JPA returns empty objects

Question

My User model

@Entity
@Table(name="users")
data class User(
        @Id @GeneratedValue(strategy = GenerationType.IDENTITY)
        val id: Long = -1,
        @Column(unique=true)
        val username: String) : Serializable

Two records in the database

The query is fine.

[2018-02-23T14:32:07.066+0100] [Payara 4.1] [FINE] [] [org.eclipse.persistence.session./file:/Users/youri/Downloads/payara41/glassfish/domains/domain1/applications/Kwetter-1.0-SNAPSHOT/WEB-INF/classes/_kwetter.sql] [tid: _ThreadID=28 _ThreadName=http-thread-pool::http-listener-1(4)] [timeMillis: 1519392727066] [levelValue: 500] [[ SELECT ID, USERNAME FROM users]]

But it outputs two empty objects, instead of two User objects

[{},{}]

Abstract Dao where I use the Entity Manager

@Stateless
abstract class Abstract<T : Serializable> {
    @PersistenceContext
    private lateinit var entityManager: EntityManager

    abstract fun getEntityClass(): Class<T>

    open fun find(id: Long): T {
        return entityManager.find(getEntityClass(), id)
    }

    // this returns the weird two empty objects
    open fun all(): List<T> {
        val builder = entityManager.criteriaBuilder
        val c = builder.createQuery(getEntityClass())
        c.from(getEntityClass())
        val query = entityManager.createQuery(c)

        return query.resultList
    }
}

It should print two serialized User objects, but it prints two empty objects... — yooouuri
Yes, but string representations are not the most reliable debugging method. What is the class of those elements? i.e. what is the result of item::class? They don't seem to be of type User. — Salem
You defined the attributes as val but they must be mutable. Use var instead of val — Simon Martinelli

Murf Murf · Accepted Answer · 2018-02-23T14:23:52

You should use var instead of val since Kotlin won't make a setter for val fields. JPA needs to have mutable properties (which means getters & setters).

TLDR: use var instead of val

JPA returns empty objects

1 Answers