在上一節(jié)的《Spring JdbcTemplate使用實(shí)例》中，我們?cè)诓樵?xún)返回對(duì)象的時(shí)候，使用了匿名類(lèi)來(lái)創(chuàng)建對(duì)象，看上去非常地不友好，代碼顯得很是冗長(zhǎng)難懂。那么本文就來(lái)先解釋下什么是Row Mapper，我們?cè)撊绾握_地使用。

    @Override
    public Student getStudentByName(String name) {
        String sql = "select name, gender from test_student where name = ?";
        Student student = this.jdbcTemplate.queryForObject(sql, new Object[]{name}, new RowMapper<Student>() {
            @Override
            public Student mapRow(ResultSet rs, int i) throws SQLException {
                Student s = new Student();
                s.setName(rs.getString("name"));
                s.setGender(rs.getString("gender"));
                return s;
            }
        });
        return student;
    }

    @Override
    public List<Student> getStudentsByName(String name) {
        String sql = "select name, gender from test_student where name = ?";
        List<Student> students = this.jdbcTemplate.query(sql, new Object[]{name}, new RowMapper<Student>() {
            @Override
            public Student mapRow(ResultSet rs, int i) throws SQLException {
                Student s = new Student();
                s.setName(rs.getString("name"));
                s.setGender(rs.getString("gender"));
                return s;
            }
        });
        return students;
    }

一、RowMapper的基本使用

使用過(guò)原生JDBC的朋友應(yīng)該知道，從數(shù)據(jù)庫(kù)查詢(xún)出來(lái)的記錄全都被保存在ResultSet結(jié)果集中，我們需要將結(jié)果集中的數(shù)據(jù)一條條地獲取并設(shè)置到具體的實(shí)體類(lèi)上，如此，該實(shí)體類(lèi)才能在接下來(lái)的程序中使用。然而問(wèn)題是，每次都要這么操作實(shí)在是太麻煩了，Spring就不應(yīng)該提供什么功能來(lái)替我們做這些事情嗎？答案當(dāng)然是有的，那就是本文的主角——RowMapper。

Spring JDBC中目前有兩個(gè)主要的RowMapper實(shí)現(xiàn)，使用它們應(yīng)該能解決大部分的場(chǎng)景了。

1.1 SingleColumnRowMapper

通過(guò)名字我們就能大概了解，在查詢(xún)返回單列數(shù)據(jù)的時(shí)候，就該使用這個(gè)RowMapper，下面我們來(lái)看看具體的代碼：

    @Override
    public String getStudentNameById(String id) {
        String sql = "select name from test_student where id = ?";
        return this.jdbcTemplate.queryForObject(sql, new Object[]{id},
                new SingleColumnRowMapper<>(String.class));
    }

    @Override
    public List<String> getStudentNamesByGrade(Integer grade) {
        String sql = "select name from test_student where grade = ?";
        return this.jdbcTemplate.query(sql, new Object[]{grade},
                new SingleColumnRowMapper<>(String.class));
    }

    @Test
    public void getStudentNameById(){
        String name = studentService.getStudentNameById("3");
        assertEquals("李清照", name);
    }

    @Test
    public void getStudentNamesByGrade(){
        List<String> names = studentService.getStudentNamesByGrade(1);
        assertTrue(2 == names.size());
    }

1.2 BeanPropertyRowMapper

當(dāng)查詢(xún)數(shù)據(jù)庫(kù)返回的是多列數(shù)據(jù)，且你需要將這些多列數(shù)據(jù)映射到某個(gè)具體的實(shí)體類(lèi)上，那么就該使用這個(gè)Row Mapper，下面是具體的使用代碼：

    @Override
    public Student getStudentByName2(String name) {
        String sql = "select name, gender from test_student where name = ?";
        return this.jdbcTemplate.queryForObject(sql, new Object[]{name},
                new BeanPropertyRowMapper<>(Student.class));
    }

    @Override
    public List<Student> getStudentsByName2(String name) {
        String sql = "select name, gender from test_student where name = ?";
        return this.jdbcTemplate.query(sql, new Object[]{name},
                new BeanPropertyRowMapper<>(Student.class));
    }

    @Test
    public void getStudentByName2(){
        Student student = studentService.getStudentByName2("杜甫");
        assertEquals("杜甫",student.getName());
        assertEquals("男",student.getGender());
    }

    @Test
    public void getStudentsByName2(){
        List<Student> studentList = studentService.getStudentsByName("Jack");
        assertTrue(2 == studentList.size());
    }

這種使用方式有一個(gè)前提，那就是數(shù)據(jù)庫(kù)SQL查出來(lái)的數(shù)據(jù)其列名與實(shí)體類(lèi)中的屬性名是一致的，當(dāng)然個(gè)數(shù)和順序可以不一致。比如數(shù)據(jù)庫(kù)SQL查出來(lái)的姓名列叫name，那么對(duì)應(yīng)的實(shí)體類(lèi)中的姓名也必須叫name，而不能叫studentName或者其它。

二、定義自己的RowMapper

當(dāng)然，如果你SQL查詢(xún)出來(lái)的數(shù)據(jù)列名就是和實(shí)體類(lèi)的屬性名不一樣，或者想按照自己的規(guī)則來(lái)裝配實(shí)體類(lèi)，那么就可以定義并使用自己的Row Mapper。

public class StudentRowMapper implements RowMapper<Student> {

    @Override
    public Student mapRow(ResultSet rs, int i) throws SQLException {
        Student student = new Student();
        student.setName(rs.getString("name"));
        student.setGender(rs.getString("gender"));
        student.setEmail(rs.getString("email"));
        return student;
    }
}

    @Override
    public Student getStudentByName3(String name) {
        String sql = "select name, gender, email from test_student where name = ?";
        return this.jdbcTemplate.queryForObject(sql, new Object[]{name}, new StudentRowMapper());
    }

    @Override
    public List<Student> getStudentsByName3(String name) {
        String sql = "select name, gender, email from test_student where name = ?";
        return this.jdbcTemplate.query(sql, new Object[]{name}, new StudentRowMapper());
    }

    @Test
    public void getStudentByName3(){
        Student student = studentService.getStudentByName3("杜甫");
        assertEquals("杜甫",student.getName());
        assertEquals("男",student.getGender());
        assertEquals("dufu@tang", student.getEmail());
    }

    @Test
    public void getStudentsByName3(){
        List<Student> studentList = studentService.getStudentsByName3("李白");
        assertTrue(2 == studentList.size());
    }