为什么在处理图像文件时出现此错误？

Question

I am following a tutorial on a book that has a lot of typos/errors. I have been able to keep up with it by examining and fixing the obvious as I go whenever the author makes a mistake.

Currently I am stuck. I have a form that uploads an image file, and when the page reloads (after the upload), the image is supposed to show above the form (blank image before upload). The img src attribute is generated after the upload and handled by Thymleaf and a URL (/uploadedPicture) in the controller, but it seems my code went wrong somewhere since it's not wokring. I'd like to continue with this somewhat great book.

Also, I'm not sure if I'm importing the right Path since there are a few available and the book didn't specify like in other chapters.

NOTE: When I look at /uploadedPicture on the Chrome console after uploading an image, it threw a 500 error with this message:

{
"timestamp":1454561252135,
"status":500,
"error":"Internal Server Error",
"exception":"org.springframework.beans.ConversionNotSupportedException",
"message":"Failed to convert value of type [org.springframework.core.io.FileSystemResource] to required type [java.nio.file.Path]; nested exception is java.lang.IllegalStateException: Cannot convert value of type [org.springframework.core.io.FileSystemResource] to required type [java.nio.file.Path]: no matching editors or conversion strategy found",
"path":"/uploadedPicture"
}

PictureUploadController.java

package masterSpringMvc.profile;

import masterSpringMvc.config.PictureUploadProperties;
import org.apache.tomcat.util.http.fileupload.IOUtils;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.core.io.FileSystemResource;
import org.springframework.core.io.Resource;
import org.springframework.stereotype.Controller;
import org.springframework.ui.Model;
import org.springframework.web.bind.annotation.ModelAttribute;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RequestMethod;
import org.springframework.web.bind.annotation.SessionAttributes;
import org.springframework.web.multipart.MultipartFile;
import org.springframework.web.servlet.mvc.support.RedirectAttributes;

import javax.servlet.http.HttpServletResponse;
import java.io.*;
import java.net.URLConnection;
import java.nio.file.Path;

@Controller
@SessionAttributes("picturePath")
public class PictureUploadController {
    private final Resource picturesDir;
    private final Resource anonymousPicture;

    @Autowired
    public PictureUploadController(PictureUploadProperties uploadProperties) {
        picturesDir = uploadProperties.getUploadPath();
        anonymousPicture = uploadProperties.getAnonymousPicture();
    }

    @ModelAttribute("picturePath")
    public Resource picturePath() {
        return anonymousPicture;
    }

    @RequestMapping("/upload")
    public String uploadPage() {
        return "profile/uploadPage";
    }

    @RequestMapping(value = "/upload", method = RequestMethod.POST)
    public String onUpload(MultipartFile file,
                           RedirectAttributes redirectAttributes,
                           Model model) throws IOException {
        if (file.isEmpty() || !isImage(file)) {
            redirectAttributes.addFlashAttribute("error",
                    "Incorrect file. Please upload a picture.");
            return "redirect:/upload";
        }

        Resource picturePath = copyFileToPictures(file);
        model.addAttribute("picturePath", picturePath);

        return "profile/uploadPage";
    }

    @RequestMapping(value = "/uploadedPicture")
    public void getUploadedPicture(HttpServletResponse response,
                                   @ModelAttribute("picturePath") Path picturePath)
            throws IOException {
        response.setHeader("Content-Type", URLConnection
                .guessContentTypeFromName(picturePath.toString()));
        IOUtils.copy(anonymousPicture.getInputStream(), response.getOutputStream());
    }

    private Resource copyFileToPictures(MultipartFile file) throws IOException {
        String fileExtension = getFileExtension(file.getOriginalFilename());
        File tempFile = File.createTempFile("pic", fileExtension,
                picturesDir.getFile());

        try (InputStream in = file.getInputStream();
             OutputStream out = new FileOutputStream(tempFile)) {
            IOUtils.copy(in, out);
        }

        return new FileSystemResource(tempFile);
    }

    private boolean isImage(MultipartFile file) {
        return file.getContentType().startsWith("image");
    }

    private static String getFileExtension(String name) {
        return name.substring(name.lastIndexOf("."));
    }
}

uploadPage.html

<!DOCTYPE html>
<html xmlns:th="http://www.thymeleaf.org"
      xmlns:layout="http://www.ultraq.net.nz/thymeleaf/layout"
      layout:decorator="layout/default">
<head lang="en">
    <title>Profile Picture Upload</title>
</head>
<body>
<div class="row" layout:fragment="content">
    <h2 class="indigo-text center">Upload</h2>

    <div class="col s12 center red-text" th:text="${error}" th:if="${error}">
        Error during upload
    </div>

    <div class="col m8 s12 offset-m2">
        <img th:src="@{/uploadedPicture}" width="100" height="100"/>
    </div>

    <form th:action="@{/upload}" method="post" enctype="multipart/form-data" class="col m8 s12 offset-m2">
        <div class="input-field col s6">
            <input type="file" id="file" name="file"/>
        </div>
        <div class="col s6 center">
            <button class="btn indigo waves-effect waves-light" type="submit" name="save" th:text="#{submit}">
                Submit<i class="mdi-content-send right"></i>
            </button>
        </div>
    </form>
</div>
</body>
</html>

Answer 1

Inject Resource instead of Path as follow:

@RequestMapping(value = "/uploadedPicture")
public void getUploadedPicture(HttpServletResponse response, @ModelAttribute("picturePath") Resource picturePath) throws IOException  {
    response.setHeader("Content-Type", URLConnection.guessContentTypeFromName(picturePath.toString()));
    Path path = Paths.get(picturePath.getURI());
    Files.copy(path, response.getOutputStream());
}