maven optional可選依賴

應用場景：projectA 依賴projectB,  projectB 依賴projectC時

<dependency>
  <groupId>com.itear.projectC</groupId>
  <artifactId>projectC</artifactId>
  <version>0.0.1-SNAPSHOT</version>
  <optional>true</optional>
</dependency>

當projectB的<optional>true</optional>時, projectA中如果沒有顯式的引入projectC, 則projectA不依賴projectC, 即projectA可以自己選擇是否依賴projectC

默認<optional>的值為false, 及子項目必須依賴