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<a name="navskip"></a><td height="25" colspan="4"><img alt="6.JavaMail API 的使用" src="imagemaster/titlebar6.jpg" border="0" height="25" width="562"></td>
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<td width="90%"><font size="4" face="Verdana, Arial, Helvetica"><b>附件的處理</b></font></td><td width="200" align="right"><font size="1" face="Verdana, Arial, Helvetica"><nobr> 第 8 頁（共9 頁）</nobr></font></td>
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<p>附件是郵件消息的相關資源，如通常不包含在消息正文里文本文件、電子表格或圖像等。常見的郵件程序，如 Eudora 和 pine 之類，可以用 JavaMail API 將資源 <i>attach（附加）</i> 到您的消息上，就可以在收到消息時得到。</p>
<p>
<b>附件的發送：</b>
<br>
發送附件非常像轉發消息。您建立各部分以組成完整消息。完成第一部件，即消息正文后，您添加其它部件，其中每個 <code>DataHandler</code> 都代表附件，而不是轉發消息情況下的共享處理程序。如果從文件中讀附件，附件的數據源是 <code> <a href="http://java.sun.com/products/javabeans/glasgow/javadocs/javax/activation/FileDataSource.html">FileDataSource</a></code>。而如果從 URL 中讀時，附件的數據源是 <code> <a href="http://java.sun.com/products/javabeans/glasgow/javadocs/javax/activation/URLDataSource.html">URLDataSource</a></code>。一旦存在 <code>DataSource</code>，只要先把它傳遞給 <code><a href="http://java.sun.com/products/javabeans/glasgow/javadocs/javax/activation/DataHandler.html">DataHandler</a></code> 構造器，最后再用 <code>setDataHandler()</code> 把它附加到 <code>BodyPart</code>。假定您要保留附件的原始文件名，最終要做的是用 <code>BodyPart</code> 的 <code>setFileName()</code> 方法設置與附件相關的文件名。如下所示：</p>
<pre>
<code style="font-family: Courier New, Courier, monospace; font-size: 12">
// Define message
Message message = new MimeMessage(session);
message.setFrom(new InternetAddress(from));
message.addRecipient(Message.RecipientType.TO,
new InternetAddress(to));
message.setSubject("Hello JavaMail Attachment");
// Create the message part
BodyPart messageBodyPart = new MimeBodyPart();
// Fill the message
messageBodyPart.setText("Pardon Ideas");
Multipart multipart = new MimeMultipart();
multipart.addBodyPart(messageBodyPart);
// Part two is attachment
messageBodyPart = new MimeBodyPart();
DataSource source = new FileDataSource(filename);
messageBodyPart.setDataHandler(new DataHandler(source));
messageBodyPart.setFileName(filename);
multipart.addBodyPart(messageBodyPart);
// Put parts in message
message.setContent(multipart);
// Send the message
Transport.send(message);
</code>
</pre>
<p>就消息引入附件時，若程序是個 servlet （小服務程序），除告知消息發送到何處外，還必需上載附件。可以將 <code>multipart/form-data</code> 表單編碼類型（form encoding type）用于每個上載文件的處理。</p>
<pre>
<code style="font-family: Courier New, Courier, monospace; font-size: 12">
<FORM ENCTYPE="multipart/form-data"
method=post action="/myservlet">
<INPUT TYPE="file" NAME="thefile">
<INPUT TYPE="submit" VALUE="Upload">
</FORM>
</code>
</pre>
<p>注意：消息大小由 SMTP 服務器而不是 JavaMail API 來限制。如果您碰到問題，可以考慮用設置 ms 和 mx 參數的方法增大 Java 堆大小。</p>
<p>
<b>練習：</b>
<br>
<a href="j-javamail-8-10.html">練習 5. 如何發送附件</a>
</p>
<p>
<b>附件的獲取：</b>
<br>
從消息中獲取附件比發送它們棘手些，因為 MIME 沒有簡單的關于附件的概念。當消息包含附件時，消息的內容是個 <code>Multipart</code> 對象。接著，您需要處理每個 <code>Part</code>，獲取主要內容和附件。標有從 <code>part.getDisposition()</code> 獲得的 <code>Part.ATTACHMENT</code> 配置（disposition）的部件（Part）無疑就是附件。但是，沒有配置（以及一個非文本 MIME 類型）和帶 <code>Part.INLINE</code> 配置的部件也可能是附件。當配置要么是 <code>Part.ATTACHMENT</code>，要么是 <code>Part.INLINE</code> 時，這個消息部件的內容就能被保存。只要用 <code>getFileName()</code> 和
<code>getInputStream()</code> 就能分別得到原始文件名和輸入流。</p>
<pre>
<code style="font-family: Courier New, Courier, monospace; font-size: 12">
Multipart mp = (Multipart)message.getContent();
for (int i=0, n=multipart.getCount(); i<n; i++) {
Part part = multipart.getBodyPart(i));
String disposition = part.getDisposition();
if ((disposition != null) &&
((disposition.equals(Part.ATTACHMENT) ||
(disposition.equals(Part.INLINE))) {
saveFile(part.getFileName(), part.getInputStream());
}
}
</code>
</pre>
<p>
<code>saveFile()</code> 方法僅依據文件名創建了一個 <code>File</code>，它從輸入流中將字節讀出，然后寫入到文件中。萬一文件已經存在，就在文件名后添加一個數字作為新文件名，如果這個文件名仍存在，則繼續添，直到找不到這樣的文件名為止。</p>
<pre>
<code style="font-family: Courier New, Courier, monospace; font-size: 12">
// from saveFile()
File file = new File(filename);
for (int i=0; file.exists(); i++) {
file = new File(filename+i);
}
</code>
</pre>
<p>上面的代碼涵蓋了最簡單的情況 — 消息中各部件恰當的標記了。要涵蓋所有情況，還要在配置為空時進行處理，并且獲取部件的 MIME 類型來進行相應處理。</p>
<pre>
<code style="font-family: Courier New, Courier, monospace; font-size: 12">
if (disposition == null) {
// Check if plain
MimeBodyPart mbp = (MimeBodyPart)part;
if (mbp.isMimeType("text/plain")) {
// Handle plain
} else {
// Special non-attachment cases here of image/gif, text/html, ...
}
...
}
</code>
</pre>
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