springboot新增修改上传图片 关于Springboot在新增和修改下上传图片并显示的问题

<form id="blog-form" action="#" th:object="${blog}" th:action="@{/admin/blogs}" method="post" enctype="multipart/form-data" class="ui form">
 
              <!--该部分内容省略，以下为重点--> 
 
               <div class="required field">
                    <div class="ui left labeled input">
                        <label class="ui teal basic label">首图</label>
                        <img src="" th:src="@{*{firstPicture}}" alt="" style="width: 500px !important;">
<!--                        不能给input type="file"文件赋值-->
                        <input type="file" name="picture">
                       <!--<input type="text" name="firstPicture" th:value="*{firstPicture}" placeholder="首图引用地址">-->
                    </div>
                </div>
 
                <!--该部分内容省略，以上为重点--> 
 
                <div class="ui right aligned container">
                    <button type="button" class="ui button" onclick="window.history.go(-1)">返回</button>
                    <button type="button" id="save-btn" class="ui secondary button">保存</button>
                    <button type="button" id="publish-btn" class="ui teal button">发布</button>
                </div>
            </form>

package com.hdq.blog_3.util;
 
 
import org.springframework.web.multipart.MultipartFile;
 
import java.io.File;
import java.io.IOException;
import java.util.UUID;
 
public class UploadImageUtils {
 
 
//    文件上传
    public static String upload(MultipartFile file){
        if(file.isEmpty()){
            return "";
        }
        String fileName = file.getOriginalFilename();//获取文件名
        String suffixName = fileName.substring(fileName.lastIndexOf("."));//获取文件后缀名
       //重命名文件
        fileName = UUID.randomUUID().toString().replaceAll("-","") + suffixName;
//        windows下
//        String filePath="E:/picture/";
        //centos下
        String filePath="/opt/findBugWeb/picture/";
        File dest = new File(filePath+fileName);
        if(!dest.getParentFile().exists()){
            dest.getParentFile().mkdirs();
        }
        try{
            file.transferTo(dest);
        }catch (IOException e){
            e.printStackTrace();
        }
        return filePath.substring(filePath.indexOf("/"))+fileName;
    }
}

package com.hdq.blog_3.sourceMapper;
 
import org.springframework.beans.factory.annotation.Value;
import org.springframework.context.annotation.Configuration;
import org.springframework.web.servlet.config.annotation.ResourceHandlerRegistry;
import org.springframework.web.servlet.config.annotation.WebMvcConfigurer;
 
//配置文件访问路径
@Configuration
public class SourceMapperConfig implements WebMvcConfigurer {
 
//    private String fileSavePath = "E:/picture/";
      String fileSavePath="/opt/findBugWeb/picture/";
    /**
     * 配置资源映射
     * 意思是：如果访问的资源路径是以“/images/”开头的，
     * 就给我映射到本机的“E:/images/”这个文件夹内，去找你要的资源
     * 注意：E:/images/ 后面的 “/”一定要带上
     */
    @Override
    public void addResourceHandlers(ResourceHandlerRegistry registry) {
        registry.addResourceHandler("/opt/findBugWeb/picture/**").addResourceLocations("file:"+fileSavePath);
    }
}

package com.hdq.blog_3.web.admin;
 
import com.hdq.blog_3.po.Blog;
import com.hdq.blog_3.po.User;
import com.hdq.blog_3.service.BlogService;
import com.hdq.blog_3.service.TagService;
import com.hdq.blog_3.service.TypeService;
import com.hdq.blog_3.util.UploadImageUtils;
import com.hdq.blog_3.vo.BlogQuery;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.data.domain.Pageable;
import org.springframework.data.domain.Sort;
import org.springframework.data.web.PageableDefault;
import org.springframework.stereotype.Controller;
import org.springframework.ui.Model;
import org.springframework.web.bind.annotation.*;
import org.springframework.web.multipart.MultipartFile;
import org.springframework.web.servlet.mvc.support.RedirectAttributes;
 
import javax.servlet.http.HttpSession;
 
 
@Controller
@RequestMapping("/admin")
public class BlogController {
 
    private static final String INPUT="admin/blogs-input";
    private static final String LIST="admin/blogs";
    private static final String REDIRECT_LIST="redirect:/admin/blogs";
 
    @Autowired
    private BlogService blogService;
 
    @Autowired
    private TypeService typeService;
 
    @Autowired
    private TagService tagService;
 
    private void setTypeAndTag(Model model){
        model.addAttribute("types",typeService.listType());
        model.addAttribute("tags",tagService.listTag());
    }
 
    //新增 or 修改
    @PostMapping("/blogs")
    public String post(@RequestParam("picture") MultipartFile picture, Blog blog, RedirectAttributes attributes, HttpSession session){
        blog.setUser((User) session.getAttribute("user"));
        blog.setType(typeService.getType(blog.getType().getId()));
        blog.setTags(tagService.listTag(blog.getTagIds()));
        //1.修改：picture为空，则不改变FirstPicture，否则改变FirstPicture。
        //2.新增：直接添加图片文件
        Blog b;
        if(blog.getId() == null){
            blog.setFirstPicture(UploadImageUtils.upload(picture));//重点
            b=blogService.saveBlog(blog);
        }else{
//            isEmpty()与null的区别：前者表示内容是否为空，后者表示对象是否实例化，在这里前端发送请求到后端时，就实例化了picture对象
            if(picture.isEmpty()){
                
           blog.setFirstPicture(blogService.getBlog(blog.getId()).getFirstPicture());//重点
            }else {
                blog.setFirstPicture(UploadImageUtils.upload(picture));//重点
            }
            b=blogService.updateBlog(blog.getId(),blog);
        }
        if(b == null){
            attributes.addFlashAttribute("message","操作失败!");
        }else{
            attributes.addFlashAttribute("message","操作成功!");
        }
        return REDIRECT_LIST;
    }
}