JDBC: can't set transaction isolation level

Hi,
   I need to set transaction isolation level to SERIALIZABLE in a java program.
Calling the method setTransationIsolation of class Connection has no effect: the
isolation level still remains READ_COMMITTED (the default).   I tried to create a Statement and set the isolation level
asan ordinary SQL
 
- stmt.executeUpdate("set transaction isolation level serializable"); - and it
works fine.   And finally, the method Connection.getTransationIsolation() always returns
2, whatever the isolation level is set to.   These problems happen with PostgreSQL 7.0.3. Can you help me?

                                                            Thanks.