專案A的POM檔案有如下依賴宣告，optional表示該依賴隻影響專案A, 如果有專案B依賴了A，則這兩個依賴不會被傳遞；

因此如果B依賴了A,並且只用到了mysql，則需在B的POM中顯示宣告依賴mysql-connector-java即可

<dependencies>
<dependency>
<groupId>mysql</groupId>
<artifactId>mysql-connector-java</artifactId>
<version>5.1.10</version>
<optional>true</optional>
<dependency>
<dependency>
<groupId>postgresql</groupId>
<artifactId>postgresql</artifactId>
<version>8.4-701.jdbc3</version>
<optional>true</optional>
<dependency>
</dependencies>