I want to lead an easy life.

try (InputStreamReader is = new InputStreamReader(multipartFile.getInputStream(), Constants.ENCODING);
        CSVReader reader = new CSVReaderBuilder(is).withSkipLines(1).build()) {
    ColumnPositionMappingStrategy strat = new ColumnPositionMappingStrategy<>();
    strat.setType(User.class);
    strat.setColumnMapping("username", "password", "firstName", "lastName", "email");

    CsvToBean<User> csv = new CsvToBean<>();
    csv.setCsvReader(reader);
    csv.setMappingStrategy(strat);
    return csv.parse();
} catch (IOException e) {
    // 適宜例外処理
} catch (IllegalStateException e) {
    // 適宜例外処理
}

CSVReader reader = new CSVReaderBuilder(is).withSkipLines(1).build();

try (OutputStreamWriter os = new OutputStreamWriter(response.getOutputStream(), Constants.ENCODING);
        CSVWriter writer = new CSVWriter(os)) {
    // ヘッダー行を追加
    writer.writeNext(new String[] {"ユーザ名","パスワード","名字","名前","ｅメール"});

    ColumnPositionMappingStrategy<User> strat = new ColumnPositionMappingStrategy<>();
    strat.setType(User.class);
    strat.setColumnMapping("username", "password", "firstName", "lastName", "email");

    StatefulBeanToCsv<User> beanToCsv = new StatefulBeanToCsvBuilder<User>(writer)
        .withMappingStrategy(strat)
        .build();
    beanToCsv.write(userList);
} catch (IOException e) {
    // 適宜例外処理
} catch (CsvException e) {
    // 適宜例外処理
}