MyBatis:注解开发

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使用注解开发可以完全省略掉XXXMapper.xml文件
UserMapper(接口)

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public interface UserMapper {

@Select("select * from user")
List<User> getUser();

@Select("select * from user where id = #{id}")
User getUserById(@Param("id") int id);

// 与实体类属性保持一致，虽然数据库里的字段是pwd，但是实体类中是password所以#{password}
@Insert("insert into user(id,name,pwd) values (#{id},#{name},#{password})")
int addUser(User user);

@Update("update user set name=#{name},pwd=#{password} where id=#{id}")
int updateUser(User user);
}

Test.java:

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public class UserMapperTest {
    @Test
    public void getUserTest() {
        SqlSession sqlSession = MybatisUtil.getSqlSession();
        UserMapper mapper = sqlSession.getMapper(UserMapper.class);
        List<User> user = mapper.getUser();
        for (User user1 : user) {
            System.out.println(user1);
        }
    }
    @Test
    public void getUserByIdTest() {
        SqlSession sqlSession = MybatisUtil.getSqlSession();
        UserMapper mapper = sqlSession.getMapper(UserMapper.class);
        User userById = mapper.getUserById(1);
        System.out.println(userById);
    }
    @Test
    public void addUserTest() {
        SqlSession sqlSession = MybatisUtil.getSqlSession();
        UserMapper mapper = sqlSession.getMapper(UserMapper.class);
        mapper.addUser(new User(5,"leo","666"));
        sqlSession.commit();
        sqlSession.close();
    }
    @Test
    public void updateTest() {
        SqlSession sqlSession = MybatisUtil.getSqlSession();
        UserMapper mapper = sqlSession.getMapper(UserMapper.class);
        mapper.updateUser(new User(1,"xiaoke","666"));
        sqlSession.commit();
        sqlSession.close();
    }
}

注解开发的底层实现是应用反射。
但是这个例子中有个问题，实体类中是password但是数据库字段为pwd，获取数据时，pwd会为null，所以官网上说注解开发只适用于简单场景，如果场景复杂还是建议xml

#与$的区别

#{} 的作用主要是替换预编译语句(PrepareStatement)中的占位符?:

对于 : INSERT INTO user (name) VALUES (#{name}); ==> INSERT INTO user (name) VALUES (?);

${} 符号的作用是直接进行字符串替换:

对于 : INSERT INTO user (name) VALUES (‘${name}’); ==> INSERT INTO user (name) VALUES (‘tianshozhi’);

为了防止SQL注入，建议使用#{}.

关于@Param()注解

  • 基本类型的参数或者String类型，需要加上
  • 引用类型不需加
  • 如果只有一个基本类型的话，可以忽略，但是建议大家都加上
  • 我们在SQL中引用的就是我们这里@Param()中设定的属性名
    例如
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    @Select("select * from user where id = #{id}")
    User getUserById(@Param("id") int id);

如果吧@Param改成(“id2”),上面的#{}也要改：

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@Select("select * from user where id = #{id2}")
User getUserById(@Param("id2") int id);