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>