Mybatis核心配置文件优化

现在我们的配置文件是这样的，但是这样存在一些问题

• 配置文件显得很长
• 无法动态修改

<?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>
    <environments default="development">
        <environment id="development">
            <transactionManager type="JDBC"/>
            <dataSource type="POOLED">
                <property name="driver" value="com.mysql.jdbc.Driver"/>
                <property name="url" value="jdbc:mysql://localhost:3306/mybaits?useSSL=true&amp;useUnicode=true&amp;characterEncoding=UTF-8"/>
                <property name="username" value="root"/>
                <property name="password" value="000000"/>
            </dataSource>
        </environment>
    </environments>
    <mappers>
        <mapper resource="dao/UserMapper.xml" />
    </mappers>
</configuration>

properties属性优化

我们可以通过peoperties标签引入一个外部配置文件，拿的时候通过${}拿就可以了
写一个配置文件

driver=com.mysql.jdbc.Driver
url=jdbc:mysql://localhost:3306/mybaits?useSSL=true&useUnicode=true&characterEncoding=UTF-8
username=root
password=000000

然后Mybatis核心配置文件

<?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="db.properties" />

    <environments default="development">
        <environment id="development">
            <transactionManager type="JDBC"/>
            <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="dao/UserMapper.xml" />
    </mappers>
</configuration>

我们还可以这样，外部内部写一半

driver=com.mysql.jdbc.Driver
url=jdbc:mysql://localhost:3306/mybaits?useSSL=true&useUnicode=true&characterEncoding=UTF-8

<?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="db.properties">
        <property name="username" value="root"/>
        <property name="password" value="000000"/>
    </properties>

    <environments default="development">
        <environment id="development">
            <transactionManager type="JDBC"/>
            <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="dao/UserMapper.xml" />
    </mappers>
</configuration>

需要注意的是，假如我们内部外部都写的情况下，外部配置文件优先级>内部配置

driver=com.mysql.jdbc.Driver
url=jdbc:mysql://localhost:3306/mybaits?useSSL=true&useUnicode=true&characterEncoding=UTF-8
username=root
password=000000

<?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="db.properties">
        <property name="username" value="root"/>
        <property name="password" value="000000"/>
    </properties>

    <environments default="development">
        <environment id="development">
            <transactionManager type="JDBC"/>
            <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="dao/UserMapper.xml" />
    </mappers>
</configuration>

typeAlias起别名

我们可以给Java bean起别名，有三种方式
第一种
给一个类起别名

<typeAliases>
        <typeAlias type="pojo.User" alias="User" />
    </typeAliases>

第二种
直接扫描一个包

<typeAliases>
        <package name="pojo"/>
    </typeAliases>

第三种
给Java bean直接添加一个注解Alias(别名)

package pojo;

import org.apache.ibatis.type.Alias;

@Alias("User")
public class User {
    private int id;
    private String name;
    private String password;

    public User() {
    }

    public User(int id, String name, String password) {
        this.id = id;
        this.name = name;
        this.password = password;
    }

    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;
    }
}

Mapper映射器
有四种方式
第一种，推荐使用

<mappers>
  <mapper resource="org/mybatis/builder/AuthorMapper.xml"/>
  <mapper resource="org/mybatis/builder/BlogMapper.xml"/>
  <mapper resource="org/mybatis/builder/PostMapper.xml"/>
</mappers>

第二种，请不要使用这种

<!-- 使用完全限定资源定位符（URL） -->
<mappers>
  <mapper url="file:///var/mappers/AuthorMapper.xml"/>
  <mapper url="file:///var/mappers/BlogMapper.xml"/>
  <mapper url="file:///var/mappers/PostMapper.xml"/>
</mappers>

第三种

<!-- 使用映射器接口实现类的完全限定类名 -->
<mappers>
  <mapper class="org.mybatis.builder.AuthorMapper"/>
  <mapper class="org.mybatis.builder.BlogMapper"/>
  <mapper class="org.mybatis.builder.PostMapper"/>
</mappers>

第四种
扫描一整个包

<!-- 将包内的映射器接口实现全部注册为映射器 -->
<mappers>
  <package name="org.mybatis.builder"/>
</mappers>

需要注意的是，第三种和第四种方法在使用的时候会有问题，那就是接口和它的配置文件必须在同一个包下，否则就会报错！！