在前面的文章<Mybatis配置之<properties>属性配置元素详述>,我们讲述了<properties>标签元素的配置和使用方法。在这篇文章中,我们来说说<typeAliases>标签元素,这个元素主要是用于对类型进行别名控制,具体什么意思呢?我们下面用一个示例说明,看了之后我相信你就会明白了。
这里我们贴出之前的UserDao对应的mapper文件,如下所示:
- <span style="font-family:'KaiTi_GB2312';font-size:18px;"><?xml version="1.0" encoding="UTF-8" ?>
- <!DOCTYPE mapper
- PUBLIC "-//ibatis.apache.org//DTD Mapper 3.0//EN"
- "http://ibatis.apache.org/dtd/ibatis-3-mapper.dtd">
- <mapper namespace="com.majing.learning.mybatis.dao.UserDao">
- <select id="findUserById" resultType="com.majing.learning.mybatis.entity.User">
- select * from user where id = #{id}
- </select>
- <insert id="addUser" parameterType="com.majing.learning.mybatis.entity.User" useGeneratedKeys="true" keyProperty="id">
- insert into user(name,password,age) values(#{name},#{password},#{age})
- </insert>
- <delete id="deleteUser" parameterType="int">
- delete from user where id = #{id}
- </delete>
- <update id="updateUser" parameterType="com.majing.learning.mybatis.entity.User">
- update user set name = #{name}, password = #{password}, age = #{age} where id = #{id}
- </update>
- </mapper></span>
从这个配置文件中,我们可以看到<select>、<insert>和<update>三个标签元素的resultType都是User对象,需要设置这个User对象的类全限定名,即packname.classname。
我们发现一个问题,那就是这个类名,我们需要写多次,如果要改这个类名的话,我们需要在多个地方进行修改。很明显,这样配置的话很容易造成修改上的遗漏,同时也书写上也比较麻烦。因此,MyBatis为我们提供了一个简单方便的配置方法,那就是使用<typeAliases>标签元素,给实体类设置一个别名。
如上所示,我们在原来的mybatis配置文件中增加了<typeAliases>标签,并将com.majing.learning.mybatis.entity.User这个实体类重命名为User,然后我们在mapper配置文件中就可以如下使用了。
备注:这里需要注意的是,typeAliases配置需要放置在settings之后,否则会出异常!!!
<?xml version="1.0" encoding="UTF-8"?> <!DOCTYPE configuration PUBLIC "-//mybatis.org//DTD Config 3.0//EN" "http://mybatis.org/dtd/mybatis-3-config.dtd"> <configuration> <properties resource="mysql.properties"> <property name="username" value="root"/> <property name="password" value="root"/> <property name="driver" value="com.mysql.jdbc.Driver"/> <property name="url" value="jdbc:mysql://localhost:3306/mybatis?characterEncoding=utf-8"/> </properties> <settings> <setting name="logImpl" value="LOG4J" /> </settings> <typeAliases> <typeAlias alias="User" type="com.majing.learning.mybatis.entity.User"/> </typeAliases></span> <!-- 和spring整合后 environments配置将废除 --> <environments default="development"> <environment id="development"> <!-- 使用jdbc事务管理 --> <transactionManager type="JDBC" /> <!-- 数据库连接池 --> <dataSource type="POOLED"> <property name="driver" value="${driver}" /> <property name="url" value="${url}" /> <property name="username" value="${username}" /> <property name="password" value="${password}" /> </dataSource> </environment> </environments> <mappers> <mapper resource="com\majing\learning\mybatis\dao\UserDaoMapper.xml" /> </mappers> </configuration>
- <span style="font-family:'KaiTi_GB2312';font-size:18px;"><?xml version="1.0" encoding="UTF-8" ?>
- <!DOCTYPE mapper
- PUBLIC "-//ibatis.apache.org//DTD Mapper 3.0//EN"
- "http://ibatis.apache.org/dtd/ibatis-3-mapper.dtd">
- <mapper namespace="com.majing.learning.mybatis.dao.UserDao">
- <select id="findUserById" resultType="User">
- select * from user where id = #{id}
- </select>
- <insert id="addUser" parameterType="User" useGeneratedKeys="true" keyProperty="id">
- insert into user(name,password,age) values(#{name},#{password},#{age})
- </insert>
- <delete id="deleteUser" parameterType="int">
- delete from user where id = #{id}
- </delete>
- <update id="updateUser" <span style="color:#FF0000;"><span style="color:#000000;">parameterType="User"</span></span>>
- update user set name = #{name}, password = #{password}, age = #{age} where id = #{id}
- </update>
- </mapper></span>
这样即使实体类名修改了,所需要修改的地方也只有一处,便于集中管理。
也许你会有疑问,如果实体类比较多怎么办?还不是要配置很多实体类和别名,NO,NO,NO!下面跟大家说说另一种配置方法。
- <span style="font-family:'KaiTi_GB2312';font-size:18px;"><?xml version="1.0" encoding="UTF-8"?>
- <!DOCTYPE configuration
- PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
- "http://mybatis.org/dtd/mybatis-3-config.dtd">
- <configuration>
- <properties resource="mysql.properties">
- <property name="username" value="root"/>
- <property name="password" value="root"/>
- <property name="driver" value="com.mysql.jdbc.Driver"/>
- <property name="url" value="jdbc:mysql://localhost:3306/mybatis?characterEncoding=utf-8"/>
- </properties>
- <settings>
- <setting name="logImpl" value="LOG4J" />
- </settings>
- <typeAliases>
- <package name="com.majing.learning.mybatis.entity"/>
- </typeAliases>
- <!-- 和spring整合后 environments配置将废除 -->
- <environments default="development">
- <environment id="development">
- <!-- 使用jdbc事务管理 -->
- <transactionManager type="JDBC" />
- <!-- 数据库连接池 -->
- <dataSource type="POOLED">
- <property name="driver" value="${driver}" />
- <property name="url" value="${url}" />
- <property name="username" value="${username}" />
- <property name="password" value="${password}" />
- </dataSource>
- </environment>
- </environments>
- <mappers>
- <mapper resource="com\majing\learning\mybatis\dao\UserDaoMapper.xml" />
- </mappers>
- </configuration></span>
在这里,我们不再使用<typeAliases>标签下<typeAliase>,而是使用<package>标签,表示扫描该包名下的所有类(除了接口和匿名内部类),如果类名上有注解,则使用注解指定的名称作为别名,如果没有则使用类名首字母小写作为别名,如com.majing.learning.mybatis.entity.User这个类如果没有设置@Alias注解,则此时会被关联到user这个别名上。
因此,按照上面的配置,我们还需要将实体类做一下调整,如下两种方式所示:
(1)给实体类添加@Alias注解
- <span style="font-family:'KaiTi_GB2312';font-size:18px;">package com.majing.learning.mybatis.entity;
- import org.apache.ibatis.type.Alias;
- @Alias(value="User")
- public class User {
- private int id;
- private String name;
- private String password;
- private int age;
- public int getId() {
- return id;
- }
- public void setId(int id) {
- this.id = id;
- }
- public String getName() {
- return name;
- }
- public void setName(String name) {
- this.name = name;
- }
- public String getPassword() {
- return password;
- }
- public void setPassword(String password) {
- this.password = password;
- }
- @Override
- public String toString() {
- return "User [id=" + id + ", name=" + name + ", password=" + password + ", age=" + age + "]";
- }
- public int getAge() {
- return age;
- }
- public void setAge(int age) {
- this.age = age;
- }
- }
- </span>
(2)实体类不加注解的情况下,修改mapper文件中引用的类型别名,改为小写,如下所示:
- <span style="font-family:'KaiTi_GB2312';font-size:18px;"><?xml version="1.0" encoding="UTF-8" ?>
- <!DOCTYPE mapper
- PUBLIC "-//ibatis.apache.org//DTD Mapper 3.0//EN"
- "http://ibatis.apache.org/dtd/ibatis-3-mapper.dtd">
- <mapper namespace="com.majing.learning.mybatis.dao.UserDao">
- <select id="findUserById" resultType="user">
- select * from user where id = #{id}
- </select>
- <insert id="addUser" parameterType="user" useGeneratedKeys="true" keyProperty="id">
- insert into user(name,password,age) values(#{name},#{password},#{age})
- </insert>
- <delete id="deleteUser" parameterType="int">
- delete from user where id = #{id}
- </delete>
- <update id="updateUser" parameterType="user">
- update user set name = #{name}, password = #{password}, age = #{age} where id = #{id}
- </update>
- </mapper></span>
最后想说,mybatis为我们已经实现了很多别名,已经为许多常见的 Java 类型内建了相应的类型别名。它们都是大小写不敏感的,需要注意的是由基本类型名称重复导致的特殊处理。
别名 | 映射的类型 |
---|---|
_byte | byte |
_long | long |
_short | short |
_int | int |
_integer | int |
_double | double |
_float | float |
_boolean | boolean |
string | String |
byte | Byte |
long | Long |
short | Short |
int | Integer |
integer | Integer |
double | Double |
float | Float |
boolean | Boolean |
date | Date |
decimal | BigDecimal |
bigdecimal | BigDecimal |
object | Object |
map | Map |
hashmap | HashMap |
list | List |
arraylist | ArrayList |
collection | Collection |
iterator | Iterator |