MySQL读取JSON转换的方式

Mysql5.7+开始支持存储JSON，后续不断优化，应用也越来越广泛
你可以自己将数据转换成json String后插入，也可以选择使用工具，
而mybatis-plus就为此提供了非常简便的方式，
只需要在字段上加上 @TableField(typeHandler = XxxTypeHandler.class)，
mybatis-plus就会自动帮你做转换，通用一般就两个：
   - com.baomidou.mybatisplus.extension.handlers.JacksonTypeHandler
   - com.baomidou.mybatisplus.extension.handlers.FastjsonTypeHandler

@Data
@TableName(autoResultMap = true)
public class Department implements Serializable {
    private static final long serialVersionUID = 203788572415896870L;
    @TableField(typeHandler = FastjsonTypeHandler.class)
    private List<Integer> userIds;
}

如果使用通用处理器，那对于基础类型以及对象来说没有什么问题。
但如果存储的字段类型是对象集合，那么当你取出来时，会发现集合中的对象都是JSONObject类型。
最常见的情况就拿出来进行遍历操作时，会抛出强转异常：
    java.lang.ClassCastException: com.alibaba.fastjson.JSONObject cannot be cast to ...
因为处理器帮你转换时，并不会存储你集合的泛型，所以统统都按照Object类型来转换了：
    @Override
    protected Object parse(String json) {
        return JSON.parseObject(json, type);
    }

@Data
@TableName(autoResultMap = true)
public class Department implements Serializable {

    private static final long serialVersionUID = 203788572415896870L;
    @TableId(value = "id", type = IdType.AUTO)
    private Integer id;
    @TableField(typeHandler = FastjsonTypeHandler.class)
    private List<User> users;
    @Data
    public static class USer implements Serializable {
        // ...
    }
}

@MappedTypes({Object.class})
@MappedJdbcTypes(JdbcType.VARCHAR)
public class ListFastJsonTypeHandler extends FastjsonTypeHandler {

    private final Class<? extends Object> type;
    public ListFastJsonTypeHandler(Class<?> type) {
        super(type);
        this.type = type;
    }
    
    @Override
    protected Object parse(String json) {
        return JSON.parseArray(json, this.type);
}

<mapper namespace="com.xxx.cn.mapper.DepartmentMapper">
    <resultMap id="BaseResultMap" type="com.xxx.cn.domain.Department">
        <id property="id" column="id"/>
        <result property="users" column="users" jdbcType="VARCHAR"
                javaType="com.xxx.cn.domain.Department.User"
                typeHandler="com.baomidou.mybatisplus.extension.handlers.FastjsonTypeHandler"/>
    </resultMap>
</mapper>

@Component
@Intercepts({
        @Signature(type = ResultSetHandler.class, method = "handleResultSets", args = {Statement.class})
})
public class ResultSetInterceptor implements Interceptor {

    
    private final SerializerFeature[] serializerFeatures = {
            SerializerFeature.WriteMapNullValue,
            SerializerFeature.WriteNullListAsEmpty,
            SerializerFeature.WriteNullStringAsEmpty
    };
    @Override
    public Object intercept(Invocation invocation) throws Throwable {
        Object proceed = invocation.proceed();
        if (containJSONObject(proceed)) {
            if (proceed instanceof Collection) {
                return JSON.parseArray(JSON.toJSONString(proceed, serializerFeatures), ((Collection<?>) proceed).toArray()[0].getClass());
            }
            return JSON.parseObject(JSON.toJSONString(proceed, serializerFeatures), proceed.getClass());
        }
//        if (proceed instanceof Collection) {
//            for (Object obj : ((Collection<?>) proceed)) {
//                parseJSON2Object(obj, obj.getClass());
//            }
//        } else {
//            parseJSON2Object(proceed, proceed.getClass());
//        }
        return proceed;
    }
     * 将返回数据中心的JSONObject对象转换成正常的对象
     *
     * @param obj
     * @param typeClass
     * @throws IllegalAccessException
     * @throws ClassNotFoundException
    private void parseJSON2Object(Object obj, Class<?> typeClass) throws IllegalAccessException, ClassNotFoundException {
        for (Field declaredField : typeClass.getDeclaredFields()) {
            declaredField.setAccessible(true);
            Object value = declaredField.get(obj);
            if (isNullValueField(value)) {
                continue;
            Type genericType = declaredField.getGenericType();
            String fieldClassName = genericType.getTypeName();
            if (genericType instanceof ParameterizedType) {
                fieldClassName = ((ParameterizedType) genericType).getActualTypeArguments()[0].getTypeName();
            if (containJSONObject(value)) {
                if (value instanceof Collection) {
                    declaredField.set(obj, JSON.parseArray(JSON.toJSONString(value, serializerFeatures), Class.forName(fieldClassName)));
                } else {
                    declaredField.set(obj, JSON.parseObject(JSON.toJSONString(value, serializerFeatures), Class.forName(fieldClassName)));
                }
     * 判断是否跳过字段
     * @param value
     * @return
    private Boolean isNullValueField(Object value) {
        return null == value || "".equals(String.valueOf(value).trim())
                || (value instanceof Collection && ((Collection<?>) value).size() == 0);
     * 判断值是否包含JSONObject对象
    private boolean containJSONObject(Object value) throws IllegalAccessException {
        if (isNullValueField(value)) {
            return false;
        if (value instanceof Collection) {
            for (Object obj : (Collection<?>) value) {
                if (obj instanceof JSONObject) {
                    return true;
                if (obj instanceof Collection && containJSONObject(obj)) {
                    for (Field declaredField : obj.getClass().getDeclaredFields()) {
                        declaredField.setAccessible(true);
                        Object fieldValue = declaredField.get(obj);
                        if (isNullValueField(fieldValue)) {
                            continue;
                        }
                        if (fieldValue instanceof JSONObject) {
                            return true;
                        if (fieldValue instanceof Collection && containJSONObject(fieldValue)) {
                    }
        return value instanceof JSONObject;
}