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在本教程中

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<img alt="用 J2ME 构建您的股票系统" src="imagemaster/masthead.jpg"></td><td width="*" valign="bottom" align="right"><a border="0" href="j-stock.zip"><img alt="下载 ZIP 文件" border="0" src="../i/icon-zip.gif"></a><a target="_blank" href="../tutorial_eng/index.html" border="0"><img alt="英文原文" border="0" src="../i/icon-source.gif"></a>
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<td width="90%"><font size="4" face="Verdana, Arial, Helvetica"><b>使用 HttpConnection</b></font></td><td width="200" align="right"><font size="1" face="Verdana, Arial, Helvetica"><nobr>   第 2 页（共5 页）</nobr></font></td>
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<p>HTTP 是一种&ldquo;请求-响应&rdquo;协议，在该协议中，必须在发送请求之前设置请求参数。在 UniStocks 中，我们使用下列代码来检索股票的实时和历史数据：
         </p>
<pre>
<code style="font-family: Courier New, Courier, monospace; font-size: 12">
// private void getInfo() throws Exception:

HttpConnection http = null;
InputStream is = null;
boolean ret = false;

// Form URL
if(formAdd != null) {
  URL = baseURL + "?E=" + formAdd.choiceExchanges.getSelectedIndex() +
        "&amp;S=" + formAdd.textCode.getString() + "&amp;T=0";
}else if(formView != null &amp;&amp; formView.s != null) {
  // Create view baseURL....
  ...
}

if(UniStock.DEBUG)
  UniStock.debug("Connecting to: " + URL);

try{
  http = (HttpConnection) Connector.open(URL);
  http.setRequestMethod(HttpConnection.GET);
  if(hasListener)

     dl.setProgress(1, 10);

  // Query the server and try to retrieve the response
  is = http.openInputStream();
  if(hasListener)
    dl.setProgress(2, 10);

  String str;		 		 // Temp buffer.
  int length = (int) http.getLength();
  if(hasListener)
    dl.setProgress(3, 10);

  if(length != -1) { // Length available.
    byte data[] = new byte[length];
    is.read(data);
    str = new String(data);
  }else{		 // Length not available.
    ByteArrayOutputStream bs = new ByteArrayOutputStream();

    int ch;
    while((ch = is.read()) != -1)
      bs.write(ch);

    str = new String(bs.toByteArray());


    bs.close();
  }


  if(UniStock.DEBUG)
    UniStock.debug("Got Data:&gt;" + str + "&lt;");

  // String downloaded.....
  // Process string here.
  ...

  if(hasListener)
    dl.setProgress(10, 10);

  // Alert the user.
  // AlertType.INFO.playSound(midlet.display);

} catch (IOException e) {
  if(midlet.DEBUG)
    midlet.debug("Downloading error: " + e.toString());
  if(formAdd != null) {
    formAdd.stockOK = false;
  }else if(formView != null) {
  }
} finally {
  if(formAdd != null)
    formAdd.process();
  if(formView != null)
    ;		 // Do something.

  /// Clean up.
  if(is != null)
    is.close();

  if(http != null)
    http.close();

  if(dl != null)
    dl.onFinish();
}

         </code>
</pre>
<p>连接应处于三种状态之一：
         </p>
<p>
<ol>
         
<li>
<b>设置：</b>在这个状态中，还未建立到服务器的连接。<code>http = (HttpConnection) Connector.open(URL)</code> 只是创建了一个尚未连接的 <code>HttpConnection</code>。
         </li>
         
<li>
<b>已连接：</b>在设置了所有必需的头之后，调用 <code>http.openInputStream()</code> 会产生服务器连接。发送请求参数，并等待响应。应用程序下载后，连接将转入下一个状态。         </li>
         
<li>
<b>已关闭：</b>连接已被关闭，并且如果调用了方法，则方法将会抛出 <code>IOException</code>。在我们的示例中，在退出定制的下载方法之前我们关闭了所有数据流和连接。
         </li>
         
</ol>
         
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