public class UserDaoJdbcImpl implements UserDao {
 public void update(User u) {
 Connection conn = null;
 PreparedStatement ps = null;
 ResultSet rs = null;
 try {
  conn = JdbcUtils.getConnection();
  String sql = "update user set name = ?, birthday = ?, money = ? where id=?";
  ps = conn.prepareStatement(sql);
  // 首先得到该记录
  User user = getUserById(u.getId());
  // 判断字段是否需要修改
  if (u.getName() == null) {
  u.setName(user.getName());
  }
  if (u.getBirthday() == null) {
  u.setBirthday(user.getBirthday());
  }
  if (u.getMoney() == 0) {
  u.setMoney(user.getMoney());
  }
  ps.setString(1, u.getName());
  ps.setDate(2, new java.sql.Date(u.getBirthday().getTime()));
  ps.setDouble(3, u.getMoney());
  ps.setInt(4, u.getId());
  int i = ps.executeUpdate();
  System.out.println("成功向user表中更新" + i + "条记录");
 } catch (SQLException e) {
  e.printStackTrace();
 } finally {
  JdbcUtils.free(rs, ps, conn);
 }
 }
 public User getUserById(int id) {
 Connection conn = null;
 PreparedStatement ps = null;
 ResultSet rs = null;
 User user = null;
 try {
  conn = JdbcUtils.getConnection();
  String sql = "select * from user where id = ?";
  ps = conn.prepareStatement(sql);
  ps.setInt(1, id);
  rs = ps.executeQuery();
  if (rs.next()) {
  user = new User();
  user.setId(rs.getInt("id"));
  user.setName(rs.getString("name"));
  user.setBirthday(rs.getDate("birthday"));
  user.setMoney(rs.getDouble("money"));
  }
 } catch (SQLException e) {
  e.printStackTrace();
 } finally {
  JdbcUtils.free(rs, ps, conn);
 }
 return user;
 }
}

public static void main(String[] args) {
 UserDao ud = new UserDaoJdbcImpl();
 User user = new User();
 user.setId(9);
 user.setName("老师");//只修改name和birthday属性
 Date d = null;
 try {
  SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd");
  d = sdf.parse("1999-9-14");
 } catch (ParseException e) {
  e.printStackTrace();
 }
 user.setBirthday(d);
 //user.setMoney(1234);不修改money属性
 ud.update(user);
}