文件链接下载

1.Html文件，使用超链接，指向处理下载请求的servlet类，并附带上文件路径和文件名
(就像自动用GET那样，在URL上使用?,&来使用和连接参数)

2.servlet要处理：

0.获取request获取URL的参数，文件路径filepath和文件名filename
1.使用ServletContext获取文件名filename的MIME类型,并设置为响应头的ContentType
2.使用ServletContext获取文件路径filepath的真实路径realpath,并为之创建出File对象
3.设置响应头的content-disposition为 attachment;filename=文件名,相当于弹窗附件下载,命为文件名
4.使用BufferedInputStream，将文件读入内存，输入到byte[],并同时使用response字节输出流输出

Download.html

<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>Insert title here</title>
</head>
<body>
<!-- 使用超链接，指向处理下载请求的servlet类，并附带上文件路径和文件名 -->
<a href = "http://localhost:8080/WebServlet/ResponseDemo3?filepath=./WEB-INF/bb.jpg&filename=bb.jpg" >图片</a>
<a href = "http://localhost:8080/WebServlet/ResponseDemo3?filepath=./b.txt&filename=b.txt" >文本</a>

</body>
</html>

ResponseDemo3_servlet

@WebServlet("/ResponseDemo3")
public class ResponseDemo3 extends HttpServlet {
	private static final long serialVersionUID = 1L;
   
	protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		doPost(request, response);	
	}
	protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

		//0.获取request获取URL的参数，文件路径filepath和文件名filename
		String filepath = request.getParameter("filepath");
		String filename = request.getParameter("filename");
		
		//1.使用ServletContext获取文件名filename的MIME类型,并设置为响应头的ContentType
		ServletContext servletContext = this.getServletContext();
		response.setContentType(servletContext.getMimeType(filename));
		
		//2.使用ServletContext获取文件路径filepath的真实路径realpath,并为之创建出File对象
		String realPath = servletContext.getRealPath(filepath);
		File file = new File(realPath);
		
		//3.设置响应头的content-disposition为 attachment;filename=文件名,相当于弹窗附件下载,命为文件名
		response.setHeader("content-disposition", "attachment;filename="+filename);
		
		//4.使用BufferedInputStream，将文件读入内存，输入到byte[],并同时使用response字节输出流输出
		BufferedInputStream bufferedInputStream = new BufferedInputStream(new FileInputStream(file));
		ServletOutputStream outputStream = response.getOutputStream();
		byte[] bs = new byte[4096];
		while(bufferedInputStream.read(bs) != -1) {
			outputStream.write(bs);
		}
	}
}