try {
   EntityA entityA = buildEntityA(criteria);
   persistEntity(entityA);
}
catch(Throwable th) {
  logger.error("", th);
}

private EntityA buildEntityA(criteria) {
  EntityA entityA = new EntityA();

  EntityB entityB = resolveEntityB(criteria);

  entityA.setEntityB(entityB);  // <-- location of when the error is thrown 
}

@Entity
public class EntityA implements Serializable {

  private Long id;
  private EntityB entityB;

... accessors/mutators
}

@Entity
public class EntityB implements Serializable {

  private Long id;

... accessors/mutators
}