一：实现原理
二：代码实现
jsp页面：
<%@ page language="java" contentType="text/html; charset=UTF-8"
pageEncoding="UTF-8"%>
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<title>Insert title here</title>
</head>
<body>
<!-- jsp 页面部分 -->
<!-- 文件上传时必须要设置表单的enctype="multipart/form-data"
让表单中的数据以二进制的方式提交，在servlet中以字节流获取
-->
<form action="/fileUpload/UploadFile" method="post" enctype="multipart/form-data">
请选择图片：<input id="myfile" name="myfile" type="file" />
<input type="submit" value="提交" />${result}
</form>
</body>
</html>
servlet类：
package com.fileupload;
import java.io.File;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.RandomAccessFile;
import javax.servlet.RequestDispatcher;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
/**
* Servlet implementation class UploadServlet
*/
public class UploadServlet extends HttpServlet {
public void doGet(HttpServletRequest req, HttpServletResponse resp)
throws ServletException, IOException {
doPost(req,resp);
}
public void doPost(HttpServletRequest req, HttpServletResponse resp)
throws ServletException, IOException {
//从request当中获取流信息
InputStream fileSource = req.getInputStream();
String tempFileName = "E:/tempFile";
//tempFile指向临时文件
File tempFile = new File(tempFileName);
//outputStram文件输出流指向这个临时文件
FileOutputStream outputStream = new FileOutputStream(tempFile);
byte b[] = new byte[1024];
int n;
while(( n = fileSource.read(b)) != -1){
outputStream.write(b, 0, n);
}
//关闭输出流、输入流
outputStream.close();
fileSource.close();
//获取上传文件的名称
RandomAccessFile randomFile = new RandomAccessFile(tempFile,"r");
randomFile.readLine();
String str = randomFile.readLine();
System.out.println(str);
int beginIndex = str.lastIndexOf("\\") + 1;
int endIndex = str.lastIndexOf("\"");
String filename = str.substring(beginIndex, endIndex);
System.out.println("filename:" + filename);
//重新定位文件指针到文件头
randomFile.seek(0);
long startPosition = 0;
int i = 1;
//获取文件内容 开始位置
while(( n = randomFile.readByte()) != -1 && i <=4){
if(n == '\n'){
startPosition = randomFile.getFilePointer();
i ++;
}
}
startPosition = randomFile.getFilePointer() -1;
//获取文件内容 结束位置
randomFile.seek(randomFile.length());
long endPosition = randomFile.getFilePointer();
int j = 1;
while(endPosition >=0 && j<=2){
endPosition--;
randomFile.seek(endPosition);
if(randomFile.readByte() == '\n'){
j++;
}
}
endPosition = endPosition -1;
//设置保存上传文件的路径
String realPath = "E:/图片";
System.out.println(realPath);
File fileupload = new File(realPath);
if(!fileupload.exists()){
fileupload.mkdir();
}
File saveFile = new File(realPath,filename);
RandomAccessFile randomAccessFile = new RandomAccessFile(saveFile,"rw");
//从临时文件当中读取文件内容（根据起止位置获取）
randomFile.seek(startPosition);
while(startPosition < endPosition){
randomAccessFile.write(randomFile.readByte());
startPosition = randomFile.getFilePointer();
}
//关闭输入输出流、删除临时文件
randomAccessFile.close();
randomFile.close();
tempFile.delete();
req.setAttribute("result", "上传成功！");
RequestDispatcher dispatcher = req.getRequestDispatcher("jsp/FileUpload.jsp");
dispatcher.forward(req, resp);
}
}
三：总结
在jsp页面中，通过设置form表单的enctype="multipart/form-date"提交，在servlet中最关键的一步就是通过InputStream fileSource = req.getInputStream();来以获取请求的字节流。然后通过字节流将获取的信息保存到一个临时文件下面，因为获取到的信息不都是所需要的。然后再选择需要的信息。