create table users (
  user_name         varchar(75) not null primary key,
  user_pass         varchar(75) not null
);
CREATE TABLE `person` (
`personId` int(11) NOT NULL auto_increment,
`name` varchar(75),
`user_name` varchar(75) not null,
  PRIMARY KEY  (`personId`),
  FOREIGN KEY (`user_name`) REFERENCES `users` (`user_name`)
);

package rain;

import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.Id;
import javax.persistence.OneToOne;
import javax.persistence.Table;

/**
* Users generated by hbm2java
*/
@Entity
@Table(name = "users", catalog = "dbex91")
public class Users implements java.io.Serializable {

   private String userName;
   private String userPass;
   private Person person;

   public Users() {
   }

   public Users(String userName, String userPass) {
      this.userName = userName;
      this.userPass = userPass;
   }

   @Id
   @Column(name = "user_name", unique = true, nullable = false, length = 75)
   public String getUserName() {
      return this.userName;
   }

   public void setUserName(String userName) {
      this.userName = userName;
   }

   @Column(name = "user_pass", nullable = false, length = 75)
   public String getUserPass() {
      return this.userPass;
   }

   public void setUserPass(String userPass) {
      this.userPass = userPass;
   }

   @OneToOne(mappedBy="user")
   public Person getPerson() {
      return person;
   }

   public void setPerson(Person person) {
      this.person = person;
   }
   

}

package rain;

import javax.persistence.*;

@Entity
@Table(name="PERSON")
public class Person {


  private int id;

 
  private Users user;
 
  private String name;

  public Person(){
    
  }
  public Person(Users user,String name) {
     this.user=user;
     this.name = name;
  }
 
  @Id
  @GeneratedValue(strategy = GenerationType.AUTO)
  @Column(name="personId")
  public int getId() {
      return id;
  }
 
  public void setId(int id) {
      this.id = id;
  }
 
  public String getName() {
      return name;
  }

  public void setName(String name) {
      this.name = name;
  }
 
  @OneToOne(cascade=CascadeType.ALL)
  @JoinColumn(name="user_name")
  public Users getUser() {
   return user;
  }
  public void setUser(Users user) {
   this.user = user;
  }
}

Users user = new Users("root","password");
Person person = new Person(user,"vik");
getHibernateTemplate().save(person);

Exception in thread "main" org.springframework.dao.DataIntegrityViolationException: could not insert: [rain.Person]; nested exception is org.hibernate.exception.ConstraintViolationException: could not insert: [rain.Person]
Caused by: org.hibernate.exception.ConstraintViolationException: could not insert: [rain.Person]
...
Caused by: java.sql.SQLException: Column 'user_name' cannot be null