why???

为什么必须把WebAppApplication启动类放到com.echo目录下才能正确使用到common模块并扫描到mapper，放com.echo.webapp下就扫描不到了

pom.xml

<dependencies>
    <dependency>
        <groupId>com.echo</groupId>
        <artifactId>model</artifactId>
        <version>1.0-SNAPSHOT</version>
        <scope>compile</scope>
    </dependency>

    <dependency>
        <groupId>com.echo</groupId>
        <artifactId>common</artifactId>
        <version>1.0-SNAPSHOT</version>
        <scope>compile</scope>
    </dependency>
</dependencies>