Chapter 18. MVC Templates
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## Chapter 18. MVC Templates

Jersey provides an extension to support the Model-View-Controller (MVC) design pattern. In the context of Jersey components, the Controller from the MVC pattern corresponds to a resource class or method, the View to a template bound to the resource class or method, and the model to a Java object (or a Java bean) returned from a resource method (Controller).

### Note

Some of the passages/examples from this chapter have been taken from MVCJ blog article written by Paul Sandoz.

In Jersey 2, the base MVC API consists of two classes (`org.glassfish.jersey.server.mvc` package) that can be used to bind model to view (template), namely Viewable and @Template. These classes determine which approach (explicit/implicit) you would be taking when working with Jersey MVC templating support.

## 18.1. Viewable

In this approach a resource method explicitly returns a reference to a view template and the data model to be used. For this purpose the Viewable class has been introduced in Jersey 1 and is also present (under a different package) in Jersey 2. A simple example of usage can be seen in Example 18.1, “Using `Viewable` in a resource class”.

Example 18.1. Using `Viewable` in a resource class

    package com.example;

    @Path("foo")
    public class Foo {

        @GET
        public Viewable get() {
            return new Viewable("index.foo", "FOO");
        }
    }

In this example, the `Foo` JAX-RS resource class is the controller and the `Viewable` instance encapsulates the provided data model (`FOO` string) and a named reference to the associated view template (`index.foo`).

### Tip

All HTTP methods may return `Viewable` instances. Thus a `POST` method may return a template reference to a template that produces a view as a result of processing an HTML Form.

## 18.2. @Template

### 18.2.1. Annotating Resource methods

There is no need to use `Viewable` every time you want to bind a model to a template. To make the resource method more readable (and to avoid verbose wrapping of a template reference and model into `Viewable`) you can simply annotate a resource method with @Template annotation. An updated example, using `@Template`, from previous section is shown in Example 18.2, “Using `@Template` on a resource method” example.

Example 18.2. Using `@Template` on a resource method

    package com.example;

    @Path("foo")
    public class Foo {

        @GET
        @Template("index.foo")
        public String get() {
            return "FOO";
        }
    }

In this example, the `Foo` JAX-RS resource class is still the controller as in previous section but the MVC model is now represented by the return value of annotated resource method.

The processing of such a method is then essentially the same as if the return type of the method was an instance of the Viewable class. If a method is annotated with `@Template` and is also returning a `Viewable` instance then the values from the `Viewable` instance take precedence over those defined in the annotation. Producible media types are for both cases, `Viewable` and `@Template`, determined by the method or class level `@Produces` annotation.

### 18.2.2. Annotating Resource classes

A resource class can have templates implicitly associated with it via @Template annotation. For example, take a look at the resource class listing in Example 18.3, “Using `@Template` on a resource class”.

Example 18.3. Using `@Template` on a resource class

    @Path("foo")
    @Template
    public class Foo {

        public String getFoo() {
            return "FOO";
        }
    }

The example relies on Jersey MVC conventions a lot and requires more explanation as such. First of all, you may have noticed that there is no resource method defined in this JAX-RS resource. Also, there is no template reference defined. In this case, since the `@Template` annotation placed on the resource class does not contain any information, the default relative template reference `index` will be used (for more on this topic see Section 18.3, “Absolute vs. Relative template reference”). As for the missing resource methods, a default `@GET` method will be automatically generated by Jersey for the `Foo` resource (which is the MVC Controller now). The implementation of the generated resource method performs the equivalent of the following explicit resource method:

    @GET
    public Viewable get() {
        return new Viewable("index", this);
    }

You can see that the resource class serves in this case also as the model. Producible media types are determined based on the `@Produces` annotation declared on the resource class, if any.

### Note

In case of "resource class"-based implicit MVC view templates, the controller is also the model. In such case the template reference `index` is special, it is the template reference associated with the controller instance itself.

In the following example, the MVC controller represented by a JAX-RS `@GET` sub-resource method, is also generated in the resource class annotated with `@Template`:

    @GET
    @Path("{implicit-view-path-parameter}")
    public Viewable get(@PathParameter("{implicit-view-path-parameter}") String template) {
        return new Viewable(template, this);
    }

This allows Jersey to support also implicit sub-resource templates. For example, a JAX-RS resource at path `foo/bar` will try to use relative template reference `bar` that resolves to an absolute template reference `/com/foo/Foo/bar`.

In other words, a HTTP `GET` request to a `/foo/bar` would be handled by this auto-generated method in the `Foo` resource and would delegate the request to a registered template processor supports processing of the absolute template reference `/com/foo/Foo/bar`, where the model is still an instance of the same JAX-RS resource class `Foo`.

## 18.3. Absolute vs. Relative template reference

As discussed in the previous section, both @Template and Viewable provide means to define a reference to a template. We will now discuss how these values are interpreted and how the concrete template is found.

### 18.3.1. Relative template reference

Relative reference is any path that does not start with a leading '`/`' (slash) character (i.e. `index.foo`). This kind of references is resolved into absolute ones by pre-pending a given value with a fully qualified name of the last matched resource.

Consider the Example 18.3, “Using `@Template` on a resource class” from the previous section, the template name reference `index` is a relative value that Jersey will resolve to its absolute template reference using a fully qualified class name of `Foo` (more on resolving relative template name to the absolute one can be found in the JavaDoc of Viewable class), which, in our case, is:

    "/com/foo/Foo/index"

Jersey will then search all the registered template processors (see Section 18.7, “Writing Custom Templating Engines”) to find a template processor that can resolve the absolute template reference further to a "processable" template reference. If a template processor is found then the "processable" template is processed using the supplied data model.

### Note

If none or empty template reference is provided (either in `Viewable` or via `@Template`) then the `index` reference is assumed and all further processing is done for this value.

### 18.3.2. Absolute template reference

Let's change the resource `GET` method in our `Foo` resource a little:

Example 18.4. Using absolute path to template in `Viewable`

    @GET
    public Viewable get() {
        return new Viewable("/index", "FOO");
    }

In this case, since the template reference begins with `"/"`, Jersey will consider the reference to be absolute already and will not attempt to absolutize it again. The reference will be used "as is" when resolving it to a "processable" template reference as described earlier.

Absolute template references start with leading '`/`' (i.e. `/com/example/index.foo`) character and are not further resolved (with respect to the resolving resource class) which means that the template is looked for at the provided path directly.

Note, however, that template processors for custom templating engines may modify (and the supported ones do) absolute template reference by pre-pending 'base template path' (if defined) and appending template suffix (i.e. `foo`) if the suffix is not provided in the reference.

For example assume that we want to use Mustache templates for our views and we have defined 'base template path' as `pages`. For the absolute template reference `/com/example/Foo/index` the template processor will transform the reference into the following path: `/pages/com/example/Foo/index.mustache`.

## 18.4. Handling errors with MVC

In addition to @Template a @ErrorTemplate annotation has been introduced in Jersey 2.3. The purpose of this annotation is to bind the model to an error view in case an exception has been raised during processing of a request. This is true for any exception thrown after the resource matching phase (i.e. this not only applies to JAX-RS resources but providers and even Jersey runtime as well). The model in this case is the thrown exception itself.

Example 18.5, “Using `@ErrorTemplate` on a resource method” shows how to use `@ErrorTemplate` on a resource method. If all goes well with the method processing, then the `/short-link` template is used to as page sent to the user. Otherwise if an exception is raised then the `/error-form` template is shown to the user.

Example 18.5. Using `@ErrorTemplate` on a resource method

    @POST
    @Produces({"text/html”})
    @Consumes(MediaType.APPLICATION_FORM_URLENCODED)
    @Template(name = "/short-link")
    @ErrorTemplate(name = "/error-form")
    public ShortenedLink createLink(@FormParam("link") final String link) {
        // ...
    }

Note that @ErrorTemplate can be used on a resource class or a resource method to merely handle error states. There is no need to use @Template or Viewable with it.

The annotation is handled by custom ExceptionMapper<E extends Throwable> which creates an instance of `Viewable` that is further processed by Jersey. This exception mapper is registered automatically with a `MvcFeature`.

### 18.4.1. MVC & Bean Validation

`@ErrorTemplate` can be used in also with Bean Validation to display specific error pages in case the validation of input/output values fails for some reason. Everything works as described above except the model is not the thrown exception but rather a list of ValidationErrors. This list can be iterated in the template and all the validation errors can be shown to the user in a desirable way.

Example 18.6. Using `@ErrorTemplate` with Bean Validation

    @POST
    @Produces({"text/html”})
    @Consumes(MediaType.APPLICATION_FORM_URLENCODED)
    @Template(name = "/short-link”) @ErrorTemplate(name = "/error-form")
    @Valid
    public ShortenedLink createLink(@NotEmpty @FormParam("link") final String link) {
        // ...
    }

Example 18.7. Iterating through `ValidationError` in JSP

    <c:forEach items="${model}" var="error">
        ${error.message} "<strong>${error.invalidValue}</strong>"<br/>
    </c:forEach>

Support for Bean Validation in Jersey MVC Templates is provided by a `jersey-mvc-bean-validation` extension module. The JAX-RS Feature provided by this module (`MvcBeanValidationFeature`) has to be registered in order to use this functionality (see Section 18.5, “Registration and Configuration”).

Maven users can find this module at coordinates

    <dependency>
        <groupId>org.glassfish.jersey.ext</groupId>
        <artifactId>jersey-mvc-bean-validation</artifactId>
        <version>2.5.1</version>
    </dependency>

and for non-Maven users the list of dependencies is available at jersey-mvc-bean-validation.

## 18.5. Registration and Configuration

To use the capabilities of Jersey MVC templating support in your JAX-RS/Jersey application you need to register specific JAX-RS Features provided by the MVC modules. For `jersey-mvc` module it is MvcFeature for others it could be, for example, FreemarkerMvcFeature (`jersey-mvc-freemarker`).

Example 18.8. Registering `MvcFeature`

    new ResourceConfig()
        .register(org.glassfish.jersey.server.mvc.MvcFeature.class)
        // Further configuration of ResourceConfig.
        .register( ... );

Example 18.9. Registering `FreemarkerMvcFeature`

    new ResourceConfig()
        .register(org.glassfish.jersey.server.mvc.freemarker.FreemarkerMvcFeature.class)
        // Further configuration of ResourceConfig.
        .register( ... );

### Note

Modules that uses capabilities of the base Jersey MVC module register `MvcFeature` automatically, so you don't need to register this feature explicitly in your code.

Almost all of the MVC modules are further configurable and either contain a `*Properties` (e.g. `FreemarkerMvcProperties`) class describing all the available properties which could be set in a JAX-RS `Application` / `ResourceConfig`. Alternatively, the properties are listed directly in the module `*Feature` class.

Example 18.10. Setting `MvcProperties.TEMPLATE_BASE_PATH` value in `ResourceConfig`

    new ResourceConfig()
        .property(MvcProperties.TEMPLATE_BASE_PATH, "templates")
        .register(MvcFeature.class)
        // Further configuration of ResourceConfig.
        .register( ... );

Example 18.11. Setting `FreemarkerMvcProperties.TEMPLATE_BASE_PATH` value in `web.xml`

    <servlet>
        <servlet-name>org.glassfish.jersey.examples.freemarker.MyApplication</servlet-name>
        <servlet-class>org.glassfish.jersey.servlet.ServletContainer</servlet-class>
        <init-param>
            <param-name>javax.ws.rs.Application</param-name>
            <param-value>org.glassfish.jersey.examples.freemarker.MyApplication</param-value>
        </init-param>
        <init-param>
            <param-name>jersey.config.server.mvc.templateBasePath.freemarker</param-name>
            <param-value>freemarker</param-value>
        </init-param>
        <load-on-startup>1</load-on-startup>
    </servlet>

## 18.6. Supported templating engines

Jersey provides extension modules that enable support for several templating engines. This section lists all the supported engines and their modules as well as discusses any module-specific details.

### 18.6.1. Mustache

An integration module for Mustache-based templating engine.

Mustache template processor resolves absolute template references to processable template references represented as Mustache templates as follows:

Procedure 18.1. Resolving Mustache template reference

1. if the absolute template reference does not end in `.mustache` append this suffix to the reference; and

2. if `ServletContext.getResource`, `Class.getResource` or `File.exists`returns a non-`null` value for the reference then return the reference as the processable template reference otherwise return `null` (to indicate the absolute reference has not been resolved by the Mustache template processor).

Thus the absolute template reference `/com/foo/Foo/index` would be resolved as `/com/foo/Foo/index.mustache`, provided there exists a `/com/foo/Foo/index.mustache` Mustache template in the application.

Available configuration properties:

- `MustacheMvcFeature.TEMPLATE_BASE_PATH` - `jersey.config.server.mvc.templateBasepath.mustache`

  The base path where Mustache templates are located.

- `MustacheMvcFeature.CACHE_TEMPLATES` - `jersey.config.server.mvc.caching.mustache`

  Enables caching of Mustache templates to avoid multiple compilation.

- `MustacheMvcFeature.TEMPLATE_OBJECT_FACTORY` - `jersey.config.server.mvc.factory.mustache`

  Property used to pass user-configured `MustacheFactory`.

Maven users can find this module at coordinates

    <dependency>
        <groupId>org.glassfish.jersey.ext</groupId>
        <artifactId>jersey-mvc-mustache</artifactId>
        <version>2.5.1</version>
    </dependency>

and for non-Maven users the list of dependencies is available at jersey-mvc-mustache.

### 18.6.2. Freemarker

An integration module for Freemarker-based templating engine.

Freemarker template processor resolves absolute template references to processable template references represented as Freemarker templates as follows:

Procedure 18.2. Resolving Freemarker template reference

1. if the absolute template reference does not end in `.ftl` append this suffix to the reference; and

2. if `ServletContext.getResource`, `Class.getResource` or `File.exists`returns a non-`null` value for the reference then return the reference as the processable template reference otherwise return `null` (to indicate the absolute reference has not been resolved by the Freemarker template processor).

Thus the absolute template reference `/com/foo/Foo/index` would be resolved to `/com/foo/Foo/index.ftl`, provided there exists a `/com/foo/Foo/index.ftl` Freemarker template in the application.

Jersey will assign the model instance to an attribute named `model`. So it is possible to reference the `foo` key from the provided `Map` (MVC Model) resource from the Freemarker template as follows:

    <h1>${model.foo}</h1>

Available configuration properties:

- `FreemarkerMvcFeature.TEMPLATE_BASE_PATH` - `jersey.config.server.mvc.templateBasepath.freemarker`

  The base path where Freemarker templates are located.

- `FreemarkerMvcFeature.CACHE_TEMPLATES` - `jersey.config.server.mvc.caching.freemarker`

  Enables caching of Freemarker templates to avoid multiple compilation.

- `FreemarkerMvcFeature.TEMPLATE_OBJECT_FACTORY` - `jersey.config.server.mvc.factory.freemarker`

  Property used to pass user-configured `FreemarkerFactory`.

Maven users can find this module at coordinates

    <dependency>
        <groupId>org.glassfish.jersey.ext</groupId>
        <artifactId>jersey-mvc-freemarker</artifactId>
        <version>2.5.1</version>
    </dependency>

and for non-Maven users the list of dependencies is available at jersey-mvc-freemarker.

### 18.6.3. JSP

An integration module for JSP-based templating engine.

### Limitations of Jersey JSP MVC Templates

Jersey web applications that want to use JSP templating support should be registered as Servlet filters rather than Servlets in the application's `web.xml`. The `web.xml`-less deployment style introduced in Servlet 3.0 is not supported at the moment for web applications that require use of Jersey MVC templating support.

JSP template processor resolves absolute template references to processable template references represented as JSP pages as follows:

Procedure 18.3. Resolving JSP template reference

1. if the absolute template reference does not end in `.jsp` append this suffix to the reference; and

2. if `ServletContext.getResource` returns a non-`null` value for the reference then return the reference as the processable template reference otherwise return `null` (to indicate the absolute reference has not been resolved by the JSP template processor).

Thus the absolute template reference `/com/foo/Foo/index` would be resolved to `/com/foo/Foo/index.jsp`, provided there exists a `/com/foo/Foo/index.jsp` JSP page in the web application.

Jersey will assign the model instance to the attribute named `model` or `it`. So it is possible to reference the `foo` property on the `Foo` resource from the JSP template as follows:

    <h1>${model.foo}</h1>

or

    <h1>${it.foo}</h1>

To include another JSP page in the currently processed one a custom `include` tag can be used. Mandatory parameter `page` represents a relative template name which would be absolutized using the same resolving resource class as the parent JSP page template.

Example 18.12. Including JSP page into JSP page

    <%@page contentType="text/html"%>
    <%@page pageEncoding="UTF-8"%>

    <%@taglib prefix="rbt" uri="urn:org:glassfish:jersey:servlet:mvc" %>

    <html>
        <body>

        <rbt:include page="include.jsp"/>

        </body>
    </html>

Available configuration properties:

- `JspMvcFeature.TEMPLATE_BASE_PATH` - `jersey.config.server.mvc.templateBasepath.jsp`

  The base path where JSP templates are located.

Maven users can find this module at coordinates

    <dependency>
        <groupId>org.glassfish.jersey.ext</groupId>
        <artifactId>jersey-mvc-jsp</artifactId>
        <version>2.5.1</version>
    </dependency>

and for non-Maven users the list of dependencies is available at jersey-mvc-jsp.

## 18.7. Writing Custom Templating Engines

To add support for other (custom) templating engines into Jersey MVC Templating facility, you need to implement the TemplateProcessor and register this class into your application.

### Tip

When writing template processors it is recommend that you use an appropriate unique suffix for the processable template references, in which case it is then possible to easily support mixing of multiple templating engines in a single application without conflicts.

Example 18.13. Custom TemplateProcessor

    @Provider
    class MyTemplateProcessor implements TemplateProcessor<String> {

        @Override
        public String resolve(String path, final MediaType mediaType) {
            final String extension = ".testp";

            if (!path.endsWith(extension)) {
                path = path + extension;
            }

            final URL u = this.getClass().getResource(path);
            return u == null ? null : path;
        }

        @Override
        public void writeTo(String templateReference,
                            Viewable viewable,
                            MediaType mediaType,
                            OutputStream out) throws IOException {
            final PrintStream ps = new PrintStream(out);
            ps.print("path=");
            ps.print(templateReference);
            ps.println();
            ps.print("model=");
            ps.print(viewable.getModel().toString());
            ps.println();
        }

    }

Example 18.14. Registering custom TemplateProcessor

    new ResourceConfig()
        .register(MyTemplateProcessor.class)
        // Further configuration of ResourceConfig.
        .register( ... );

### Note

In a typical set-up projects using the Jersey MVC templating support would depend on the base module that provides the API and SPI and a single templating engine module for the templating engine of your choice. These modules need to be mentioned explicitly in your `pom.xml` file.

If you want to use just templating API infrastructure provided by Jersey for the MVC templating support in order to implement your custom support for a templating engine other than the ones provided by Jersey, you will need to add the base jersey-mvc module into the list of your dependencies:

    <dependency>
        <groupId>org.glassfish.jersey.ext</groupId>
        <artifactId>jersey-mvc</artifactId>
        <version>2.5.1</version>
    </dependency>

## 18.8. Other Examples

To see an example of MVC (JSP) templating support in Jersey refer to the MVC (Bookstore) example.

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