<chapter id="MicroProfile_Rest_Client">
<title>MicroProfile Rest Client</title>

<para>
As the microservices style of system architecture (see, for example,
<ulink url="https://martinfowler.com/articles/microservices.html">Microservices</ulink> by Martin Fowler)
gains increasing traction, new API standards are coming along to support it. One set of such standards comes from
the <ulink url="https://microprofile.io/">Microprofile Project</ulink> supported by the Eclipse Foundation, and among 
those is one, <ulink url="https://microprofile.io/project/eclipse/microprofile-rest-client">MicroProfile Rest Client</ulink>,
of particular interest to RESTEasy and JAX-RS. In fact, it is intended to be based on, and consistent with, JAX-RS,
and it includes ideas already implemented in RESTEasy. For a more detailed description of MicroProfile Rest Client,
see <ulink url="https://github.com/eclipse/microprofile-rest-client">https://github.com/eclipse/microprofile-rest-client</ulink>.
In particular, the API code is in 
<ulink url="https://github.com/eclipse/microprofile-rest-client/tree/master/api">https://github.com/eclipse/microprofile-rest-client/tree/master/api</ulink>.
and the specification is in
<ulink url="https://github.com/eclipse/microprofile-rest-client/tree/master/spec">https://github.com/eclipse/microprofile-rest-client/tree/master/spec</ulink>.
</para>

<sect1>
<title>Client proxies</title>

<para>
One of the central ideas in MicroProfile Rest Client is a version of <emphasis>distributed object communication</emphasis>, a concept
implemented in, among other places, <ulink url="http://www.corba.org/orb_basics.htm">CORBA</ulink>, Java RMI, the JBoss
Remoting project, and RESTEasy. Consider the resource
</para>

<programlisting>
@Path("resource")
public class TestResource {

   @Path("test")
   @GET
   String test() {
      return "test";
   }
}
</programlisting>

<para>
The JAX-RS native way of accessing <classname>TestResource</classname> looks like
</para>

<programlisting>
Client client = ClientBuilder.newClient();
String response = client.target("http://localhost:8081/test").request().get(String.class);
</programlisting>

<para>
The call to <methodname>TestResource.test()</methodname> is not particularly onerous, but calling 
<methodname>test()</methodname> directly allows a more natural syntax. That is exactly what Microprofile
Rest Client supports:
</para>

<programlisting>
@Path("resource")
public interface TestResourceIntf {

   @Path("test")
   @GET
   public String test();
}
   
TestResourceIntf service = MicroprofileClientBuilderResolver.instance()
                              .newBuilder()
                              .baseUrl(http://localhost:8081/))
                              .build(TestResourceIntf.class);
String s = service.test();
</programlisting>

<para>
The first four lines of executable code are spent creating a proxy, <code>service</code>, that implements
<classname>TestResourceIntf</classname>, but once that is done, calls on <classname>TestResource</classname>
can be made very naturally in terms of <classname>TestResourceIntf</classname>, as illustrated by the call
<code>service.test()</code>. 
</para>

<para>
Beyond the natural syntax, another advantage of proxies is the way the proxy construction process quietly
gathers useful information from the implemented interface and makes it available for remote invocations.
Consider a more elaborate version of <classname>TestResourceIntf</classname>:
</para>

<programlisting>
@Path("resource")
public interface TestResourceIntf2 {

   @Path("test/{path}")
   @Consumes("text/plain")
   @Produces("text/html")
   @POST
   public String test(@PathParam("path") String path, @QueryParam("query") String query, String entity);
}
</programlisting>

<para>
Calling <methodname>service.test("p", "q", "e")</methodname> results in an HTTP message that looks like
</para>

<programlisting>
POST /resource/test/p/?query=q HTTP/1.1
Accept: text/html
Content-Type: text/plain
Content-Length: 1

e
</programlisting>

<para>
The HTTP verb is derived from the <code>@POST</code> annotation, the request URI is derived from the
two instances of the <classname>@Path</classname> annotation (one on the class, one on the method) plus the
first and second parameters of <methodname>test()</methodname>, the Accept header is derived from the
<classname>@Produces</classname> annotation, and the Content-Type header is derived from the
<classname>@Consumes</classname> annotation, 
</para>

<para>
Using the JAX-RS API, <code>service.test("p", "q", "e")</code> would look like the more verbose
</para>

<programlisting>
Client client = ClientBuilder.newClient();
String response = client.target("http://localhost:8081/resource/test/p")
                     .queryParam("query", "q")
                     .request()
                     .accept("text/html")
                     .post(Entity.entity("e", "text/plain"), String.class);
</programlisting>

<para>
One other basic facility offered by MicroProfile Rest Client is the ability to configure the client environment
by registering providers:
</para>

<programlisting id="listing1">
TestResourceIntf service = MicroprofileClientBuilderResolver.instance()
                              .newBuilder()
                              .baseUrl(http://localhost:8081/))
                              .register(MyClientResponseFilter.class)
                              .register(MyMessageBodyReader.class)
                              .build(TestResourceIntf.class);
</programlisting>

<para>
Naturally, the registered providers should be relevant to the client environment, rather than, say, a
<classname>ContainerResponseFilter</classname>.
</para>

<note>
<para>
So far, the MicroProfile Rest Client should look familiar to anyone who has used the RESTEasy client proxy facility
(<link linkend='proxies'>Section ""RESTEasy Proxy Framework"</link>). The construction in 
the previous listing would look like
</para>

<programlisting>
ResteasyClient client = (ResteasyClient) ResteasyClientBuilder.newClient();
TestResourceIntf service = client.target("http://localhost:8081/")
                              .register(MyClientResponseFilter.class)
                              .register(MyMessageBodyReader.class)
                              .proxy(TestResourceIntf.class);
</programlisting>

<para>
in RESTEasy.
</para>
</note>
</sect1>

<sect1>
<title>Beyond RESTEasy</title>

<para>
There are a few concepts in MicroProfile Rest Client that do not appear in RESTEasy.
</para>

<sect1>
<title>Declarative registration of providers</title>

<para>
In addition to programmatic registration of providers as illustrated above, it is also possible to
register providers declaratively with annotations introduced in MicroProfile Rest Client. In particular,
providers can be registered by adding the <classname>org.eclipse.microprofile.rest.client.annotation.RegisterProvider</classname>
annotation to the target interface:
</para>

<programlisting>
@Path("resource")
@RegisterProvider(MyClientResponseFilter.class)
@RegisterProvider(MyMessageBodyReader.class)
public interface TestResourceIntf2 {

   @Path("test/{path}")
   @Consumes("text/plain")
   @Produces("text/html")
   @POST
   public String test(@PathParam("path") String path, @QueryParam("query") String query, String entity);
}
</programlisting>

<para>
Declaring <classname>MyClientResponseFilter</classname> and <classname>MyMessageBodyReader</classname>  with
annotations eliminates the need to call <methodname>RestClientBuilder.register()</methodname>.
</para>
</sect1>

<sect1>
<title>ResponseExceptionMapper</title>

<para>
The <classname>org.eclipse.microprofile.rest.client.ext.ResponseExceptionMapper</classname> is the
client side inverse of the <classname>javax.ws.rs.ext.ExceptionMapper</classname> defined in JAX-RS. That is,
where <methodname>ExceptionMapper.toResponse()</methodname> turns an <classname>Exception</classname> thrown
during server side processing into a <classname>Response</classname>,
<methodname>ResponseExceptionMapper.toThrowable()</methodname> turns a 
<classname>Response</classname> received on the client side with an HTTP error status into
an <classname>Exception</classname>. <classname>ResponseExceptionMapper</classname>s can be registered
in the same manner as other providers, that is, either programmatically or declaratively. In the absence
of a registered <classname>ResponseExceptionMapper</classname>, a default <classname>ResponseExceptionMapper</classname>
will map any response with status >= 400 to a <classname>WebApplicationException</classname>.
</para>

</sect1>

<sect1>
<title>Proxy injection by CDI</title>

<para>
MicroProfile Rest Client mandates that implementations must support CDI injection of proxies. At first, the
concept might seem odd in that CDI is more commonly available on the server side. However, the idea is very
consistent with the microservices philosophy. If an application is composed of a number of small services,
then it is to be expected that services will often act as clients to other services.
</para>

<para>
CDI (Contexts and Dependency Injection) is a fairly rich subject and beyond the scope of this Guide. For more information, see 
<ulink url="https://www.jcp.org/en/jsr/detail?id=365">JSR 365: Contexts and Dependency Injection for JavaTM 2.0</ulink> 
(the specification), 
<ulink url="https://docs.oracle.com/javaee/7/tutorial/cdi-basic.htm">Java EE 7 Tutorial</ulink>, or
<ulink url="https://docs.jboss.org/weld/reference/latest-master/en-US/html/">WELD - CDI Reference Implementation</ulink>.
</para>

<para>
The fundamental thing to know about CDI injection is that annotating a variable with 
<classname>javax.inject.Inject</classname> will lead the CDI runtime (if it is present and enabled) to
create an object of the appropriate type and assign it to the variable. For example, in
</para>

<programlisting>
   public interface Book {
      public String getTitle();
      public void setTitle(String title);
   }

   public class BookImpl implements Book {
      
      private String title;

      @Override
      public String getTitle() {
         return title;
      }
      
      @Override
      public void setTitle(String title) {
         this.title = title;
      }
   }
   
   public class Author {
      
      @Inject private Book book; 
      
      public Book getBook() {
         return book;
      }
   }
</programlisting>

<para>
The CDI runtime will create an instance of <classname>BookImpl</classname> and assign it to the private field
<code>book</code> when an instance of <classname>Author</classname> is created;
</para>

<para>
In this example, the injection is done because <classname>BookImpl</classname> is assignable to <code>book</code>, but
greater discrimination can be imposed by annotating the interface and the field with <emphasis role="bold">qualifier</emphasis>
annotations. For the injection to be legal, every qualifier on the field must be present on the injected interface.
For example:</para>

<programlisting>
   @Qualifier
   @Target({ElementType.TYPE, ElementType.METHOD, ElementType.PARAMETER, ElementType.FIELD})
   @Retention(RetentionPolicy.RUNTIME)
   public @interface Text {}
   
   @Qualifier
   @Target({ElementType.TYPE, ElementType.METHOD, ElementType.PARAMETER, ElementType.FIELD})
   @Retention(RetentionPolicy.RUNTIME)
   public @interface Graphic {}
   
   @Text
   public class TextBookImpl extends BookImpl { }
   
   @Graphic
   public class GraphicNovelImpl extends BookImpl { }
   
   public class Genius {
      
      @Inject @Graphic Book book;
   }

</programlisting>

<para>
Here, the class <classname>TextBookImpl</classname> is annotated with the <classname>@Text</classname> qualifier and
<classname>GraphicNovelImpl</classname> is annotated with <classname>@Graphic</classname>. It follows that an instance
of <classname>GraphicNovelImpl</classname> is eligible for assignment to the field <code>book</code> in the 
<classname>Genius</classname> class, but an instance of <classname>TextBookImpl</classname> is not.
</para>

<para>
Now, in MicroProfile Rest Client, any interface that is to be managed as a CDI bean must be annotated with
<classname>@RegisterRestClient</classname>:
</para>

<programlisting>
   @Path("resource")
   @RegisterProvider(MyClientResponseFilter.class)
   public static class TestResourceImpl {

      @Inject TestDataBase db;
      
      @Path("test/{path}")
      @Consumes("text/plain")
      @Produces("text/html")
      @POST
      public String test(@PathParam("path") String path, @QueryParam("query") String query, String entity) {
         return db.getByName(query);
      }
   }

   @Path("database")
   @RegisterRestClient
   public interface TestDataBase {
      
      @Path("")
      @POST
      public String getByName(String name);
   }
</programlisting>

<para>
Here, the MicroProfile Rest Client implementation creates a proxy for a <classname>TestDataBase</classname> service,
allowing easy access by <classname>TestResourceImpl</classname>. Notice, though, that there's no indication of
where the <classname>TestDataBase</classname> implementation lives. That information can be supplied externally with
the system variable
</para>

<programlisting>
            &lt;fqn of TestDataBase&gt;/mp-rest/url=&lt;URL&gt;
</programlisting>

<para>
 For example,
</para>

<programlisting>
com.bluemonkeydiamond.TestDatabase/mp-rest/url=https://localhost:8080/webapp
</programlisting>

<para>
indicates that an implementation of <classname>com.bluemonkeydiamond.TestDatabase</classname> can be
accessed at https://localhost:8080/webapp.
</para>

</sect1>

</sect1>
</chapter>