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<td width="90%"><font size="4" face="Verdana, Arial, Helvetica"><b>注冊 JDBC 驅(qū)動程序</b></font></td><td width="200" align="right"><font size="1" face="Verdana, Arial, Helvetica"><nobr>   第 2 頁（共9 頁）</nobr></font></td>
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<p>
在 Java 應(yīng)用程序和數(shù)據(jù)庫之間創(chuàng)建連接的過程的第一步是用&ldquo;Java 虛擬機(jī)&rdquo;（Java virtual machine（JVM））注冊 JDBC 驅(qū)動程序，Java 應(yīng)用程序就是在此 JVM 中運(yùn)行的。在傳統(tǒng)的連接機(jī)制（相對于后面中討論的 <code>DataSource</code> 連接機(jī)制）中，連接和所有數(shù)據(jù)庫通訊是由 <code>DriverManager</code> 對象控制的。要建立一個連接，必須用 <code>DriverManager</code> 對象注冊針對目標(biāo)數(shù)據(jù)庫的適當(dāng)?shù)?JDBC 驅(qū)動程序。
</p>
<p>
JDBC 規(guī)范（更多細(xì)節(jié)請參閱 <i>JDBC API 教程和參考</i><a href="j-jdbc-8-2.html">參考資料</a>），規(guī)定了 JDBC 驅(qū)動程序在裝入 JVM 時應(yīng)該自動用 <code>DriverManager</code> 對象注冊自身。例如，下面的代碼段使用一個靜態(tài)的初始化程序來首先創(chuàng)建一個 <code>persistentjava</code> JDBC 驅(qū)動程序的實(shí)例，然后用 <code>DriverManager</code> 將其注冊。</p>
<pre>
<code style="font-family: Courier New, Courier, monospace; font-size: 12">
static {
  java.sql.DriverManager.registerDriver(new com.persistentjava.JdbcDriver()) ;
}
</code>
</pre>
<p>
注冊驅(qū)動程序就是將驅(qū)動程序類裝入 JVM 的工作，它可以用幾種不同的方法完成。一種方法就是使用類調(diào)用器（ClassLoader）<code>Class.forName(com.persistentjava.JdbcDriver);</code>。另一種方法不太常見，是使用 <code>jdbc.drivers</code> 系統(tǒng)屬性。這種方法可以有三種不同的方式：</p>
<ul>
<li>從命令行：
<br>
<code>java -Djdb.drivers=com.persistentjava.JdbcDriver Connect</code>
<br>
<br>
</li>
<li>在 java 應(yīng)用程序中：<br>
<code>System.setProperty("jdbc.drivers", "com.persistentjava.JdbcDriver") ;</code>
<br>
<br>
</li>
<li>通過在系統(tǒng)屬性文件中設(shè)置 <code>jdbc.drivers</code> 屬性，這通常和系統(tǒng)有關(guān)。</li>
</ul>
<p>通過用冒號隔離驅(qū)動程序，多個驅(qū)動程序可以使用上面提到的系統(tǒng)屬性方法注冊。使用系統(tǒng)屬性方法的一個好處是驅(qū)動程序很容易在 JVM 中調(diào)入和調(diào)出，而不用修改任何代碼（或至少是修改最少的代碼）。如果注冊了多個驅(qū)動程序，它們的優(yōu)先順序是：1）JVM 初始化時通過 <code>jdbc.drivers</code> 屬性注冊的 JDBC 驅(qū)動程序，2）動態(tài)裝入的 JDBC 驅(qū)動程序。因?yàn)?<code>jdbc.drivers</code> 屬性只在 <code>DriverManager()</code> 方法的第一次調(diào)用時檢查一次，確保所有驅(qū)動程序在建立數(shù)據(jù)庫連接之前被正確注冊是很重要的。
</p>
<p>
然而，不是所有的 JVM 都平等創(chuàng)建，一些 JVM 并不符合 JVM 規(guī)范。結(jié)果是，靜態(tài)初始化程序并不如預(yù)期的那樣運(yùn)行。這導(dǎo)致了注冊 JDBC 驅(qū)動程序的多種方法，包括：</p>
<ul>
<li>
<code>Class.forName("com.persistentjava.JdbcDriver").newInstance() ; </code>
</li>
<li>
<code>DriverManager.registerDriver(new com.persistentjava.JdbcDriver()) ; </code>
</li>
</ul>
<p>這些替代方案應(yīng)該在所有 JVM 中都運(yùn)行良好，所以您應(yīng)該放心地在眾多可能出現(xiàn)的 JVM 中使用它們工作。最后一個問題是 <code>Class.forname()</code> 可能拋出一個 <code>ClassNotFoundException</code>，所以您需要在合適的異常處理程序中包裝注冊代碼。
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