一般来说文件下载需要的步骤为：

(1)获得客户端请求的文件的名称

String fileName = req.getParameter("fileName");

(2)获得服务器端的真实的物理路径

 File dir = new File(super.getServletContext().getRealPath("/temp"));

(3)创建目标文件的输入流对象

 File targetFile = new File(dir, fileName);

(4) 执行下载
<1>设置返回内容的类型

resp.setContentType("application/msword");//Content-Type

<2>设置返回内容的长度

resp.setContentLength((int)targetFile.length());//Content-Length

<3>设置怎样处理文件，征求客户的意见

 resp.setHeader("Content-Disposition","attachment;filename=test.doc");

<4>利用tomcat的FileUtils.copyFile方法进行文件的下载

FileUtils.copyFile(targetFile, resp.getOutputStream());

FileDownloadServlet.java

package com.xixi.servlet;

import java.io.File;
import java.io.IOException;

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

import org.apache.commons.io.FileUtils;

@WebServlet(urlPatterns="/downloadServlet")
public class FileDownoloadServlet extends HttpServlet {

    private static final long serialVersionUID = 1L;

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) 
        throws ServletException, IOException {
        //设置字符编码
        req.setCharacterEncoding("utf-8");
        //获得请求的fileName的名称 
        String fileName = req.getParameter("fileName");
        if (fileName!=null && fileName.length() > 0) {
            //获得服务器端的真实的物理路径
            File dir = new File(super.getServletContext().getRealPath("/temp"));
            File targetFile = new File(dir, fileName);
            if (targetFile.exists()) {
                //执行下载
                //设置返回内容的类型
                resp.setContentType("application/msword");//Content-Type
                //设置返回内容的长度
                resp.setContentLength((int)targetFile.length());//Content-Length
                //设置怎样处理文件，征求客户的意见
                resp.setHeader("Content-Disposition", "attachment;filename=test.doc");
                //利用tomcat的FileUtils.copyFile方法进行文件的下载
                FileUtils.copyFile(targetFile, resp.getOutputStream());
            }
        }
    }
}