The org.jboss.resteasy.spi.ContextInjector interface must be implemented on any custom asynch injector object. The implementation class must be tagged with the @Provider annotation.

Notice the names of the interface's generic type parameters. Designators WrappedType, UnwrappedType are used to indicate that java primitives are not to be used as type parameters.

public interface ContextInjector<WrappedType, UnwrappedType> {
/**
 * This interface allows users to create custom injectable asynchronous types.
 *
 * Asynch injection is only attempted at points where asynchronous injection is
 * permitted, such as on resource creation and resource method invocation. It
 * is not enabled at points where the API does not allow for suspending the
 * request
 *
 * The rawType and genericType can not be java primitives.  If the primitive is
 * desired its wrapped type may be used.
 *
 * @param rawType
 * @param genericType
 * @param annotations The annotation list is useful to parametrize the injection.
 * @return
 */
 public WrappedType resolve(
            Class<? extends WrappedType> rawType,
            Type genericType,
            Annotation[] annotations);
  }
        

package my.test;

public class Foo {
   private String value = "PRE-SET-VALUE";

   public void setValue(String s) {
      this.value = s;
   }

   public String getValue() {
      return this.value;
   }
}
        

package my.test.asyc.resources;

import io.reactivex.Single;
import javax.ws.rs.ext.Provider;
import org.jboss.resteasy.spi.ContextInjector;
import my.test.Foo;

@Provider
public class FooAsychInjectorProvider implements
            ContextInjector<Single<Foo>,Foo> {

   public Single<Foo> resolve(Class<? extends Single<Foo>> rawType,
            Type genericType,
            Annotation[] annotations)
   {
      Foo value = new Foo();
      return Single.just(value.setValue("made it"));
   }
}
        

A convenience interface to provide annotation parameter designators

@Retention(RUNTIME)
@Target({ FIELD, METHOD, PARAMETER })
public @interface AsyncInjectionPrimitiveInjectorSpecifier
{
   public enum Type {
      VALUE, NULL, NO_RESULT;
   }

   Type value() default Type.VALUE;
}
    

@Provider
public class AsyncInjectionFloatInjector implements
            ContextInjector<CompletionStage<Float>, Float>
{

   @Override
   public CompletionStage<Float> resolve(
      Class<? extends CompletionStage<Float>> rawType,
            Type genericType,
            Annotation[] annotations)
    {
       for (Annotation annotation : annotations)
       {
           if(annotation.annotationType() ==
              AsyncInjectionPrimitiveInjectorSpecifier.class) {
             AsyncInjectionPrimitiveInjectorSpecifier.Type value =
               ((AsyncInjectionPrimitiveInjectorSpecifier)annotation).value();
             switch(value) {
               case NO_RESULT:
                  return null;
               case NULL:
                  return CompletableFuture.completedFuture(null);
               case VALUE:
                  return CompletableFuture.completedFuture(4.2f);
            }
            break;
          }
       }
       return CompletableFuture.completedFuture(4.2f);
    }
}