Erklären Sie die Antwort-Download-Methode von Java Web

@Controller
@RequestMapping("/user")
public class UploadController {
@RequestMapping(value="/courseTab",method=RequestMethod.GET)
	public void courseTab(HttpServletResponse response,HttpServletRequest request) throws IOException{
		String path = request.getSession().getServletContext().getRealPath("/courseTab/课表上传模板.xls");
		DownUtil.downMb(response, path, "课表模板"+DateFormat.formatSimple(new Date()));
}
}

package org.cxxy.base.cxsc.util;

import java.io.BufferedInputStream;
import java.io.BufferedOutputStream;
import java.io.File;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.IOException;
import java.io.UnsupportedEncodingException;

import javax.servlet.http.HttpServletResponse;

/**
 * @Title: DownUtil.java
 * @Package org.cxxy.base.cxsc.util
 * @Description:文件下载工具类
 * @author ChoviWu
 * @date 2017年6月18日 下午2:44:17
 * @version V1.0
 */
public class DownUtil {

	/**
	 * 
	 * @Description:
	 * @param @param response
	 * @param @param url 文件在数据库的路径
	 * @param @param base 文件存放的基础路径
	 * @param @param folderPath 上传所在的文件夹
	 * @param @return
	 * @param @throws IOException
	 * @return int
	 * @throws
	 */
	@SuppressWarnings("unused")
	public static int downFile(HttpServletResponse response, String url,
			Integer down, String base, String folderPath) throws IOException {
		// 文件的名称
		String fileName = url.split("/")[1];
		System.out.println(fileName);
		// 文件的后缀
		String last = url.substring(url.lastIndexOf(".") + 1);
		System.out.println(last);
		// 文件路径
		String downFilePath = base + folderPath + fileName;

		Long fileLength = new File(downFilePath).length();// 文件的长度
		if (fileLength != 0) {
			response.reset();
			response.setContentType("application/octet-stream;charset=utf-8"); // 改成输出excel文件
			try {
				response.setHeader(
						"Content-disposition",
						"attachment; filename="
								+ new String(fileName.getBytes("utf-8"),
										"ISO8859-1"));
				response.setHeader("Content-Length", String.valueOf(fileLength));
			} catch (UnsupportedEncodingException e) {
				e.printStackTrace();
			}
			BufferedInputStream bis = null;
			BufferedOutputStream bos = null;
			FileInputStream fis = null;
			try {
				fis = new FileInputStream(downFilePath);
				bis = new BufferedInputStream(fis);
				// 输出流
				bos = new BufferedOutputStream(response.getOutputStream());
				byte[] buff = new byte[2048];
				int bytesread;
				// 写文件
				while (-1 != (bytesread = bis.read(buff, 0, buff.length))) {
					bos.write(buff, 0, bytesread);
				}
				// 跳转的路径
				fis.close();
				bis.close();
				bos.close();

			} catch (FileNotFoundException e) {
				System.out.println("File is Not Exsist!");
			}

		} else {
			// 抛异常
			response.getWriter()
					.write("<script charset=&#39;utf-8&#39; type=&#39;text/javascript&#39;>alert(&#39;该资源不存在！&#39;);history.go(-1);</script>");
			return down;
		}
		down++;
		return down;
	}

	/**
	 * 
	 * @Description: 下载的模板
	 * @param @param response
	 * @param @param path 路径名
	 * @param @param name 模板名称
	 * @param @throws IOException
	 * @return void
	 * @throws
	 */
	@SuppressWarnings("unused")
	public static void downMb(HttpServletResponse response, String path,
			String name) throws IOException {

		Long fileLength = new File(path).length();// 文件的长度
		System.out.println("文件的长度：" + fileLength);
		if (fileLength != 0) {
			response.reset();
			response.setContentType("application/octet-stream;charset=utf-8"); // 改成输出excel文件
			try {
				response.setHeader(
						"Content-disposition",
						"attachment; filename="
								+ new String(name.getBytes("utf-8"),
										"ISO8859-1"));
				response.setHeader("Content-Length", String.valueOf(fileLength));
			} catch (UnsupportedEncodingException e) {
				e.printStackTrace();
			}
			BufferedInputStream bis = null;
			BufferedOutputStream bos = null;
			FileInputStream fis = null;
			try {
				fis = new FileInputStream(path);
				bis = new BufferedInputStream(fis);
				// 输出流
				bos = new BufferedOutputStream(response.getOutputStream());
				byte[] buff = new byte[2048];
				int bytesread;
				// 写文件
				while (-1 != (bytesread = bis.read(buff, 0, buff.length))) {
					bos.write(buff, 0, bytesread);
				}
				fis.close();
				bis.close();
				bos.close();

			} catch (FileNotFoundException e) {
				System.out.println("File is Not Exsist!");

			}
		}
	}
}

response.setHeader("Content-Length", String.valueOf(fileLength));

response.setContentType("application/octet-stream;charset=utf-8"); // 改成输出excel文件

BufferedInputStream bis = null;
BufferedOutputStream bos = null;

byte[] buff = new byte[2048];
	int bytesread;
	// 写文件
	while (-1 != (bytesread = bis.read(buff, 0, buff.length))) {
		bos.write(buff, 0, bytesread);
	}