原理： 使用html 的 标签，提交form 的几个属性必须为： method=post encType=multipart/form-data;

组件:smartUpload或者commons fileupload

smartUpload代码实现

1。文件预览

function showImage(obj)

{

var str=obj.value;

$("#id").html("");

}

2.无插件实现上传

//从request中获取文件上传流信息

InputStream file=request.getInputStream();

//把流信息写入磁盘文件

String str="G:/";

File temp=new File("str");//写入文件

//获得输出流

FileOutputStream out=new FileOutputStream(temp);

//循环写入

byte[] b=new Byte[1024];

int n;

while((n=file.read(b))!=-1)

{

out.write(b,0,n);

}

file.close();

out.close();

//获取文件名称和内容

RandomAccessFile f=new RandomAccessFile("temp","r");

f.readLine();

String string=f.readLine();

int beginIndex=string.lastIndexOf("\")+1;

int lastIndex=string.lastIndexOf("\");

String name=string.substr(beginIndex,lastIndex);

getServletContext.getRealPath("/");//getServletContext().getRealPath("/")获取项目路径

文件下载

//定义保存路径

String f=getServletContext().getRealPath("/")+"images/";

String filename=request.getParamter("filename");

response.setContentType("appliaction/x-msdownload");//application/octet-stream;

response.setHeader("Context-Dispoinition","attachment:filename=\"+filename+"\"");

/*

解决各浏览器的中文乱码问题

*/

String userAgent = request.getHeader("User-Agent");

String userAgent = request.getHeader("User-Agent");

byte[] bytes = userAgent.contains("MSIE") ? fileName.getBytes()

: fileName.getBytes("UTF-8"); // fileName.getBytes("UTF-8")处理safari的乱码问题

fileName = new String(bytes, "ISO-8859-1"); // 各浏览器基本都支持ISO编码

response.setHeader("Content-disposition",

String.format("attachment; filename=\"%s\"", fileName));

response.setHeader("Content-Length", String.valueOf(fileLength));

byte[] buff = new byte[2048];

int bytesRead;

while (-1 != (bytesRead = bis.read(buff, 0, buff.length))) {

bos.write(buff, 0, bytesRead);

}

bis.close();

bos.close();

}