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<P><STRONG><FONT
color=#0080FF size=3 >EJB開發(fā)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(guī)范正式地定義了針對應(yīng)用服務(wù)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(wù)器端組件和編程接口。開發(fā)人員構(gòu)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(yè)的商業(yè)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(yùn)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(wù)器上，由后者為其提供事務(wù)管理和安全保證。開發(fā)人員無須對這些底層的復(fù)雜的服務(wù)編程，而可以將精力集中于將組織或系統(tǒng)的商業(yè)規(guī)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(wù)器會提供相應(yīng)服務(wù)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(guī)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(quán)威的，但它主要對開發(fā)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(gòu)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(wù)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(fā)人員學(xué)習(xí)開發(fā)</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(chuàng)建應(yīng)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(jīng)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(fā)，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(chuàng)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(jìn)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(fā)</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>
</b></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"">在應(yīng)用程序開發(fā)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(wù)器端模型相當(dāng)流行?？蛻舳藨?yīng)用程序駐留于本地機(jī)器訪問數(shù)據(jù)存儲庫比如關(guān)系數(shù)據(jù)管理系統(tǒng)（</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(shù)據(jù)。這一模型在僅有少量用戶的系統(tǒng)中工作良好。但當(dāng)越來越多的用戶需要訪問數(shù)據(jù)時，這些應(yīng)用程序并不能很好的滿足要求。因?yàn)榭蛻舳税虡I(yè)邏輯，它必須安裝在每臺客戶機(jī)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(yīng)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(wù)器端模型分割為多于兩層的優(yōu)勢變得明顯了。在一個多層應(yīng)用程序中，只將用戶接口駐留在客戶機(jī)上，而應(yīng)用程序的邏輯運(yùn)行于服務(wù)器上的中間層上。最終的一層仍然存儲著數(shù)據(jù)。當(dāng)一個應(yīng)用程序的邏輯需要更新時，只需修改服務(wù)器上的中間層的軟件，這使得更新管理變得相當(dāng)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(chuàng)建可靠、安全、易于管理的分布式應(yīng)用程序的難度是眾人皆知的。比如對分布式系統(tǒng)的事務(wù)處理是一項(xiàng)主要任務(wù)。幸運(yùn)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(guī)范運(yùn)用組件來構(gòu)建分布式系統(tǒng)能減輕許多負(fù)擔(dān)：</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(tǒng)的開發(fā)分割為明確的任務(wù)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(yīng)用程序是會計系統(tǒng)，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(fā)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(yè)務(wù)。系統(tǒng)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(fù)責(zé)應(yīng)用程序的監(jiān)測部署和運(yùn)行情況。每一個專家都假設(shè)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(wù)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(yīng)用程序開發(fā)人員提供服務(wù)。</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(wù)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(fā)商）處理許多更難的任務(wù)，使得開發(fā)人員不必關(guān)心這些細(xì)節(jié)。比如，運(yùn)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(wù)和安全服務(wù)。</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(biāo)準(zhǔn)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(wù)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(fā)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(yīng)用程序開發(fā)中的角色</span></font><span lang="EN-US"><o:p>
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