JPA/Hibernate Static Metamodel Attributes not Populated -- NullPointerException

I had the same issue and it was fixed by putting the Model and Model_ class into the same package.

2019-04-24

The usual issue for unpopulated metamodel class attributes, is when the metamodel classes are in a different package than the corresponding managed classes.

The latest, JPA 2.2 specification still requires to have your metamodel classes in the same package as your corresponding managed classes.

Reference: Page 238, §6.2.1.1 Canonical Metamodel

FYI, I encountered a case where Hibernate creates a metamodel attribute but never initializes it, leading to a NullPointerException's when trying to use it.

public class Upper {
    public String getLabel() { return this.label; }
    public void setLabel(String label) { this.label = label; }
}

public class Lower extends Upper {
   @Override
   public String getLabel() { return super.getLabel(); }
}

Hibernate generate a label attribute declaration in both classes:

@Generated(value = "org.hibernate.jpamodelgen.JPAMetaModelEntityProcessor")
@StaticMetamodel(Upper.class)
public abstract class Upper_ {
    public static volatile SingularAttribute<Upper, String> label;
}

@Generated(value = "org.hibernate.jpamodelgen.JPAMetaModelEntityProcessor")
@StaticMetamodel(Lower.class)
public abstract class Lower_ {
    public static volatile SingularAttribute<Lower, String> label;
}

...and it will initialize Upper_.label but leave Lower_.label equal to null.

Boom.

The class and the metaModel should be in the same package, i.e.

Folder entities:

• Eje
• Eje_
• Element
• Element_

I attached one example of the metamodel code

import javax.annotation.Generated;
import javax.persistence.metamodel.SetAttribute;
import javax.persistence.metamodel.SingularAttribute;
import javax.persistence.metamodel.StaticMetamodel;
import java.util.Date;

@Generated(value = "org.hibernate.jpamodelgen.JPAMetaModelEntityProcessor")
@StaticMetamodel(Eje.class)
public abstract class Eje_ {
    public static volatile SingularAttribute<Eje, Integer> id;
    public static volatile SingularAttribute<Eje, String> name;
    public static volatile SingularAttribute<Eje, Integer> users;
    public static volatile SingularAttribute<Eje, Date> createdAt;
    public static volatile SingularAttribute<Eje, Date> updatedAt;
    public static volatile SetAttribute<Eje, FactorCritico> factorCriticos;
}

If nothing of above resolve this NPE issue, you also can check whether you are using List in your Entities relationships instead of Set.

I found out that using List's it is needed to declare ListAttribute instead of SetAttribute in the metamodel, otherwise, it provoke a NullPointerException and if you don't see the whole stack trace you will not notice that the metamodel was not initialized by your JPA specification.

Debbie's answer got me half of the way there.

There is another "name matching" voodoo gotcha.

Short version:

The "names" have to match between Model and the MetaModel for the "properties".

Longer version:

I was using non-easy-peezy names.

First the entities:

import javax.persistence.CascadeType;
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.FetchType;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.OneToMany;
import javax.persistence.Table;
import java.io.Serializable;
import java.time.OffsetDateTime;
import java.util.LinkedHashSet;
import java.util.Set;

@Entity
@Table(name = "DepartmentTable")
public class DepartmentJpaEntity implements Serializable {

    @Id
    @Column(name = "DepartmentKey", unique = true)
    @GeneratedValue(strategy = GenerationType.AUTO)
    private long departmentKey;
    @Column(name = "DepartmentName", unique = true)
    private String departmentName;
    @Column(name = "CreateOffsetDateTime", columnDefinition = "TIMESTAMP WITH TIME ZONE")
    private OffsetDateTime createOffsetDateTime;
    @OneToMany(
            mappedBy = "parentDepartmentJpaEntity",
            cascade = CascadeType.PERSIST,
            orphanRemoval = true,
            fetch = FetchType.LAZY /* Lazy or Eager here */
    )

    private Set<EmployeeJpaEntity> employeeJpaEntities = new LinkedHashSet<>();

}

and

import javax.persistence.CascadeType;
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.FetchType;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.JoinColumn;
import javax.persistence.ManyToOne;
import javax.persistence.Table;
import java.io.Serializable;
import java.time.OffsetDateTime;

@Entity
@Table(name = "EmployeeTable")
public class EmployeeJpaEntity implements Serializable {

    @Id
    @Column(name = "EmployeeKey", unique = true)
    @GeneratedValue(strategy = GenerationType.AUTO)
    private long employeeKey;

    @Column(name = "Ssn")
    private String ssn;

    @Column(name = "LastName")
    private String lastName;

    @Column(name = "FirstName")
    private String firstName;

    @Column(name = "CreateOffsetDateTime", columnDefinition = "TIMESTAMP WITH TIME ZONE")
    private OffsetDateTime createOffsetDateTime;

    //region Navigation
    
    @ManyToOne(fetch = FetchType.LAZY, targetEntity = DepartmentJpaEntity.class, cascade = CascadeType.PERSIST )
    @JoinColumn(name = "DepartmentForeignKey")
        private DepartmentJpaEntity parentDepartmentJpaEntity;
    //endregion

}

Note, my somewhat not default names. pay attention to the OnetoMany and ManyToOne object names.

Now, my meta-models:

import javax.persistence.metamodel.SetAttribute;
import javax.persistence.metamodel.SingularAttribute;

@javax.persistence.metamodel.StaticMetamodel(DepartmentJpaEntity.class)

public class DepartmentJpaEntity_ {
  public static volatile SingularAttribute<DepartmentJpaEntity, Long> departmentKey;
  public static volatile SetAttribute<DepartmentJpaEntity, EmployeeJpaEntity> employees; /* DOES NOT WORK :(  */
}

but this (below) will work, because I'm magic voodoo'ing the same names:

import javax.persistence.metamodel.SetAttribute;
import javax.persistence.metamodel.SingularAttribute;

@javax.persistence.metamodel.StaticMetamodel(DepartmentJpaEntity.class)

public class DepartmentJpaEntity_ {
  public static volatile SingularAttribute<DepartmentJpaEntity, Long> departmentKey;
  public static volatile SetAttribute<DepartmentJpaEntity, EmployeeJpaEntity> employeeJpaEntities;
}

and the other one:

import javax.persistence.metamodel.SingularAttribute;

@javax.persistence.metamodel.StaticMetamodel(EmployeeJpaEntity.class)

public class EmployeeJpaEntity_ {
  public static volatile SingularAttribute<EmployeeJpaEntity, Long> employeeKey;
  public static volatile SingularAttribute<EmployeeJpaEntity, DepartmentJpaEntity> parentDepartment;  /*does NOT work...its null at run time...no voodoo name matching */
}

But this does work:

import javax.persistence.metamodel.SingularAttribute;

@javax.persistence.metamodel.StaticMetamodel(EmployeeJpaEntity.class)

public class EmployeeJpaEntity_ {
  public static volatile SingularAttribute<EmployeeJpaEntity, Long> employeeKey;
  public static volatile SingularAttribute<EmployeeJpaEntity, DepartmentJpaEntity> parentDepartmentJpaEntity;
}

One of the reasons I purposely use non "standard" names for things is to flush out these voodoo setup issues at the beginning......in order to try and fail early...vs a failLater (hopefully in QA, not in production) issue(s) because someone renamed a property which seems benign at the time.