spring Security.

To mimic the behaviour of the built-in Spring Security login form, you also need to be able to redirect to a "saved request" that the user tried to access before login. Spring Security has an AuthenticationSuccessHandler abstraction for that, and a simple implementation that knows about the saved request. So the authenticate method can use that (it needs the servlet request and response, which you can add those as method parameters, and Spring MVC will inject them):

LoginController.java
private AuthenticationSuccessHandler handler = new SavedRequestAwareAuthenticationSuccessHandler();

@PostMapping
public void authenticate(@RequestParam Map<String, String> map,
    HttpServletRequest request, HttpServletResponse response) throws Exception {
  // ... authenticate user from request parameters
  handler.onAuthenticationSuccess(request, response, result);
}

The Login Form

Now you are ready to accept authentication requests, you need a form for users to fill in and submit. The LoginController renders the "login" template, so you need to add a "login.html" to your templates folder. For example:

login.html
<!doctype html>
<html lang="en">
<body>
  <h1>Login</h1>
  <form action="/login" method="post">                            (1)
    <label for="username">Username:</label>
    <input type="text" name="username" />                         (2)
    <label for="password">Password:</label>
    <input type="password" name="password" />                     (3)
    <input type="hidden" name="_csrf" value="{{_csrf.token}}" />  (4)
    <button type="submit" class="btn btn-primary">Submit</button>
  </form>
</body>
</html>
  1. a form, with a submit button to send the contents to "POST /login"

  2. username field input

  3. password field input

  4. CSRF token, in the format required by Spring Security.

The CSRF token is your first piece of dynamic content, and it shows you how Mustache works, and incidentally why it is called "Mustache". Variables from the "context" (in this case the Spring MVC model object) can be rendered using double braces, or "mustaches" ({{ and }}). JMustache also navigates the object graph inside variables, so _csrf.token resolves as the "token" property of the "_csrf" object.

Spring Security puts the "_csrf" object into request attributes. To get it copied to the MVC model you need a setting in your application.properties:

application.properties
spring.mustache.expose-request-attributes=true

With all that in place, you should find that on visiting the application in a browser will first redirect to "/login". Because of the weak (non-existent) authentication logic in your handler, you can put anything you like in the form and submit it to see the home page.

Note
in the sample app we have some stylesheets imported via webjars to make the app look a little bit nicer, but they don’t add anything to the functionality.

The sample code has a "base" tag which is an application with all the features we have seen so far.

Layout Abstractions: Using Includes

There are only 2 pages in our application, but even with such a small code base there is going to be a quite a bit of duplication in the HTML. It is useful to extract some common elements of all pages into re-usable templates. One way to do this is with "includes". So we could extract the top matter and bottom matter into "header.html":

header.html
<!doctype html>
<html lang="en">
<body>

and "footer.html"

footer.html
</body>
</html>

(These are intentionally trivial examples. In a real app they would probably have a lot of stylesheets, scripts, and meta tags.)

With those templates we can re-write the home page:

index.html
{{>header}}
    <h1>Demo</h1>
    <div>Hello World</div>
{{>footer}}

and the login form would look similar (just the body of the HTML). In these examples you can see the Mustache syntax for "includes", which is a bit like a variable, but with an extra ">" in the opening tag. The name of the template is resolved in the same way as the view templates (so "footer" is mapped to "footer.html" in the "templates" directory).

"Natural" Templates

Some people like to use HTML templates that render on their own and can be viewed in a browser. It’s kind of neat to be able to edit the templates and be able to see the result independent of any server or application logic. Mustache isn’t a perfect language for such "natural" templates, but it does have one feature that you can use to get something approximating it. That feature is "comments".

So, for example, you could add a static header and footer to your home page template, so that it renders in the browser (almost) as if it was in the application. Just surround the static content with Mustache comment tags ({{! and }}). For example:

index.html
{{!
<!doctype html>
<html lang="en">
<body>
}}
{{>header}}
    <h1>Demo</h1>
    <div>Hello World</div>
{{>footer}}
{{!
</body>
</html>
}}

The browser will still render the Mustache tags as literal braces, but you can squint and ignore those, and the rest of the content will be layed out exactly as it would be in the application. Obviously, with such basic content there isn’t a huge benefit, but when the content is more complex and has styling and scripts it might make more sense.

The sample code has a tag in GitHub called "includes", which is an application with all the features we have seen so far.

Layout Abstractions: Using a Lambda

Some people will be perfectly happy with a header and a footer in separate templates, but others will moan. To be honest it does feel a little awkward to be laying out hierarchical content (HTML), and be forced to break elements (like the <body> tag in the sample) across multiple files. It would be nicer if we could control the layout in a single file, something like this:

layout.html
<!doctype html>
<html lang="en">
<body>
  {{{layout.body}}}
</body>
</html>

and then somehow generate the "body" content in our home page and login page.

Mustache allows you to insert generic "executable" content into your templates. This is a really powerful feature, and you can use it to extract the layout into its own template, as well as to do other things that involve a bit of logic. The syntax for that is a generic Mustache tag that resolves to something executable. The home page would look something like this:

index.html
{{#layout}}
    <h1>Demo</h1>
    <div>Hello World</div>\
{{/layout}}

To make this work you first need an object called "layout" of type Mustache.Lambda in our MVC model. You could do this in your controller methods, or (better) use a @ControllerAdvice to add model attributes to all views. For example:

LayoutAdvice.java
@ControllerAdvice
class LayoutAdvice {

  @ModelAttribute("layout")
  public Mustache.Lambda layout() {
    return new Layout();
  }

}

class Layout implements Mustache.Lambda {
  String body;
  @Override
  public void execute(Fragment frag, Writer out) throws IOException {
    body = frag.execute();
  }
}

Notice that the "layout" attribute renders its body using Fragment.execute() and assigns it to a property called "body", which can be referenced as a variable in Mustache. The "layout.html" template already contains the code to pull in the body, {{{layout.body}}}, so all that remains is to actually render the layout (so far we have only rendered the body). We can do this, in a first pass, by importing the layout explicitly into the home page:

index.html
{{#layout}}
    <h1>Demo</h1>
    <div>Hello World</div>\
{{/layout}}
{{>layout}}

Do the same with the login template:

login.html
{{#layout}}
  <h1>Login</h1>
  <form action="/login" method="post">
    <label for="username">Username:</label>
    <input type="text" name="username" />
    <label for="password">Password:</label>
    <input type="password" name="password" />
    <input type="hidden" name="_csrf" value="{{_csrf.token}}" />
    <button type="submit" class="btn btn-primary">Submit</button>
  </form>
{{/layout}}
{{>layout}}

and you are good to go. Everything works, and the app shows the login page and home page with the same layout.

Tip
you might have noticed the triple mustaches ({{{ and }}}) in the "layout.html". This is a JMustache feature: all content is escaped by default, but this content is going to be rendered twice, so we only need it escaped the first time, so we use triple mustaches.

Rendering the Layout in the Lambda

To remove the need for the explicit {{>layout}} include in every page that uses {{#layout}}, you can do that part inside the lambda. You’ll need a reference to the Mustache compiler, and then you just need to compile a template which includes the layout and execute it:

Layout.java
class Layout implements Mustache.Lambda {

  String body;

  private Compiler compiler;

  public Layout(Compiler compiler) {
    this.compiler = compiler;
  }

  @Override
  public void execute(Fragment frag, Writer out) throws IOException {
    body = frag.execute();
    compiler.compile("{{>layout}}").execute(frag.context(), out);
  }

}

The compiler is wired into the Layout in its constructor, and it can be injected into the controller advice using @Autowired:

LayoutAdvice.java
@ControllerAdvice
class LayoutAdvice {
	private final Mustache.Compiler compiler;

	@Autowired
	public LayoutAdvice(Compiler compiler) {
		this.compiler = compiler;
	}

	@ModelAttribute("layout")
	public Mustache.Lambda layout(Map<String, Object> model) {
		return new Layout(compiler);
	}
}

That’s it. You can remove the include from the view templates. E.g. this works for the home page:

index.html
{{#layout}}
    <h1>Demo</h1>
    <div>Hello World</div>\
{{/layout}}

The last line of the old version of the template has effectively been moved into the Layout lambda.

More Dynamic Content

It’s quite common for layout templates like the one that we are developing to have content that varies between uses. For instance you might want the "title" on the home page to be different to that on the login page, but it is part of the HTML header, not the body, so logically it is part of the layout. Let’s make that explicit, by adding the title to the header of the layout:

layout.html
<!doctype html>
<html lang="en">
<head>
  <title>{{{layout.title}}}</title>
</head>
<body>
  {{{layout.body}}}
</body>
</html>

This is a strong hint about how you can implement this feature: the layout has a new property called "title", and you can give it a default value in the class declaration:

Layout.java
class Layout implements Mustache.Lambda {

  String body;

  String title = "Demo Application";

  ...

}

Now, all that remains is to populate that property. Logically, setting the title is part of the page view, not the layout, so you’d like to set it in the same place you declare the rest of the page content. Other template languages have "parameterised fragments", but Mustache is too minimalistic for that. The minimalism is a feature, and actually it leads to quite an elegant solution to this problem.

All you have is tags, so you might want to do something like this:

index.html
{{#layout}}{{#title}}Home Page{{/title}}
    <h1>Demo</h1>
    <div>Hello World</div>\
{{/layout}}

That looks like it might work. All you need to do is provide a lambda to capture the title. In the layout advice you can do this:

LayoutAdvice.java
@ControllerAdvice
class LayoutAdvice {

  ...

  @ModelAttribute("title")
  public Mustache.Lambda defaults(@ModelAttribute Layout layout) {
    return (frag, out) -> {
      layout.title = frag.execute();
    };
  }

}

and as long as the call to {{#title}} is nested inside the call to {{#layout}} everything will work out just fine. You cleaned up your templates and moved a tiny piece of logic to Java, where it belongs.

The sample code is tagged with "layout" at this point, if you want to check it out and compare notes.

Logout: Menus and Spring Boot Configuration

You can load a home page and log into your application using a form. The user can’t yet log out, so you probably want to add that feature, ideally as a link on all pages, so that makes it part of the layout. To show how that works, let’s add a generic, declarative menu bar to the application, and make one part of it a logout button.

The logout link is actually pretty easy. We only need a form with the CSRF token and a link to submit it, e.g:

layout.html
<!doctype html>
<html lang="en">
<head>
  <title>{{{layout.title}}}</title>
</head>
<body>
  <form id="logout" action="/logout" method="post">
    <input type="hidden" name="_csrf" value="{{_csrf.token}}" />
    <button type="submit" class="btn btn-primary">Logout</button>
  </form>
  {{{layout.body}}}
</body>
</html>

That already should work. But lets incorporate the logout into a more generic set of menu links. A list of elements in HTML can be represented as a <ul/> with nested <li/>, so the menus for your application can be rendered that way. In Mustache you do iteration just like lambdas, using a tag, so let’s invent a new one called {{#menus}}:

layout.html
<!doctype html>
<html lang="en">
<head>
  <title>{{{layout.title}}}</title>
</head>
<body>
  <ul class="nav nav-pills" role="tablist">
    {{#menus}}<li><a href="{{path}}">{{name}}</a></li>{{/menus}}
    <li><a href="#" onclick="document.getElementById('#logout').submit()">Logout</a></li>
  </ul>
  {{{layout.body}}}
  <form id="logout" action="/logout" method="post">
    <input type="hidden" name="_csrf" value="{{_csrf.token}}" />
  </form>
</body>
</html>

Notice that inside the {{#menus}} tag we pull out variables, "name" and "path" using the normal Mustache syntax.

Now you have to define the tag in your controller advice (or equivalently in the controllers), so that "menus" resolves to an iterable:

LayoutAdvice.java
@ModelAttribute("menus")
public Iterable<Menu> menus() {
  return application.getMenus();
}

So this new code introduced a Menu type that contains the static content for each menu in the UI. The layout calls for "name" and "path", so you need those properties:

Menu.java
class Menu {
  private String name;
  private String path;
  // ... getters and setters
}

In the layout advice above the menus came from an application object. That wasn’t strictly necessary: you could have declared the list of menus inline in the menus() method, but extracting it into another object gives us the chance to use a nice Spring Boot feature, where we can declare the menus in a config file in a compact format.

So now you need to create the Application object to hold the menus, and inject it into the layout advice:

Layout.java
private Application application;

@Autowired
public LayoutAdvice(Compiler compiler, Application application) {
  this.compiler = compiler;
  this.application = application;
}

where in Application you have something like this

Application.java
@Component
@ConfigurationProperties("app")
class Application {
  private List<Menu> menus = new ArrayList<>();
  // .. getters and setters
}

The @ConfigurationProperties tells Spring Boot to bind to this bean from the environment. Switching from application.properties to application.yml you could create a "Home" and a "Login" menu like this:

application.yml
app.menus:
  - name: Home
    path: /
  - name: Login
    path: /login

With this in place, the "layout.html" that you already defined now has all it needs to work.

The sample code is tagged with "menus" at this point in github, if you want to check it out and compare notes. It’s also the final state, so it’s the same code in master, possibly with bug fixes and updates to libraries. I hope you enjoy using Mustache as much as I do.

Footnote

The sample has one or two extra features on top of the code in the text. One of which is that the "active" menu is rendered differently to the others using a CSS style. For that to work, you need to add a flag to the Menu and reset it in the layout advice. The logic is natural and easy to add to the advice. Another is that the title for the page is part of the menu definition instead of being a separate lambda.

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