CDI (Contexts and Dependency Injection) is a fairly rich subject and beyond the scope of this Guide. For more information, see JSR 365: Contexts and Dependency Injection for JavaTM 2.0 (the specification), Java EE 7 Tutorial, or WELD - CDI Reference Implementation.

The fundamental thing to know about CDI injection is that annotating a variable with javax.inject.Inject 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

   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;
      }
   }

The CDI runtime will create an instance of BookImpl and assign it to the private field book when an instance of Author is created;

In this example, the injection is done because BookImpl is assignable to book, but greater discrimination can be imposed by annotating the interface and the field with qualifier annotations. For the injection to be legal, every qualifier on the field must be present on the injected interface. For example:

   @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;
   }

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

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

   @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);
   }

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

            <fqn of TestDataBase>/mp-rest/url=<URL>

For example,

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

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