mybatis的@MappedTypes

“Given enough eyeballs, all bugs are shallow.” — Linus’s Law, Eric S. Raymond

看到com.baomidou.mybatisplus.extension.handlers.JacksonTypeHandler的源码

/*
 * Copyright (c) 2011-2023, baomidou (jobob@qq.com).
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package com.baomidou.mybatisplus.extension.handlers;

import com.baomidou.mybatisplus.core.toolkit.Assert;
import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.databind.ObjectMapper;
import lombok.extern.slf4j.Slf4j;
import org.apache.ibatis.type.JdbcType;
import org.apache.ibatis.type.MappedJdbcTypes;
import org.apache.ibatis.type.MappedTypes;

import java.io.IOException;

/**
 * Jackson 实现 JSON 字段类型处理器
 *
 * @author hubin
 * @since 2019-08-25
 */
@Slf4j
@MappedTypes({Object.class})
@MappedJdbcTypes(JdbcType.VARCHAR)
public class JacksonTypeHandler extends AbstractJsonTypeHandler<Object> {
    private static ObjectMapper OBJECT_MAPPER;
    private final Class<?> type;

    public JacksonTypeHandler(Class<?> type) {
        if (log.isTraceEnabled()) {
            log.trace("JacksonTypeHandler(" + type + ")");
        }
        Assert.notNull(type, "Type argument cannot be null");
        this.type = type;
    }

    @Override
    protected Object parse(String json) {
        try {
            return getObjectMapper().readValue(json, type);
        } catch (IOException e) {
            throw new RuntimeException(e);
        }
    }

    @Override
    protected String toJson(Object obj) {
        try {
            return getObjectMapper().writeValueAsString(obj);
        } catch (JsonProcessingException e) {
            throw new RuntimeException(e);
        }
    }

    public static ObjectMapper getObjectMapper() {
        if (null == OBJECT_MAPPER) {
            OBJECT_MAPPER = new ObjectMapper();
        }
        return OBJECT_MAPPER;
    }

    public static void setObjectMapper(ObjectMapper objectMapper) {
        Assert.notNull(objectMapper, "ObjectMapper should not be null");
        JacksonTypeHandler.OBJECT_MAPPER = objectMapper;
    }
}

这里有个@MappedTypes({Object.class})

研究了下，发现它用于指定一个或多个 Java 类型到数据库类型的映射

例如

@MappedTypes(UUID.class)
public class UUIDTypeHandler extends BaseTypeHandler<UUID> {
    // 实现类型转换逻辑
}

这样就会默认使用这个typeHandler而不需要额外在每个字段上指定