如何用Java实现浏览器文件下载功能

一、了解ServletContext对象

代表整个web应用，可以和程序的容器(服务器)来通信。

1. 概念

        ServletContext是Servlet中最大的一个接口，呈现了web应用的Servlet视图。服务器会为每一个工程创建一个对象，这个对象就是ServletContext对象。这个对象全局唯一，而且工程内部的所有servlet都共享这个对象。所以叫全局应用程序共享对象。

2. 获取

1. 通过request对象获取

request.getServletContext();

2. 通过HttpServlet获取

this.getServletContext();

3. 测试

import javax.servlet.*;
import javax.servlet.http.*;
import javax.servlet.annotation.*;
import java.io.IOException;

@WebServlet(name = "servletContextDemo1", value = "/servletContextDemo1")
public class servletContextDemo1 extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        ServletContext context1 = request.getServletContext();
        ServletContext context2 = this.getServletContext();               //httpServlet下的servletContext
        System.out.println(context1);
        System.out.println(context2);
    }

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

 3. 功能

1. 获取MIME类型：

• MIME类型:在互联网通信过程中定义的一种文件数据类型
• 格式： 大类型/小类型 text/html image/jpeg
• 获取：String getMimeType(String file)

@WebServlet(name = "servletContextDemo2", value = "/servletContextDemo2")
public class servletContextDemo2 extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        ServletContext context = this.getServletContext();               //httpServlet下的servletContext
        String filename = "a.jpg";
        String mimeType = context.getMimeType(filename);               //获取mime类型
        System.out.println(mimeType);                //mime类型为  image/jpeg
    }

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

2. 域对象：共享数据

• 设置数据域：setAttribute(String name,Object value)

@WebServlet(name = "servletContextDemo3", value = "/servletContextDemo3")
public class servletContextDemo3 extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        ServletContext context = this.getServletContext();               //httpServlet下的servletContext
        context.setAttribute("msg","ohmyga");            //设置共享数据,给整个服务器共享
    }

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

• 获取数据域：getAttribute(String name)

@WebServlet(name = "servletContextDemo4", value = "/servletContextDemo4")
public class servletContextDemo4 extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        ServletContext context = this.getServletContext();               //httpServlet下的servletContext
        Object msg = context.getAttribute("msg");//设置共享数据
        System.out.println(msg);
    }

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

 二、如何获取文件真实路径

String getRealPath(String path)；

1.  web目录下资源访问

String a = context.getRealPath("/a.txt");
System.out.println(a);

2. WEB-INF目录下的资源访问

String b = context.getRealPath("/WEB-INF/b.txt");
System.out.println(b);

3. src目录下的资源访问

String c = context.getRealPath("/WEB-INF/classes/c.txt");
System.out.println(c);

@WebServlet(name = "servletContextDemo5", value = "/servletContextDemo5")
public class 获取文件真实路径 extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        ServletContext context = this.getServletContext();               //httpServlet下的servletContext
        String a = context.getRealPath("/WEB-INF/classes/a.txt");           //src目录下的a.txt
        System.out.println(a);
    }

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

 三、实现文件下载

1. html文件

        1. 编写一段简单的html代码，比如下载这个 1.jpg 文件

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>下载</title>
</head>
<body>
<a href="/Web/servletContextDemo6?filename=1.jpg">图片下载</a>
</body>
</html>

         2. 预览效果

 2. web 服务器代码

这里注意需要设置response响应头的mime类型和响应头的打开方式！！！

response.setHeader(String name, String value)；

import javax.servlet.*;
import javax.servlet.http.*;
import javax.servlet.annotation.*;
import java.io.FileInputStream;
import java.io.IOException;

@WebServlet(name = "servletContextDemo6", value = "/servletContextDemo6")
public class 文件下载 extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        String filename = request.getParameter("filename");           //获取请求参数

        //使用字节输入流加载文件进内存
        ServletContext servletContext = this.getServletContext();
        String realPath = servletContext.getRealPath("/WEB-INF/classes/servletContext/" + filename);  //找到文件的服务器路径
        FileInputStream fis = new FileInputStream(realPath);                    //使用字节输入流读取文件

        //设置response响应头
        String mimeType = servletContext.getMimeType(filename);                   //获取mime类型
        response.setHeader("content-type",mimeType);                              //设置响应头类型
        response.setHeader("content-disposition","attachment;filename=" + filename);    //设置响应头打开方式

        //将输入流的事件写出到输出事件
        ServletOutputStream sos = response.getOutputStream();
        byte[] buffer = new byte[1024 * 8];
        int length = 0;
        while((length = fis.read(buffer)) != -1){
            sos.write(buffer,0,length);
        }
        sos.close();
    }

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

 3. 预览效果

点击下载

 文件就被默认（也可以用浏览器设置到其他地方）的下载到了这个位置