Improving GET request performance in JAX-RS by using the ETag header

BooksResource_2.java

@Path("books")
@Stateless
public class BooksResource_2{
  
  @PersistenceContext
  EntityManager em;
  
  @Context
  Request request;
  
  @Path("{id}")
  @GET
  @Produces("application/json")
  public Response getById(@PathParam("id") String id){
    Book book = em.find(Book.class, id); //get book info from backend DB
    String uniqueHashForBook = uniqueHashForBook(book); //calculate tag value based on your custom implementation
    EntityTag etag = new EntityTag(uniqueHashForBook)  //instantiate the object
    
    ResponseBuilder evaluationResultBuilder = request.evaluatePreconditions(etag); //let JAX-RS do the math!
    
    if(evaluationResultBuilder == null){
      evaluationResultBuilder = Response.ok(book); //resource was modified, send latest info (and HTTP 200 status)
    }else{
      System.out.println("Resource not modified - HTTP 304 status");
    }
    CacheControl caching = ...; //decide caching semantics
    evaluationResultBuilder.cacheControl(caching)
                           .tag(etag); //add metadata

    return evaluationResultBuilder.build();
  }
}