spring webflux mono excel dowonlad(poi 라이브러리)

excel poi 라이브러리

 <dependency>
    <groupId>org.apache.poi</groupId>
    <artifactId>poi-ooxml</artifactId>
    <version>5.2.3</version>
</dependency>

spring webflux 에서는 HttpServletResponse 를 사용하지 않아 header 에 xlsx 정보를 넣어주는부분에서 spring 일반 샘플대로 하면 오류가 생길 것이다. 
HttpHeaders 에 set 해주고 Mono.just 에 headers 를 세팅해주면 된다. 

import org.apache.poi.ss.usermodel.*;
import org.apache.poi.xssf.usermodel.XSSFWorkbook;
import org.springframework.core.io.ByteArrayResource;
import org.springframework.http.HttpHeaders;
import org.springframework.http.MediaType;
import org.springframework.http.ResponseEntity;
import reactor.core.publisher.Mono;

import java.io.ByteArrayOutputStream;
import java.io.IOException;

@RestController
public class ExcelController {

    @GetMapping("/download-excel")
    public Mono<ResponseEntity<ByteArrayResource>> downloadExcel() {
        Workbook workbook = new XSSFWorkbook();
        Sheet sheet = workbook.createSheet("Sheet1");

        // Create some sample data
        Row headerRow = sheet.createRow(0);
        headerRow.createCell(0).setCellValue("Name");
        headerRow.createCell(1).setCellValue("Email");

        Row dataRow = sheet.createRow(1);
        dataRow.createCell(0).setCellValue("John Doe");
        dataRow.createCell(1).setCellValue("john.doe@example.com");

        // Generate Excel file bytes
        ByteArrayOutputStream outputStream = new ByteArrayOutputStream();
        try {
            workbook.write(outputStream);
            workbook.close();
        } catch (IOException e) {
            e.printStackTrace();
        }

        // Set up the HTTP response headers
        HttpHeaders headers = new HttpHeaders();
        headers.setContentType(MediaType.APPLICATION_OCTET_STREAM);
        headers.setContentDispositionFormData("attachment", "data.xlsx");

        // Create a ByteArrayResource from the file bytes
        ByteArrayResource resource = new ByteArrayResource(outputStream.toByteArray());

        // Return the response entity with the file bytes and headers
        return Mono.just(ResponseEntity.ok()
                .headers(headers)
                .contentLength(outputStream.size())
                .body(resource));
    }
}