<dependencies>
    <dependency>
      <groupId>org.springframework.boot</groupId>
      <artifactId>spring-boot-starter-web</artifactId>
    </dependency>

    <dependency>
      <groupId>org.springframework.boot</groupId>
      <artifactId>spring-boot-starter-test</artifactId>
      <scope>test</scope>
    </dependency>

    <dependency>
      <groupId>org.springframework.boot</groupId>
      <artifactId>spring-boot-starter-thymeleaf</artifactId>
    </dependency>
  </dependencies>

@Controller
public class FileUploadController {
  private final StorageService storageService;

  @Autowired
  public FileUploadController(StorageService storageService) {
    this.storageService = storageService;
  }

  //展示上傳過的文件
  @GetMapping("/")
  public String listUploadedFiles(Model model) throws IOException {

    model.addAttribute("files", storageService.loadAll().map(path ->
            MvcUriComponentsBuilder.fromMethodName(FileUploadController.class, "serveFile", path.getFileName().toString())
            .build().toString())
        .collect(Collectors.toList()));

    return "uploadForm";
  }

  //下載選定的上傳的文件
  @GetMapping("/files/{filename:.+}")
  @ResponseBody
  public ResponseEntity<Resource> serveFile(@PathVariable String filename) {

    Resource file = storageService.loadAsResource(filename);
    return ResponseEntity
        .ok()
        .header(HttpHeaders.CONTENT_DISPOSITION, "attachment; filename=\""+file.getFilename()+"\"")
        .body(file);
  }

  //上傳文件
  @PostMapping("/")
  public String handleFileUpload(@RequestParam("file") MultipartFile file,
                  RedirectAttributes redirectAttributes) {

    storageService.store(file);
    redirectAttributes.addFlashAttribute("message",
        "You successfully uploaded " + file.getOriginalFilename() + "!");

    return "redirect:/";
  }

  @ExceptionHandler(StorageFileNotFoundException.class)
  public ResponseEntity<?> handleStorageFileNotFound(StorageFileNotFoundException exc) {
    return ResponseEntity.notFound().build();
  }
}

@Service
public class FileSystemStorageService implements StorageService {

  private final Path rootLocation;

  @Autowired
  public FileSystemStorageService(StorageProperties properties) {
    this.rootLocation = Paths.get(properties.getLocation());
  }

  @Override
  public void store(MultipartFile file) {
    try {
      if (file.isEmpty()) {
        throw new StorageException("Failed to store empty file " + file.getOriginalFilename());
      }
      Files.copy(file.getInputStream(), this.rootLocation.resolve(file.getOriginalFilename()));
    } catch (IOException e) {
      throw new StorageException("Failed to store file " + file.getOriginalFilename(), e);
    }
  }

  @Override
  public Stream<Path> loadAll() {
    try {
      return Files.walk(this.rootLocation, 1)
          .filter(path -> !path.equals(this.rootLocation))
          .map(path -> this.rootLocation.relativize(path));
    } catch (IOException e) {
      throw new StorageException("Failed to read stored files", e);
    }

  }

  @Override
  public Path load(String filename) {
    return rootLocation.resolve(filename);
  }

  @Override
  public Resource loadAsResource(String filename) {
    try {
      Path file = load(filename);
      Resource resource = new UrlResource(file.toUri());
      if(resource.exists() || resource.isReadable()) {
        return resource;
      }
      else {
        throw new StorageFileNotFoundException("Could not read file: " + filename);

      }
    } catch (MalformedURLException e) {
      throw new StorageFileNotFoundException("Could not read file: " + filename, e);
    }
  }

  @Override
  public void deleteAll() {
    FileSystemUtils.deleteRecursively(rootLocation.toFile());
  }

  @Override
  public void init() {
    try {
      Files.createDirectory(rootLocation);
    } catch (IOException e) {
      throw new StorageException("Could not initialize storage", e);
    }
  }
}