J道的老大banq曾經說過設計模式是衡量一個程式員水平高低最重要的標準。個人非常贊同這個觀點，從這個角度看學好J2EE第一個要瞭解的就是作為一個龐大複雜的系統，它是如何由各個模組拼裝並協同有效地運作。
先來幾個定義暖暖身 Definitions of Different Dimensions in J2EE
Tiers: A logical or physical organization of components into an ordered chain of service providers and consumers. Components within a tier typically consume the services of those in an "adjacent" provider tier and provide services to one or more "adjacent" consumer tiers. Within a tier, services are grouped to like requirements, such as functionality, security, or load distribution.
Layers: The hardware and software stack that hosts services within a given tier. Physical, network, and software platforms and standard API sets support the components that provide a service. Layers, like tiers, represent a well-ordered relationship across boundaries that are mediated by interfaces. Whereas tiers represent processing chains across components, layers represent container/component relationships in implementation and deployment of services.
Tiers層的概念是我們最常聽到的，一般來說它可以分為 (客戶) <---互動---> [ Client Interface --- Presentation Tier --- BizLogic Tier --- Integration Tier --- Resource Tier ]
Client Interface 典型的客戶介面為IE, Swing, Applet, WAP等，客戶與之直接互動
Presentation Tier 負責接受客戶請求並返回相應的處理結果。登陸，許可權檢查，會話管理在此進行，從技術角度就是JSP HTML等
Business Logic Tier 負責處理商務邏輯和資料，是J2EE的核心，它也同時管理事務Transaction，EJB等在這一層
Integration Tier 負責串連商務邏輯層與系統資源層，最常見的JDBC資料庫連接就是一個例子
Resource Tier 負責提供上層所需要的資源，比如資料庫, File System, Legacy System等等
結合實際應用和設計模式，我們可以將層次進一步劃分。多層次會降低效率，也會導致出現很多看似“無用”的代碼，但正是這些代碼實現了低耦合從而降低了後期維護成本。
1) Presentation Tier應用MVC模式, 將程式流程式控制制器(Controller) 視圖(View)和資料三者分離，如Struts就是其中一員大將。Filtering和User-Role-Priviledge-Resource也常在使用(具體還會開新貼細談)
2) Business Logic Tier 和Presentation Tier的低耦合由Business Delegate層實現，很典型的例子 BusinessDelegate -> BusinessMgr (EJB) -> ...而Facade模式也經常在這裡被用來做封裝。
主要參考文獻：軟體體系架構模式在J2EE中的應用
主要參考項目：Medlog Logistic PACE2 SCS
