Spring JdbcTemplate中关于RowMapper的

在上一节的《Spring JdbcTemplate使用实例》中，我们在查询返回对象的时候，使用了匿名类来创建对象，看上去非常地不友好，代码显得很是冗长难懂。那么本文就来先解释下什么是Row Mapper，我们该如何正确地使用。

    @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

使用过原生JDBC的朋友应该知道，从数据库查询出来的记录全都被保存在ResultSet结果集中，我们需要将结果集中的数据一条条地获取并设置到具体的实体类上，如此，该实体类才能在接下来的程序中使用。然而问题是，每次都要这么操作实在是太麻烦了，Spring就不应该提供什么功能来替我们做这些事情吗？答案当然是有的，那就是本文的主角——RowMapper。

Spring JDBC中目前有两个主要的RowMapper实现，使用它们应该能解决大部分的场景了。

1.1 SingleColumnRowMapper

通过名字我们就能大概了解，在查询返回单列数据的时候，就该使用这个RowMapper，下面我们来看看具体的代码：

    @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

当查询数据库返回的是多列数据，且你需要将这些多列数据映射到某个具体的实体类上，那么就该使用这个Row Mapper，下面是具体的使用代码：

    @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());
    }

这种使用方式有一个前提，那就是数据库SQL查出来的数据其列名与实体类中的属性名是一致的，当然个数和顺序可以不一致。比如数据库SQL查出来的姓名列叫name，那么对应的实体类中的姓名也必须叫name，而不能叫studentName或者其它。

二、定义自己的RowMapper

当然，如果你SQL查询出来的数据列名就是和实体类的属性名不一样，或者想按照自己的规则来装配实体类，那么就可以定义并使用自己的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());
    }