I am using Hibernate as the ORM for a database that has a number of foreign key relationships. The problem is that sometimes I want to fetch these related datasets and sometimes I do not, so on these collections I have set “fetch” to “lazy”. Unfortunately, every time I try to serialize these objects Hibernate will throw a LazyInitializationException, because the session is closed. Using an OpenSessionInView filter simply causes Hibernate to populate these collections anyway, thus defeating the whole purpose of having a lazy collection in the first place.
Is there a simple way to serialize or otherwise extract the data populated in the POJO without triggering the LIE, and without having to populate all of the lazy collections?
EDIT: Here is some example code I am trying to get working, dealing with two tables, “Departments” and “Employees,” which is the child in a one-to-many relationship with Departments. I want to be able to view the Departments listed in the database, without having to load all of the Employees that belong to said Departments:
Departments:
package com.test.model;
// Generated Apr 7, 2012 7:10:28 PM by Hibernate Tools 3.4.0.CR1
import java.util.HashSet;
import java.util.Set;
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.FetchType;
import javax.persistence.GeneratedValue;
import static javax.persistence.GenerationType.IDENTITY;
import javax.persistence.Id;
import javax.persistence.OneToMany;
import javax.persistence.Table;
/**
* Departments generated by hbm2java
*/
@Entity
@Table(name="Departments"
,catalog="test"
)
public class Departments implements java.io.Serializable {
private Integer id;
private String name;
private Set<Employees> employeeses = new HashSet(0);
public Departments() {
}
public Departments(String name) {
this.name = name;
}
public Departments(String name, Set employeeses) {
this.name = name;
this.employeeses = employeeses;
}
@Id @GeneratedValue(strategy=IDENTITY)
@Column(name="Id", unique=true, nullable=false)
public Integer getId() {
return this.id;
}
public void setId(Integer id) {
this.id = id;
}
@Column(name="Name", nullable=false)
public String getName() {
return this.name;
}
public void setName(String name) {
this.name = name;
}
@OneToMany(fetch=FetchType.LAZY, mappedBy="departments")
public Set<Employees> getEmployeeses() {
return this.employeeses;
}
public void setEmployeeses(Set employeeses) {
this.employeeses = employeeses;
}
}
Employees:
package com.test.model;
// Generated Apr 7, 2012 7:10:28 PM by Hibernate Tools 3.4.0.CR1
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.FetchType;
import javax.persistence.GeneratedValue;
import static javax.persistence.GenerationType.IDENTITY;
import javax.persistence.Id;
import javax.persistence.JoinColumn;
import javax.persistence.ManyToOne;
import javax.persistence.Table;
/**
* Employees generated by hbm2java
*/
@Entity
@Table(name="Employees"
,catalog="test"
)
public class Employees implements java.io.Serializable {
private Integer id;
private Departments departments;
private String firstName;
private String lastName;
public Employees() {
}
public Employees(Departments departments, String firstName, String lastName) {
this.departments = departments;
this.firstName = firstName;
this.lastName = lastName;
}
@Id @GeneratedValue(strategy=IDENTITY)
@Column(name="Id", unique=true, nullable=false)
public Integer getId() {
return this.id;
}
public void setId(Integer id) {
this.id = id;
}
@ManyToOne(fetch=FetchType.LAZY)
@JoinColumn(name="DepartmentsId", nullable=false)
public Departments getDepartments() {
return this.departments;
}
public void setDepartments(Departments departments) {
this.departments = departments;
}
@Column(name="FirstName", nullable=false)
public String getFirstName() {
return this.firstName;
}
public void setFirstName(String firstName) {
this.firstName = firstName;
}
@Column(name="LastName", nullable=false)
public String getLastName() {
return this.lastName;
}
public void setLastName(String lastName) {
this.lastName = lastName;
}
}
My action class (which gets serialized by the Struts2 XSLT result):
package com.test.view;
import java.util.List;
import java.util.Iterator;
import com.opensymphony.xwork2.ActionSupport;
import com.test.controller.DepartmentsManager;
import com.test.model.Departments;
import com.test.util.HibernateUtil;
public class DepartmentsAction extends ActionSupport {
private DepartmentsManager departmentsManager;
private List<Departments> departmentsList;
public DepartmentsAction() {
this.departmentsManager = new DepartmentsManager();
}
public String list() {
this.departmentsList = departmentsManager.list();
System.out.println("Execute called");
HibernateUtil.createDTO(departmentsList);
return SUCCESS;
}
public List<Departments> getDepartmentsList() {
return departmentsList;
}
public void setDepartmentsList(List<Departments> departmentsList) {
this.departmentsList = departmentsList;
}
}
My Manager class (which the Action class calls to populate the list of Departments):
package com.test.controller;
import java.util.List;
import java.util.Iterator;
import org.hibernate.Criteria;
import org.hibernate.Hibernate;
import org.hibernate.HibernateException;
import org.hibernate.Query;
import org.hibernate.Session;
import com.test.model.Departments;
import com.test.util.HibernateUtil;
public class DepartmentsManager {
public List<Departments> list() {
Session session = HibernateUtil.getSessionFactory().getCurrentSession();
session.beginTransaction();
List<Departments> set = null;
try {
Query q = session.createQuery("FROM Departments");
/*Query q = session.createQuery("FROM Departments d JOIN FETCH d.employeeses e");*/
q.setResultTransformer(Criteria.DISTINCT_ROOT_ENTITY);
set = (List<Departments>) q.list();
} catch (HibernateException e) {
e.printStackTrace();
session.getTransaction().rollback();
}
session.getTransaction().commit();
return set;
}
}
Lazy collections work only within the scope of the transaction (where the owning entity was retrieved from a DB). In other words, you should not pass a Hibernate entity with non-loaded lazy sub-entities or collections outside the transaction scope.
You need either to build another entity or use lazy=”false” if you want to pass an entity to JSP, or serialization code or anything else.