文件上传需要注意的问题

package cn.itcast.web.servlet;

import java.io.File;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.util.Arrays;
import java.util.List;
import java.util.UUID;

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

import org.apache.commons.fileupload.FileItem;
import org.apache.commons.fileupload.FileUploadBase;
import org.apache.commons.fileupload.ProgressListener;
import org.apache.commons.fileupload.disk.DiskFileItemFactory;
import org.apache.commons.fileupload.servlet.ServletFileUpload;

public class UploadServlet3 extends HttpServlet {

public void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {

List types = Arrays.asList("jpg","gif","avi","txt");

try{
DiskFileItemFactory factory = new DiskFileItemFactory();  //10k
factory.setSizeThreshold(1024*1024);
factory.setRepository(new File(this.getServletContext().getRealPath("/temp")));

ServletFileUpload upload = new ServletFileUpload(factory);
upload.setProgressListener(new ProgressListener(){
public void update(long pBytesRead, long pContentLength, int pItems) {
System.out.println("当前已解析：" + pBytesRead);
}
});

upload.setFileSizeMax(1024*1024*5);
if(!upload.isMultipartContent(request)){
//按照传统方式获取表单数据
request.getParameter("username");
return;
}
upload.setHeaderEncoding("UTF-8");
List<FileItem> list = upload.parseRequest(request);

for(FileItem item : list){
if(item.isFormField()){
//为普通输入项
String inputName = item.getFieldName();
String inputValue = item.getString("UTF-8");
//inputValue = new String(inputValue.getBytes("iso8859-1"),"UTF-8");
System.out.println(inputName + "="  + inputValue);
}else{
String filename = item.getName().substring(item.getName().lastIndexOf("\\")+1);  //""
if(filename==null || filename.trim().equals("")){
continue;
}

/*String ext = filename.substring(filename.lastIndexOf(".")+1);
if(!types.contains(ext)){
request.setAttribute("message", "本系统不支持" + ext + "这种类型");
request.getRequestDispatcher("/message.jsp").forward(request, response);
return;
}*/
InputStream in = item.getInputStream();
int len = 0;
byte buffer[] = new byte[1024];
String saveFileName = generateFileName(filename);
String savepath = generateSavePath(this.getServletContext().getRealPath("/WEB-INF/upload"),saveFileName);
FileOutputStream out = new FileOutputStream(savepath + File.separator + saveFileName);
while((len=in.read(buffer))>0){
out.write(buffer, 0, len);
}
in.close();
out.close();
item.delete();  //删除临时文件
}
}
}catch (FileUploadBase.FileSizeLimitExceededException e) {
request.setAttribute("message", "文件大小不能超过5m");
request.getRequestDispatcher("/message.jsp").forward(request, response);
return;
}catch (Exception e) {
throw new RuntimeException(e);
}
request.setAttribute("message", "上传成功！！");
request.getRequestDispatcher("/message.jsp").forward(request, response);
}

//
public String generateSavePath(String path,String filename){
int hashcode = filename.hashCode();  //121221
int dir1 = hashcode&15;
int dir2 = (hashcode>>4)&0xf;

String savepath = path + File.separator + dir1 + File.separator + dir2;
File file = new File(savepath);
if(!file.exists()){
file.mkdirs();
}
return savepath;
}

public String generateFileName(String filename){
//83434-83u483-934934
return UUID.randomUUID().toString() + "_" + filename;
}

public void doPost(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
doGet(request, response);
}

}

1.上传文件的中文乱码

1.1 解决文件的乱码

ServletFileUpload.setHeaderEncoding("UTF-8")

1.2 解决普通输入项的乱码（注意，表单类型为multipart/form-data的时候，设置request的编码是无效的）

FileItem.setString("UTF-8"); //解决乱码

2.在处理表单之前，要记得调用：

ServletFileUpload.isMultipartContent方法判断提交表单的类型，如果该方法返回true，则按上传方式处理，否则按照传统方式处理表单即可。

3.设置解析器缓冲区的大小，以及临时文件的删除

设置解析器缓冲区的大小 DiskFileItemFactory.setSizeThreshold(1024*1024);

临时文件的删除：在程序中处理完上传文件后，一定要记得调用item.delete()方法，以删除临时文件

4.在做上传系统时，千万要注意上传文件的保存目录，这个上传文件的保存目录绝对不能让外界直接访问到。

5.限制上传文件的类型

在处理上传文件时，判断上传文件的后缀名是不是允许的

6.限制上传文件的大小

调用解析器的ServletFileUpload.setFileSizeMax(1024*1024*5);就可以限制上传文件的大小，如果上传文件超出限制，则解析器会抛FileUploadBase.FileSizeLimitExceededException异常，程序员通过是否抓到这个异常，进而就可以给用户友好提示。

7.如何判断空的上传输入项

String filename = item.getName().substring(item.getName().lastIndexOf("\\")+1); //""

if(filename==null || filename.trim().equals("")){

continue;

}

8、为避免上传文件的覆盖，程序在保存上传文件时，要为每一个文件生成一个唯一的文件名

public String generateFileName(String filename){

//83434-83u483-934934

return UUID.randomUUID().toString() + "_" + filename;

}

9、为避免在一个文件夹下面保存超过1000个文件，影响文件访问性能，程序应该把上传文件打散后存储。

public String generateSavePath(String path,String filename){

int hashcode = filename.hashCode(); //121221

int dir1 = hashcode&15;

int dir2 = (hashcode>>4)&0xf;

String savepath = path + File.separator + dir1 + File.separator + dir2;

File file = new File(savepath);

if(!file.exists()){

file.mkdirs();

}

return savepath;

}

10、监听上传进度

ServletFileUpload upload = new ServletFileUpload(factory);

upload.setProgressListener(new ProgressListener(){

public void update(long pBytesRead, long pContentLength, int pItems) {

System.out.println("当前已解析：" + pBytesRead);

}

});

11、在web页面中添加动态上传输入项