1、将响应的内容类型设置为文件的内容类型。标头Content-type用来规定实体主体中的数据类型，包含媒体类型和子类型标识符。
2、添加一个名为Content-Disposition的HTTP响应头，给它赋值attachment；filename=filename，这里的fileName是指在文件下载对话框中显示出来的默认文件名。它通常与文件名相同，但是也可以不同。
例如，以下就是将一个文件发送到浏览器的代码范例。
FileInputStream fis = new FileInputStream(file) ;
BufferedInputStream bis = new BufferedInputStream(fis) ;
byte[] bytes = new byte[bis.available()] ;
response.setContentType(contentType) ;
OutputStream os = response.getOutputStream() ;
bis.read(bytes) ;
os.write(bytes) ;
警告：一定要确保你没有在无意中发送超出实际文件内容以外的任何字符。这有可能在你毫不知情的情况下发生。例如，如果需要在JSP页面中使用page指令，可以这么写：
<%@ page import=”java.io.FileInputStream”%>
<jsp:useBean id=”DBBeanId” scope=”page” class=”MyBean”>
在你毫不察觉的情况下，page指令后面的回车换行符就会被发送给浏览器。为了防止发送多余的字符，需要像下面这样编写这个指令：
<%@ page import=”java.io.FileInputStream”
%><jsp:useBean id=”DBBeanId” scope=”page” class=”MyBean”>
在下面这个程序中，我们用一个FileDownloadServlet servlet将secret.pdf文件发送到浏览器。但是，只有授权用户才能浏览。如果用户没有登录，应用程序就会跳转到Login页面。在这里，用户可以在表单中输入用户名和密码，这些内容都将被提交给另一个Servlet：LoginServlet。
LoginServlet.java
package filedownloaded;
import java.io.IOException;
import javax.servlet.RequestDispatcher;
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 javax.servlet.http.HttpSession;
@WebServlet(urlPatterns = {"/login"})
public class LoginServlet extends HttpServlet{
private static final long serialVersionUID = 1L;
public void doPost(HttpServletRequest request,
HttpServletResponse response) throws ServletException,
IOException {
String userName = request.getParameter("userName") ;
String password = request.getParameter("password") ;
if(userName != null && userName.equals("ken")
&& password != null && password.equals("secret")){
HttpSession session = request.getSession(true) ;
session.setAttribute("loggedIn", Boolean.TRUE);
response.sendRedirect("download");
return ;
}else{
RequestDispatcher dispatcher =
request.getRequestDispatcher("/login.jsp") ;
dispatcher.forward(request, response);
}
}
}
<%@ 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 'login.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="styles.css">
-->
</head>
<body>
<form action="login" method="post">
<table>
<tr>
<td>User name: </td>
<td><input name="userName"/></td>
</tr>
<tr>
<td>Password: </td>
<td><input name="password" type="password"/></td>
</tr>
<tr>
<td colspan="2">
<input type="submit" value="login"/>
</td>
</tr>
</table>
</form>
</body>
</html>
FileDownloadServlet.java
package filedownloaded;
import java.io.BufferedInputStream;
import java.io.File;
import java.io.FileInputStream;
import java.io.IOException;
import java.io.OutputStream;
import javax.servlet.RequestDispatcher;
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 javax.servlet.http.HttpSession;
@WebServlet(urlPatterns = {"/download"})
public class FileDownloadServlet extends HttpServlet{
private static final long serialVersionUID = 1L;
@Override
public void doGet(HttpServletRequest req, HttpServletResponse resp)
throws ServletException, IOException {
HttpSession session = req.getSession() ;
if(session == null || session.getAttribute("loggedIn") == null){
RequestDispatcher dispatcher = req.getRequestDispatcher("/login.jsp") ;
dispatcher.forward(req, resp);
return ;
}
String dataDirectory = req.getServletContext().getRealPath("/WEB-INF/data") ;
File file = new File(dataDirectory, "secret.pdf") ;
if(file.exists()){
resp.setContentType("application/pdf");
resp.addHeader("Content-Disposition", "attachment; filename=secret.pdf");
byte[] buffer = new byte[1024] ;
try(FileInputStream fis = new FileInputStream(file) ;
BufferedInputStream bis = new BufferedInputStream(fis);
OutputStream os = resp.getOutputStream()){
int i = bis.read(buffer) ;
while(i != -1){
os.write(buffer, 0, i);
i = bis.read(buffer) ;
}
}catch(IOException e){
e.printStackTrace();
}
}
}
}
原文：http://blog.csdn.net/benjamin_whx/article/details/38851795