Implementation streaming download with spring webflux + fastexcel + databuffer + flow

StreamingHandler.kt

package com.emotimer.presentation.handler

import kotlinx.coroutines.flow.flow
import kotlinx.coroutines.reactive.asPublisher
import kotlinx.coroutines.reactor.awaitSingle
import org.dhatim.fastexcel.Workbook
import org.reactivestreams.Publisher
import org.springframework.core.io.buffer.DataBuffer
import org.springframework.core.io.buffer.DefaultDataBufferFactory
import org.springframework.http.ContentDisposition
import org.springframework.http.MediaType
import org.springframework.web.reactive.function.BodyInserters
import org.springframework.web.reactive.function.server.ServerRequest
import org.springframework.web.reactive.function.server.ServerResponse
import java.io.ByteArrayOutputStream
import java.util.*

class SampleHandler {
    suspend fun downloadCsv(request: ServerRequest): ServerResponse {
        val buffers: Publisher<DataBuffer> = flow {
            val factory = DefaultDataBufferFactory()
            val os = ByteArrayOutputStream()
            os.use {
                val wb = Workbook(it, "excel", "1.0")
                val ws = wb.newWorksheet("sheet1")

                // Generate large data set
                (0..100000).forEach { row ->
                    (0..100).forEach { col ->
                        ws.value(row, col, UUID.randomUUID().toString())
                    }

                    if (row % 1000 == 0) {
                        ws.flush()
                        if (os.size() > 0) {
                            println("PROCESS : $row, ${os.size()}bytes")
                            val buffer = factory.wrap(os.toByteArray())
                            emit(buffer)
                            os.reset()
                        }
                    }
                }

                wb.finish()
                emit(factory.wrap(os.toByteArray()))
            }
        }.asPublisher()

        return ServerResponse.ok()
            .headers {
                it.contentDisposition = ContentDisposition.attachment().filename("test.xlsx").build()
                it.contentType = MediaType.APPLICATION_OCTET_STREAM
            }
            .body(
                BodyInserters.fromDataBuffers(
                    buffers
                )
            )
            .awaitSingle()
    }
}