Java Database Connectivity (JDBC)

Hi, i'm working on a webservice with GlassFish v2 and Oracle 10g as database. When I get a connection from the connection pool I set it autocommit to false after that I execute a preparedstatement and i don't commit and i don't rollback (because i have to wait for more PreparedStatements). The problem is that the transaction actually commits. Does any one know what could be the problem?

This is the code when a i get the connection:

Context initCtx = null;
Connection conn;

// Connection conn;
try
{
initCtx = new InitialContext();
DataSource ds = (javax.sql.DataSource)initCtx.lookup("jdbc/personalizador");
conn = ds.getConnection(); 
conn.setAutoCommit(false);
}

catch (Exception e)
{
throw new JDBCException();
}

Thanks. Ramiro.