How to do a Inner-Join in JPA Criteria? - sql

Question

I'm using Netbeans to program a webservice REST that returns a JSON response, I am also using JPA Criteria to create the querys. I have two Entities that looks like this:
public class User implements Serializable {
private static final long serialVersionUID = 1L;
#Id
#Basic(optional = false)
#NotNull
#Column(name = "id_user")
private Integer idUser;
#Size(max = 45)
#Column(name = "username")
private String username;
#Size(max = 45)
#Column(name = "password")
private String password;
#Size(max = 45)
#Column(name = "email")
private String email;
#OneToMany(mappedBy = "idUser")
private Collection<Comment> commentCollection;
public class Comment implements Serializable {
private static final long serialVersionUID = 1L;
#Id
#Basic(optional = false)
#NotNull
#Column(name = "id_comment")
private Integer idComment;
#Column(name = "id_thesis")
private Integer idThesis;
#Size(max = 250)
#Column(name = "comment")
private String comment;
#Column(name = "cdate")
#Temporal(TemporalType.DATE)
private Date cdate;
#JoinColumn(name = "id_user", referencedColumnName = "id_user")
#ManyToOne
private User idUser;
}
Both entities with sets and gets. I want to do a query like this:
SELECT * FROM Comments c INNER JOIN User u WHERE c.id_user = u.id_user;
but in the JPA Criteria language, I've had many problems trying to make it work but i don't get it yet.
This is the code that I'm using for the join
AbstractFacade.java
public Join<User, Comment> getCommentInfo() {
CriteriaBuilder cb = getEntityManager().getCriteriaBuilder();
CriteriaQuery q = cb.createQuery();
Root<User> r = q.from(User.class);
Join<User, Comment> j = r.join("commentCollection", JoinType.INNER);
Query query = getEntityManager().createQuery(q);
return (Join<User, Comment>) query.getResultList();
}
UserFacadeREST.java
#GET
#Path("test")
#Produces({"application/json"})
public Join<User, Comment> getCommentInfoREST() {
return getCommentInfo();
}
This error is shown when I test the method:
java.util.Vector cannot be cast to javax.persistence.criteria.Join
Please help me with that, I do not know if the sentence join is wrong or how to solve the cast properly.
Edit: I add the next lines to the getCommentInfo() method to see the content of the list.
q.select(j.get("username"));
List results = query.getResultList();
Iterator iter = results.iterator();
while (iter.hasNext()){
System.out.println(iter.next());
}
Error: The attribute [username] is not present in the managed type [EntityTypeImpl#1000979996:Comment.

Answer 1

In the case of getResultList(), the javadocs state that it returns an java.util.List (see here: http://docs.oracle.com/javaee/5/api/javax/persistence/Query.html#getResultList%28%29 ), that Vector implements.
The result type, aka what's in the list, depends on the criteria projection or, in a JPQL Query, of the from statement.
In your case, because you don't do projection, I think it should return a List<User>.
For your information, and if you are using JPA 2.0, you can also use TypedQuery which could avoid that (ugly !) cast : http://www.javabeat.net/typedquery-jpa-2/