Locking & updating table in one transaction

Question

I have a method in legacy code which generates unique ID for application.Everytime it generates 40-50 numbers depeding on blocksize and updates the DB with the last id generated.
try{
   DataSource ds = serviceLocator.getDataSource("java:MYDB");
   con = ds.getConnection();
   //1. add conn.setAutoCommit(false);
   ps = con.prepareStatement("LOCK TABLE  IDGEN IN EXCLUSIVE   MODE");
   ps.execute();
   st = con.createStatement();
   rs = st.executeQuery("SELECT ID FROM IDGEN");
   if (rs.next()) {
           startid = rs.getInt(1);
           endid =  startid + blocksize;
   // set last number for next call
   ps = con.prepareStatement("UPDATE IDGEN SET ID = ?);
   ps.setLong(1,endid);
   ps.executeUpdate();          
   // 2. add  con.commit();
  //3. add conn.setAutoCommit(true);
}catch (SQLException e){
    //log message
  }finally {
       //  close rs , stmt, conn
      // returns startid   which will be used to genearte next set of  numbers.
  }I think we need to add autocommit statements here. (1,2,3)
since autocommit is "true" by default , thread 1 locks table, this lock is released after ps.execute(); so "SELECT ID FROM IDGEN" and rest of the queries are not really executed on locked table.
Am I correct?
Thanks in advance.

Answer 1

DataSource ds = serviceLocator.getDataSource("foo");
Connection conn = null;
PreparedStatement stmtSelect = null;
PreparedStatement stmtUpdate = null;
ResultSet resultsSelect = null;
try {
  conn = ds.getConnection();
  conn.setAutoCommit(false);  // IMO, this should always be done
  // FOR UPDATE clause will lock the table
  stmtSelect = conn.prepareStatement("SELECT id FROM idgen FOR UPDATE");
  resultsSelect = stmtSelect.executeQuery();
  int maxValue = results.next() ? results.getInt(1) + 1 : 1;
  stmtUpdate = conn.prepareStatement("UPDATE idgen SET id = ?");
  stmtUpdate.setInt(1, maxValue);
  stmtUpdate.executeUpdate();
  conn.commit();  // must be done to release lock (rollback will also do so)
catch (SQLException e) {
   if (conn != null) try { conn.rollback(); } catch (SQLException fatal) { fatal.printStackTrace(); }
   throw new RuntimeException("Error fetching next sequence", e);
finally {
  // also close the prepared statements and result sets here
   if (conn != null) {
      try { conn.close(); } catch (SQLException e) { e.printStackTrace() };