servlet实现图片预览、文件上传下载（转载）

//文件上传
package com.imooc.servlet;

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;

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();
        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 = getServletContext().getRealPath("/") + "images";
        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/01.jsp");
        dispatcher.forward(req, resp);
    }
}

　　

//文件下载
package com.imooc.servlet;

import java.io.File;
import java.io.FileInputStream;
import java.io.IOException;
import java.io.InputStream;

import javax.servlet.RequestDispatcher;
import javax.servlet.ServletException;
import javax.servlet.ServletOutputStream;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

public class DownloadServlet extends HttpServlet {

    public void doGet(HttpServletRequest req, HttpServletResponse resp)
            throws ServletException, IOException {
        //获取文件下载路径
        String path = getServletContext().getRealPath("/") + "images/";
        String filename = req.getParameter("filename");
        File file = new File(path + filename);
        if(file.exists()){
            //设置相应类型application/octet-stream
            resp.setContentType("application/x-msdownload");
            //设置头信息
            resp.setHeader("Content-Disposition", "attachment;filename=\"" + filename + "\"");
            InputStream inputStream = new FileInputStream(file);
            ServletOutputStream ouputStream = resp.getOutputStream();
            byte b[] = new byte[1024];
            int n ;
            while((n = inputStream.read(b)) != -1){
                ouputStream.write(b,0,n);
            }
            //关闭流、释放资源
            ouputStream.close();
            inputStream.close();


        }else{
            req.setAttribute("errorResult", "文件不存在下载失败！");
            RequestDispatcher dispatcher = req.getRequestDispatcher("jsp/01.jsp");
            dispatcher.forward(req, resp);
        }


    }

    public void doPost(HttpServletRequest req, HttpServletResponse resp)
            throws ServletException, IOException {
        doGet(req,resp);
    }

}

jsp页面
<%@ page language="java" import="java.util.*" pageEncoding="UTF-8"%>
<%
String path = request.getContextPath();
String basePath = request.getScheme()+"://"+request.getServerName()+":"+request.getServerPort()+path+"/";
%>

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
  <head>
    <base href="<%=basePath%>">

    <title>My JSP '01.jsp' starting page</title>

    <meta http-equiv="pragma" content="no-cache">
    <meta http-equiv="cache-control" content="no-cache">
    <meta http-equiv="expires" content="0">    
    <meta http-equiv="keywords" content="keyword1,keyword2,keyword3">
    <meta http-equiv="description" content="This is my page">
    <link rel="stylesheet" type="text/css" href="css/common.css" />
    <script type="text/javascript" src="js/jquery-1.11.1.js"></script>
    <script type="text/javascript">
        $(function(){
            $(".thumbs a").click(function(){
                var largePath  = $(this).attr("href");
                var largeAlt = $(this).attr("title");
                $("#largeImg").attr({
                    src : largePath,
                    alt : largeAlt
                });
                return false;
            });

            $("#myfile").change(function(){
                $("#previewImg").attr("src","file:///" + $("#myfile").val());
            });

            var la = $("#large");
            la.hide();

            $("#previewImg").mousemove(function(e){
                la.css({
                    top : e.pageY,
                    left : e.pageX
                }).html('<img src = "' + this.src + '" />').show();
            }).mouseout(function(){
                la.hide();
            });
        }); 
        /*  //使用js实现文件上传图片的预览
        function showPreview(obj){
            var str = obj.value;
            document.getElementById("previewImg").innerHTML = 
                "<img src = '" + str + "' />";
        }
        */
    </script>
  </head>

  <body>

     <img id="previewImg" src="images/preview.jpg" width="80" height="80" />
     <form action="uploadServlet.do" method="post" enctype="multipart/form-data">
        请选择图片：<input id="myfile" name="myfile" type="file" onchange="showPreview(this)"/>
        <input type="submit" value="提交"  />${result}
    </form>
    下载：<a href="downloadServlet.do?filename=test.txt">test.txt</a>    ${errorResult}

    <div id="large"></div>

    <hr>
    <h2>图片预览</h2>
    <p><img id="largeImg" src="images/img1-lg.jpg" alt="Large Image"/></p>
    <p class="thumbs">
        <a href="images/img2-lg.jpg" title="Image2"><img src="images/img2-thumb.jpg"></a>
        <a href="images/img3-lg.jpg" title="Image3"><img src="images/img3-thumb.jpg"></a>
        <a href="images/img4-lg.jpg" title="Image4"><img src="images/img4-thumb.jpg"></a>
        <a href="images/img5-lg.jpg" title="Image5"><img src="images/img5-thumb.jpg"></a>
        <a href="images/img6-lg.jpg" title="Image6"><img src="images/img6-thumb.jpg"></a>
    </p>
  </body>
</html>　　

慕课网《easy实现文件上传下载（David）》视频教程；http://www.imooc.com/video/5566

下篇就发布在FastDfs上实现图片上传下载的Demo