public abstract class BaseDomain implements IPersistentObject {

   protected String oid = OidGenerator.createId(); // oid: this just returns a UUID string.. is the ID for any object and any row in the table.
   protected Integer version;                      // version: HIBERNATE unsaved-value="null" AND optimistic-lock="version"
   private boolean removed = false;

   @Override
   public void setOid(String anOid) {
      this.oid = anOid;
   }

   @Override
   public String getOid() {
      return oid;
   }

   @Override
   public void setVersion(Integer aVersion) {
      this.version = aVersion;
   }

   @Override
   public Integer getVersion() {
      return version;
   }

   public void setRemoved(boolean removed) {
      this.removed = removed;
   }

   public boolean isRemoved() {
      return removed;
   }

   @Override
   public boolean equals(Object obj) {
      if (this == obj)
         return true;
      if (obj == null || !(this.isAPersistenObject(obj))) {
         return false;
      }

      IPersistentObject other = (IPersistentObject) obj;

      if (oid == null)
         return false;
      return oid.equals(other.getOid());
   }

   private boolean isAPersistenObject(Object obj) {
      return obj instanceof IPersistentObject;
   }

   public boolean equalsToDTO(AbstractDTO aBaseDTO) {
      return oid.equals(aBaseDTO.getOid());
   }

   @Override
   public int hashCode() {
      if (oid != null) {
         return oid.hashCode();
      } else {
         return super.hashCode();
      }
   }

   @Override
   public String toString() {
      return this.getClass().getName() + " [id=" + oid + "]";
   }
}

public class User extends BaseDomain{

   protected String userName;
   protected String password;
   
   public User() {
   }
   
   public User(String anUserName, String aPassword) {
      this.userName = anUserName;
      this.password = aPassword;
   }
   
   public String getUserName() {
      return userName;
   }

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

   public String getPassword() {
      return password;
   }

   public void setPassword(String aPassword) {
      this.password = aPassword;
   }
}

<hibernate-mapping>
   <class name="user.domain.User" table="USUARIO" optimistic-lock="version">  // I tried select-before-update="true"
      <id name="oid" column="OID">
         <generator class="assigned" />
      </id>
      <version column="version" name="version" unsaved-value="null"/>      
      <property name="userName" column="user_name" unique="true" />
      <property name="password" column="password" />
      <property name="removed" column="removed" />
   </class>

   <!-- Queries for User -->
   <query name="getUserByUserName">
        <![CDATA[from User user where user.userName = :anUserName]]>
   </query>
   <query name="getUsersByOids">
        <![CDATA[from User user where user.removed = :isRemoved and user.oid IN (:oids)]]>
   </query>
</hibernate-mapping>

public class Team extends BaseDomain {

   private String name;
   private final Collection<User> users = new HashSet<User>();

   public Team() {
   }

   public Team(String aName, Collection<User> users) {
      this.name = aName;
      this.users.addAll(users);
   }

   public String getName() {
      return name;
   }

   public void setName(String name) {
      this.name = name;
   }

   public Collection<User> getUsers() {
      return users;
   }

   protected void setUsers(Collection<User> users) {
      this.users.addAll(users);
   }

   public void addUser(User anUser) {
      this.users.add(anUser);
   }
}

<hibernate-mapping>
   <class name="user.domain.team.Team" table="TEAM" optimistic-lock="version">  // I tried select-before-update="true"
      <id name="oid" column="OID">
         <generator class="assigned" />
      </id>
      
      <version column="version" name="version" unsaved-value="null"/>      
      <property name="name" column="name" unique="true" />
      
      <bag name="users" table="TEAM_USER">   // <--- THIS IS THE MANY TO MANY RELATION!
         <key column="oid_team" />
         <many-to-many column="oid_user" class="user.domain.User" />
      </bag>
   </class>

   <!-- Queries for User -->
   <query name="getTeamByName">
        <![CDATA[from Team team where team.name = :aName]]>
   </query>
</hibernate-mapping>

public class TeamServiceImpl extends AbstractServiceImpl implements TeamServiceBI {

   @Override
   public TeamDTO getTeam(String sessionToken, TeamDTO aTeamDTO) throws UnknownTeamException {
   
      // get the Team from a respository.
      Team aTeam = this.getTeamRespository().getTeamByDTO(aTeamDTO);
      // create a new Data transfer object with data from the real team.
      return (TeamDTO) TeamDTOFactory.getInstance().getDTO(aTeam);
   }
   
   @Override
   public void removeTeam(String sessionToken, TeamDTO aTeamDTOToRemove) throws DTOConcurrencyException,
         UnknownTeamException {
      ItemTracker theItemTracker = this.getItemTrackerRespository().getItemTracker();
      Team aTeam = this.getTeamRespository().getTeamByDTO(aTeamDTOToRemove);
      this.checkDTOConcurrency(aTeamDTOToRemove, aTeam);   // CHECK THIS... WHERE IS A PART OF THE PROBLEM.
      theItemTracker.removeTeam(aTeam);

   }
   
   // I add this method here so you can see it.
   final protected void checkDTOConcurrency(AbstractDTO aDTO, BaseDomain aBaseDomianObject)
         throws DTOConcurrencyException {
      if (!aDTO.getVersion().equals(aBaseDomianObject.getVersion())) {
         throw new DTOConcurrencyException("The entity that you what to modifier had been used by other user.");
      }
   }   
}

   <bean id="teamServiceTarget" class="user.service.team.TeamServiceImpl"
      parent="abstractServiceImpl">
   </bean>

   <bean id="teamService"
      class="org.springframework.transaction.interceptor.TransactionProxyFactoryBean">
      <property name="target" ref="teamServiceTarget" />
      <property name="transactionManager" ref="transactionManager" />
      <property name="transactionAttributes">
         <props>
            <prop key="*">PROPAGATION_REQUIRED</prop>
         </props>
      </property>
      <property name="preInterceptors">
         <list>
            <bean class="base.security.ServiceSecurityPreInterceptor" /> // THIS DOES NOT MODIFIES AN OBJECT
         </list>
      </property>         
   </bean>