Jackson ObjectWriter only writes first entry from stream - spring

Question

I want to create a Spring Boot controller which creates a CSV file using data from a stream. I use Jackson CSV (jackson-dataformat-csv 2.12.1) to write the data stream from the DB to a StreamingResponseBody.
To keep it simple, I replaced the actual data from the DB with a list containing 1, 2, 3. I want a CSV file which looks like this:
1
2
3
But it only contains the first entry (1). Can someone help me to identify the problem?
Please not that I don't want to create the file somewhere on the server, I want to stream the content directly to the user.
My code looks like this:
import com.fasterxml.jackson.dataformat.csv.CsvMapper
import org.springframework.http.MediaType
import org.springframework.http.ResponseEntity
import org.springframework.web.bind.annotation.GetMapping
import org.springframework.web.bind.annotation.RequestMapping
import org.springframework.web.bind.annotation.RestController
import org.springframework.web.servlet.mvc.method.annotation.StreamingResponseBody
import javax.servlet.http.HttpServletResponse
#RestController
#RequestMapping(value = ["/test"], produces = [MediaType.TEXT_HTML_VALUE])
class TestController {
#GetMapping("/download", produces = [MediaType.TEXT_EVENT_STREAM_VALUE])
fun download(response: HttpServletResponse): ResponseEntity<StreamingResponseBody>? {
response.setHeader("Content-Disposition", "attachment;filename=download.csv")
response.status = HttpServletResponse.SC_OK
val mapper = CsvMapper()
val schema = mapper.schemaFor(Int::class.java)
val writer = mapper.writer(schema)
val input = listOf(1, 2, 3).stream()
val stream = StreamingResponseBody { outputStream ->
input.forEach { entity ->
writer.writeValue(outputStream, entity)
}
}
return ResponseEntity
.ok()
.header("Content-Disposition", "attachment;filename=download.csv")
.body(stream)
}
}

Answer 1

With the help of Andriy's comment I was able to find the cause and the solution. Jackson closes the stream when it's finished writing to it, see: ObjectMapper._writeValueAndClose.
To change this behavior you have to set JsonGenerator.Feature.AUTO_CLOSE_TARGET to false like this:
val jsonFactory = CsvFactory().configure(JsonGenerator.Feature.AUTO_CLOSE_TARGET, false)
val mapper = ObjectMapper(jsonFactory)
val writer = mapper.writer(CsvMapper().schemaFor(Int::class.java))
Note: There is no AUTO_CLOSE_TARGET option for the CsvGeneratorbut using the JsonGenerator setting also works for the CsvFactory.

Answer 2

Didn't see the column formatter assigned while writing to the stream in your code. Can you try the following :-
CsvMapper mapper = new CsvMapper();
CsvSchema schema = mapper.schemaFor(Dymmy.class);
schema = schema.withColumnSeparator('\t');

Answer 3

There is a better alternative then setting AUTO_CLOSE_TARGET to false, which is to use SequenceWritter.
val stream = StreamingResponseBody { outputStream ->
val mapper = CsvMapper()
val sequenceWriter = mapper.writer(mapper.schemaFor(Int::class.java).withHeader())
.writeValues(outputStream)
input.forEach { entity ->
sequenceWriter.write(entity)
}
}