You don't need HttpServletResponse to set a header on the response. You can do it using javax.ws.rs.core.Response. Just make your method to return Response instead of entity:
return Response.ok(entity).header("Content-Disposition", "attachment; filename=\"" + fileName + "\"").build()
If you still want to use HttpServletResponse you can get it either injected to one of the class fields, or using property, or to method parameter:
@Path("/resource")
class MyResource {
// one way to get HttpServletResponse
@Context
private HttpServletResponse anotherServletResponse;
// another way
Response myMethod(@Context HttpServletResponse servletResponse) {
// ... code
}
}