通过HttpURLConnection调用文件服务，实现文件下载

通过HttpURLConnection调用文件服务，实现文件下载

前端请求A服务controller，A服务通过HttpURLConnection方式请求到B服务，B服务再将文件返回给A服务，A服务用文件流的方式写入下载
一、A服务通过HttpURLConnection调用B服务接口获取File文件
参数介绍：
（1）urlStr：调用服务地址：http://xxx:8080/xxx/xxx?fileId = xxxx
（2）downloadDir：文件下载路径

public static File fileDownload(String urlStr, String downloadDir){
    File file = null;
     try {
         // 统一资源
         URL url = new URL(urlStr);
         // 连接类的父类，抽象类
         URLConnection urlConnection = url.openConnection();
         // http的连接类
         HttpURLConnection httpURLConnection = (HttpURLConnection) urlConnection;
         // 设定请求的方法，默认是GET
         httpURLConnection.setRequestMethod("GET");
         // 设置字符编码
         httpURLConnection.setRequestProperty("Charset", CHARSET);
         // 打开到此 URL 引用的资源的通信链接（如果尚未建立这样的连接）。
         httpURLConnection.connect();
         // 文件大小
         int fileLength = httpURLConnection.getContentLength();
         System.out.println("file length---->" + fileLength);
         URLConnection con = url.openConnection();
         BufferedInputStream bin = new BufferedInputStream(httpURLConnection.getInputStream());
         file = new File(downloadDir);
         if (!file.exists()) {
             OutputStream out = new FileOutputStream(file);
             int size = 0;
             int len = 0;
             byte[] buf = new byte[1024];
             while ((size = bin.read(buf)) != -1) {
                 len += size;
                 out.write(buf, 0, size);
             }
             out.flush();
             out.close();
         }
         bin.close();
     } catch (MalformedURLException e) {
         // TODO Auto-generated catch block
         e.printStackTrace();
     } catch (IOException e) {
         // TODO Auto-generated catch block
         e.printStackTrace();
     } finally {
         return file;
     }
}

二、B服务端controller

@RequestMapping(value = "downloadFile",method = {RequestMethod.GET})
public void downloadFile(HttpServletRequest request, HttpServletResponse response) throws Exception{
	response.setCharacterEncoding("utf-8");
	String fileId = request.getParameter("fileId");
	//通过查询数据库获取到保存的文件路劲，或者之前用接口传来文件路径查找也行
	File file = new File(filePath);
	//获取
	InputStream inputStream = new FileInputStream(file)
	String fileName = new String(file.getFileName().getBytes(), "UTF-8");
	//设置返回头
	response.setContentType("multipart/form-data");
	response.setHeader("Content-Disposition","attachment;filename=" + fileName);
	OutputStream os = response.getOutputStream();
		byte[] b = new byte[2048];
		int length;
		while ((length = inputStream.read(b)) > 0) {
			os.write(b, 0, length);
			os.flush();
		}
	os.close();
	inputStream.close();
}

三、A服务controller
参数：fileId：文件Id

@RequestMapping(value = "downloadFile",method = {RequestMethod.POST})
public String downloadFile(@RequestBody String fileId , HttpServletRequest request , HttpServletResponse response) throws Exception{
	//调用fileDownload方法，详见第一步：客户端通过HttpURLConnection获取B服务中的文件
	String urlStr = "http://xxx:8080/xxx/xxx?fileId = xxxx";
	String downloadDir = "E/xxxx/";
	File file = fileDownload(urlStr, downloadDir);
	// 如果文件名存在，则进行下载
    if (file.exists()) {
        // 配置文件下载
        response.setHeader("content-type", "application/octet-stream");
        response.setContentType("application/octet-stream");
        // 下载文件能正常显示中文
        response.setHeader("Content-Disposition", "attachment;filename=" + new String(filename.getBytes(), "iso8859-1"));
        // 实现文件下载
        byte[] buffer = new byte[1024];
        FileInputStream fis = null;
        BufferedInputStream bis = null;
        try {
            fis = new FileInputStream(file);
            bis = new BufferedInputStream(fis);
            OutputStream os = response.getOutputStream();
            int i = bis.read(buffer);
            while (i != -1) {
                os.write(buffer, 0, i);
                i = bis.read(buffer);
            }
            System.out.println("Download the song successfully!");
        } catch (Exception e) {
            System.out.println("Download the song failed!");
        } finally {
            if (bis != null) {
                try {
                    bis.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
            if (fis != null) {
                try {
                    fis.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
        }
    } else {//对应文件不存在
        try {
            //设置响应的数据类型是html文本，并且告知浏览器，使用UTF-8 来编码。
            response.setContentType("text/html;charset=UTF-8");
            //String这个类里面， getBytes()方法使用的码表，是UTF-8，跟tomcat的默认码表没关系。tomcat iso-8859-1
            String csn = Charset.defaultCharset().name();
            System.out.println("默认的String里面的getBytes方法使用的码表是： " + csn);
            //1. 指定浏览器看这份数据使用的码表
            response.setHeader("Content-Type", "text/html;charset=UTF-8");
            OutputStream os = response.getOutputStream();
            os.write("对应文件不存在".getBytes());
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

总结，代码亲测有效，希望能帮助大家，注意的是我用postman测试，下载下来的文件名有乱码，但在浏览器上能正常显示，不确定是不是postmain的坑。。。。。。