{ User user = null; try { QueryRunner queryRunner = new QueryRunner(DBUtils.getDataSource()); String sql = "select * from user where username=?"; user = queryRunner.query(sql, new BeanHandler<User>(User.class), username); if (user != null) { return "true"; } else { return "false"; } } catch (Exception e) { e.printStackTrace(); } return "none"; }将此段代码中的queryRunner替换

可以使用PreparedStatement对queryRunner进行替换，示例代码如下：

User user = null;
try {
    Connection conn = DBUtils.getConnection();
    String sql = "select * from user where username=?";
    PreparedStatement ps = conn.prepareStatement(sql);
    ps.setString(1, username);
    ResultSet rs = ps.executeQuery();
    if (rs.next()) {
        user = new User();
        user.setId(rs.getInt("id"));
        user.setUsername(rs.getString("username"));
        user.setPassword(rs.getString("password"));
        user.setAge(rs.getInt("age"));
        // ...
    }
    rs.close();
    ps.close();
    conn.close();
} catch (Exception e) {
    e.printStackTrace();
}
if (user != null) {
    return "true";
} else {
    return "false";
}

需要注意的是，此处需要手动关闭数据库连接，避免资源泄露。同时，代码中也需要手动从ResultSet中读取数据，再赋值给User对象，相对于BeanHandler来说，代码量稍微多一些。