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<!DOCTYPE chapter [<!ENTITY % ents SYSTEM "jersey.ent" > %ents; ]>
<chapter xmlns="http://docbook.org/ns/docbook"
         version="5.0"
         xml:lang="en"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xmlns:xi="http://www.w3.org/2001/XInclude"
         xmlns:xlink="http://www.w3.org/1999/xlink"
         xsi:schemaLocation="http://docbook.org/ns/docbook http://docbook.org/xml/5.0/xsd/docbook.xsd
                             http://www.w3.org/1999/xlink http://www.w3.org/1999/xlink.xsd"
         xml:id="declarative-linking">

    <title>Declarative Hyperlinking</title>

    <para>
        <link xlink:href="http://roy.gbiv.com/untangled/2008/rest-apis-must-be-hypertext-driven">RESTful APIs must be
            hypertext-driven</link>. JAX-RS currently offers &jaxrs.core.UriBuilder; to simplify URI
        creation but Jersey adds an additional annotation-based alternative that is described here.
    </para>
    <important>
        <para>
            This API is currently under development and experimental so it is subject to change at any time.
        </para>
    </important>

    <section>
        <title>Dependency</title>

        <para>
            To use Declarative Linking you need to add &lit.jersey-declarative-linking; module to your &lit.pom.xml; file:

            <programlisting language="xml" linenumbering="unnumbered">&lt;dependency&gt;
    &lt;groupId&gt;org.glassfish.jersey.ext&lt;/groupId&gt;
    &lt;artifactId&gt;jersey-declarative-linking&lt;/artifactId&gt;
    &lt;version&gt;&version;&lt;/version&gt;
&lt;/dependency&gt;</programlisting>

            Additionally you will need to add the following dependencies, if you are not deploying
            into a container that is already including them:

            <programlisting language="xml" linenumbering="unnumbered">&lt;dependency&gt;
    &lt;groupId&gt;jakarta.el&lt;/groupId&gt;
    &lt;artifactId&gt;jakarta.el-api&lt;/artifactId&gt;
    &lt;version&gt;&jakarta.el.version;&lt;/version&gt;
&lt;/dependency&gt;</programlisting>

            <programlisting language="xml" linenumbering="unnumbered">&lt;dependency&gt;
    &lt;groupId&gt;org.glassfish.web&lt;/groupId&gt;
    &lt;artifactId&gt;jakarta.el&lt;/artifactId&gt;
    &lt;version&gt;&jakarta.el-impl.version;&lt;/version&gt;
&lt;/dependency&gt;</programlisting>

            If you're not using Maven make sure to have all needed dependencies (see &jersey.ext.declarative-linking.deps.link;) on
            the classpath.
        </para>
    </section>


    <section>
        <title>Links in Representations</title>

        <para>
            Links are added to representations using the <literal>@InjectLink</literal> annotation on
            entity class fields. The Jersey runtime is responsible for injecting the appropriate URI
            into the field prior to serialization by a message body writer. E.g. consider the
            following resource and entity classes:
        </para>

        <programlisting language="java">@Path("widgets")
public class WidgetsResource {
    @GET
    public Widgets get() {
        return new Widgets();
    }
}

public class Widgets {
    @InjectLink(resource=WidgetsResource.class)
    URI u;
}</programlisting>

        <para>
            After a call to <literal>WidgetsResource#get</literal>, the Jersey runtime will inject the value
            <literal>"/context/widgets"</literal>
            <footnote>
                <para>
                    Where <literal>/context</literal> is the application deployment context.
                </para>
            </footnote>
            into the returned <literal>Widgets</literal> instance. If an absolute URI is
            desired instead of an absolute path then the annotation can be changed to
            <literal>@InjectLink(resource=WidgetsResource.class, style=ABSOLUTE)</literal>.
        </para>
        <para>
            The above usage works nicely when there's already a URI template on a class that you
            want to reuse. If there's no such template available then you can use a literal value
            instead of a reference. E.g. the following is equivalent to the earlier example:
            <literal>@InjectLink(value="widgets", style=ABSOLUTE)</literal>.
        </para>
    </section>

    <section>
        <title>List of Link Injection</title>
        <para>
            You can inject multiple links into an array or a List collection type. E.g.:
            <programlisting language="java">@InjectLinks({@InjectLink(resource=WidgetsResource.class, rel = "self")})
List&lt;Link&gt; links</programlisting>

            The field doesn't need to be initialized. However, if it already contains a collection with manually created links,
            then it will merge those with the generated links into a new collection which then replaces the field value.
        </para>
    </section>

    <section>
        <title>Links from Resources</title>

        <para>
            As an alternative to defining the links in the entity class, they can also be defined in the resource classes by
            annotating the resource methods with <literal>@ProvideLink</literal>. This has the benefit, that the target
            method is already known and doesn't need to be referenced. Other than that it has the same parameters and behaviors
            as <literal>@InjectLink</literal>. The entity classes need to have a field annotated with
            <literal>@InjectLinks</literal>, which can be empty.
        </para>

        <para>
            The <literal>@ProvideLink</literal> annotation can be repeated to add links to different entities using different
            options. Entities are defined via the <literal>value</literal> property. If the entities are similar in structure they
            can also be declared as an array. <literal>@ProvideLink</literal> also works with class hierarchies, e.g., contributions
            defined for a superclass will also be injected into the derived classes (interfaces are not supported).

            <programlisting language="java">@ProvideLink(value = Order.class,rel = "self",
     bindings = @Binding(name = "orderId", value = "${instance.id}"))
@ProvideLink(value = PaymentConfirmation.class, rel = "order",
     bindings = @Binding(name = "orderId", value = "${instance.orderId}"))
@GET
@Path("/{orderId}")
public Response get(@PathParam("orderId") String orderId)</programlisting>
        </para>
    </section>

    <section>
        <title>Binding Template Parameters</title>

        <para>
            Referenced or literal templates may contain parameters. Two forms of parameters are
            supported:
            <itemizedlist>
                <listitem><para>
                    URI template parameters, e.g. <literal>widgets/{id}</literal> where <literal>{id}</literal> represents
                    a variable part of the URI.
                </para></listitem>
                <listitem><para>
                    EL expressions, e.g. <literal>widgets/${instance.id}</literal> where <literal>${instance.id}</literal>
                    is an EL expression.
                </para></listitem>
            </itemizedlist>
        </para>

        <para>
            Parameter values can be extracted from three implicit beans:
            <variablelist>
                <varlistentry>
                    <term>
                        <literal>instance</literal>
                    </term>
                    <listitem>
                        <para>Represents the instance of the class that contains the annotated field.</para>
                    </listitem>
                </varlistentry>
                <varlistentry>
                    <term>
                        <literal>entity</literal>
                    </term>
                    <listitem>
                        <para>Represents the entity class instance returned by the resource method.</para>
                    </listitem>
                </varlistentry>
                <varlistentry>
                    <term>
                        <literal>resource</literal>
                    </term>
                    <listitem>
                        <para>Represents the resource class instance that returned the entity.</para>
                    </listitem>
                </varlistentry>
            </variablelist>
        </para>

        <para>
            By default URI template parameter values are extracted from the implicit <literal>instance</literal> bean,
            i.e. the following two annotations are equivalent:

            <programlisting language="java">@InjectLink("widgets/{id}")

@InjectLink("widgets/${instance.id}")</programlisting>
        </para>

        <para>
            The source for URI template parameter values can be changed using the <literal>@Binding</literal>
            annotation, E.g. the following three annotations are equivalent:
            <programlisting language="java">@InjectLink(value="widgets/{id}", bindings={
    @Binding(name="id" value="${resource.id}"}
)

@InjectLink(value="widgets/{value}", bindings={
    @Binding("${resource.id}")})
@InjectLink("widgets/${resource.id}")</programlisting>
        </para>

    </section>

    <section>
        <title>Conditional Link Injection</title>
        <para>
            Link value injection can be made conditional by use of the <literal>condition</literal> property.
            The value of this property is a boolean EL expression and a link will only be injected if the condition
            expression evaluates to true. E.g.:

            <programlisting language="java">@InjectLink(value="widgets/${instance.id}/offers",
    condition="${instance.offers}")
URI offers;</programlisting>
        </para>

        <para>
            In the above, a URI will only be injected into the <literal>offers</literal> field if the
            <literal>offers</literal> property of the instance is <literal>true</literal>.
        </para>
    </section>

    <section>
        <title>Link Headers</title>
        <para>
            <link xlink:href="http://tools.ietf.org/html/rfc5988#section-5">HTTP Link headers</link> can also be added
            to responses using annotations. Instead of annotating the fields of an entity class with
            <literal>@InjectLink</literal>, you instead annotate the entity class itself with
            <literal>@InjectLinks</literal>. E.g.:
            <programlisting language="java">@InjectLinks(
    @InjectLink(value="widgets/${resource.nextId}", rel="next")
)</programlisting>
        </para>

        <para>
            The above would insert a HTTP Link header into any response whose entity was thus annotated.
            The <literal>@InjectLink</literal> annotation contains properties that map to the parameters
            of the HTTP Link header. The above specifies the link relation as <literal>next</literal>.
            All properties of the <literal>@InjectLink</literal> annotation may be used as described above.
        </para>

        <para>
            Multiple link headers can be added by use of the <literal>@InjectLinks</literal> annotation
            which can contain multiple <literal>@InjectLink</literal> annotations.
        </para>

        <para>
            Resource links via <literal>@ProvideLink</literal> are currently not supported for link headers.
        </para>
    </section>

    <section>
        <title>Prevent Recursive Injection</title>
        <para>
            By default, Jersey will try to recursively find all <literal>@InjectionLink</literal> annotations in
            the members of your object unless this member is annotated with <literal>@XmlTransient</literal>.

            But in some cases, you might want to control which member will be introspected regardless of the
            <literal>@XmlTransient</literal> annotation.

            You can prevent Jersey to look into an object by adding <literal>@InjectLinkNoFollow</literal> to a field.

            <programlisting language="java">@InjectLinkNoFollow
Context context;</programlisting>
        </para>
    </section>


    <section>
        <title>Meta-annotation support</title>
        <para>
            The <literal>@ProvideLink</literal> annotation can be used as a meta-annotation, i.e., annotating your own annotation.
            This enables you to create custom annotations to reuse <literal>@ProvideLink</literal> configurations instead of
            copy pasting them on each method. There is a special marker class <literal>ProvideLink.InheritFromAnnotation</literal>
            that can be used in place of the actual entity class, this indicates that the <literal>Class&lt;?> value()</literal>
            from the custom annotation should be used instead.

            Repeated annotations are currently unsupported for this feature. Also the  <literal>Class&lt;?> value()</literal>
            method must return a single class and not an array of classes.
        </para>

        <para>

            Here is an example (getter/setter omitted for brevity) of how a meta annotation can be used.
            The example app uses a <literal>Page</literal> class as a base class for all entities that contain paged data.

            <programlisting language="java">public class Page {
    private int number;

    private int size;

    private boolean isPreviousPageAvailable;

    private boolean isNextPageAvailable;

    @InjectLinks
    private List&lt;Link> links;
}</programlisting>

            Instead of duplicating the <literal>@ProvideLink</literal> annotations for the  next and previous links on every
            method, we create the following <literal>@PageLinks</literal> annotation.

            <programlisting language="java">@ProvideLink(value = ProvideLink.InheritFromAnnotation.class, rel = "next",
    bindings = {
        @Binding(name = "page", value = "${instance.number + 1}"),
        @Binding(name = "size", value = "${instance.size}"),
    }, condition = "${instance.nextPageAvailable}")
@ProvideLink(value = ProvideLink.InheritFromAnnotation.class, rel = "prev",
    bindings = {
        @Binding(name = "page", value = "${instance.number - 1}"),
        @Binding(name = "size", value = "${instance.size}"),
    }, condition = "${instance.previousPageAvailable}")
@Target({ElementType.METHOD})
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface PageLinks {
  Class&lt;?> value();
}</programlisting>

            The annotation can the then be used on the resource methods with the actual entity class as <literal>value</literal>.

            <programlisting language="java">@PageLinks(OrderPage.class)
@GET
public Response list(@QueryParam("page") @DefaultValue("0") int page,
                     @QueryParam("size") @DefaultValue("20") int size)</programlisting>

            The entity just extends from <literal>Page</literal> and declares its content. It is necessary to use distinct classes
            instead of just a generic page to have a unique target for <literal>@ProvideLink</literal>, otherwise every method
            annotated with <literal>@ProvideLink(value=Page.class)</literal> would contribute to the entity.

            <programlisting language="java">public class OrderPage extends Page {
    private List&lt;Order> orders;
}</programlisting>
        </para>
    </section>

    <section>
        <title>Configure and register</title>

        <para>
            In order to add the Declarative Linking feature register &jersey.linking.DeclarativeLinkingFeature;

            <example>
                <title>Creating JAX-RS application with Declarative Linking feature enabled.</title>

                <programlisting language="java">// Create JAX-RS application.
final Application application = new ResourceConfig()
        .packages("org.glassfish.jersey.examples.linking")
        .register(DeclarativeLinkingFeature.class);</programlisting>
            </example>
        </para>
    </section>
</chapter>