什么是自動(dòng)映射？

介紹自動(dòng)映射之前先看一下手動(dòng)映射，如下：

<resultMap type='com.javacode2018.chat05.demo7.model.OrderModel'> <id column='id' property='id'/> <result column='userId' property='userId' /> <result column='createTime' property='createTime' /> <result column='upTime' property='upTime' /></resultMap> <select resultMap='orderModelMap1'> <![CDATA[ SELECT a.id, a.user_id userId, a.create_time createTime, a.up_time upTime FROM t_order a WHERE a.id = #{value} ]]></select>

注意上面的resultMap元素中有4行配置，如下：

<id column='id' property='id'/><result column='userId' property='userId' /><result column='createTime' property='createTime' /><result column='upTime' property='upTime' />

這4行代碼用于配置sql結(jié)果的列和OrderModel對(duì)象中字段的映射關(guān)系。

大家有沒有注意到，映射規(guī)則中column和property元素的值都是一樣，mybatis中支持自動(dòng)映射配置，當(dāng)開啟自動(dòng)映射之后，當(dāng)sql的列名和Model中的字段名稱是一樣的時(shí)候（不區(qū)分大小寫），mybatis內(nèi)部會(huì)進(jìn)行自動(dòng)映射，不需要我們手動(dòng)去寫上面的4行映射規(guī)則。

下面我們將上面的示例改成自動(dòng)映射的方式，如下：

<resultMap type='com.javacode2018.chat05.demo7.model.OrderModel' autoMapping='true'></resultMap><select resultMap='orderModelMap2'> <![CDATA[ SELECT a.id, a.user_id userId, a.create_time createTime, a.up_time upTime FROM t_order a WHERE a.id = #{value} ]]></select>

注意上面的resultMap中的autoMapping屬性，是否開啟自動(dòng)映射，我們?cè)O(shè)置為true，這樣mybatis會(huì)自動(dòng)按照列名和Model中同名的字段進(jìn)行映射賦值。

上面兩個(gè)配置最后查詢結(jié)果是一樣的，都會(huì)將查詢結(jié)果對(duì)應(yīng)的4個(gè)字段的值自動(dòng)賦值給OrderModel中同名的屬性。

自動(dòng)映射開關(guān)

mybatis中自動(dòng)映射主要有2種配置，一種是全局的配置，對(duì)應(yīng)用中所有的resultMap起效，這個(gè)是在mybatis配置文件中進(jìn)行設(shè)置的；另外一種是通過(guò)resultMap的autoMapping屬性進(jìn)行配置。

mybatis判斷某個(gè)resultMap是否開啟自動(dòng)映射配置的時(shí)候，會(huì)先查找自身的autoMapping屬性，如果這個(gè)屬性設(shè)置值了，就直接用這個(gè)屬性的值，如果resultMap元素的autoMapping屬性沒有配置，則走全局配置的自動(dòng)映射規(guī)則。

下面我們來(lái)詳解介紹一下這款的內(nèi)容。

mybatis自動(dòng)映射全局配置

在mybatis全局配置文件中加入下面配置：

<settings> <setting name='autoMappingBehavior' value='自動(dòng)映射規(guī)則'/></settings>

autoMappingBehavior值來(lái)源于枚舉：org.apache.ibatis.session.AutoMappingBehavior，源碼：

public enum AutoMappingBehavior { /** * Disables auto-mapping. */ NONE, /** * Will only auto-map results with no nested result mappings defined inside. */ PARTIAL, /** * Will auto-map result mappings of any complexity (containing nested or otherwise). */ FULL} NONE：關(guān)閉全局映射開關(guān) PARTIAL：對(duì)除在內(nèi)部定義了嵌套結(jié)果映射（也就是連接的屬性）以外的屬性進(jìn)行映射，這個(gè)也是默認(rèn)值。 FULL：自動(dòng)映射所有屬性。

小提示：settings元素中有很多配置，這些配置最后都會(huì)被解析成org.apache.ibatis.session.Configuration的屬性，源碼位于org.apache.ibatis.builder.xml.XMLConfigBuilder#settingsElement方法中。

下面我們來(lái)演示一下autoMappingBehavior每種配置的效果。

NONE

mybatis-config.xml加入配置

<settings> <!-- 關(guān)閉自動(dòng)映射開關(guān) --> <setting name='autoMappingBehavior' value='NONE'/></settings>

OrderMapper.xml

<resultMap type='com.javacode2018.chat05.demo7.model.OrderModel'></resultMap><select resultMap='orderModelMap4'> <![CDATA[ SELECT a.id, a.user_id userId, a.create_time createTime, a.up_time upTime FROM t_order a WHERE a.id = #{value} ]]></select>

OrderMapper.java加入

OrderModel getById4(int id);

測(cè)試用例

com.javacode2018.chat05.demo7.Demo7Test#getById4@Testpublic void getById4() throws IOException { this.before('demo7/mybatis-config1.xml'); try (SqlSession sqlSession = this.sqlSessionFactory.openSession(true);) { OrderMapper mapper = sqlSession.getMapper(OrderMapper.class); OrderModel orderModel = mapper.getById4(2); log.info('{}', orderModel); }}

運(yùn)行結(jié)果

21:58.821 [main] DEBUG c.j.c.d.mapper.OrderMapper.getById4 - ==> Preparing: SELECT a.id, a.user_id userId, a.create_time createTime, a.up_time upTime FROM t_order a WHERE a.id = ? 21:58.850 [main] DEBUG c.j.c.d.mapper.OrderMapper.getById4 - ==> Parameters: 2(Integer)21:58.868 [main] DEBUG c.j.c.d.mapper.OrderMapper.getById4 - <== Total: 121:58.868 [main] INFO c.j.chat05.demo7.Demo7Test - null

從輸出中可以看到最后一樣輸出結(jié)果為null，sql實(shí)際上返回的是有結(jié)果的，但是結(jié)果映射的時(shí)候返回的是空。

結(jié)果解釋

由于mybatis全局配置中將autoMappingBehavior的值置為了NONE,表示全局自動(dòng)映射被關(guān)閉了，而resultMapper中的orderModelMap4沒有配置autoMapping屬性，所以最終這個(gè)查詢結(jié)果不會(huì)自動(dòng)映射，所以最后查詢結(jié)果為null。

PARTIAL

對(duì)除在內(nèi)部定義了嵌套結(jié)果映射（也就是連接的屬性）以外的屬性進(jìn)行映射，這個(gè)也是autoMappingBehavior的默認(rèn)值。

mybatis-config.xml加入配置

<settings> <!-- 對(duì)除在內(nèi)部定義了嵌套結(jié)果映射（也就是連接的屬性）以外的屬性進(jìn)行映射，這個(gè)也是autoMappingBehavior的默認(rèn)值。 --> <setting name='autoMappingBehavior' value='PARTIAL'/></settings>

OrderMapper.xml

<resultMap type='com.javacode2018.chat05.demo7.model.OrderModel'></resultMap><select resultMap='orderModelMap5'> <![CDATA[ SELECT a.id, a.user_id userId, a.create_time createTime, a.up_time upTime FROM t_order a WHERE a.id = #{value} ]]></select>

OrderMapper.java加入

OrderModel getById5(int id);

測(cè)試用例

com.javacode2018.chat05.demo7.Demo7Test#getById5@Testpublic void getById5() throws IOException { this.before('demo7/mybatis-config2.xml'); try (SqlSession sqlSession = this.sqlSessionFactory.openSession(true);) { OrderMapper mapper = sqlSession.getMapper(OrderMapper.class); OrderModel orderModel = mapper.getById5(2); log.info('{}', orderModel); }}

運(yùn)行結(jié)果

28:32.612 [main] DEBUG c.j.c.d.mapper.OrderMapper.getById5 - ==> Preparing: SELECT a.id, a.user_id userId, a.create_time createTime, a.up_time upTime FROM t_order a WHERE a.id = ? 28:32.648 [main] DEBUG c.j.c.d.mapper.OrderMapper.getById5 - ==> Parameters: 2(Integer)28:32.664 [main] DEBUG c.j.c.d.mapper.OrderMapper.getById5 - <== Total: 128:32.665 [main] INFO c.j.chat05.demo7.Demo7Test - OrderModel(id=2, userId=1, createTime=1577947790, upTime=1577947790, userModel=null)

OrderModel中的4個(gè)屬性被自動(dòng)映射成功了。

結(jié)果解釋

orderModelMap5中沒有指定autoMapping屬性，所以自動(dòng)映射會(huì)走全局配置的規(guī)則，即PARTIAL，會(huì)進(jìn)行自動(dòng)映射。

我們?cè)賮?lái)看看PARTIAL的解釋：對(duì)除在內(nèi)部定義了嵌套結(jié)果映射（也就是連接的屬性）以外的屬性進(jìn)行映射。這句話是什么意思？

有些復(fù)雜的查詢映射會(huì)在resultMap中嵌套一些映射（如：association，collection），當(dāng)使用PARTIAL的時(shí)候，如果有嵌套映射，則這個(gè)嵌套映射不會(huì)進(jìn)行自動(dòng)映射了。

通過(guò)訂單id查詢出訂單以及訂單用戶的信息，sqlmap如下：

<resultMap type='com.javacode2018.chat05.demo7.model.OrderModel'> <association property='userModel'> </association></resultMap><select resultMap='orderModelMap6'> <![CDATA[ SELECT a.id, a.user_id userId, a.create_time createTime, a.up_time upTime b.id as user_id, b.name FROM t_order a,t_user b WHERE a.user_id = b.id AND a.id = #{value} ]]></select>

OrderModel.java

package com.javacode2018.chat05.demo7.model;import lombok.*;import java.util.List;@Getter@Setter@Builder@ToString@NoArgsConstructor@AllArgsConstructorpublic class OrderModel { private Integer id; private Integer userId; private Long createTime; private Long upTime; private UserModel userModel;}

內(nèi)部有個(gè)userModel屬性引用用戶對(duì)象。

UserModel.java

package com.javacode2018.chat05.demo7.model;import lombok.*;@Getter@Setter@Builder@ToString@NoArgsConstructor@AllArgsConstructorpublic class UserModel { private Integer id; private String name;}

OrderMapper.java中加入

OrderModel getById6(int id);

測(cè)試用例

com.javacode2018.chat05.demo7.Demo7Test#getById6@Testpublic void getById6() throws IOException { this.before('demo7/mybatis-config2.xml'); try (SqlSession sqlSession = this.sqlSessionFactory.openSession(true);) { OrderMapper mapper = sqlSession.getMapper(OrderMapper.class); OrderModel orderModel = mapper.getById6(2); log.info('{}', orderModel); }}

運(yùn)行輸出

52:49.037 [main] DEBUG c.j.c.d.mapper.OrderMapper.getById6 - ==> Preparing: SELECT a.id, a.user_id userId, a.create_time createTime, a.up_time upTime, b.id as user_id, b.name FROM t_order a,t_user b WHERE a.user_id = b.id AND a.id = ? 52:49.066 [main] DEBUG c.j.c.d.mapper.OrderMapper.getById6 - ==> Parameters: 2(Integer)52:49.087 [main] DEBUG c.j.c.d.mapper.OrderMapper.getById6 - <== Total: 152:49.088 [main] INFO c.j.chat05.demo7.Demo7Test - null

sql查詢實(shí)際上是有一條記錄的，但是最后返回的是null，說(shuō)明沒有進(jìn)行自動(dòng)映射。

FULL

自動(dòng)映射所有屬性。

這次Mapper我們不動(dòng)，還是下面這樣，沒有手動(dòng)指定映射規(guī)則。

<resultMap type='com.javacode2018.chat05.demo7.model.OrderModel'> <association property='userModel'> </association></resultMap><select resultMap='orderModelMap6'> <![CDATA[ SELECT a.id, a.user_id userId, a.create_time createTime, a.up_time upTime b.id as user_id, b.name FROM t_order a,t_user b WHERE a.user_id = b.id AND a.id = #{value} ]]></select>

修改一下autoMappingBehavior的值為FULL，看看效果。

mybatis配置

<settings> <!-- 自動(dòng)映射所有屬性 --> <setting name='autoMappingBehavior' value='FULL'/></settings>

測(cè)試用例

com.javacode2018.chat05.demo7.Demo7Test#getById6_0@Testpublic void getById6_0() throws IOException { this.before('demo7/mybatis-config3.xml'); try (SqlSession sqlSession = this.sqlSessionFactory.openSession(true);) { OrderMapper mapper = sqlSession.getMapper(OrderMapper.class); OrderModel orderModel = mapper.getById6(2); log.info('{}', orderModel); }}

運(yùn)行輸出

56:05.127 [main] DEBUG c.j.c.d.mapper.OrderMapper.getById6 - ==> Preparing: SELECT a.id, a.user_id userId, a.create_time createTime, a.up_time upTime, b.id as user_id, b.name FROM t_order a,t_user b WHERE a.user_id = b.id AND a.id = ? 56:05.155 [main] DEBUG c.j.c.d.mapper.OrderMapper.getById6 - ==> Parameters: 2(Integer)56:05.186 [main] DEBUG c.j.c.d.mapper.OrderMapper.getById6 - <== Total: 156:05.186 [main] INFO c.j.chat05.demo7.Demo7Test - OrderModel(id=2, userId=1, createTime=1577947790, upTime=1577947790, userModel=UserModel(id=2, name=張學(xué)友))

輸出中可以看到OrderModel所有屬性都是有值的，userModel的2個(gè)屬性也有值，userModel.id是2，我們運(yùn)行一下sql看看，用戶id是多少，如下：

mysql> SELECT a.id, a.user_id userId, a.create_time createTime, a.up_time upTime, b.id as user_id, b.name FROM t_order a,t_user b WHERE a.user_id = b.id AND a.id = 2;+----+--------+------------+------------+---------+-----------+| id | userId | createTime | upTime | user_id | name |+----+--------+------------+------------+---------+-----------+| 2 | 1 | 1577947790 | 1577947790 | 1 | 張學(xué)友 |+----+--------+------------+------------+---------+-----------+1 row in set (0.00 sec)

user_id實(shí)際上是1，mybatis中按照sql字段和model結(jié)果字段同名進(jìn)行自動(dòng)映射，所以將訂單的id賦值給userModel的id屬性了。

此時(shí)需要我們orderModelMap6的配置，手動(dòng)指定一下user_id和userModel.id的映射規(guī)則，如下：

<resultMap type='com.javacode2018.chat05.demo7.model.OrderModel'> <association property='userModel'> <id column='user_id' property='id'/> </association></resultMap>

再次運(yùn)行測(cè)試用例

com.javacode2018.chat05.demo7.Demo7Test#getById6_0

輸出

15:02.751 [main] DEBUG c.j.c.d.mapper.OrderMapper.getById6 - ==> Preparing: SELECT a.id, a.user_id userId, a.create_time createTime, a.up_time upTime, b.id as user_id, b.name FROM t_order a,t_user b WHERE a.user_id = b.id AND a.id = ? 15:02.783 [main] DEBUG c.j.c.d.mapper.OrderMapper.getById6 - ==> Parameters: 2(Integer)15:02.801 [main] DEBUG c.j.c.d.mapper.OrderMapper.getById6 - <== Total: 115:02.801 [main] INFO c.j.chat05.demo7.Demo7Test - OrderModel(id=2, userId=1, createTime=1577947790, upTime=1577947790, userModel=UserModel(id=1, name=張學(xué)友))

這次userModel中的id正確了。

autoMapping使用

上面我們有說(shuō)過(guò)，當(dāng)在resultMap中指定了autoMapping屬性之后，這個(gè)resultMap的自動(dòng)映射就受autoMapping屬性的控制，和mybatis中全局映射配置（autoMappingBehavior）行為無(wú)關(guān)了。

案例1

這個(gè)核心配置主要在sqlmap中，如下：

<resultMap type='com.javacode2018.chat05.demo7.model.OrderModel' autoMapping='true'> <association property='userModel' autoMapping='true'> <id column='user_id' property='id'/> </association></resultMap><select resultMap='orderModelMap7'> <![CDATA[ SELECT a.id, a.user_id userId, a.create_time createTime, a.up_time upTime, b.id as user_id, b.name FROM t_order a,t_user b WHERE a.user_id = b.id AND a.id = #{value} ]]></select>

對(duì)應(yīng)測(cè)試用例

com.javacode2018.chat05.demo7.Demo7Test#getById7@Testpublic void getById7() throws IOException { this.before('demo7/mybatis-config1.xml'); try (SqlSession sqlSession = this.sqlSessionFactory.openSession(true);) { OrderMapper mapper = sqlSession.getMapper(OrderMapper.class); OrderModel orderModel = mapper.getById7(2); log.info('{}', orderModel); }}

運(yùn)行輸出

24:37.544 [main] DEBUG c.j.c.d.mapper.OrderMapper.getById7 - ==> Preparing: SELECT a.id, a.user_id userId, a.create_time createTime, a.up_time upTime, b.id as user_id, b.name FROM t_order a,t_user b WHERE a.user_id = b.id AND a.id = ? 24:37.589 [main] DEBUG c.j.c.d.mapper.OrderMapper.getById7 - ==> Parameters: 2(Integer)24:37.610 [main] DEBUG c.j.c.d.mapper.OrderMapper.getById7 - <== Total: 124:37.610 [main] INFO c.j.chat05.demo7.Demo7Test - OrderModel(id=2, userId=1, createTime=1577947790, upTime=1577947790, userModel=UserModel(id=1, name=張學(xué)友))

OrderModel中所有屬性都自動(dòng)映射成功。

自動(dòng)裝配并不是那么好玩，玩不轉(zhuǎn)可能帶來(lái)一些隱患，我們看一個(gè)案例，見下面的示例2。

示例2

根據(jù)訂單編號(hào)，查詢出訂單信息，順便查詢出訂單明細(xì)列表。這個(gè)我們使用mybatis中的一對(duì)多查詢。

OrderDetaiMapper.xml加入

<select resultType='com.javacode2018.chat05.demo7.model.OrderDetailModel'> <![CDATA[ SELECT a.id, a.order_id AS orderId, a.goods_id AS goodsId, a.num, a.total_price AS totalPrice FROM t_order_detail a WHERE a.order_id = #{value} ]]></select>

這個(gè)可以根據(jù)訂單的id，查詢出訂單關(guān)聯(lián)的明細(xì)列表。

OrderMapper.xml加入

<resultMap type='com.javacode2018.chat05.demo7.model.OrderModel' autoMapping='true'> <collection property='orderDetailModelList' select='com.javacode2018.chat05.demo7.mapper.OrderDetailMapper.getListByOrderId1' column='id'/></resultMap> <select resultMap='orderModelMap8'> <![CDATA[ SELECT a.id, a.user_id userId, a.create_time createTime, a.up_time upTime FROM t_order a WHERE a.id = #{value} ]]></select>

測(cè)試用例

com.javacode2018.chat05.demo7.Demo7Test#getById8@Testpublic void getById8() throws IOException { this.before('demo7/mybatis-config.xml'); try (SqlSession sqlSession = this.sqlSessionFactory.openSession(true);) { OrderMapper mapper = sqlSession.getMapper(OrderMapper.class); OrderModel orderModel = mapper.getById8(1); log.info('{}', orderModel); }}

運(yùn)行輸出

11:06.193 [main] DEBUG c.j.c.d.mapper.OrderMapper.getById8 - ==> Preparing: SELECT a.id, a.user_id userId, a.create_time createTime, a.up_time upTime FROM t_order a WHERE a.id = ? 11:06.229 [main] DEBUG c.j.c.d.mapper.OrderMapper.getById8 - ==> Parameters: 1(Integer)11:06.250 [main] DEBUG c.j.c.d.m.O.getListByOrderId1 - ====> Preparing: SELECT a.id, a.order_id AS orderId, a.goods_id AS goodsId, a.num, a.total_price AS totalPrice FROM t_order_detail a WHERE a.order_id = ? 11:06.251 [main] DEBUG c.j.c.d.m.O.getListByOrderId1 - ====> Parameters: 1(Integer)11:06.255 [main] DEBUG c.j.c.d.m.O.getListByOrderId1 - <==== Total: 211:06.256 [main] DEBUG c.j.c.d.mapper.OrderMapper.getById8 - <== Total: 111:06.256 [main] INFO c.j.chat05.demo7.Demo7Test - OrderModel(id=null, userId=2, createTime=1577947790, upTime=1577947790, userModel=null, orderDetailModelList=[OrderDetailModel(id=1, orderId=1, goodsId=1, num=2, totalPrice=17.76), OrderDetailModel(id=2, orderId=1, goodsId=1, num=1, totalPrice=16.66)])

注意輸出中OrderModel的id屬性，怎么是null值？主要是下面這行配置導(dǎo)致的

<collection property='orderDetailModelList' select='com.javacode2018.chat05.demo7.mapper.OrderDetailMapper.getListByOrderId1' column='id'/>

上面這個(gè)配置中有個(gè)column屬性，指定的是id，此時(shí)mybatis認(rèn)為你對(duì)id字段手動(dòng)指定了映射關(guān)系，就跳過(guò)了對(duì)id字段到OrderModel.id屬性的自動(dòng)映射，所以導(dǎo)致OrderModel對(duì)象的id屬性沒有賦值，此時(shí)需要我們?cè)趏rderModelMap8手動(dòng)指定id的映射規(guī)則，如下：

<resultMap type='com.javacode2018.chat05.demo7.model.OrderModel' autoMapping='true'> <id column='id' property='id' /> <collection property='orderDetailModelList' select='com.javacode2018.chat05.demo7.mapper.OrderDetailMapper.getListByOrderId1' column='id'/></resultMap>

再去運(yùn)行測(cè)試用例就正常了。

總結(jié)一下

對(duì)于咱們開發(fā)來(lái)說(shuō)，自動(dòng)映射確實(shí)可以幫助我們節(jié)省一些代碼，不過(guò)也存在一些隱患，我們希望自己開發(fā)的系統(tǒng)是健壯的，建議大家寫mapper xml的時(shí)候，還是花點(diǎn)時(shí)間將映射的配置都給寫上去，這樣能夠杜絕一些隱患，使我們的系統(tǒng)更穩(wěn)定。

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