workbook转MultipartFile

将Workbook转换为MultipartFile需要进行以下步骤：
1. 将Workbook转换为ByteArrayOutputStream。
2. 将ByteArrayOutputStream转换为ByteArrayResource。
3. 将ByteArrayResource转换为MultipartFile。

具体实现可以参考以下代码：

import org.springframework.core.io.ByteArrayResource;
import org.springframework.web.multipart.MultipartFile;
import org.apache.poi.ss.usermodel.Workbook;
import org.apache.poi.ss.usermodel.WorkbookFactory;
import java.io.ByteArrayOutputStream;
import java.io.IOException;

public class WorkbookToMultipartFileConverter {

    public static MultipartFile convert(Workbook workbook, String filename) throws IOException {
        ByteArrayOutputStream bos = new ByteArrayOutputStream();
        workbook.write(bos);
        ByteArrayResource resource = new ByteArrayResource(bos.toByteArray()) {
            @Override
            public String getFilename() {
                return filename;
            }
        };
        return new CustomMultipartFile(resource);
    }

    private static class CustomMultipartFile implements MultipartFile {

        private final ByteArrayResource resource;

        public CustomMultipartFile(ByteArrayResource resource) {
            this.resource = resource;
        }

        @Override
        public String getName() {
            return resource.getFilename();
        }

        @Override
        public String getOriginalFilename() {
            return resource.getFilename();
        }

        @Override
        public String getContentType() {
            return "application/vnd.ms-excel";
        }

        @Override
        public boolean isEmpty() {
            return resource.contentLength() == 0;
        }

        @Override
        public long getSize() {
            return resource.contentLength();
        }

        @Override
        public byte[] getBytes() throws IOException {
            return resource.getByteArray();
        }

        @Override
        public InputStream getInputStream() throws IOException {
            return resource.getInputStream();
        }

        @Override
        public void transferTo(File file) throws IOException, IllegalStateException {
            resource.transferTo(file);
        }
    }
}