/**
*
* @author nikita
*
* @hibernate.class table="clients" dynamic-update="true" dynamic-insert="true"
*/
public class Client extends Persistent{
   
    private String name;
   
    /** Creates a new instance of Client */
    public Client() {
    }
   
    /**
     * guaranteed to return a Client object with given name:
     * -if a row w/ name = aName exist return it
     * -if such row does not exist, insert one and return it
     */
    public static Client findOrCreate(String aName) {
        Client c = null;
       
        //get a matching row from DB
        String hql = "select * from Client as client where client.name = ?";
        Query q = HibernateUtil.getCurrentSession().createQuery(hql);
        q.setParameter(0, aName);
        c = (Client) q.uniqueResult();
       
        //if row does not exist, create & insert one
        if(c == null) {
            c = new Client();
            c.setName(aName);
            HibernateUtil.getCurrentSession().persist(c); //may result in constraint violation exception!!!
            //PROBLEM: what if another DB tx does an INSERT after this TX did a SELECT,
            //but before this TX could commit an INSERT?
        }       
       
        return c;
    }
   
    /**
     * @hibernate.property unique="true" not-null="true"
     */
    public String getName() {
        return name;
    }
   
    public void setName(String name) {
        this.name = name;
    }
   
}

abstract public class Persistent implements Serializable{
    private String id = UUID.randomUUID().toString();
   
    transient Logger logger = Logger.getLogger(Persistent.class.getName());
   
    private Integer hibernateVersion;
    private Date creationDate = new Date();;
   
    /**
     * @hibernate.id  generator-class="assigned"
     */
    public String getId() {
        return id;
    }
   
    void setId(String id) {
        this.id = id;
    }
   
    Logger getLogger() {
        return this.logger;
    }
   
    public String toString() {
        String s = "";
       
        s += this.getClass().getName();
        s += ": ";
        s += "id="+this.getId();
        s +=" ";
        return s;
    }
   
   
    /**
     *do NOT manipulate directly
     *
     * @hibernate.version unsaved-value="null"
     */
    public Integer getHibernateVersion() {
        return hibernateVersion;
    }
   
    private void setHibernateVersion(Integer hibernateVersion) {
        this.hibernateVersion = hibernateVersion;
    }
   
    /**
     * @hibernate.property
     */
    public Date getCreationDate() {
        return creationDate;
    }
   
    public void setCreationDate(Date creationDate) {
        this.creationDate = creationDate;
    }
   
    /**
     * equal if IDs are equal
     */
    public boolean equals(Object obj) {
        if(this == obj) {
            return true;
        }
       
        if(! (obj instanceof Persistent)){
            return false;
        }
       
        final Persistent other = (Persistent)obj;
        return this.getId().equals(other.getId());
    }
   
    /**
     * accompanying equals()
     */
    public int hashCode() {
        return this.getId().hashCode();
    }
   

T1: BEGIN TX;

T2: BEGIN TX;

T1: SELECT w/ name = "joe"; //returns nothing

T2: SELECT w/ name = "joe"; //returns nothing

T1: INSERT w/ name = "joe";
T1: COMMIT; //row inserted

T2: INSERT w/ name = "joe";
T2: COMMIT; //constraint violation

<?xml version="1.0" encoding="ISO-8859-1"?>
<!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN" "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">

<hibernate-mapping>
  <class dynamic-update="true" table="clients" name="net.eviltwinstudios.common.beans.Client" dynamic-insert="true">
    <id name="id">
      <generator class="assigned"/>
    </id>
    <version unsaved-value="null" name="hibernateVersion"/>
    <property name="name" not-null="true" unique="true"/>
    <property name="creationDate"/>
  </class>
</hibernate-mapping>