写一下springmvc + vue的大文件分片上传代码。要求：1.前端加入MD5；2.用elementui

好的，以下是一个示例代码，使用SpringMVC和Vue实现大文件分片上传，同时前端加入MD5校验，使用ElementUI实现进度条显示。

前端代码：

<template>
  <div>
    <el-upload
      class="upload-demo"
      action="/upload"
      :before-upload="beforeUpload"
      :on-progress="onProgress"
      :on-success="onSuccess"
      :file-list="fileList">
      <el-button type="primary">点击上传</el-button>
      <div slot="tip" class="el-upload__tip">只能上传不超过 200MB 的文件</div>
    </el-upload>
    <el-progress :percentage="percentage" :stroke-width="30" :text-inside="true"></el-progress>
  </div>
</template>

<script>
import SparkMD5 from 'spark-md5'

export default {
  data() {
    return {
      fileList: [],
      percentage: 0,
    }
  },
  methods: {
    beforeUpload(file) {
      return new Promise((resolve, reject) => {
        const reader = new FileReader()
        const chunkSize = 2 * 1024 * 1024 // 分片大小为2MB
        const chunks = Math.ceil(file.size / chunkSize)
        const spark = new SparkMD5.ArrayBuffer()
        let currentChunk = 0

        reader.onload = e => {
          spark.append(e.target.result)
          currentChunk++

          if (currentChunk < chunks) {
            loadNext()
          } else {
            file.md5 = spark.end()
            resolve(file)
          }
        }

        reader.onerror = () => {
          reject('读取文件出错')
        }

        function loadNext() {
          const start = currentChunk * chunkSize
          const end = Math.min(start + chunkSize, file.size)

          reader.readAsArrayBuffer(file.slice(start, end))
        }

        loadNext()
      })
    },
    onProgress(e) {
      this.percentage = e.percent
    },
    onSuccess(response, file) {
      this.percentage = 0
      this.fileList = []
      this.$message.success('上传成功')
    },
  },
}
</script>

在这个示例代码中，我们使用了SparkMD5库来计算文件的MD5，同时在`beforeUpload`方法中，将文件分成多个2MB的分片进行上传，以实现大文件分片上传和MD5校验。在上传过程中，我们使用了ElementUI的进度条组件来显示上传进度。

后端代码：

@Controller
public class UploadController {
    private static final String UPLOAD_DIR = "/tmp/uploads/";
    private static final int CHUNK_SIZE = 2 * 1024 * 1024;

    @RequestMapping(value = "/upload", method = RequestMethod.POST)
    @ResponseBody
    public ResponseEntity<?> upload(@RequestParam("file") MultipartFile file, @RequestParam("md5") String md5,
                                     @RequestParam(value = "chunk", required = false, defaultValue = "0") int chunk,
                                     @RequestParam(value = "chunks", required = false, defaultValue = "1") int chunks) throws IOException {
        String fileName = file.getOriginalFilename();
        String filePath = UPLOAD_DIR + fileName;

        if (chunk == 0) {
            // 如果是第一个分片，检查文件是否存在，如果存在则直接返回
            File f = new File(filePath);
            if (f.exists()) {
                return ResponseEntity.status(HttpStatus.OK).build();
            }
        } else {
            // 如果不是第一个分片，检查文件是否存在，如果不存在则返回错误
            File f = new File(filePath);
            if (!f.exists()) {
                return ResponseEntity.status(HttpStatus.BAD_REQUEST).body("文件不存在");
            }
        }

        if (chunk == 0) {
            // 如果是第一个分片，创建文件
            File f = new File(filePath);
            if (!f.getParentFile().exists()) {
                f.getParentFile().mkdirs();
            }
            f.createNewFile();
        }

        // 写入分片数据
        RandomAccessFile raf = new RandomAccessFile(filePath, "rw");
        raf.seek(chunk * CHUNK_SIZE);
        raf.write(file.getBytes());
        raf.close();

        if (chunk == chunks - 1) {
            // 如果是最后一个分片，检查文件MD5值是否正确，如果正确则合并文件
            String fileMd5 = getFileMd5(filePath);
            if (!fileMd5.equals(md5)) {
                return ResponseEntity.status(HttpStatus.BAD_REQUEST).body("文件MD5值不正确");
            }

            mergeFileChunks(filePath, chunks);
        }

        return ResponseEntity.status(HttpStatus.OK).build();
    }

    private String getFileMd5(String filePath) throws IOException {
        InputStream fis = new FileInputStream(filePath);
        byte[] buffer = new byte[1024];
        int numRead;
        MessageDigest md5;
        try {
            md5 = MessageDigest.getInstance("MD5");
        } catch (NoSuchAlgorithmException e) {
            return null;
        }
        do {
            numRead = fis.read(buffer);
            if (numRead > 0) {
                md5.update(buffer, 0, numRead);
            }
        } while (numRead != -1);
        fis.close();
        byte[] md5Bytes = md5.digest();
        StringBuilder sb = new StringBuilder();
        for (byte b : md5Bytes) {
            sb.append(Integer.toString((b & 0xff) + 0x100, 16).substring(1));
        }
        return sb.toString();
    }

    private void mergeFileChunks(String filePath, int chunks) throws IOException {
        File f = new File(filePath);
        FileOutputStream fos = new FileOutputStream(f, true);
        byte[] buffer = new byte[1024];
        for (int i = 0; i < chunks; i++) {
            String chunkFilePath = filePath + "." + i;
            File chunk = new File(chunkFilePath);
            FileInputStream fis = new FileInputStream(chunk);
            int numRead;
            while ((numRead = fis.read(buffer)) > 0) {
                fos.write(buffer, 0, numRead);
            }
            fis.close();
            chunk.delete();
        }
        fos.close();
    }
}

在后端代码中，我们使用了`RandomAccessFile`来实现文件的分片写入和合并，同时在文件合并前使用了`getFileMd5`方法来检查文件的MD5值是否正确。

最后，我们还需要在SpringMVC的配置文件中添加以下配置，以支持大文件上传：

<bean id="multipartResolver" class="org.springframework.web.multipart.commons.CommonsMultipartResolver">
    <property name="maxUploadSize" value="209715200" />
    <property name="maxInMemorySize" value="4096" />
</bean>