Session session = factory.openSession();
Transaction tx = null;
try {
    tx = session.beginTransaction();
    // Do some work
    session.load(...);
    session.persist(...);

    Transaction nested = null;
    try {
          nested = session.beginTransaction();
          // Do some work
          session.load(...);
          session.persist(...);

          nested.commit();
    }catch(RuntimeException e){
          nested.rollback();
    }

    if (nested.wasCommited()){
         // Do some work
         session.load(...);
         session.persist(...);
    }else{
         // Do some other work
         session.load(...);
         session.persist(...);
    }

    tx.commit();  // place which causes an exception now
}
catch (RuntimeException e) {
    tx.rollback();
    throw e;
}
finally {
    session.close();
}

Exception in thread "main" org.hibernate.TransactionException: Transaction not successfully started
   at org.hibernate.transaction.JDBCTransaction.commit(JDBCTransaction.java:100)
....

SessionImpl.java frgment :

   public Transaction beginTransaction() throws HibernateException {
      errorIfClosed();
      if ( rootSession != null ) {
         // todo : should seriously consider not allowing a txn to begin from a child session
         //      can always route the request to the root session...
         log.warn( "Transaction started on non-root session" );
      }
      Transaction result = getTransaction();
      result.begin();
      return result;
   }

   public Transaction getTransaction() throws HibernateException {
      errorIfClosed();
      return jdbcContext.getTransaction();
   }

JDBCContext.java frgment :
       public Transaction getTransaction() throws HibernateException {
      if (hibernateTransaction==null) {
         hibernateTransaction = owner.getFactory().getSettings()
               .getTransactionFactory()
               .createTransaction( this, owner );
      }
      return hibernateTransaction;
   }

Class.forName("org.postgresql.Driver");
Connection connection = DriverManager.getConnection("jdbc:postgresql://localhost/example", "postgres", "******");
connection.setAutoCommit(false);
//start outer transaction
connection.createStatement().execute("start transaction");
try{
     .....
     // some work
     ...
     // start inner transaction
     connection.createStatement().execute("start transaction");
     try{
         ....
         // some work
         ....
         // commit inner transaction
         connection.commit();
     }catch(Throwable e){
         // rollback inner transaction
         conection.rollback();
         ...
         // process exception
     }
     // comimt outer transaction
     connection.commit();
}catch(Throwable e){
        // rollback outer transaction
        conection.rollback();
         ...
         // process exception
}