GAE (1.7.4): 取得上傳日期 String applicationVersion = SystemProperty . applicationVersion . get (); Date uploadDate = new Date ( Long . parseLong ( applicationVersion . substring ( applicationVersion . lastIndexOf ( "." )+ 1 ))/ ( 2 << 27 ) * 1000 ); Ref: SystemProperty JPA 2 限制： Simple Join Entity Group Transaction An entity’s parent, parent’s parent, and so on recursively, are its ancestors; its children, children’s children, and so on, are its descendants. An entity and its descendants are said to belong to the same **entity group**_._ 一般我們常常在一個transaction內大量修改同一類型物件，例如修改多個User的資料，這就算 Cross Group Transaction操作，最多不能超過5個entity group。 參考資料： Google App Engine JPA test case simple join works on unowned one-to-many relationship. limitation: Problem with query : Joins can only be sorted by the join column in ascending order (in this case ‘attendeeList’) 開發GAE會遇到的問題 Maven 若使用JPA，datanucleus plugin 的設定官方文件未提及，不方便，很容易跟project 依賴的版本不合。我參