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<td width="90%"><font size="4" face="Verdana, Arial, Helvetica"><b>从服务器请求一个文档</b></font></td><td width="200" align="right"><font size="1" face="Verdana, Arial, Helvetica"><nobr>   第 5 页（共6 页）</nobr></font></td>
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<font size="2" face="Verdana, Arial, Helvetica">
<p>
<code>getDocumentAt()</code> 方法处理获取 Web 上的文档的实际工作：</p>
<pre>
<code style="font-family: Courier New, Courier, monospace; font-size: 12">
public String getDocumentAt(String urlString) {
    StringBuffer document = new StringBuffer();
    try {
        URL url = new URL(urlString);
        URLConnection conn = url.openConnection();
        BufferedReader reader = new BufferedReader(new InputStreamReader(conn.getInputStream()));

        String line = null;
        while ((line = reader.readLine()) != null)
	      document.append(line + "\n");
        reader.close();
    } catch (MalformedURLException e) {
        System.out.println("Unable to connect to URL: " + urlString);
    } catch (IOException e) {
        System.out.println("IOException when connecting to URL: " + urlString);
    }
    return document.toString();
}
</code>
</pre>
<p>
<code>getDocumentAt()</code> 方法有一个 <code>String</code> 参数，该参数包含我们想获取的文档的 URL。我们在开始时创建一个 <code>StringBuffer</code> 来保存文档的行。然后我们用我们传进去的 <code>urlString</code> 创建一个新 <code>URL</code>。接着创建一个 <code>URLConnection</code> 并打开它：</p>
<pre>
<code style="font-family: Courier New, Courier, monospace; font-size: 12">
URLConnection conn = url.openConnection();
</code>
</pre>
<p>一旦有了一个 <code>URLConnection</code>，我们就获取它的 <code>InputStream</code> 并包装进 <code>InputStreamReader</code>，然后我们又把 <code>InputStreamReader</code> 包装进 <code>BufferedReader</code> 以使我们能够读取想从服务器上获取的文档的行。在 Java 代码中处理套接字时，我们将经常使用这种包装技术，但我们不会总是详细讨论它。在我们继续往前讲之前，您应该熟悉它：</p>
<pre>
<code style="font-family: Courier New, Courier, monospace; font-size: 12">
BufferedReader reader =
     new BufferedReader(new InputStreamReader(conn.getInputStream()));
</code>
</pre>
<p>有了 <code>BufferedReader</code>，就使得我们能够容易地读取文档内容。我们在 <code>while</code> 循环中调用 <code>reader</code> 上的 <code>readLine()</code>：</p>
<pre>
<code style="font-family: Courier New, Courier, monospace; font-size: 12">
String line = null;
while ((line = reader.readLine()) != null)
    document.append(line + "\n");
</code>
</pre>
<p>对 <code>readLine()</code> 的调用将直至碰到一个从 <code>InputStream</code> 传入的行终止符（例如换行符）时才<i>阻塞</i>。如果没碰到，它将继续等待。只有当连接被关闭时，它才会返回 <code>null</code>。在这个案例中，一旦我们获取一个行（line），我们就把它连同一个换行符一起附加（append）到名为 <code>document</code> 的 <code>StringBuffer</code> 上。这保留了服务器端上读取的文档的格式。</p>
<p>我们在读完行之后关闭 <code>BufferedReader</code>：</p>
<pre>
<code style="font-family: Courier New, Courier, monospace; font-size: 12">
reader.close();
</code>
</pre>
<p>如果提供给 <code>URL</code> 构造器的 <code>urlString</code> 是无效的，那么将抛出 <code>MalformedURLException</code>。如果发生了别的错误，例如当从连接上获取 <code>InputStream</code> 时，那么将抛出 <code>IOException</code>。</p>
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