Query about save(), saveorupdate(), update(), lock() and merge() methods

[Vaibhav G Garg]

I am relatively new to Hibernate framework. I have written the following piece of code and changed the method calls to : save(), saveOrUpdate(), update(), lock() and merge() methods. Surprisingly, I am getting the same result by calling each and every method. The DB is updated with the newly set values in the Emp Objects. I am confused about the usage of these methods. I have gone through the Hibernate Community documentation as well but couldn't understand.

SessionTest.java:


import org.hibernate.LockMode;
import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.cfg.Configuration;

/*
 * To change this template, choose Tools | Templates
 * and open the template in the editor.
 */

/**
 *
 * @author vaibhav_garg01
 */
public class SessionTest {

    public static void main(String args[]){

        try{
            Configuration cfg = new Configuration().configure();

            SessionFactory sf = cfg.buildSessionFactory();

            Session ssn = sf.openSession();
            ssn.beginTransaction();

            Emp emp1 = new Emp("Emp1", 1, 10000);

            emp1.setEmpId(11);

            ssn.save(emp1);
            ssn.flush();
            ssn.getTransaction().commit();
            ssn.close();

            Session ssn2 = sf.openSession();
            Emp emp2 = (Emp) ssn2.get(Emp.class, 1);
            emp2.setEmpAge(22);

            Emp emp3 = (Emp) ssn2.get(Emp.class, 1);
            emp3.setEmpName("VaibhavG2");
            
            ssn2.beginTransaction();
            System.out.println("emp2 : " + emp2);
            System.out.println("emp3 : " + emp3);
            //ssn2.merge(emp1);

            ssn2.lock(emp1, LockMode.NONE);

            ssn2.flush();
            ssn2.getTransaction().commit();

            ssn2.close();

        }
        catch(Exception e){
            System.out.println("Exception is : " + e);
        }
    }

}


Emp.java:

/*
 * To change this template, choose Tools | Templates
 * and open the template in the editor.
 */

/**
 *
 * @author vaibhav_garg01
 */
public class Emp {

    private int empId;

    private String empName;

    private int empAge;

    private int empSalary;

    public int getEmpAge() {
        return empAge;
    }

    public void setEmpAge(int empAge) {
        this.empAge = empAge;
    }

    public int getEmpId() {
        return empId;
    }

    public void setEmpId(int empId) {
        this.empId = empId;
    }

    public String getEmpName() {
        return empName;
    }

    public void setEmpName(String empName) {
        this.empName = empName;
    }

    public int getEmpSalary() {
        return empSalary;
    }

    public void setEmpSalary(int empSalary) {
        this.empSalary = empSalary;
    }

    public Emp(String empName, int empAge, int empSalary) {
        this.empName = empName;
        this.empAge = empAge;
        this.empSalary = empSalary;
    }

    public Emp(){}

    @Override
    public String toString() {
        return "Emp{" + "empId=" + empId + "empName=" + empName + "empAge=" + empAge + "empSalary=" + empSalary + '}';
    }

    

}


Emp.hbm.xml:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN" "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<hibernate-mapping>
  <class name="Emp" table="EMP">

      <id name="empId">
          <generator class="increment"/>
      </id>

      <property name="empName" />
      <property name="empSalary" />
      <property name="empAge" />

  </class>
</hibernate-mapping>

Thanks in advance.

[Bill Gorder]

Most of your questions can be answered by reading this thread:

http://stackoverflow.com/questions/161224/what-are-the-differences-between-the-different-saving-methods-in-hibernate

lock(emp1, LockMode.NONE) was used to reassociate a transient instance with a session. If you call update on a detached object it triggers an update whether the object has changed or not. To avoid this lock was used to re associate the instance with a session so this would not happen. Note that lock is now deprecated.

I prefer to just use JPA2

[Vaibhav G Garg]

Thanks Bill for your inputs.

I am trying to grasp the basics of following functions:
1. save()
2. saveOrUpdate()
3. update()
4. merge()
5. lock()

Considering the first method i.e. save() method, please refer the code snippet CODE SNIPPET I below.

I. I inserted some data in the DB with empId = 1 and then get this object in emp1 in session instance ssn1. Now, I called ssn1.save(emp1) but it doesn't save the object again into the db nor is any exception thrown. Now, if I open a new session, and then call ssn2.save(emp1), it inserts a new record into the db. Why it is behaving in this manner?

II. Also, I if I call saveOrUpdate(emp1) method on ssn2, then it doesn't insert a new record. Please explain this behavior.

CODE SNIPPET I

//CODE SNIPPET I
import org.hibernate.LockMode;
import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.cfg.Configuration;

/*
* Code for save() and saveOrUpdate() Methods
 */

/**
 *
 * @author vaibhav_garg01
 */
public class SessionTest {

    public static void main(String args[]){

        try{
            
            Configuration cfg = new Configuration().configure();
            SessionFactory sf = cfg.buildSessionFactory();
            Session ssn1 = sf.openSession();
            ssn1.beginTransaction();
            Emp emp1 = new Emp("Emp1", 12, 1112);
            emp1 = (Emp) ssn1.get(Emp.class, 1);

            if(emp1 != null) emp1.setEmpAge(3);

            //It doesn't save the emp1 to the DB associated with ssn1
            ssn1.save(emp1);
           
            ssn1.flush();
            ssn1.getTransaction().commit();
            ssn1.close();

            Session ssn2 = sf.openSession();

            ssn2.beginTransaction();
            //It saves the emp1 to the DB associated with ssn2
            ssn2.save(emp1);

            //It doesn't save a new record, rather, it updates the same record
            ssn2.saveOrUpdate(emp1);

            ssn2.getTransaction().commit();
            ssn2.close();
        }
        catch(Exception e){
            System.out.println("Exception is : " + e);
            e.printStackTrace();
        }
    }
}

CODE SNIPPET II

import org.hibernate.LockMode;
import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.cfg.Configuration;

/*
 * To change this template, choose Tools | Templates
 * and open the template in the editor.
 */

/**
 *
 * @author vaibhav_garg01
 */
public class SessionTest {

    public static void main(String args[]){

        try{
            
            Configuration cfg = new Configuration().configure();
            SessionFactory sf = cfg.buildSessionFactory();
            Session ssn1 = sf.openSession();
            Emp emp1 = new Emp("Emp1", 12, 1112);
            emp1 = (Emp) ssn1.get(Emp.class, 1);
            if(emp1 != null) emp1.setEmpAge(33);        
            ssn1.close();
            Session ssn2 = sf.openSession();
            ssn2.beginTransaction();
            ssn2.update(emp1);

//Below statements has the same result as ssn2.update(emp1) -> WHY?
//            ssn2.saveOrUpdate(emp1);
//            sssn2.merge(emp1);
            ssn2.getTransaction().commit();
            ssn2.close();
        }
        catch(Exception e){
            System.out.println("Exception is : " + e);
            e.printStackTrace();
        }
    }
}

Considering CODE SNIPPET II, I have the following queries:

saveOrUpdate: It either inserts a new record if one doesn't exist in db or updates the existing one if one with the given id exist in the db. But, how will it behave in case of detached objects?

update : It will reattach the object to the session. Suppose, I get an object say object1 using session1 and then session1 is closed, Now, the object1 is detached. And, I opened a new sessoin session2 and call update with object1 on this session session2, what will happen? What will happen if I call saveOrUpdate in this case rather than update method?

merge: what will happen if I call merge method rather than above two methods in above scenario.


Thanks for your patience and your time. I am really sorry if I am missing on some basics of the topic as I am a learner in this area.

[Bill Gorder]

    //CODE SNIPPET I  
    import org.hibernate.LockMode;  
    import org.hibernate.Session;  
    import org.hibernate.SessionFactory;  
    import org.hibernate.cfg.Configuration;  
      
    /* 
    * Code for save() and saveOrUpdate() Methods 
     */  
      
    /** 
     * 
     * @author vaibhav_garg01 
     */  
    public class SessionTest {  
      
        public static void main(String args[]){  
      
            try{  
                  
                Configuration cfg = new Configuration().configure();  
                SessionFactory sf = cfg.buildSessionFactory();  
                Session ssn1 = sf.openSession();  
                ssn1.beginTransaction(); 
                //This is meaningless since you reassign it from the  database select on the next line
                //Emp emp1 = new Emp("Emp1", 12, 1112);  
                emp1 = (Emp) ssn1.get(Emp.class, 1);  
      
                //here you make a modification to the attached (managed) entity
                if(emp1 != null) emp1.setEmpAge(3);  
      
                //It doesn't save the emp1 to the DB associated with ssn1
                //This does not do anything you are passing it a managed entity
                ssn1.save(emp1);  
                
                //this will flush the changes made which will result in an update.  However commenting out this line
                //will result in the same as on the next line you commit the transaction which will also flush.
                ssn1.flush();  
                ssn1.getTransaction().commit();  
                ssn1.close();  
      
                Session ssn2 = sf.openSession();  
      
                ssn2.beginTransaction();  
                //It saves the emp1 to the DB associated with ssn2
                //Nope actually this is not doing what you think it is.  emp1 is now detached, it thinks you are giving it a new emp and assigning it a new id
                //If emp1 had an id of 1 before this after this line it will have an id of 2 
                ssn2.save(emp1);  
      
                //It doesn't save a new record, rather, it updates the same record
                //actually with the above save() this line will have no effect
                //If you comment out the above save() it will re-attach the emp1 placing it in the persistence context and marking it as dirty.  In this case
                //It will result in an update on the object when the transaction is committed (even though it has not changed, it is marked as dirty when it is re-attached)
                ssn2.saveOrUpdate(emp1);  
                
                //With the save() this will be an insert.  If you comment out the save it will be an update.
                ssn2.getTransaction().commit();  
                ssn2.close();  
            }  
            catch(Exception e){  
                System.out.println("Exception is : " + e);  
                e.printStackTrace();  
            }  
        }  
    }  

In your example update saveOrUpdate and merge behave the same but merge is very different. Please read:
http://docs.jboss.org/hibernate/core/3.6/reference/en-US/html_single/#objectstate-saveorupdate

[Vaibhav G Garg]

Thanks Bill for your patience and time. I have gone through the link provided but still things are not very much clear to me. Can you please update my sample code so that it produces different results for merge, saveOrUpdate and lock methods and I can visualize the difference among these methods. Thanks again for your time.