背景

公司最近需要把A服务的数据传到B服务，数据中包含图片信息，图片是存在mongoDB,mysql数据库存的是mongoDB的文件id,所以读取图片、文件需要通过文件id查mongoDB，返回的是byte[]。现在需要把byte[]转为文件流传给B服务。

代码实现

A服务
先通过id从mongoDB获取文件字节：

public Optional<FileDocument> getFileById(String id) {
        FileDocument fileDocument = mongoTemplate.findById(id, FileDocument.class);
        if (fileDocument != null) {
            Query gridQuery = new Query().addCriteria(Criteria.where("filename").is(fileDocument.getGridfsId()));
            try {
                //根据id查询文件
                GridFSFile fsFile = gridFsTemplate.findOne(gridQuery);
                //打开流下载对象
                GridFSDownloadStream in = gridFSBucket.openDownloadStream(fsFile.getObjectId());
                if (in.getGridFSFile().getLength() > 0) {
                    //获取流对象
                    GridFsResource resource = new GridFsResource(fsFile, in);
                    //获取数据
                    fileDocument.setContent(getBytes(resource.getInputStream()));
                    return Optional.of(fileDocument);
                } else {
                    fileDocument = null;
                    return Optional.empty();
                }`在这里插入代码片`
            } catch (IOException ex) {
                ex.printStackTrace();
            }
        }
        return Optional.empty();
    }

文件对象：FileDocument

    @Id  // 主键
    private String id;
    private String name;        // 文件名称
    private long size;          // 文件大小
    private Date uploadDate;    // 上传时间
    private String md5;         // 文件MD5值
    private byte[] content;     // 文件内容
    private String contentType; // 文件类型
    private String suffix;      // 文件后缀名
    private String description; // 文件描述

发送http请求：

```java
 public static String  httpFilePost(byte[] bytes, String fileName,String url,long size) {
            DataOutputStream out = null;
            BufferedReader in = null;
            StringBuilder result = new StringBuilder();
            try {
                URL console = new URL(url);
                HttpURLConnection conn = (HttpURLConnection) console.openConnection();
                conn.addRequestProperty("fileName", new String(fileName.getBytes("GBK")));
                conn.addRequestProperty("fileSize", String.valueOf(size));
                conn.setConnectTimeout(30000);
                conn.setReadTimeout(30000);
                conn.setUseCaches(false);
                conn.setDoOutput(true);
                conn.setDoInput(true);
                conn.setRequestMethod("POST");
                conn.connect();
                out = new DataOutputStream(conn.getOutputStream());
                out.write(bytes);
                out.flush();
                out.close();
                in = new BufferedReader(new InputStreamReader(conn.getInputStream(), "utf-8"));
                String line;
                while ((line = in.readLine()) != null)
                {
                    result.append(line);
                }
                conn.disconnect();
            } catch (Exception e) {
                log.error("Upload failed: {}", e);
            }    finally {
                try
                {
                    if (out != null)
                    {
                        out.close();
                    }
                    if (in != null)
                    {
                        in.close();
                    }
                }
                catch (IOException ex)
                {
                    log.error("调用in.close Exception, url=" + url + ",param=" + fileName, ex);
                }
            }
            return result.toString();
        }

参数为文件对象的属性，到此A服务代码结束。
B服务

 /**
     *
     * 接收文件信息
     * @param request
     * @return
     */
    @ResponseBody
    @RequestMapping("/receiveFile")
    public Json receiveFile(HttpServletRequest request) {
        Json json = new Json();
        try {
            String fileName = new String(request.getHeader("fileName").getBytes("GBK"));
            String fileSize = new String(request.getHeader("fileSize").getBytes("GBK"));
            ServletInputStream in = request.getInputStream();
            byte[] bytes = IOUtils.toByteArray(in);
            UploadFile uploadFile = new UploadFile();
            uploadFile.setId(UUID.randomUUID().toString().replaceAll("-",""));
            uploadFile.setName(fileName);
            uploadFile.setCreateTime(new Date());
            uploadFile.setContent(new Binary(bytes));
            //uploadFile.setContentType(file.getContentType());
            uploadFile.setSize(Long.valueOf(fileSize));
            //存mongoDb ，到此文件就从A服务传到B服务了。
            mongoTemplate.save(uploadFile, Constants.MONGDB_IMAGE_FHXC);
            json.setObj(uploadFile.getId());
            json.setSuccess(true);
        }catch (Exception e){
            e.printStackTrace();
        }
        return json;
    }

备注：1、如果文件是存在磁盘或者云存储（阿里云、七牛云等），都是先把文件读到再进行操作。
2、如果是在页面上传文件后就同步的话，可以参考其他文献。
参考文献：https://blog.csdn.net/MachineRandy/article/details/112490604