1. is defined by a parent POM referencing one or more submodules
2. The appropriate packaging for a project likesimple-parent that simply provides a Project Object Model is pom.
3. Maven knows to look in these directories for pom.xml files，maven知道如何去找子模块的pom.xml
4. Using POM inheritance allows you to add common dependencies for universal dependencies
那为什么要多模块呢？ 要明白多个模块仍然在一个项目中，启动这个项目后，多个模块之间就可以相互调用。多个模块方面分工开发和功能聚合。