Watching a Directory for Changes

簡介： 這是JDK 7低新功能主要ㄌㄟ對於資料夾監聽是否有刪除修改新增等等!!

需要NIO2的基礎優~~我的Blog有基礎的一些教學也可以去java看教學！
作法如下
1 先建立一個你要監聽的對象如我想監聽src資料夾可以這麼做

 Path path = Paths.get("..\\src");

2  建立一個WatchService,可由FileSystems.getDefault().newWatchService();取得建立時記得要try catch

3 將WatchService與需監聽的狀態 註冊到 Path上 程式碼如下
因為Path有實作Watchable介面才可註冊WatchService

try {

watchService = FileSystems.getDefault().newWatchService();

path.register(watchService, StandardWatchEventKinds.ENTRY_MODIFY,

StandardWatchEventKinds.ENTRY_CREATE,

StandardWatchEventKinds.ENTRY_DELETE);

} catch (IOException e) {

// TODO Auto-generated catch block

e.printStackTrace();

}

4 建立一個無窮迴圈他取得資料夾修改資訊watchService有三種方式可以取得修改資訊

1 poll 會返回WatchKey,如果沒有值會回傳null

2 poll(timeout, unit) 可指定等待時間返回WatchKey

3 take 如序列中沒資料時會wait也返回WatchKey

5 透過WatchKey當中的pollEvents 使用For回圈取得所有註冊狀態的回應

6 最後記得需要呼叫WatchKey的rest 將狀態變成Ready,不然會收不到新的訊息
7 你的的程序中可能會收到一個OVERFLOW事件，甚至如果該程序被沒有針對此事件註冊。
8 如果你註冊在根目錄其子目錄並不會有監聽效果！

程式下載

補充:
可監聽的狀態有:

• ENTRY_CREATE – A directory entry is created.
• ENTRY_DELETE – A directory entry is deleted.
• ENTRY_MODIFY – A directory entry is modified.
• OVERFLOW – Indicates that events might have been lost or discarded. You do not have to register for the OVERFLOW event to receive it.

WatchKey的狀態

• Ready indicates that the key is ready to accept events. When first created, a key is in the ready state.
• path.register(watchService, StandardWatchEventKinds.ENTRY_MODIFY,這時會回傳WatchKey，此時的狀態事Ready 
• Signaled indicates that one or more events are queued. Once the key has been signaled, it is no longer in the ready state until the reset method is invoked.
• WatchKey watchKey = watchService.take();透過watchService取的WatchKey因已經有Event在內的就變成了Signaled 
• Invalid indicates that the key is no longer active. This state happens when one of the following events occurs:
• The process explicitly cancels the key by using the cancel method.
• The directory becomes inaccessible.
• The watch service is closed.
• 當呼叫 watchKey.reset()或是watchKey.isValid()都會回傳false

 You do not need to specify the OVERFLOW event while registering a watchable with a WatchService  to receive this event.

  The counts for ENTRY_CREATE and ENTRY_DELETE are always 1.

  This is as per the API description of these events. However, the count for  ENTRY_MODIFY and  OVERFLOW      can be 1 or greater.

 The count for ENTRY_MODIFY and  OVERFLOW  the count can be greater than 1.    可能會大於1