攔截器應該可以說是一個很經典的設計模式，它有點類似于過濾器，當某信息從一個地方流向目的地的過程中，可能需要統一對信息進行處理，如果考慮到系統的可擴展性和靈活性通常就會使用攔截器模式，它就像一個個關卡被設置在信息流動的通道中，并且可以按照實際需要添加和減少關卡。Tribes為了在應用層提供對源消息統一處理的渠道引入通道攔截器，用戶在應用層只需要根據自己需要添加攔截器即可，例如，壓縮解壓攔截器、消息輸出輸入統計攔截器、異步消息發送器等等。
攔截器的數據流向示意圖可以參考前面的tribes簡介章節，數據從IO層流向應用層，中間就會經過一個攔截器棧，應用層處理完就會返回一個ack給發送端表示已經接收并處理完畢（消息可靠級別為SYNC_ACK），下面嘗試用最簡單一些代碼和偽代碼說明tribes的攔截器實現，旨在領會攔截器如何設計而并非具體的實現。最終實現的功能如圖所示，最底層的協調者ChannelCoordinator永遠作為第一個加入攔截器棧的攔截器，往上則是按照添加順序排列，且每個攔截器的previous、next分別指向前一個攔截器和下一個攔截器。
?
① 定義攔截器接口
~~~
public?interface?ChannelInterceptor{
????public?void?setNext(ChannelInterceptor?next)?;
????public?ChannelInterceptor?getPrevious();
????public?void?sendMessage(ChannelMessage?msg);
????public?void?messageReceived(ChannelMessage?msg);
}
~~~
② 定義一個基礎攔截器，提供一些公共的操作，由于攔截器執行完后要觸發下個攔截器，所以把觸發工作統一抽離到基礎類里面完成，當然里面必須包含前一個和后一個攔截器的引用。
~~~
public?class?ChannelInterceptorBase?implements?ChannelInterceptor?{
????private?ChannelInterceptor?next;
????private?ChannelInterceptor?previous;
????public?ChannelInterceptorBase()?{
????}
????public?final?void?setNext(ChannelInterceptor?next)?{
????????this.next?=?next;
????}
????public?final?ChannelInterceptor?getNext()?{
????????return?next;
????}
????public?final?void?setPrevious(ChannelInterceptor?previous)?{
????????this.previous?=?previous;
????}
????public?final?ChannelInterceptor?getPrevious()?{
????????return?previous;
????}
????public?void?sendMessage(ChannelMessage?msg)?{
????????if?(getNext()?!=?null)?getNext().sendMessage(msg,);
????}
????public?void?messageReceived(ChannelMessage?msg)?{
????????if?(getPrevious()?!=?null)?getPrevious().messageReceived(msg);
????}
}
~~~
③ 壓縮解壓攔截器，此攔截器負責按一定算法壓縮和解壓處理。
~~~
public?class?GzipInterceptor?extends?ChannelInterceptorBase?{
????public?void?sendMessage(ChannelMessage?msg){
????????????compress?the?msg;
????????????getNext().sendMessage(msg);
????}
????public?void?messageReceived(ChannelMessage?msg)?{
????????????decompress?the?msg;
????????????getPrevious().messageReceived(msg);
????}
}
~~~
④ 最底層的協調器，直接與網絡IO做交互
~~~
public?class?ChannelCoordinator?extends?ChannelInterceptorBase{
????public?ChannelCoordinator()?{
????}
????public?void?sendMessage(ChannelMessage?msg)?throws?ChannelException?{
????????Network?IO?Send
????}
public?void?messageReceived(ChannelMessage?msg)?{
????Network?IO?READ
????????super.messageReceived(msg);
????}
}
~~~
⑤ 測試類
~~~
public?class?Test{
public?void?main(String[]?args){
ChannelCoordinator?coordinator?=?new?ChannelCoordinator();
GzipInterceptor?gzipInterceptor?=?new?GzipInterceptor();
coordinator.setNext(null);
coordinator.setPrevious(gzipInterceptor);
gzipInterceptor.setPrevious(null);
gzipInterceptor?.setNext(coordinator);
gzipInterceptor.sendMessage(msg);
coordinator.messageReceived(msg);
}?
}
~~~
????Tribes的攔截器整體設計就如上面，整個攔截器的執行順序如下，當執行寫操作時，數據流向GzipInterceptor?->?ChannelCoordinator?->?Network?IO；當執行讀操作時，數據流向則為Network?IO?->?ChannelCoordinator?->?GzipInterceptor。理解了整個設計原理后對于tribes的整體把握將會更加深入。
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