像图片或者HTML文件这样的静态资源,在浏览器中打开正确的URL即可下载。只要该资源是放在应用程序的目录下,或者放在应用程序目录的子目录下,而不是放在WEB-INF下,Servlet/JSP容器就会将该资源发送到浏览器。然而,有时静态资源是保存在应用程序目录外,或者是保存在某一个数据库中,或者有时需要控制它的访问权限,防止其他网站交叉引用它。如果出现以上任意一种情况,都必须通过编程来发送资源。
简言之,通过编程进行的文件下载,使你可以有选择地将文件发送到浏览器。
1.文件下载概览
为了将像文件这样的资源发送到浏览器,需要在控制器中完成以下工作:
(1)对请求处理方法使用void返回类型,并在方法中添加HttpServletResponse参数。
(2)将响应的内容类型设为文件的内容类型。Content-Type标题在某个实体的body中定义数据的类型,并包含媒体类型和子类型标识符。
(3)添加一个名为Content-Disposition的HTTP响应标题,并赋值attachment; filename=fileName,这里的fileName是默认文件名,应该出现在File Download(文件下载)对话框中。它通常与文件同名,但是也并非一定如此。
2.范例1:隐藏资源
@RequestMapping(value = "/resource_download")
public String downloadResource(HttpSession session, HttpServletRequest request,
HttpServletResponse response)
{
if (session == null || session.getAttribute("LoggedIn") == null)
{
return "LoginForm";
}
String dataDirectory = request.getServletContext().getRealPath("/WEB-INF/data");
File file = new File(dataDirectory, "secret.pdf");
if (file.exists())
{
response.setContentType("application/pdf");
response.addHeader("Content-Disposition", "attachment; filename=secret.pdf");
byte[] buffer = new byte[1024];
FileInputStream fis = null;
BufferedInputStream bis = null;
try
{
fis = new FileInputStream(file);
bis = new BufferedInputStream(fis);
OutputStream os = response.getOutputStream();
int i = bis.read(buffer);
while (i != -1)
{
os.write(buffer, 0, i);
i = bis.read(buffer);
}
}
catch (IOException e)
{
}
finally
{
if (bis != null)
{
try
{
bis.close();
}
catch (IOException e)
{}
}
if (fis != null)
{
try
{
fis.close();
}
catch (IOException e)
{}
}
}
}
return null;
}
Main.jsp页面中包含了一个链接,用户可以单击它来下载文件。
<%@ taglib prefix="c" uri="http://java.sun.com/jsp/jstl/core"%>
<!DOCTYPE HTML>
<html>
<head>
<title>Download Page</title>
</head>
<body>
<div id="global">
<h4>Please click the link below.</h4>
<p>
<a href="resource_download">Download</a>
</p>
</div>
</body>
</html>
范例2:防止交叉引用
springmvc-config.xml
<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:mvc="http://www.springframework.org/schema/mvc"
xmlns:context="http://www.springframework.org/schema/context"
xsi:schemaLocation="http://www.springframework.org/schema/beans
http://www.springframework.org/schema/beans/spring-beans.xsd
http://www.springframework.org/schema/mvc
http://www.springframework.org/schema/mvc/spring-mvc.xsd
http://www.springframework.org/schema/context
http://www.springframework.org/schema/context/spring-context.xsd">
<mvc:annotation-driven />
<context:component-scan base-package="app12a.controller" />
<mvc:resources location="/" mapping="/*.html" />
<mvc:resources location="/image/" mapping="/image/**" />
<bean id="viewResolver"
class="org.springframework.web.servlet.view.InternalResourceViewResolver">
<property name="prefix" value="/WEB-INF/jsp/" />
<property name="suffix" value=".jsp" />
</bean>
</beans>
Controller
package app12a.controller;
import java.io.BufferedInputStream;
import java.io.File;
import java.io.FileInputStream;
import java.io.IOException;
import java.io.OutputStream;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;
import org.springframework.stereotype.Controller;
import org.springframework.web.bind.annotation.PathVariable;
import org.springframework.web.bind.annotation.RequestHeader;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RequestMethod;
@Controller
public class ImageController
{
private static final Log logger = LogFactory.getLog(ImageController.class);
@RequestMapping(value = "/image_get/{name}", method = RequestMethod.GET)
public void getImage(@PathVariable String name, HttpServletRequest request,
HttpServletResponse response, @RequestHeader String referer)
{
if (referer != null)
{
String imageDirectory = request.getServletContext().getRealPath("/WEB-INF/image");
File file = new File(imageDirectory, name + ".jpg");
if (file.exists())
{
response.setContentType("image/jpg");
byte[] buffer = new byte[1024];
FileInputStream fis = null;
BufferedInputStream bis = null;
try
{
fis = new FileInputStream(file);
bis = new BufferedInputStream(fis);
OutputStream os = response.getOutputStream();
int i = bis.read(buffer);
while (i != -1)
{
os.write(buffer, 0, i);
i = bis.read(buffer);
}
}
catch (IOException e)
{}
finally
{
if (bis != null)
{
try
{
bis.close();
}
catch (IOException e)
{}
}
if (fis != null)
{
try
{
fis.close();
}
catch (IOException e)
{}
}
}
}
}
}
}