What does an underscore concatenated to a class name mean?

Question

I was reading the "Dynamic, typesafe queries in JPA 2.0" article and stumbled upon this example:

EntityManager em = ... CriteriaBuilder qb = em.getCriteriaBuilder(); CriteriaQuery<Person> c = qb.createQuery(Person.class); Root<Person> p = c.from(Person.class); Predicate condition = qb.gt(p.get(Person_.age), 20); //                                     ^^ --- this one c.where(condition); TypedQuery<Person> q = em.createQuery(c);  List<Person> result = q.getResultList(); 

I was wondering, what exactly does the underscore here mean?

Since an underscore it is a valid part of a classname I don't understand why this can be used in JPA. I checked this with an existing entity in my code and of course my class couldn't be resolved as ClassName_

Answer 1

That is the metamodel for the persistance. It is how you can do type safe JPA queries in Java. It allows queries to staticly check your queries because classBar_ describes your JPA Bar. In HQL, you can easily mistype a query and not know it until it is run.

So technically, the _ does not mean anything, but it is the convention used by JPA to name a metamodel class of a JPA persistent model class. Model_ is the metamodel of Model, and it provides the names of the queryable fields and their types.

Answer 2

I found this way to declare the metamodel in this article.

/**  * A  meta model class used to create type safe queries from person  * information.  * @author Petri Kainulainen  */ @StaticMetamodel(Person.class) public class Person_ {     public static volatile SingularAttribute<Person, String> lastName; }