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首先描述一下问题，spring boot 使用的是内嵌的tomcat， 所以不清楚文件上传到哪里去了， 而且spring boot 把静态的文件全部在启动的时候都会加载到classpath的目录下的，所以上传的文件不知相对于应用目录在哪，也不知怎么写访问路径合适，对于新手的自己真的一头雾水。
后面想起了官方的例子，没想到一开始被自己找到的官方例子，后面太依赖百度谷歌了，结果发现只有官方的例子能帮上忙，而且帮上大忙，直接上密码的代码
package hello;
import static org.springframework.hateoas.mvc.ControllerLinkBuilder.linkTo;
import static org.springframework.hateoas.mvc.ControllerLinkBuilder.methodOn;
import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Paths;
import java.util.stream.Collectors;
import javax.servlet.http.HttpServletRequest;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.core.io.ResourceLoader;
import org.springframework.http.ResponseEntity;
import org.springframework.stereotype.Controller;
import org.springframework.ui.Model;
import org.springframework.web.bind.annotation.PathVariable;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RequestMethod;
import org.springframework.web.bind.annotation.RequestParam;
import org.springframework.web.bind.annotation.ResponseBody;
import org.springframework.web.multipart.MultipartFile;
import org.springframework.web.servlet.mvc.support.RedirectAttributes;
@Controller
public class FileUploadController {
private static final Logger log = LoggerFactory.getLogger(FileUploadController.class);
public static final String ROOT = "upload-dir";
private final ResourceLoader resourceLoader;
@Autowired
public FileUploadController(ResourceLoader resourceLoader) {
this.resourceLoader = resourceLoader;
}
@RequestMapping(method = RequestMethod.GET, value = "/")
public String provideUploadInfo(Model model) throws IOException {
model.addAttribute("files", Files.walk(Paths.get(ROOT))
.filter(path -> !path.equals(Paths.get(ROOT)))
.map(path -> Paths.get(ROOT).relativize(path))
.map(path -> linkTo(methodOn(FileUploadController.class).getFile(path.toString())).withRel(path.toString()))
.collect(Collectors.toList()));
return "uploadForm";
}
//显示图片的方法关键 匹配路径像 localhost:8080/b7c76eb3-5a67-4d41-ae5c-1642af3f8746.png
@RequestMapping(method = RequestMethod.GET, value = "/{filename:.+}")
@ResponseBody
public ResponseEntity<?> getFile(@PathVariable String filename) {
try {
return ResponseEntity.ok(resourceLoader.getResource("file:" + Paths.get(ROOT, filename).toString()));
} catch (Exception e) {
return ResponseEntity.notFound().build();
}
}
//上传的方法
@RequestMapping(method = RequestMethod.POST, value = "/")
public String handleFileUpload(@RequestParam("file") MultipartFile file,
RedirectAttributes redirectAttributes, HttpServletRequest request) {
System.out.println(request.getParameter("member"));
if (!file.isEmpty()) {
try {
Files.copy(file.getInputStream(), Paths.get(ROOT, file.getOriginalFilename()));
redirectAttributes.addFlashAttribute("message",
"You successfully uploaded " + file.getOriginalFilename() + "!");
} catch (IOException|RuntimeException e) {
redirectAttributes.addFlashAttribute("message", "Failued to upload " + file.getOriginalFilename() + " => " + e.getMessage());
}
} else {
redirectAttributes.addFlashAttribute("message", "Failed to upload " + file.getOriginalFilename() + " because it was empty");
}
return "redirect:/";
}
} 
