用maven管理庫依賴，有個好處就是連同庫的依賴的全部jar檔案一起下載，免去手工新增的麻煩，但同時也帶來了同一個jar會被下載了不同版本的問題，好在pom的配置裡面允許用<exclusion>來排除一些不需要同時下載的依賴jar 。

<!-- Struts2 -->

<dependency>

<groupId>org.apache.struts</groupId>

<artifactId>struts2-core</artifactId>

<version>${struts.version}</version>

<exclusions>

<exclusion>

<groupId>javassist</groupId>

<artifactId>javassist</artifactId>

</exclusion>

<exclusion> <!-- we prefer our explicit version, though it should be the same -->

<groupId>asm</groupId>

<artifactId>asm</artifactId>

</exclusion>

<exclusion> <!-- we prefer our explicit version, though it should be the same -->

<groupId>asm</groupId>

<artifactId>asm-commons</artifactId>

</exclusion>

<exclusion> <!-- we prefer our explicit version, though it should be the same -->

<groupId>org.ow2.asm</groupId>

<artifactId>asm</artifactId>

</exclusion>

<exclusion> <!-- we prefer our explicit version, though it should be the same -->

<groupId>org.ow2.asm</groupId>

<artifactId>asm-commons</artifactId>

</exclusion>

</exclusions>

</dependency>