參考轉載：
https://www.cnblogs.com/EasonJim/p/6554669.html
https://my.oschina.net/Barudisshu/blog/150026
原理：
利用getInputStream()讀取整個上傳表單內容，然後通過分析body中的filename字符索引，獲取到文件名稱以及跟在後面的文件內容的起始位置，然後讀取到服務器。
讀取到的內容形式一般如下，使用--boundary分割每個表單項。
代碼：
1、upload.jsp
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
<title>upload</title>
</head>
<body>
<p>選擇要上傳的文件</p>
<form action="accept.jsp" method="post" enctype="multipart/form-data">
<input type="text" name="girl" value="susan">
<input type="file" name="boy" size="38">
<br>
<input type="submit" value="提交">
</form>
</body>
</html>
2、accept.jsp
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
<title>接收文件</title>
</head>
<body>
<%!
class Position {
int begin;
int end;
public Position(int begin, int end) {
this.begin = begin;
this.end = end;
}
}
%>
<%
// try {
// InputStream in = request.getInputStream();
// File dir = new File(application.getRealPath("/"),"upload");
// dir.mkdir();
// File f=new File(dir,"b.txt");
// FileOutputStream outstream=new FileOutputStream(f);
// byte[] b= new byte[100];
// int n=0;
// while ((n = in.read(b)) != -1) {
// outstream.write(b,0,n);
// }
// in.close();outstream.close();
// out.print("文件已上傳");
// } catch (IOException e) {
// e.printStackTrace();
// out.print("上傳失敗");
// }
int formDataLength = request.getContentLength();
DataInputStream dataStream = new DataInputStream(request.getInputStream());
byte[] body = new byte[formDataLength];
int totalBytes = 0;
while (totalBytes < formDataLength) {
int bytes = dataStream.read(body, totalBytes, formDataLength);
totalBytes += bytes;
}
String textBody = new String(body, "ISO-8859-1");
String fileName = textBody.substring(textBody.indexOf("filename=\"") + 10);//獲取文件名
fileName = fileName.substring(0, fileName.indexOf("\n"));
fileName = fileName.substring(fileName.indexOf("\n") + 1, fileName.indexOf("\""));
fileName= new String( fileName.getBytes("ISO-8859-1"),"UTF-8");
String contentType = request.getContentType();
String boundaryText = contentType.substring(contentType.lastIndexOf("=") + 1, contentType.length());
int pos = textBody.indexOf("filename=\"");//從上圖看需要幾個換行後纔是文件內容
pos = textBody.indexOf("\n", pos) + 1;
pos = textBody.indexOf("\n", pos) + 1;
pos = textBody.indexOf("\n", pos) + 1;
int boundaryLoc = textBody.indexOf(boundaryText, pos) - 4;
int begin = (textBody.substring(0, pos).getBytes("ISO-8859-1")).length;
int end = (textBody.substring(0, boundaryLoc).getBytes("ISO-8859-1")).length;
Position p = new Position(begin, end);
FileOutputStream fileOutputStream = new FileOutputStream(request.getServletContext().getRealPath("/" + fileName));
fileOutputStream.write(body, p.begin, (p.end - p.begin));
fileOutputStream.flush();
fileOutputStream.close();
%>
</body>
</html>