I have a table with a primary key in the format M001, M002 etc (lets not think about what happens after M999 for now). I’m using Hibernate Annotations, and I found a great way of generating the Primary Key value for new Records:

First I created a database sequence to use. Then I implemented org.hibernate.id.IdentifierGenerator;

public class StockCodeGenerator implements IdentifierGenerator {

    private static Logger log = Logger.getLogger(StockCodeGenerator.class);

    public Serializable generate(SessionImplementor session, Object object)
            throws HibernateException {

        String prefix = "M";
        Connection connection = session.connection();
        try {

            PreparedStatement ps = connection
                    .prepareStatement("SELECT nextval ('seq_stock_code') as nextval");

            ResultSet rs = ps.executeQuery();
            if (rs.next()) {
                int id = rs.getInt("nextval");
                String code = prefix + StringUtils.leftPad("" + id,3, '0');
                log.debug("Generated Stock Code: " + code);
                return code;
            }

        } catch (SQLException e) {
            log.error(e);
            throw new HibernateException(
                    "Unable to generate Stock Code Sequence");
        }
        return null;
    }
}

Then, in my entity class, I simply annotate the id field like this:

@Id
@GenericGenerator(name="seq_id", strategy="my.package.StockCodeGenerator")
@GeneratedValue(generator="seq_id")
@Column(name = "stock_code", unique = true, nullable = false, length = 20)
public String getStockCode() {
    return this.stockCode;
}

It works really well!

[Thanks to Jejomar Dimayuga for this post http://blog.dagitab.com/htsrv/trackback.php?tb_id=30 which I have modified slightly to use a sequence rather than a table]