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<td width="90%"><font size="4" face="Verdana, Arial, Helvetica"><b>類元數(shù)據(jù)</b></font></td><td width="200" align="right"><font size="1" face="Verdana, Arial, Helvetica"><nobr>   第 7 頁(yè)（共15 頁(yè)）</nobr></font></td>
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<code style="font-family: Courier New, Courier, monospace; font-size: 12">
&lt;?xml version="1.0"?&gt;
&lt;!DOCTYPE jdo SYSTEM "jdo.dtd"&gt;
&lt;jdo&gt;
   &lt;package name="com.stereobeacon.jdo.tutorial"&gt;
     &lt;class name="AddressImpl"&gt;
         &lt;field name="addressLine1"/&gt;
         &lt;field name="addressLine2"/&gt;
         &lt;field name="city"/&gt;
         &lt;field name="state"/&gt;
         &lt;field name="zip"/&gt;
     &lt;/class&gt;
   &lt;/package&gt;
&lt;/jdo&gt;
</code>
</pre>
<p>注：該元數(shù)據(jù)（XML 文件）包含 DTD。還請(qǐng)注意 <code>package</code> 標(biāo)記允許一個(gè)包中的類組合成一個(gè)主體。<code>class</code> 標(biāo)記標(biāo)識(shí)該包中應(yīng)具有持久性的類。最后，標(biāo)識(shí)類中具有持久性的每個(gè)字段。在本例中，我們只使用使 <code>AddressImpl</code> 具有持久性所需的最少元數(shù)據(jù)。JDO 規(guī)范提供了類的許多附加選項(xiàng)，例如指定主鍵以及字段是否可以為空值的能力。</p>
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