在JavaWeb中实现遍历指定目录图片并实现下载功能

在JavaWeb（JSP）中实现资源下载功能只需要在响应头上加入content-disposition响应类型即可，类型的值为attachment;filename=文件名，这样就可以实现下载功能。

首先，我们需要一个引导页面，在引导页面中将指定的目录下的图片全部显示出来，然后输出到页面中，并提供下载按钮。代码如下：

<%@ page import="java.io.File" %>
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
    <title>Download2</title>
    <style type="text/css">
        a.down{
            color: #FFF;
            width: 150px;
            height: 30px;
            display: inline-block;
            line-height: 30px;
            background-color: #00b4ef;
            text-decoration: none;
            border-radius: 5px;
        }
        a.down:hover{
            opacity:0.5;
        }
        .down_img{
            border:1px dashed #ddd;
            border-radius: 5px;
        }
        .downbox{
            border:1px solid #ddd;
            padding: 10px;
            width: 260px;
            height:160px;
            box-shadow: 0 1px 3px rgba(41, 41, 41, 0.95);
            display: inline-block;
            margin:10px;
        }
    </style>
</head>
<body>
<%!
    String[] checkNames = {"jpg","png","gif"}; //可显示与下载的图片类型
    public boolean check(String name){
        String[] s = name.split(":");
        int len = s.length;
        for (String names:checkNames) {
            if(name.equals(s[len-1])){
                return true;
            }
        }
        return false;
    }

%>
<%

    String path = request.getServletContext().getRealPath("/images");
    File file = new File(path);
    File[] files = file.listFiles();
    for (File f:files) {
        if(check(f.getName())){
            out.print("<div class=\"downbox\">\n" +
                    "    <center>\n" +
                    "        <img src=\"../images/"+f.getName()+"\" class=\"down_img\" alt=\"加载中...\" width=\"200px\" height=\"105px\">\n" +
                    "    </center>\n" +
                    "    <br>\n" +
                    "    <center><a href=\"/cn/exercise/Download2?download="+f.getName()+"\" class=\"down\">下载此图</a></center>\n" +
                    "</div>");
        }
    }
%>
</body>
</html>

接着，在点击下载的时候，会向服务器的Servlet类传递一个参数，然后服务器根据传递进行的参数值做出响应，实现对参数指定图片进行下载。

package cn.exercise;

import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
@WebServlet(name = "Download2", urlPatterns = {"/cn/exercise/Download2"})
public class Download2 extends HttpServlet {
    protected void doPost(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        this.doGet(request, response);
    }

    protected void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        request.setCharacterEncoding("utf-8");
        response.setCharacterEncoding("utf-8");
        String imgName = request.getParameter("download");
        /*imgName = new String(imgName.getBytes("iso8859-1"),"utf-8");*/
        String path = "/images/"+imgName;
       /* System.out.println(path);*/
        new Tools().fileWriter(path,response);
    }
}

由上Servlet类代码可以看出，它将参数传递给了工具类中的fileWriter方法，这样可以实现业务逻辑分离，然后我们可以在该方法里面进行文件读入与写出，接着做出对客户端的响应。

package cn.exercise;

import javax.servlet.http.HttpServletResponse;
import java.io.*;
import java.net.URLEncoder;

class Tools {
    void fileWriter(String path, HttpServletResponse response) throws IOException {
        response.setCharacterEncoding("utf-8");
        path = "../../"+path;
        path = this.getClass().getClassLoader().getResource(path).getPath();
        File file = new File(path);
        InputStream in = new FileInputStream(file);
        OutputStream out = response.getOutputStream();
        response.setHeader("content-disposition","attachment;filename="+ URLEncoder.encode(file.getName(),"utf-8"));
        byte[] b = new byte[1024];
        int length = 0;
        while((length=in.read(b))!=-1){
            out.write(b,0,length);
        }
        out.close();
        in.close();
    }
}

到这里，遍历指定目录下的图片并实现下载功能就实现了。运行截图如下：

[v_tips]在该功能实现中，只进行对指定目录下的一级目录图片进行下载，没有进行对多级目录下的图片进行下载，原因是只循环了一级目录下的所有文件并进行了判断是否是指定的图片类型，没有进行多级循环（递归）来判断。另若该代码中存在Bug或者还有别的更好的实现方法，可以在下方评论区发表您的看法！ [/v_tips]