Hibernate One-to-One Mapping with JPA @OneToOne Examples

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1. Hi Lokesh,
   This article is very good, your way explanation also very good. I studied various articles on One to One and i found total 6 technical ways to define One to One mapping (all information is not defined any where, based on understanding on articles, I defined as below ways)
   1. Unidirectional with Foreign Key
   2. Unidirectional with Shared Primary Key.
   3. Bidirectional with Foreign Key
   4. Bidirectional with Shared Primary Key.
   5. Bidirectional with MapsId
   6. Join Table.

   Requesting could you please clarify my doubt, how many ways can we define One to One mapping ?

2. Thanks for this Article.. I’m having query related to lazy fetching.. how to achieve the same using the above @OneToOne mapping example. Please can you help on that..

3. Wonderful explanation!
   You wrote a great article. Earlier I spent a lot of time studying this topic but I did not understand it. Thank you!

4. User.java
   @OneToMany(fetch = FetchType.LAZY, cascade = CascadeType.ALL, mappedBy="user")
   private Set<Address> userAddresses = new HashSet<Address>();
   
   Address.java
   @ManyToOne(fetch = FetchType.LAZY)
   @JoinColumn(name="UserID")
   private User user;
   

   In OneToMany relation, the many side is the owner of the relation right ? but by specifying mappedBy=user we are saying that User is the owning entity.why?
   Can you please explain the above concept ?

   for bi directional rules check oracle documentation:https://docs.oracle.com/cd/E19798-01/821-1841/bnbqj/index.html

5. Hi. I have a question about @OneToOne(cascade = CascadeType.ALL) when you implement “Using a common join table”. In this case, the cascade is on the side of EmployeeEntity. But I wonder, if the mappedby is in AccountEntity, then EmployeeEntity has a foreign key in AccountEntity, and I expect that removing, for example, a record from AccountEntity, the removal will be cascaded also in EmployeeEntity. So, shouldn’t the cascade be on the side of AccountEntity, where you also have the mapped by? Thanks for these great tutorials.

   Valentino

6. In your “mappedBy” example no foreign key relationship maintained by EmployeeEntity. Even there is no relationship between both table . You can cross verify your sql query above. i.e.

   Hibernate: insert into ACCOUNT (ACC_NUMBER) values (?)
   Hibernate: insert into Employee (ACCOUNT_ID, EMAIL, FIRST_NAME, LAST_NAME) values (?, ?, ?, ?)

   there is no update query for foreign key.

   Please clarify?

   I think @JoinColumn instead of @PrimaryKeyJoinColumn is better choice.

7. Hi,

   I tried foreign key column model, my question is why hibernate creates join table ’employee_acccount’ and have no use in one to one association. can any one clear my doubt?

8. Superb tutorial, was of great help to implement the one to one relationships in my application’s datamodel. Thanks a lot, keep rocking.

9. thanks , good explanation. Can you please modify the same example to add one new column in newly created table.

   • Never tried this. I will try in my free time.

     • thanks but I got How to do it…
       I am pasting my example here…

       1) AccountEntity.java

       package com.hbm;
       
       import java.io.Serializable;
       import java.util.HashSet;
       import java.util.Set;
       
       import javax.persistence.Column;
       import javax.persistence.Entity;
       import javax.persistence.GeneratedValue;
       import javax.persistence.GenerationType;
       import javax.persistence.Id;
       import javax.persistence.OneToMany;
       import javax.persistence.Table;
       import javax.persistence.UniqueConstraint;
       
       
       @Entity(name = "JoinTableAccountEntity")
       @Table(name = "ACCOUNT", uniqueConstraints = {
       @UniqueConstraint(columnNames = "ID")})
       public class AccountEntity implements Serializable
       {
       
          private static final long serialVersionUID = -6790693372846798580L;
       
          @Id
          @GeneratedValue(strategy = GenerationType.IDENTITY)
          @Column(name = "ID", unique = true, nullable = false)
          private Integer accountId;
       
          @Column(name = "ACC_NUMBER", unique = true, nullable = false, length = 100)
          private String accountNumber;
          
          public AccountEntity( String accountNumber) {
       	super();
       	this.accountNumber = accountNumber;
       }
       
       @OneToMany(mappedBy = "accountentity")
          private Set empaccdetail= new HashSet();
       
          public Set getEmpaccdetail() {
       	return empaccdetail;
       }
       
       public void setEmpaccdetail(Set empaccdetail) {
       	this.empaccdetail = empaccdetail;
       }
       
       public Integer getAccountId() {
              return accountId;
          }
       
          public void setAccountId(Integer accountId) {
              this.accountId = accountId;
          }
       
          public String getAccountNumber() {
              return accountNumber;
          }
       
          public void setAccountNumber(String accountNumber) {
              this.accountNumber = accountNumber;
          }
       }
       

       2)EmployeeEntity.java

       package com.hbm;
       import java.io.Serializable;
       import java.util.Set;
       
       import javax.persistence.CascadeType;
       import javax.persistence.Column;
       import javax.persistence.Entity;
       import javax.persistence.GeneratedValue;
       import javax.persistence.GenerationType;
       import javax.persistence.Id;
       import javax.persistence.JoinColumn;
       import javax.persistence.JoinTable;
       import javax.persistence.OneToMany;
       import javax.persistence.Table;
       import javax.persistence.UniqueConstraint;
        
       @Entity
       @Table(name = "Employee", uniqueConstraints = {
       @UniqueConstraint(columnNames = "ID"),
       @UniqueConstraint(columnNames = "EMAIL") })
       public class EmployeeEntity implements Serializable
       {
           private static final long serialVersionUID = -1798070786993154676L;
        
           @Id
           @GeneratedValue(strategy = GenerationType.IDENTITY)
           @Column(name = "ID", unique = true, nullable = false)
           private Integer employeeId;
        
           public EmployeeEntity(String email, String firstName,
       			String lastName) {
       		super();
       		this.email = email;
       		this.firstName = firstName;
       		this.lastName = lastName;
       	}
       
       	@Column(name = "EMAIL", unique = true, nullable = false, length = 100)
           private String email;
        
           @Column(name = "FIRST_NAME", unique = false, nullable = false, length = 100)
           private String firstName;
        
           @Column(name = "LAST_NAME", unique = false, nullable = false, length = 100)
           private String lastName;
        
           @OneToMany( mappedBy="accountentity")
           private Set empaccdetail;
        
           public Integer getEmployeeId() {
               return employeeId;
           }
        
           public void setEmployeeId(Integer employeeId) {
               this.employeeId = employeeId;
           }
        
           public String getEmail() {
               return email;
           }
        
           public void setEmail(String email) {
               this.email = email;
           }
        
           public String getFirstName() {
               return firstName;
           }
        
           public void setFirstName(String firstName) {
               this.firstName = firstName;
           }
        
           public String getLastName() {
               return lastName;
           }
        
           public void setLastName(String lastName) {
               this.lastName = lastName;
           }
       
       	public Set getEmpaccdetail() {
       		return empaccdetail;
       	}
       
       	public void setEmpaccdetail(Set empaccdetail) {
       		this.empaccdetail = empaccdetail;
       	}
       }
       

       3)EmpAcc.java (bean for joined table)

       package com.hbm;
       
       import javax.persistence.CascadeType;
       import javax.persistence.Column;
       import javax.persistence.Entity;
       import javax.persistence.GeneratedValue;
       import javax.persistence.Id;
       import javax.persistence.JoinColumn;
       import javax.persistence.ManyToOne;
       import javax.persistence.Table;
       
       @Entity
       @Table(name = "Emp_Acc")
       public class EmpAcc 
       {
       	@Id
       	 @GeneratedValue
       	@Column(name="EMP_ACC_ID")
       		private long emp_acc_id;
       	
       	@ManyToOne(cascade = CascadeType.ALL)
       	    @JoinColumn(name = "EID")  
       	    private EmployeeEntity employeeentity;
       	 
       	  @ManyToOne(cascade = CascadeType.ALL)
       	    @JoinColumn(name = "AID")
       	    private AccountEntity accountentity;
       	    public long getEmp_acc_id() {
       		return emp_acc_id;
       	}
       	public void setEmp_acc_id(long emp_acc_id) {
       		this.emp_acc_id = emp_acc_id;
       	}
       	public EmployeeEntity getEmployeeentity() {
       		return employeeentity;
       	}
       	public void setEmployeeentity(EmployeeEntity employeeentity) {
       		this.employeeentity = employeeentity;
       	}
       	public AccountEntity getAccountentity() {
       		return accountentity;
       	}
       	public void setAccountentity(AccountEntity accountentity) {
       		this.accountentity = accountentity;
       	}
       	public int getBalance() {
       		return balance;
       	}
       	public void setBalance(int balance) {
       		this.balance = balance;
       	}
       		private int  balance;
       	
       
       }
       

       4)MainClass.java

       package com.hbm;
       
       import org.hibernate.Session;
       import org.hibernate.cfg.Configuration;
        
       public class TestJoinTable
       {
           public static void main(String[] args)
           {
               Session session = new Configuration().configure("com/hbm/hibernate.cfg.xml").buildSessionFactory().openSession();
               		session.beginTransaction();
        
               		EmployeeEntity emp=new EmployeeEntity("fdsgf@fgf"," jffksjf"," fdlskf");
               		
               		AccountEntity acc=new AccountEntity("245");
               		        		
               		EmpAcc empAcc=new EmpAcc();
               		empAcc.setEmployeeentity(emp);
               		empAcc.setAccountentity(acc);
               		empAcc.setBalance(3555);
               	
          session.save(empAcc);
           System.out.println("saved ");
               session.getTransaction().commit();
       
           }
       }
       

       In the above example balance is the new column.

       • Thanks for sharing your solution. Much appreciated.

         • Welcome….do you know gwt?

         • Never tried

10. your example is good one but i want one additional column in a newly created join table (employee_account) Can you please modify this example to add extra column

11. Hi ,
    Can you please elobarate on mappedBy attribute.
    Could not understand, even I searched so many sites.
    mappedby definition says ‘I am not the owner side and mapped by other entity’
    From the given example(Employee and Account) with below code.
    @OneToOne(mappedBy=”account”)
    private EmployeeEntity employee;
    Who is not the owner? and who is Owner and what is does actually.
    Please explains

    • This will help you : https://howtodoinjava.com/hibernate/how-to-define-association-mappings-between-hibernate-entities/

12. How to insert data into database using annotation

13. Was learning Hibernate and went through so many tutorials online. Completely misunderstood the functionality of mappedBy till i came across this article. Good stuff guys, keep up the good work (Y)

14. Its great site to learn java. Thanks a lot Keep it up…………..

15. Great Explanation….Thanks..Sir Keep it up …

16. i need a help,
    we have a list as field to store it in data base,
    for this we have to use hibernate annotations mapping class(no hibernate mapping file should be used), pojo class object contains some fields as List, those fileds will be inserted into db as string by mapping class. if possible for you, give an example

    • Similar example is discussed in one to many mapping tutorial

      private Set accounts;

17. Awsome explanation…. bookmarking site. Thanks

18. big Thank’s for your HELP

19. HI Lokesh,
    Good after noon,

    Currently i am working with hibernate 3. i have been trying to develop a simple hibernate project with one to many relationship.every thing is fine but i am getting error like. i have placed cfg.xml file under src folder.is correct. why i am getting error. please suggest me. as soon as…..

    log4j:WARN No appenders could be found for logger (org.hibernate.cfg.annotations.Version).
    log4j:WARN Please initialize the log4j system properly.
    Initial SessionFactory creation failed.org.hibernate.HibernateException: could not find file: hibernate.cgf.xml
    Exception in thread “main” java.lang.ExceptionInInitializerError
    at com.hib.pro.HibernateUtil.buildSessionFactory(HibernateUtil.java:23)
    at com.hib.pro.HibernateUtil.(HibernateUtil.java:11)
    at com.hib.pro.TestForeignKeyAssociation.main(TestForeignKeyAssociation.java:19)
    Caused by: org.hibernate.HibernateException: could not find file: hibernate.cgf.xml
    at org.hibernate.cfg.Configuration.configure(Configuration.java:1462)
    at com.hib.pro.HibernateUtil.buildSessionFactory(HibernateUtil.java:18)
    … 2 more
    Caused by: java.io.FileNotFoundException: hibernate.cgf.xml (The system cannot find the file specified)
    at java.io.FileInputStream.open(Native Method)
    at java.io.FileInputStream.(FileInputStream.java:106)
    at org.hibernate.cfg.Configuration.configure(Configuration.java:1459)
    … 3 more

    • Usually src folders have filters which cause to generate only .class files in target folder or classes folder. Copy the file in resources folder. The important thing is that file should be present in target folder (war file).

20. Hi
    Can you please help me in
    https://stackoverflow.com/questions/18895585/hibernate-version-annotation-and-object-references-an-unsaved-transient-instanc

21. Hi Lokesh,

    Good Work. But I see a flaw in the DB design of your “Using foreign key association”. In the Employee Table one can have one account shared by the multiple employees which does not make it one to one but many to one. For that you need to put Unique constraint on Account Id column in Employee Table. Please clarify.

    • Abhi, you are right. It should be implemented this way. But, also that’s not core concept for this tutorial. Mixing concepts in single post, sometimes push the core concept in back seat. Which i did not wanted to do, otherwise in any code review session, I can myself blew this code. [:-)]

22. Wonderful information captured in one place. This blog is awesome.

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