Maven optional的作用
这里的optional作用，可参考How do optional dependencies work?。
Project-A -> Project-B
The diagram above says that Project-A depends on Project-B. When A declares B as an optional dependency in its POM, this relationship remains unchanged. It's just like a normal build where Project-B will be added in Project-A's classpath.
Project-X -> Project-A
When another project (Project-X) declares Project-A as a dependency in its POM, the optional nature of the dependency takes effect. Project-B is not included in the classpath of Project-X. You need to declare it directly in the POM of Project X for B to be included in X's classpath.
简单来说，Project-X依赖Project-A，Project-A依赖Project-B，在Project-A中定义Project-B为optional的，在Project-X才会起作用并且不会引入Project-B依赖，也就是说Optional会阻止依赖传递。
比如有个项目X2，它的功能跟Hibernate类似，支持MySQL, PostgreSQL, Oracle，它会依赖各个数据库的驱动，在X2编译时，需要所有驱动的依赖；但你的项目仅使用其中一个数据库，而不需要其他依赖。X2中就可以声明所有驱动为Optional，你的项目直接声明依赖X2，X2中的驱动依赖并不会自动引入到类路径，你的项目需要显式声明目标数据库驱动依赖。