Required @QueryParam in JAX-RS (and what to do in their absence)

Good question. Unfortunately (or maybe fortunately) there is no mechanism in JAX-RS to make any params mandatory. If a parameter is not supplied it’s value will be NULL and your resource should deal with it accordingly. I would recommend to use WebApplicationException to inform your users:

@GET
@Path("/some-path")
public String read(@QueryParam("name") String name) {
  if (name == null) {
    throw new WebApplicationException(
      Response.status(Response.Status.BAD_REQUEST)
        .entity("name parameter is mandatory")
        .build()
    );
  }
  // continue with a normal flow
}