mybatis 自动映射对象为json

╰+攻爆jí腚メ 2022-11-30 05:42 323阅读 0赞

> 一般我们关系型数据库存储的字段都是从不同单一维度描述这个对象, 而随着业务的复杂和数据维度的增加, 我们有时候需要直接将一些简单维度(只作展示, 不涉及条件查询以及基本不修改)的集合封装成`json`格式放入一个大字段中(避免联合查询的额外扫表的开销)

## typeHandler的方式做对象映射 ##

> 本文只介绍简单的使用方式, 具体原理和详细解析请跳转以下链接
> 
> 参考资料
> 
> https://www.codenong.com/js92a4cfdcc700/
> 
> https://juejin.im/post/6844903997271179277

### 1. 自定义定义映射器 ###

#### 对象映射器 ####

public abstract class BaseMybatisObject2JsonHandler<T> extends BaseTypeHandler<T> { 
    
    
    	@Override
    	public void setNonNullParameter(PreparedStatement ps, int i, Object parameter,
    									JdbcType jdbcType) throws SQLException { 
    		ps.setString(i, JSON.toJSONString(parameter));
    	}
    
    	@Override
    	public T getNullableResult(ResultSet rs, String columnName)
    			throws SQLException { 
    		String data = rs.getString(columnName);
    		return StringUtils.isBlank(data) ? null : JSON.parseObject(data, (Class<T>) getRawType());
    	}
    
    	@Override
    	public T getNullableResult(ResultSet rs, int columnIndex) throws SQLException { 
    		String data = rs.getString(columnIndex);
    		return StringUtils.isBlank(data) ? null : JSON.parseObject(data, (Class<T>) getRawType());
    	}
    
    	@Override
    	public T getNullableResult(CallableStatement cs, int columnIndex)
    			throws SQLException { 
    		String data = cs.getString(columnIndex);
    		return StringUtils.isBlank(data) ? null : JSON.parseObject(data, (Class<T>) getRawType());
    	}
    }

#### 列表映射器 ####

public abstract class BaseMybatisList2JsonHandler<T> extends BaseTypeHandler<List<T>> { 
    
    	@Override
    	public void setNonNullParameter(PreparedStatement ps, int i, List<T> parameter, JdbcType jdbcType) throws SQLException { 
    		ps.setString(i, JSON.toJSONString(parameter));
    	}
    
    	@Override
    	public List<T> getNullableResult(ResultSet rs, String columnName)
    			throws SQLException { 
    		String data = rs.getString(columnName);
    		return StringUtils.isBlank(data) ? null : JSON.parseArray(data, (Class<T>) getRawType());
    	}
    
    	@Override
    	public List<T> getNullableResult(ResultSet rs, int columnIndex) throws SQLException { 
    		String data = rs.getString(columnIndex);
    		return StringUtils.isBlank(data) ? null : JSON.parseArray(data, (Class<T>) getRawType());
    	}
    
    	@Override
    	public List<T> getNullableResult(CallableStatement cs, int columnIndex)
    			throws SQLException { 
    		String data = cs.getString(columnIndex);
    		return StringUtils.isBlank(data) ? null : JSON.parseArray(data, (Class<T>) getRawType());
    	}
    }

#### 需要进行映射的继承注册一下即可 ####

@MappedTypes({ Snippet.class})
    @MappedJdbcTypes(value = JdbcType.VARCHAR, includeNullJdbcType = true)
    public class InstructionBoTypeHandler extends BaseMybatisObject2JsonHandler<InstructionBo> { 
      
    }

*  `@MappedTypes`: 对应的`java`对象
 *  `@MappedJdbcTypes`: 对应的数据库字段类型
 *  `includeNullJdbcType`: 是否包含一个`null`的`Jdbc`类型(这个类型在框架获取`typeHandler`时候用到, 详细看[跳转文章][Link 1])

### 2. 配置 ###

#### java对象内容 ####

> 注意: 实体对象类上需要加上`@TableName(autoResultMap = true)`注解, 使得`@TableField`(该注解为`mybatis-plus`的注解)生效, 否者将会默认以`mybatis`的方式通过字段类型匹配对应的`TypeHandler`

@TableName(autoResultMap = true)
    public class Snippet implements Serializable { 
    
        private static final long serialVersionUID = 1L;
    
        @TableId(value = "id", type = IdType.AUTO)
        private Integer id;
        private String name;
        private Integer homeId;
        @TableField(typeHandler = FastjsonTypeHandler.class)
        private InstructionBo commandSet;
        private Integer revision;
        private Integer state;
        private String createUser;
        @TableField(fill = FieldFill.INSERT)
        private LocalDateTime createTime;
        private String updateUser;
        @TableField(fill = FieldFill.INSERT_UPDATE)
        private LocalDateTime updateTime;
    }

#### xml配置 ####

<resultMap id="BaseResultMap" type="com.smart.life.userserver.domain.po.Snippet">
            <id column="id" property="id" />
            <result column="name" property="name" />
            <result column="home_id" property="homeId" />
            <result column="command_set" property="commandSet" javaType="com.smart.life.userserver.domain.po.Snippet" typeHandler="com.smart.life.userserver.helper.type.InstructionBoTypeHandler"/>
            <result column="revision" property="revision" />
            <result column="state" property="state" />
            <result column="create_user" property="createUser" />
            <result column="create_time" property="createTime" />
            <result column="update_user" property="updateUser" />
            <result column="update_time" property="updateTime" />
        </resultMap>

### 3. properties配置 ###

#### mybatis-spring-boot-starter方式 ####

## application.properties里配置 
    mybatis.typeHandlersPackage={BarTypeHandler所在包路径}

#### mybatis-spring 方式一 ####

> 构造一个SqlSessionFactoryBean对象，并调用其setTypeHandlersPackage方法设置类型处理器扫包路径

#### mybatis-spring 方式二 ####

## application.properties里配置 
    mybatis-plus.type-aliases-package=com.smart.life.userserver.helper.type

## 错误 ##

> 一直报下面这个错
> 
>     org.mybatis.spring.MyBatisSystemException: nested exception is org.apache.ibatis.exceptions.PersistenceException: 
>     ### Error updating database.  Cause: java.lang.IllegalStateException: Type handler was null on parameter mapping for property 'commandSet'. It was either not specified and/or could not be found for the javaType (com.smart.life.common.domain.Bo.InstructionBo) : jdbcType (null) combination.
>     ### The error may exist in com/smart/life/userserver/mapper/SnippetMapper.java (best guess)
>     ### The error may involve com.smart.life.userserver.mapper.SnippetMapper.insert
>     ### The error occurred while executing an update
>     ### Cause: java.lang.IllegalStateException: Type handler was null on parameter mapping for property 'commandSet'. It was either not specified and/or could not be found for the javaType (com.smart.life.common.domain.Bo.InstructionBo) : jdbcType (null) combination.

## 如果使用mybatis-plus ##

> 使用`mybatis-plus`的童鞋可以非常方便的使用转换器, 因为`mybatis-plus`都已经实现好了, 只需要指定他用哪种`json`

在需要进行`json`序列化与反序列化的字段上加上如下注解

> 根据自己使用使用的`json`包选择对应的注解

@TableField(typeHandler = FastjsonTypeHandler.class)
    @TableField(typeHandler = JacksonTypeHandler.class)

xml 中配置

> 需要`json`化的字段加上 `typeHandler="com.baomidou.mybatisplus.extension.handlers.FastjsonTypeHandler"`

[Link 1]: https://www.codenong.com/js92a4cfdcc700/