generic DAO in java

Question

I am trying to develop generic DAO in java. I have tried the following. Is this a good way to implement generic DAO? I don't want to use hibernate. I am trying to make it as generic as possible so that I don't have to repeat the same code over and over again.

public abstract class  AbstractDAO<T> {

    protected ResultSet findbyId(String tablename, Integer id){
        ResultSet rs= null;
        try {
           // the following lines are not working
            pStmt = cn.prepareStatement("SELECT * FROM "+ tablename+ "WHERE id = ?");
            pStmt.setInt(1, id);
            rs = pStmt.executeQuery();


        } catch (SQLException ex) {
            System.out.println("ERROR in findbyid " +ex.getMessage() +ex.getCause());
            ex.printStackTrace();
        }finally{
            return rs;
        }

    }

}

Now I have:

public class UserDAO extends AbstractDAO<User>{

  public List<User> findbyid(int id){
   Resultset rs =findbyid("USERS",id) // "USERS" is table name in DB
   List<Users> users = convertToList(rs);
   return users; 
}


 private List<User> convertToList(ResultSet rs)  {
        List<User> userList= new ArrayList();
        User user= new User();;
        try {
            while (rs.next()) {
                user.setId(rs.getInt("id"));
                user.setUsername(rs.getString("username"));
                user.setFname(rs.getString("fname"));
                user.setLname(rs.getString("lname"));
                user.setUsertype(rs.getInt("usertype"));
                user.setPasswd(rs.getString("passwd"));
                userList.add(user);
            }
        } catch (SQLException ex) {
            Logger.getLogger(UserDAO.class.getName()).log(Level.SEVERE, null, ex);
        }

        return userList;

    }
}

Answer 1

If you can live with Spring, I will suggest the following improvements:

• Let Spring do the exception handling.
• Use JdbcTemplate instead of creating prepared statements yourself.

Independent of using Spring, I will recommend the following:

• Don't send the table name as parameter. That should be done in the initialization phase.
• Use a String on the id parameter, since that's much more generic.
• Consider returning a generic object instead of a collection, since the collection should always contain only one object.

An improved AbstractDao with Spring:

import java.util.Collection;

import org.springframework.jdbc.core.JdbcTemplate;
import org.springframework.jdbc.core.RowMapper;

public abstract class AbstractDao<T> {

    protected final RowMapper<T> rowMapper;

    protected final String findByIdSql;

    protected final JdbcTemplate jdbcTemplate;

    protected AbstractDao(RowMapper<T> rowMapper, String tableName,
            JdbcTemplate jdbcTemplate) {
        this.rowMapper = rowMapper;
        this.findByIdSql = "SELECT * FROM " + tableName + "WHERE id = ?";
        this.jdbcTemplate = jdbcTemplate;
    }

    public  Collection<T> findById(final String id) {
        Object[] params = {id};
        return jdbcTemplate.query(findByIdSql, params, rowMapper);
    }
}

As you see, no exception handling or hacking with the primitive SQL classes. This templates closes the ResultSet for you, which I can't see in your code.

And the UserDao:

import java.sql.ResultSet;
import java.sql.SQLException;

import org.springframework.jdbc.core.JdbcTemplate;
import org.springframework.jdbc.core.RowMapper;

public class UserDao extends AbstractDao<User> {

    private final static String TABLE_NAME = "USERS";

    public UserDao(JdbcTemplate jdbcTemplate) {
        super(new UserRowMapper(), TABLE_NAME, jdbcTemplate);
    }

    private static class UserRowMapper implements RowMapper<User> {
        public User mapRow(ResultSet rs, int rowNum) throws SQLException {
            User user = new User();
            user.setUserName(rs.getString("username"));
            user.setFirstName(rs.getString("fname"));
            user.setLastName(rs.getString("lname"));

            return user;
        }
    }
}

Updated:

When you know the id and the id corresponds to a single row in the database, you should consider returning a generic object instead of a collection.

public T findUniqueObjectById(final String id) {
    Object[] params = {id};
    return jdbcTemplate.queryForObject(findByIdSql, params, rowMapper);
}

This makes your service code more readable, since you don't need to retrieve the user from a list, but only:

User user = userDao.findUniqueObjectById("22");