TypeHandler使用手冊(cè)

場(chǎng)景：想保存user時(shí) teacher自動(dòng)轉(zhuǎn)String ，不想每次保存都要手動(dòng)去轉(zhuǎn)String；從DB查詢出來(lái)時(shí)，也要自動(dòng)幫我們轉(zhuǎn)換成Java對(duì)象 Teacher

@Data
public class User {private Integer id;private String name;private Integer sex;//這里直接使用數(shù)據(jù)庫(kù)無(wú)法識(shí)別的自定義類型private Teacher teacher;
}

當(dāng)然Mybatis給我們提供了接口 TypeHandler（類型映射）

public interface TypeHandler<T> {//參數(shù)轉(zhuǎn)換void setParameter(PreparedStatement ps, int i, T parameter, JdbcType jdbcType) throws SQLException;//列值轉(zhuǎn)換T getResult(ResultSet rs, String columnName) throws SQLException;//列值轉(zhuǎn)換T getResult(ResultSet rs, int columnIndex) throws SQLException;//列值轉(zhuǎn)換T getResult(CallableStatement cs, int columnIndex) throws SQLException;
}

看看mybatis內(nèi)置的類型處理器，這就是為什么一些Java的數(shù)據(jù)類型不用我們手動(dòng)轉(zhuǎn)換的原因

我們可以參考String，看他是怎么處理的，發(fā)現(xiàn)都是通過(guò)原生的jdbc來(lái)處理的

public class StringTypeHandler extends BaseTypeHandler<String> {@Overridepublic void setNonNullParameter(PreparedStatement ps, int i, String parameter, JdbcType jdbcType)throws SQLException {ps.setString(i, parameter);}@Overridepublic String getNullableResult(ResultSet rs, String columnName)throws SQLException {return rs.getString(columnName);}@Overridepublic String getNullableResult(ResultSet rs, int columnIndex)throws SQLException {return rs.getString(columnIndex);}@Overridepublic String getNullableResult(CallableStatement cs, int columnIndex)throws SQLException {return cs.getString(columnIndex);}
}

實(shí)戰(zhàn)

我們結(jié)合springboot來(lái)看，啟動(dòng)容器

server:port: 8080
mybatis:mapper-locations: classpath:mappers/*.xmltype-handlers-package: com.example.ssm.demos.web.typeHandler

創(chuàng)建SqlSessionFactory的時(shí)候掃描并注冊(cè)

public class SqlSessionFactoryBean implements InitializingBean {@Overridepublic void afterPropertiesSet() throws Exception {this.sqlSessionFactory = buildSqlSessionFactory();}protected SqlSessionFactory buildSqlSessionFactory() throws Exception {if (hasLength(this.typeHandlersPackage)) {//在包路徑下找到實(shí)現(xiàn)TypeHandler的類并注冊(cè)scanClasses(this.typeHandlersPackage, TypeHandler.class).stream().filter(clazz -> !clazz.isAnonymousClass()).filter(clazz -> !clazz.isInterface()).filter(clazz -> !Modifier.isAbstract(clazz.getModifiers())).forEach(targetConfiguration.getTypeHandlerRegistry()::register);}}
}

注冊(cè)并加入緩存

public final class TypeHandlerRegistry {private final Map<Type, Map<JdbcType, TypeHandler<?>>> typeHandlerMap = new ConcurrentHashMap<>();private final Map<Class<?>, TypeHandler<?>> allTypeHandlersMap = new HashMap<>();public void register(Class<?> typeHandlerClass) {boolean mappedTypeFound = false;//如果這個(gè)類有注解@MappedTypes，根據(jù)注解的指定的類進(jìn)行有參構(gòu)造實(shí)例化MappedTypes mappedTypes = typeHandlerClass.getAnnotation(MappedTypes.class);if (mappedTypes != null) {for (Class<?> javaTypeClass : mappedTypes.value()) {register(javaTypeClass, typeHandlerClass);mappedTypeFound = true;}}//沒(méi)有注解，直接無(wú)參構(gòu)造實(shí)例化if (!mappedTypeFound) {register(getInstance(null, typeHandlerClass));}}//加入緩存 private void register(Type javaType, JdbcType jdbcType, TypeHandler<?> handler) {if (javaType != null) {Map<JdbcType, TypeHandler<?>> map = typeHandlerMap.get(javaType);if (map == null || map == NULL_TYPE_HANDLER_MAP) {map = new HashMap<>();}map.put(jdbcType, handler);typeHandlerMap.put(javaType, map);}allTypeHandlersMap.put(handler.getClass(), handler);}
}

容器啟動(dòng)完成，測(cè)試一下

@Mapper
public interface UserMapper {public User getUserListByEntity(Integer id);public int insertUser(User user);
}

定義一個(gè)Teacher 處理器

public class TeacherTypeHandler extends BaseTypeHandler<Teacher> {@Overridepublic void setNonNullParameter(PreparedStatement ps, int i, Teacher parameter, JdbcType jdbcType) throws SQLException {ps.setString(i, JSON.toJSONString(parameter));}@Overridepublic Teacher getNullableResult(ResultSet rs, String columnName) throws SQLException {return JSON.parseObject(rs.getString(columnName),Teacher.class);}@Overridepublic Teacher getNullableResult(ResultSet rs, int columnIndex) throws SQLException {return JSON.parseObject(rs.getString(columnIndex),Teacher.class);}@Overridepublic Teacher getNullableResult(CallableStatement cs, int columnIndex) throws SQLException {return JSON.parseObject(cs.getString(columnIndex),Teacher.class);}
}

mapper文件

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE mapperPUBLIC "-//mybatis.org//DTD Mapper 3.0//EN""http://mybatis.org/dtd/mybatis-3-mapper.dtd"><mapper namespace="com.example.ssm.demos.web.mapper.UserMapper"><sql id="allUserCollun">id,name,sex,teacher</sql><resultMap id="userMap" type="com.example.ssm.demos.web.pojo.User"><id column="id" property="id"></id><result property="name" column="name"></result><result property="sex" column="sex"></result><!-- 指定字段處理器，反序列化成Teacher對(duì)象 --><result property="teacher" column="teacher" typeHandler="com.example.ssm.demos.web.typeHandler.TeacherTypeHandler" ></result></resultMap><!-- 測(cè)試查詢 --><select id="getUserListByEntity" resultMap="userMap">select<include refid="allUserCollun"></include>from User<where>id = #{id}</where></select><!-- 測(cè)試新增 --><insert id="insertUser" parameterType="com.example.ssm.demos.web.pojo.User">insert into user(<include refid="allUserCollun"></include>) values (#{id},#{name},#{sex},<!-- 指定字段處理器，轉(zhuǎn)換成json格式的字符串 -->#{teacher,typeHandler=com.example.ssm.demos.web.typeHandler.TeacherTypeHandler})</insert>
</mapper>

經(jīng)過(guò)測(cè)試，完全沒(méi)有問(wèn)題，插入數(shù)據(jù)庫(kù)時(shí)，自動(dòng)轉(zhuǎn) String，查詢時(shí)，自動(dòng)轉(zhuǎn) Teacher

如果想知道調(diào)用位置，參數(shù)可以看這個(gè)類： org.apache.ibatis.scripting.defaults.DefaultParameterHandler#setParameters

返回值可以看這個(gè)類： org.apache.ibatis.executor.resultset.DefaultResultSetHandler#getPropertyMappingValue