Gof Creational - Facade Pattern

• Intent

提供一個單一且盡可能簡化的介面給client端存取各項自系統，常見的應用是J2EE Session Facade，而事實上在前面其他的章節中看到的 Abstract Factory就是一種Facade pattern的實做。

常見的狀況是，在client端直接的呼叫了business object的各項功能，這樣的情況下導致client端必須很清楚的知道整個商業流程細節，以及必須細心的維護每一次系統需求調整時，相應的business object 的呼叫方式甚至順序，這樣的狀況會使得程式在維護時，變得很難以控制，不小心就變成了大爆炸。

在自己的經驗中，過去寫過一個複雜的查詢功能程式，在jsp中我 寫了 html , javascript , scriptlets , ejb call interactions ..，寫到後來還因為要夾雜script與html輸出，在scriptlets中做了更多苦工，而這樣的code，就被我資深同事給打槍，他接手時選擇整個砍掉重練，完全不理會我原本的功能是寫啥了XD ( Orz...這打擊真大...)

所以，真的要好好善用facade，對於前端的程式碼，盡可能地不該相依於後端的business obj ，如此面臨功能變動時，異動影響的範圍就少了很多。

• Benefits

1. 在不影響既有的各項子系統提供的操作功能中，提供一個簡單的介面給予操作。


2. 屏蔽了各項子系統的複雜內容。


3. 同時也因為透過單一物件的傳遞資訊，降低了cleint與各項子系統的耦合度。

• 適用情境

1. You need to provide a simple interface to a complex subsystem.

2. Several dependencies exist between clients and the implementation classes of an abstraction.

3. Layering the subsystems is necessary or desired.

• 常見的Lib or API - java.net.URL


• (Force) How to ?

• Provide a simpler interface to the clients by hiding all the complex interactions between business components.


• Reduce the number of business objects that are exposed to the client across the service layer over the network.


• Hide from the client the underlying interactions and interdependencies between business components. This provides better manageability, centralization of interactions (responsibility), greater flexibility, and greater ability to cope with changes.


• Provide a uniform coarse-grained service layer to separate business object implementation from business service abstraction.


• Avoid exposing the underlying business objects directly to the client to keep tight coupling between the two tiers to a minimum.


• Use a session bean as a facade to encapsulate the complexity of interactions between the business objects participating in a workflow. The Session Facade manages the business objects, and provides a uniform coarse-grained service access layer to clients.

• Class Diagram


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  [caption id="attachment_196" align="alignnone" width="478" caption="SessionFacade"][/caption]