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<P><STRONG><FONT
color=#0080FF size=3 >EJB開發介紹
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譯者名字: genode(JD015)<br>
郵件地址:<a href="mailto:genode@sina.com">genode@sina.com</a>
<p class="MsoNormal" style="margin-left:.85gd;text-indent:27.1pt;mso-char-indent-count:
2.58;mso-char-indent-size:10.5pt"><span lang="EN-US">Enterprise JavaBeans(EJB)</span><span style="font-family:宋體;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"">規范正式地定義了針對應用服務器的</span><span lang="EN-US">Java</span><span style="font-family:宋體;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"">服務器端組件和編程接口。開發人員構建稱為</span><span lang="EN-US">enterprise
beans</span><span style="font-family:宋體;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"">的組件來封裝企業的商業邏輯。</span><span lang="EN-US">Enterprise
beans</span><span style="font-family:宋體;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"">運行于一個</span><span lang="EN-US">EJB</span><span style="font-family:宋體;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"">服務器上，由后者為其提供事務管理和安全保證。開發人員無須對這些底層的復雜的服務編程，而可以將精力集中于將組織或系統的商業規則封裝入</span><span lang="EN-US">beans</span><span style="font-family:宋體;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"">，只需知道在需要的時候</span><span lang="EN-US">EJB</span><span style="font-family:宋體;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"">服務器會提供相應服務的。</span></p>
<p class="MsoNormal" style="margin-left:.85gd;text-indent:27.1pt;mso-char-indent-count:
2.58;mso-char-indent-size:10.5pt"><span style="font-family:宋體;mso-ascii-font-family:
"Times New Roman";mso-hansi-font-family:"Times New Roman"">雖然</span><span lang="EN-US">Enterprise
JavaBeans</span><span style="font-family:宋體;mso-ascii-font-family:
"Times New Roman";mso-hansi-font-family:"Times New Roman"">規范對于</span><span lang="EN-US">EJB</span><span style="font-family:宋體;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"">框架的說明是最為權威的，但它主要對開發商</span><span lang="EN-US">,
</span><span style="font-family:宋體;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"">列如構建</span><span lang="EN-US">EJB</span><span style="font-family:宋體;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"">服務器和</span><span lang="EN-US">beans</span><span style="font-family:宋體;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"">容器的</span><span lang="EN-US">Borland</span><span style="font-family:宋體;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"">公司有用。本書將幫助</span><span lang="EN-US">JBuilder</span><span style="font-family:宋體;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"">開發人員學習開發</span><span lang="EN-US">enterprise
beans</span><span style="font-family:宋體;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"">的必備知識，并創建應用程序來使用它們。</span></p>
<p class="MsoNormal" style="margin-left:.85gd;text-indent:27.1pt;mso-char-indent-count:
2.58;mso-char-indent-size:10.5pt"><span style="font-family:宋體;mso-ascii-font-family:
"Times New Roman";mso-hansi-font-family:"Times New Roman"">如果你已經熟悉了</span><span lang="EN-US">EJB</span><span style="font-family:宋體;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"">的開發，想要開始用</span><span lang="EN-US">JBuilder</span><span style="font-family:宋體;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"">創建</span><span lang="EN-US">enterprise beans</span><span style="font-family:宋體;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"">，進入“用</span><span lang="EN-US">Jbuilder</span><span style="font-family:宋體;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"">開發</span><span lang="EN-US">enterprise beans</span><span style="font-family:宋體;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"">”。</span></p>
<hr>
<p><b><font size="3"><font color="#000080">為什么我們需要Enterprise
JavaBeans</font><o:p>
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</o:p>
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<p class="MsoNormal" style="margin-left:.85gd;text-indent:27.1pt;mso-char-indent-count:
2.58;mso-char-indent-size:10.5pt"><span style="font-family:宋體;mso-ascii-font-family:
"Times New Roman";mso-hansi-font-family:"Times New Roman"">在應用程序開發中，客戶端</span><span lang="EN-US">—</span><span style="font-family:宋體;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"">服務器端模型相當流行。客戶端應用程序駐留于本地機器訪問數據存儲庫比如關系數據管理系統（</span><span lang="EN-US">RDMS</span><span style="font-family:宋體;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"">）中的數據。這一模型在僅有少量用戶的系統中工作良好。但當越來越多的用戶需要訪問數據時，這些應用程序并不能很好的滿足要求。因為客戶端包含商業邏輯，它必須安裝在每臺客戶機上。這樣使管理上越來越困難。</span></p>
<p class="MsoNormal" style="margin-left:.85gd;text-indent:27.1pt;mso-char-indent-count:
2.58;mso-char-indent-size:10.5pt"><span style="font-family:宋體;mso-ascii-font-family:
"Times New Roman";mso-hansi-font-family:"Times New Roman"">逐漸的，將應用程序的客戶端</span><span lang="EN-US">—</span><span style="font-family:宋體;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"">服務器端模型分割為多于兩層的優勢變得明顯了。在一個多層應用程序中，只將用戶接口駐留在客戶機上，而應用程序的邏輯運行于服務器上的中間層上。最終的一層仍然存儲著數據。當一個應用程序的邏輯需要更新時，只需修改服務器上的中間層的軟件，這使得更新管理變得相當簡單。</span></p>
<p class="MsoNormal" style="margin-left:.85gd;text-indent:27.1pt;mso-char-indent-count:
2.58;mso-char-indent-size:10.5pt"><span style="font-family:宋體;mso-ascii-font-family:
"Times New Roman";mso-hansi-font-family:"Times New Roman"">但是創建可靠、安全、易于管理的分布式應用程序的難度是眾人皆知的。比如對分布式系統的事務處理是一項主要任務。幸運的是，按照如下的</span><span lang="EN-US">EJB</span><span style="font-family:宋體;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"">規范運用組件來構建分布式系統能減輕許多負擔：</span></p>
<p class="MsoNormal" style="margin-left:57.0pt;text-indent:-21.0pt;mso-list:l0 level1 lfo1;
tab-stops:list 57.0pt"><span lang="EN-US" style="font-family:
Wingdings">l<span style="font:7.0pt "Times New Roman""> </span></span><span style="font-family:宋體;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"">將分布式系統的開發分割為明確的任務分配給專家們。</span></p>
<p class="MsoNormal" style="margin-left:5.14gd"><span style="font-family:宋體;
mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:"Times New Roman"">例如，這個應用程序是會計系統，則</span><span lang="EN-US">EJB</span><span style="font-family:宋體;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"">組件開發者</span><span lang="EN-US">(Enterprise
Bean Developer) </span><span style="font-family:宋體;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"">需要理解會計業務。系統管理員（</span><span lang="EN-US">system<span style="mso-spacerun: yes">
</span>administrator</span><span style="font-family:宋體;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"">）必須負責應用程序的監測部署和運行情況。每一個專家都假設為一個特定角色。</span></p>
<p class="MsoNormal" style="margin-left:57.0pt;text-indent:-21.0pt;mso-list:l0 level1 lfo1;
tab-stops:list 57.0pt"><span lang="EN-US" style="font-family:
Wingdings">l<span style="font:7.0pt "Times New Roman""> </span></span><span style="font-family:宋體;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"">使得</span><span lang="EN-US">EJB</span><span style="font-family:宋體;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"">服務器和容器能為</span><span lang="EN-US">enterprise
beans</span><span style="font-family:宋體;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"">和應用程序開發人員提供服務。</span></p>
<p class="MsoNormal" style="margin-left:57.0pt"><span lang="EN-US">EJB</span><span style="font-family:宋體;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"">服務器提供者</span><span lang="EN-US">(EJB
Server Provider)</span><span style="font-family:宋體;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"">和</span><span lang="EN-US">EJB </span><span style="font-family:
宋體;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:"Times New Roman"">容器提供者</span><span lang="EN-US">(EJB
Container Provider)</span><span style="font-family:宋體;
mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:"Times New Roman"">（他們通常是同一個開發商）處理許多更難的任務，使得開發人員不必關心這些細節。比如，運行</span><span lang="EN-US">enterprise
beans</span><span style="font-family:宋體;mso-ascii-font-family:
"Times New Roman";mso-hansi-font-family:"Times New Roman"">的容器，自動為</span><span lang="EN-US">beans</span><span style="font-family:宋體;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"">提供了事務和安全服務。</span></p>
<p class="MsoNormal" style="margin-left:57.0pt;text-indent:-21.0pt;mso-list:l0 level1 lfo1;
tab-stops:list 57.0pt"><span lang="EN-US" style="font-family:
Wingdings">l<span style="font:7.0pt "Times New Roman""> </span></span><span style="font-family:宋體;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"">使得</span><span lang="EN-US">enterprise
beans</span><span style="font-family:宋體;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"">具有可移植性。</span></p>
<p class="MsoNormal" style="margin-left:54.0pt"><span style="font-family:宋體;
mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:"Times New Roman"">一旦一個</span><span lang="EN-US">bean</span><span style="font-family:宋體;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"">編寫完成，它可以被部署在符合</span><span lang="EN-US">Enterprise
JavaBeans</span><span style="font-family:宋體;mso-ascii-font-family:"Times New Roman";
mso-hansi-font-family:"Times New Roman"">標準的任何</span><span lang="EN-US">EJB</span><span style="font-family:宋體;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"">服務器上。但是每一個</span><span lang="EN-US">bean</span><span style="font-family:宋體;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"">可能包括開發商的特殊元件。</span></p>
<hr>
<p><b><span lang="EN-US"><font color="#000080" size="3">EJB</font></span><font color="#000080" size="3"><span style="font-family:宋體;mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:
"Times New Roman"">應用程序開發中的角色</span></font><span lang="EN-US"><o:p>
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