Mapping ResultSet to Java Objects in JDBC

Direct Answer

ResultSet mapping reads each row's columns, handles SQL nulls and type conversion, and constructs a Java DTO/domain value. Keep the mapping in a focused function or RowMapper so query aliases and object construction stay consistent. Spring JdbcTemplate calls a RowMapper once per row and manages iteration/resource cleanup around it.

Code Example

record User(long id, String name, String email) {}

// RowMapper: maps ONE row to a User
interface RowMapper<T> { T map(ResultSet rs) throws SQLException; }
RowMapper<User> userMapper = rs ->
    new User(rs.getLong("id"), rs.getString("name"), rs.getString("email"));

List<User> users = new ArrayList<>();
try (PreparedStatement ps = con.prepareStatement("SELECT id, name, email FROM users");
     ResultSet rs = ps.executeQuery()) {
    while (rs.next()) users.add(userMapper.map(rs));  // reuse the mapper
}
// Spring: jdbcTemplate.query("SELECT ... ", (rs, rowNum) -> new User(...));

Common Mistakes & Pitfalls

• Inlining mapping everywhere: duplicates column reads; extract a RowMapper.
• Column name typos: a wrong name throws at runtime — keep mapping centralized.
• Ignoring NULLs: use getObject/wasNull for nullable columns.
• Building objects before next(): read only after advancing the cursor.