out.println("Press&nbsp; your&nbsp; Back&nbsp; button&nbsp; and&nbsp; select&nbsp; a&nbsp; TimeZone");<br>
}<br>
&lt;/jsp:scriptlet><br>
======================================================================<br>
第二種方法（在內部使用了代碼）可能有些笨重，但允許開發者確保輸出不至于很糟糕（例如"null:null:null&nbsp; null")，假定Session&nbsp; bean還沒有被實例化以及沒有進行值的設置。&nbsp; 這種情況發生在客戶端直接調用了View頁。問題是使用腳本scriptlets可以允許更強的控制。如果你確信你可以控制url存取，那么bean方法當然更適合于開發，并使&nbsp; View頁更方便于HTML設計者的協同工作。<br>
<br>
<br>
上面的是"傳統的"&nbsp; Model&nbsp; II設計。所有的變量都包裝了并放在Session對象中。這有2個不足：<br>
<br>
1)&nbsp; 如果客戶端拒絕參與的話，Session是不可得到的。<br>
<br>
2)&nbsp; 除非Session變量被顯式地移走，否則它回一直存在，直到Session被破壞或過期。&nbsp; <br>
<br>
第一種案例很可能發生在這樣的場合，即使用了cookies作為聲明的結構（mechanism）而開發者沒有能夠提供聲明的結構的替代表單（form），即URL改寫。&nbsp; <br>
<br>
第二個案例甚至更為嚴重，因為它可能引起很大的內存消耗，如果Sessions被定義為保存比標準存留時間更長的話（(標準存留時間是30分鐘）。即使是30分鐘的Session，這種Model也可能在大的應用中引起災難性的內存泄露。為什么呢？在Session對象內部設置的對象被實例化了，并且在Session終止以前一直沒有被移去。因為它們仍然有關聯references（Session對象）&nbsp; 指向它們，所以無法被垃圾收集（garbage-collected）。在Model&nbsp; II&nbsp; 模型中，很多對象被放到Session中（要么直接地，要么通過JavaBean）。隨著Session的進行，更多的頁被存取，內存使用會增加并持續下去直到客戶端終止了Session或者Session過期。要一直等到Session變得非法，放在那的對象才能被垃圾收集，而那些損失的內存本可以用于任何其它的用途。.&nbsp; <br>
<br>
改進的方法之一是將Beans或者其它變量放到Request對象中去，并使用RequestDispatcher.include()而不是RequestDispatcher.forward()。這樣做以后，View&nbsp; 頁具有和Controller一樣的存取請求的對象。傳統的Model&nbsp; II設計的不足可以被排除。<br>
<br>
一個最后的評注：盡管有如上所述，我個人仍有些不喜歡Model&nbsp; II&nbsp; 的范例，如果它用通常方法開發的話。&nbsp; 客戶端被引送到某一個地址，然后又被轉向到另一個不同的類，我不喜歡創建這樣的系統。基于這樣的原因，我修改了設計，使它變成了以下的樣子：&nbsp; <br>
<br>
Controller:&nbsp; timeByZone2.jsp<br>
<br>
<br>
和前面一樣，controller使用Request值來取得必要的數據，并且將數據放到請求的對象中去。這回的區別是View頁將使用RequestDispatcher.include()來調用Controller。在這種方法中，客戶端再也不做重定向，請求不是“鏈接chained”的。相當于class/jsp請求了另一方來為它做一些工作，然后繼續。<br>
<br>
======================================================================<br>
&lt;xml&nbsp; version="1.0"&nbsp; ?><br>
&lt;!--Worker&nbsp; Class,&nbsp; nobody&nbsp; should&nbsp; see&nbsp; me--><br>
&lt;jsp:scriptlet><br>
//the&nbsp; parameter&nbsp; "zone"&nbsp; shall&nbsp; be&nbsp; equal&nbsp; to&nbsp; a&nbsp; number&nbsp; between&nbsp; 0&nbsp; and&nbsp; 24&nbsp; (inclusive)<br>
TimeZone&nbsp; timeZone&nbsp; =&nbsp; TimeZone.getDefault();&nbsp; //returns&nbsp; the&nbsp; default&nbsp; TimeZone&nbsp; for&nbsp; the&nbsp; server<br>
if&nbsp; (request.getParameterValues("zone")&nbsp; !=&nbsp; null)<br>
{<br>
String&nbsp; timeZoneArg&nbsp; =&nbsp; request.getParameterValues("zone")[0];<br>
timeZone&nbsp; =&nbsp; TimeZone.getTimeZone("GMT+"&nbsp; +&nbsp; timeZoneArg&nbsp; +&nbsp; ":00");&nbsp; <br>
//&nbsp; gets&nbsp; a&nbsp; TimeZone.&nbsp; For&nbsp; this&nbsp; example&nbsp; we're&nbsp; just&nbsp; going&nbsp; to&nbsp; assume&nbsp; <br>
//&nbsp; its&nbsp; a&nbsp; positive&nbsp; argument,&nbsp; not&nbsp; a&nbsp; negative&nbsp; one.<br>
}<br>
TimeBean&nbsp; timeBean&nbsp; =&nbsp; new&nbsp; TimeBean();<br>
timeBean.setHours&nbsp; =&nbsp; myCalendar.get(Calendar.HOUR_OF_DAY);<br>
timeBean.setMinutes&nbsp; =&nbsp; myCalendar.get(Calendar.MINUTE);<br>
timeBean.setSeconds&nbsp; =&nbsp; myCalendar.get(Calendar.SECOND);<br>
request.setAttribute("tempTimeBean",&nbsp; timeBean);<br>
&lt;/jsp:scriptlet><br>
======================================================================<br>
<br>
<br>
View:&nbsp; displayTime2.jsp<br>
<br>
<br>
和displayTime.jsp非常相似，但timeByZone2.jsp在也的頂部被調用。請注意&nbsp; &lt;jsp:useBean&nbsp; />&nbsp; 中的"scope"已經被換成了"request"。<br>
<br>
======================================================================<br>
&lt;xml&nbsp; version="1.0"&nbsp; ?><br>
&lt;H1>Time&nbsp; JSP&lt;/H1><br>
<br>
&lt;jsp:include&nbsp; page="timeByZone2.jsp"&nbsp; /><br>
<br>
&lt;jsp:useBean&nbsp; class="TimeBean"&nbsp; id="tempTimeBean"&nbsp; scope="request"&nbsp; />&nbsp; <br>
&lt;jsp:getProperty&nbsp; name="tempTimeBean"&nbsp; property="hours">:<br>
&lt;jsp:getProperty&nbsp; name="tempTimeBean"&nbsp; property="minutes">:<br>
&lt;jsp:getProperty&nbsp; name="tempTimeBean"&nbsp; property="seconds"><br>
&lt;!--&nbsp; these&nbsp; would&nbsp; have&nbsp; printed&nbsp; "null"&nbsp; if&nbsp; tempTimeBean&nbsp; was&nbsp; not&nbsp; instantiated&nbsp; by&nbsp; timeByZone2.jsp&nbsp; --><br>
<br>
======================================================================<br>
<br>
<br>
在一個在建系統中，我們已經使用這種方法來創建類的鏈，每一個都只對它所處理的工作負責。通過辨別公用的表示格式，我們創建了一個View對象，即使在很高層次的JSP中它也可以重復使用。我們的目標就是建立一些可重用的頁，同時減少用于表示的類的數量。&nbsp; <br>
<br>
單個的Servlet&nbsp; Model&nbsp; (A&nbsp; Model&nbsp; II&nbsp; Design)<br>
<br>
<br>
什么時候我有足夠時間來研究這個課題，我會在這里發表更多的東西。&nbsp; <br>
<br>
附原文：<br>
<br>
JSP&nbsp; Architectures<br>
An&nbsp; explanation&nbsp; and&nbsp; comparison&nbsp; of&nbsp; the&nbsp; methodologies<br>
commonly&nbsp; known&nbsp; as&nbsp; "Model&nbsp; I"&nbsp; and&nbsp; "Model&nbsp; II".<br>
Lance&nbsp; Lavandowska&nbsp; To&nbsp; Outline&nbsp; <br>
<br>
If&nbsp; you&nbsp; spend&nbsp; any&nbsp; time&nbsp; reading&nbsp; through&nbsp; Servlet&nbsp; or&nbsp; JSP&nbsp; related&nbsp; newsgroups&nbsp; or&nbsp; mailing&nbsp; lists,&nbsp; you're&nbsp; likely&nbsp; to&nbsp; encounter&nbsp; a&nbsp; discussion&nbsp; of&nbsp; Model&nbsp; I&nbsp; versus&nbsp; Model&nbsp; II&nbsp; methodologies&nbsp; .&nbsp; Which&nbsp; one&nbsp; you&nbsp; use&nbsp; depends&nbsp; on&nbsp; personal&nbsp; taste,&nbsp; team&nbsp; work&nbsp; strategies&nbsp; and&nbsp; OOP&nbsp; orthodoxy.&nbsp; <br>
<br>
Loosely&nbsp; described,&nbsp; Model&nbsp; I&nbsp; is&nbsp; an&nbsp; approach&nbsp; where&nbsp; business&nbsp; logic&nbsp; and&nbsp; presentation&nbsp; code&nbsp; can&nbsp; be&nbsp; intermixed&nbsp; with&nbsp; the&nbsp; presentation&nbsp; itself&nbsp; (HTML&nbsp; in&nbsp; our&nbsp; arena).&nbsp; Model&nbsp; II&nbsp; proscribes&nbsp; that&nbsp; all&nbsp; code,&nbsp; to&nbsp; the&nbsp; extent&nbsp; this&nbsp; is&nbsp; possible,&nbsp; be&nbsp; excluded&nbsp; from&nbsp; the&nbsp; presentation.&nbsp; <br>
<br>
Model&nbsp; I:&nbsp; Simple&nbsp; 2&nbsp; 1/2&nbsp; Tier&nbsp; Application<br>
In&nbsp; a&nbsp; team&nbsp; environment&nbsp; where&nbsp; everyone&nbsp; knows&nbsp; Java&nbsp; and&nbsp; HTML,&nbsp; or&nbsp; if&nbsp; you're&nbsp; doing&nbsp; it&nbsp; all&nbsp; yourself,&nbsp; this&nbsp; approach&nbsp; can&nbsp; work&nbsp; well,&nbsp; provided&nbsp; everyone&nbsp; maintains&nbsp; a&nbsp; clear&nbsp; coding&nbsp; structure&nbsp; (that&nbsp; discussion&nbsp; is&nbsp; outside&nbsp; the&nbsp; bounds&nbsp; of&nbsp; this&nbsp; article).&nbsp; The&nbsp; primary&nbsp; advantage&nbsp; of&nbsp; this&nbsp; approach&nbsp; is&nbsp; that&nbsp; there&nbsp; is&nbsp; only&nbsp; one&nbsp; file&nbsp; to&nbsp; maintain&nbsp; for&nbsp; changes&nbsp; to&nbsp; your&nbsp; application.&nbsp; The&nbsp; major&nbsp; disadvantage&nbsp; is&nbsp; readability!&nbsp; Unless&nbsp; great&nbsp; care&nbsp; is&nbsp; taken,&nbsp; your&nbsp; HTML&nbsp; and&nbsp; Java&nbsp; code&nbsp; can&nbsp; become&nbsp; so&nbsp; intermingled&nbsp; that&nbsp; it&nbsp; becomes&nbsp; difficult&nbsp; to&nbsp; debug&nbsp; and&nbsp; maintain&nbsp; your&nbsp; application.&nbsp; <br>
<br>
For&nbsp; this&nbsp; example,&nbsp; we&nbsp; are&nbsp; going&nbsp; to&nbsp; revisit&nbsp; the&nbsp; "Sample&nbsp; Page"&nbsp; from&nbsp; the&nbsp; JSP&nbsp; Quick&nbsp; Start&nbsp; chapter.&nbsp; I'm&nbsp; going&nbsp; to&nbsp; add&nbsp; a&nbsp; TimeZone&nbsp; element,&nbsp; so&nbsp; we'll&nbsp; have&nbsp; a&nbsp; JSP&nbsp; that&nbsp; returns&nbsp; the&nbsp; time&nbsp; based&nbsp; on&nbsp; the&nbsp; desired&nbsp; timezone.&nbsp; If&nbsp; no&nbsp; TimeZone&nbsp; is&nbsp; submitted,&nbsp; we'll&nbsp; default&nbsp; to&nbsp; that&nbsp; of&nbsp; the&nbsp; server.<br>
<br>
======================================================================<br>
&lt;xml&nbsp; version="1.0"&nbsp; ?><br>
&lt;H1>Time&nbsp; JSP&lt;/H1><br>
&lt;jsp:scriptlet><br>
//the&nbsp; parameter&nbsp; "zone"&nbsp; shall&nbsp; be&nbsp; equal&nbsp; to&nbsp; a&nbsp; number&nbsp; between&nbsp; 0&nbsp; and&nbsp; 24&nbsp; (inclusive)<br>
TimeZone&nbsp; timeZone&nbsp; =&nbsp; TimeZone.getDefault();&nbsp; //returns&nbsp; the&nbsp; default&nbsp; TimeZone&nbsp; for&nbsp; the&nbsp; server<br>
if&nbsp; (request.getParameterValues("zone")&nbsp; !=&nbsp; null)<br>
{<br>
String&nbsp; timeZoneArg&nbsp; =&nbsp; request.getParameterValues("zone")[0];<br>
timeZone&nbsp; =&nbsp; TimeZone.getTimeZone("GMT+"&nbsp; +&nbsp; timeZoneArg&nbsp; +&nbsp; ":00");&nbsp; <br>
//&nbsp; gets&nbsp; a&nbsp; TimeZone.&nbsp; For&nbsp; this&nbsp; example&nbsp; we're&nbsp; just&nbsp; going&nbsp; to&nbsp; assume&nbsp; <br>
//&nbsp; its&nbsp; a&nbsp; positive&nbsp; argument,&nbsp; not&nbsp; a&nbsp; negative&nbsp; one.<br>
}<br>
//since&nbsp; we're&nbsp; basing&nbsp; our&nbsp; time&nbsp; from&nbsp; GMT,&nbsp; we'll&nbsp; set&nbsp; our&nbsp; Locale&nbsp; to&nbsp; Brittania,&nbsp; and&nbsp; get&nbsp; a&nbsp; Calendar.<br>
Calendar&nbsp; myCalendar&nbsp; =&nbsp; Calendar.getInstance(timeZone,&nbsp; Locale.UK);<br>
&lt;/jsp:scriptlet><br>
&lt;%=&nbsp; myCalendar.get(Calendar.HOUR_OF_DAY)&nbsp; %>:<br>
&lt;%=&nbsp; myCalendar.get(Calendar.MINUTE)&nbsp; %>:<br>
&lt;%=&nbsp; myCalendar.get(Calendar.SECOND)&nbsp; %><br>
======================================================================<br>
Similarly,&nbsp; the&nbsp; data&nbsp; to&nbsp; be&nbsp; displayed&nbsp; could&nbsp; have&nbsp; been&nbsp; gotten&nbsp; from&nbsp; a&nbsp; JavaBean.&nbsp; We'll&nbsp; see&nbsp; a&nbsp; little&nbsp; of&nbsp; that&nbsp; in&nbsp; the&nbsp; next&nbsp; example.&nbsp; <br>
Model&nbsp; II:&nbsp; Redirecting&nbsp; Requests<br>
In&nbsp; a&nbsp; team&nbsp; environment&nbsp; where&nbsp; some&nbsp; members&nbsp; are&nbsp; HTML&nbsp; designers&nbsp; and&nbsp; others&nbsp; are&nbsp; Java&nbsp; programmers,&nbsp; this&nbsp; approach&nbsp; can&nbsp; be&nbsp; particularly&nbsp; strong.&nbsp; The&nbsp; Java&nbsp; programmers&nbsp; can&nbsp; focus&nbsp; on&nbsp; creating&nbsp; (re)usable&nbsp; code,&nbsp; while&nbsp; the&nbsp; HTML&nbsp; designers&nbsp; can&nbsp; focus&nbsp; on&nbsp; presentation.&nbsp; While&nbsp; the&nbsp; two&nbsp; remain&nbsp; dependant&nbsp; on&nbsp; each&nbsp; other,&nbsp; one&nbsp; or&nbsp; the&nbsp; other&nbsp; can&nbsp; change&nbsp; dramatically&nbsp; so&nbsp; long&nbsp; as&nbsp; the&nbsp; principle&nbsp; inputs&nbsp; and&nbsp; outputs&nbsp; (respectively)&nbsp; remain&nbsp; the&nbsp; same.&nbsp; <br>
<br>
Now&nbsp; we'll&nbsp; take&nbsp; the&nbsp; same&nbsp; desired&nbsp; behaviour&nbsp; from&nbsp; the&nbsp; Model&nbsp; I&nbsp; example,&nbsp; and&nbsp; present&nbsp; it&nbsp; using&nbsp; the&nbsp; Model&nbsp; II&nbsp; methodology.&nbsp; This&nbsp; methodology&nbsp; follows&nbsp; the&nbsp; Model-View-Controller&nbsp; (MVC)&nbsp; paradigm&nbsp; (cite&nbsp; Design&nbsp; Patterns&nbsp; book).&nbsp; For&nbsp; this&nbsp; example,&nbsp; we'll&nbsp; have&nbsp; one&nbsp; class&nbsp; (or&nbsp; page&nbsp; or&nbsp; servlet)&nbsp; process&nbsp; the&nbsp; request&nbsp; (Controller),&nbsp; get&nbsp; the&nbsp; TimeZone,&nbsp; set&nbsp; all&nbsp; the&nbsp; required&nbsp; variables&nbsp; for&nbsp; presentation,&nbsp; and&nbsp; pass&nbsp; control&nbsp; off&nbsp; to&nbsp; a&nbsp; presentation&nbsp; page&nbsp; (View).&nbsp; For&nbsp; simple&nbsp; apps&nbsp; like&nbsp; this,&nbsp; there&nbsp; is&nbsp; no&nbsp; "Model".&nbsp; <br>
<br>