下载文件

思路如下：

1. 要获取下载文件的路径
2. 下载的文件名是啥
3. 设置想办法让浏览器能够支持下载我们需要的东西
4. 获取下载文件的输入流
5. 创建缓冲区
6. 获取OutputStream对象
7. 将FileOutputStream流写到buffer缓冲区
8. 使用OutputStream将缓冲区中的数据输出到客户端

FileDownload.java

import javax.servlet.ServletException;
import javax.servlet.ServletOutputStream;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.FileInputStream;
import java.io.IOException;

public class FileDownload extends HttpServlet {

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {

        //获取下载文件的路径
            // getRealPath 代表的是绝对路径，target里面的项目路径，/ 代表项目路径下
        String realPath = this.getServletContext().getRealPath("/WEB-INF/classes/picture.png");
        System.out.println("下载文件的路径："+realPath);
        //下载的文件名是什么
        String fileName = realPath.substring(realPath.lastIndexOf("//") + 1);
        //设置 想办法让浏览器能够支持我们需要的东西
        resp.setHeader("Content-Disposition","attachment;filename="+fileName);

            //中文文件名 URLEncoder.encode 编码，否则有可能乱码
            //resp.setHeader("Content-Disposition","attachment;filename="+URLEncoder.encode(fileName,"UTF-8"));

        //获取下载文件的输入流
        FileInputStream in = new FileInputStream(realPath);
        //创建缓冲区
        int len = 0;
        byte[] buffer = new byte[1024];
        //获取OutputStream对象
        ServletOutputStream out = resp.getOutputStream();
        //将FileOutputStream流写到buffer缓冲区
        while ((len=in.read(buffer))>0){
            //使用OutputStream将缓冲区的数据输出到客户端
            out.write(buffer,0,len);
        }

        //关闭流
        in.close();
        out.close();
    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        super.doPost(req, resp);
    }
}

<?xml version="1.0" encoding="UTF-8"?>

<web-app xmlns="http://xmlns.jcp.org/xml/ns/javaee"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee
				http://xmlns.jcp.org/xml/ns/javaee/web-app_4_0.xsd"
         version="4.0"
         metadata-complete="true">

    <servlet>
        <servlet-name>fileDownload</servlet-name>
        <servlet-class>com.mahe666.servlet.FileDownload</servlet-class>
    </servlet>

    <servlet-mapping>
        <servlet-name>fileDownload</servlet-name>
        <url-pattern>/fileDownload</url-pattern>
    </servlet-mapping>

</web-app>