TypeHandler使用手册
场景:想保存user时 teacher自动转String ,不想每次保存都要手动去转String;从DB查询出来时,也要自动帮我们转换成Java对象 Teacher
@Data
public class User {
private Integer id;
private String name;
private Integer sex;
//这里直接使用数据库无法识别的自定义类型
private Teacher teacher;
}
当然Mybatis给我们提供了接口 TypeHandler
(类型映射)
Java
public interface TypeHandler<T> {
//参数转换
void setParameter(PreparedStatement ps, int i, T parameter, JdbcType jdbcType) throws SQLException;
//列值转换
T getResult(ResultSet rs, String columnName) throws SQLException;
//列值转换
T getResult(ResultSet rs, int columnIndex) throws SQLException;
//列值转换
T getResult(CallableStatement cs, int columnIndex) throws SQLException;
}
看看mybatis内置的类型处理器,这就是为什么一些Java的数据类型不用我们手动转换的原因
我们可以参考String,看他是怎么处理的,发现都是通过原生的jdbc来处理的
Java
public class StringTypeHandler extends BaseTypeHandler<String> {
@Override
public void setNonNullParameter(PreparedStatement ps, int i, String parameter, JdbcType jdbcType)
throws SQLException {
ps.setString(i, parameter);
}
@Override
public String getNullableResult(ResultSet rs, String columnName)
throws SQLException {
return rs.getString(columnName);
}
@Override
public String getNullableResult(ResultSet rs, int columnIndex)
throws SQLException {
return rs.getString(columnIndex);
}
@Override
public String getNullableResult(CallableStatement cs, int columnIndex)
throws SQLException {
return cs.getString(columnIndex);
}
}
实战
我们结合springboot来看,启动容器
yaml
server:
port: 8080
mybatis:
mapper-locations: classpath:mappers/*.xml
type-handlers-package: com.example.ssm.demos.web.typeHandler
创建SqlSessionFactory的时候扫描并注册
Java
public class SqlSessionFactoryBean implements InitializingBean {
@Override
public void afterPropertiesSet() throws Exception {
this.sqlSessionFactory = buildSqlSessionFactory();
}
protected SqlSessionFactory buildSqlSessionFactory() throws Exception {
if (hasLength(this.typeHandlersPackage)) {
//在包路径下找到实现TypeHandler的类并注册
scanClasses(this.typeHandlersPackage, TypeHandler.class).stream().filter(clazz -> !clazz.isAnonymousClass())
.filter(clazz -> !clazz.isInterface()).filter(clazz -> !Modifier.isAbstract(clazz.getModifiers()))
.forEach(targetConfiguration.getTypeHandlerRegistry()::register);
}
}
}
注册并加入缓存
java
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;
//如果这个类有注解@MappedTypes,根据注解的指定的类进行有参构造实例化
MappedTypes mappedTypes = typeHandlerClass.getAnnotation(MappedTypes.class);
if (mappedTypes != null) {
for (Class<?> javaTypeClass : mappedTypes.value()) {
register(javaTypeClass, typeHandlerClass);
mappedTypeFound = true;
}
}
//没有注解,直接无参构造实例化
if (!mappedTypeFound) {
register(getInstance(null, typeHandlerClass));
}
}
//加入缓存
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);
}
}
容器启动完成,测试一下
Java
@Mapper
public interface UserMapper {
public User getUserListByEntity(Integer id);
public int insertUser(User user);
}
定义一个Teacher 处理器
Java
public class TeacherTypeHandler extends BaseTypeHandler<Teacher> {
@Override
public void setNonNullParameter(PreparedStatement ps, int i, Teacher parameter, JdbcType jdbcType) throws SQLException {
ps.setString(i, JSON.toJSONString(parameter));
}
@Override
public Teacher getNullableResult(ResultSet rs, String columnName) throws SQLException {
return JSON.parseObject(rs.getString(columnName),Teacher.class);
}
@Override
public Teacher getNullableResult(ResultSet rs, int columnIndex) throws SQLException {
return JSON.parseObject(rs.getString(columnIndex),Teacher.class);
}
@Override
public Teacher getNullableResult(CallableStatement cs, int columnIndex) throws SQLException {
return JSON.parseObject(cs.getString(columnIndex),Teacher.class);
}
}
mapper文件
xml
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE mapper
PUBLIC "-//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>
<!-- 指定字段处理器,反序列化成Teacher对象 -->
<result property="teacher" column="teacher" typeHandler="com.example.ssm.demos.web.typeHandler.TeacherTypeHandler" ></result>
</resultMap>
<!-- 测试查询 -->
<select id="getUserListByEntity" resultMap="userMap">
select
<include refid="allUserCollun"></include>
from User
<where>
id = #{id}
</where>
</select>
<!-- 测试新增 -->
<insert id="insertUser" parameterType="com.example.ssm.demos.web.pojo.User">
insert into user(
<include refid="allUserCollun"></include>
) values (
#{id},#{name},#{sex},
<!-- 指定字段处理器,转换成json格式的字符串 -->
#{teacher,typeHandler=com.example.ssm.demos.web.typeHandler.TeacherTypeHandler})
</insert>
</mapper>
经过测试,完全没有问题,插入数据库时,自动转 String
,查询时,自动转 Teacher
如果想知道调用位置,参数可以看这个类: org.apache.ibatis.scripting.defaults.DefaultParameterHandler#setParameters
返回值可以看这个类: org.apache.ibatis.executor.resultset.DefaultResultSetHandler#getPropertyMappingValue