When Import Maven project:
"Java Build Path" > "Libraries" 中的 component 显示的是引用 jar, 而不是引用本 workspace. 此时就要勾选 "Resolve Workspace projects".
Resolve Workspace projects:
Dependencies opened as workspace projects will be resolved without having to install them to your local Maven repository first. This way, any changes made to one of these dependencies will have an immediate effect on other projects consuming it (compilation, refactoring, etc.).
When Resolve Workspace projects is disabled, dependencies existing in the workspace must be installed to your local Maven repository after any change (by running mvn install), in order to see effects in projects consuming them.
Window->Show view->Other...->Mave Repositories
Right-click "Workspace projects", and select "Rebuild index", and then "clean all projects", and Maven "update project" again. then problem solved. actually not sure whether solved by this.