最近项目上有一个需求，用户上传文件后，要返回外部能直接的URL，查了些资料中与搞出来了，该需求的核心点是要将上传文件的路径暴露出来，便于外部通过URL直接访问，即将虚拟路径映射问文件系统的绝对路径

现将过程整理如下：

开发环境：SpringBoot2.0以上、JDK 1.8

1.修改spring boot配置文件信息如下

spring:
	mvc:
        static-path-pattern: /**
    resources: #设置上传文件目录对外可见，这个参数很重要，不然上传文件对外是访问不到的
        static-locations: classpath:/META-INF/resources/,classpath:/resources/,classpath:/static/,classpath:/public/,file:${serverInfo.uploadpath}
    servlet:
        multipart:   #配置上传文件最大size
            max-file-size: 100MB
            max-request-size: 100MB 

serverInfo: #配置上传文件磁盘存储路径
    uploadpath: e:\uploadFile

2.上传文件代码

controller

    @PostMapping("upload")
    public ResultMsg uploadFile(Integer userId, MultipartFile file) {
        long size = (long) file.getSize();
        if (size > localConfig.getMaxFileSize()) {
            return ResultMsg.getMsg("上传文件过大，请上传小于100MB大小的文件");
        }
        ResultMsg resultMsg = fileUploadService.uploadFile(senderId,uploadType, file);
        return resultMsg;
    }

service

@Service
public class FileUploadService {
    @Autowired
    private LocalConfig localConfig;
    public ResultMsg uploadFile(Integer sendId, MultipartFile file){
        //获取操作系统文件分隔符
        String sepa = java.io.File.separator; 
        String fileName = file.getOriginalFilename();
        String fileNameSuffix = fileName.substring(fileName.lastIndexOf(".") + 1);
        String fileNamePrefix = fileName.substring(0 , fileName.lastIndexOf("."));
        String timeStr = ""+ DateUtils.getDayStr(new Date());
        //文件名为：原文件名+上传人Id+上床时间+文件后缀名
        String newFileName = fileNamePrefix + "-"+ sendId +"-"+ DateUtils.getDateStr(new Date())  + "." + fileNameSuffix;
        //从yml配置文件获取保存文件的根路径
        String baseFilePath = localConfig.getUploadFilePath();
        //保存文件的路径为：根路径/上传人ID/时间戳/
        String path = baseFilePath + sendId+ sepa + timeStr;
        //创建文件保存目录
        createDirectory(path);
        File targetFile = new File(path + sepa + newFileName);
        try {
            //将上传文件写到服务器上指定的文件
            file.transferTo(targetFile);
            //构造上传文件的对外访问的URL
            String saveUrl =localConfig.getBaseurl()+ sendId+ "/" + timeStr + "/" + newFileName;
            FileInfo fileInfo = new FileInfo(newFileName,fileNameSuffix,saveUrl);
            return ResultMsg.getMsg(fileInfo);
        } catch (IOException e) {
            e.printStackTrace();
            return ResultMsg.getFailedMsg(e.getMessage());
        }
    }
        public void createDirectory(String directoryPath){
        File targetFile = new File(directoryPath);
        if (!targetFile.exists()) {
            targetFile.mkdirs();
        }
    }
}

FileInfo.java 返回给前端的数据结构，包括文件名、URL、文件后缀名等信息

@Data  
public class FileInfo {
    public FileInfo(){}
    public FileInfo(String fileName,String fileType,String fileUrl){
        this.fileName = fileName;
        this.fileType= fileType;
        this.fileUrl = fileUrl;
    }
    public String fileName;
    public String fileType;
    public String fileUrl;
}

LocalConfig.java是配置信息的加载类

@Component
@Data
public class LocalConfig {
     public LocalConfig() {}
    private Logger logger = LoggerFactory.getLogger(getClass().getName());
    @Value("${serverInfo.uploadpath}")
    private String uploadFilePath;
    @Value("${spring.servlet.multipart.max-file-size}")
    private String maxfilesize;
    @Value("${serverInfo.baseurl}")
    private String baseurl;
    
    public Long getMaxFileSize(){
        String str = maxfilesize.substring(0,maxfilesize.length()-2);
        Long maxSize = 1024 * 1024 * Long.parseLong(str);
        return maxSize;
    }
}