38、JAVA_WEB开发基础之下载功能

是什么

下载指的是将服务器端的文件保存到客户端的磁盘中

基本实现原理

根据对应的要下载的文件在服务器端找到对应的文件，然后获取到文件流对象，将流对象发送到客户端，然后客户端通过对返回结果中文件流的解析，将流对象转换成相应的文件保存在客户端的磁盘中，以实现下载效果

实例

<%@ page language="java" contentType="text/html; charset=UTF-8" pageEncoding="UTF-8"%>
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<title>Insert title here</title>
</head>
<body>
      <form action="fileDownloadServlet"  method="post">
      				<input type="hidden" name="fileName" value="ceshixzai.txt">
                 	下载文件：下载测试文件<input type="submit" value="下载">
      </form> 
</body>
</html>

import java.io.File;
import java.io.FileInputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.net.URLEncoder;

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

@WebServlet(name = "FileDownloadServlet", urlPatterns = "/fileDownloadServlet")
public class FileDownloadServlet extends HttpServlet {

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

	@Override
	protected void doPost(HttpServletRequest request, HttpServletResponse response)
			throws ServletException, IOException {
		// 获取下载文件名
		String fileName = request.getParameter("fileName");
		fileName = URLEncoder.encode(fileName, "UTF-8");
		response.setHeader("Content-Disposition", "attachment;filename=" + fileName);
		// 设置一个存放下载文件的目录
		File file = new File("F:" + File.separator + fileName);
		// 想写通用的方法的话，可以在这里进行文件的遍历查找
		InputStream input = null;
		OutputStream output = response.getOutputStream();;
		if (file.exists()) {
			input = new FileInputStream(file);
			if (input != null && input.read() > 0) {
				byte[] buff = new byte[1024 * 5];
				int len = 0;
				while ((len = input.read(buff)) > -1) {
					output.write(buff, 0, len);
				}
			} else {
				output.write("文件已经失效！".getBytes());
			}
		}else{
			output.write("文件已经失效！".getBytes());
		}
		if (input != null) {
			input.close();
		}
		if (output != null) {
			output.close();
		}
		
	}
}

38、JAVA_WEB开发基础之下载功能

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