java 53 lines · 9 steps

Streaming a CSV export in Spring

A Spring endpoint streams orders straight to the HTTP response as CSV without buffering them all in memory.

Explained by highlit

Spring Intermediate streaming csv-export backpressure dependency-injection http-headers

1@RestController
2@RequestMapping("/api/reports")
3public class OrderReportController {
4 
  private final OrderRepository orderRepository;
6 
  public OrderReportController(OrderRepository orderRepository) {
      this.orderRepository = orderRepository;
  }
10 
  @GetMapping("/orders")
  public ResponseEntity<StreamingResponseBody> streamOrders(
          @RequestParam(defaultValue = "#{T(java.time.LocalDate).now().minusMonths(1)}") LocalDate from,
          @RequestParam(defaultValue = "#{T(java.time.LocalDate).now()}") LocalDate to) {
15 
      StreamingResponseBody body = out -> {
          var writer = new BufferedWriter(new OutputStreamWriter(out, StandardCharsets.UTF_8));
          writer.write("id,customer,placed_at,status,total\n");
19 
          try (Stream<Order> orders = orderRepository.streamByPlacedAtBetween(from.atStartOfDay(), to.atTime(LocalTime.MAX))) {
              orders.forEach(order -> {
                  try {
                      writer.write(String.join(",",
                              Long.toString(order.getId()),
                              escape(order.getCustomerName()),
                              order.getPlacedAt().format(DateTimeFormatter.ISO_LOCAL_DATE_TIME),
                              order.getStatus().name(),
                              order.getTotal().toPlainString()));
                      writer.write('\n');
                  } catch (IOException e) {
                      throw new UncheckedIOException(e);
                  }
              });
          }
          writer.flush();
      };
37 
      String filename = "orders-%s-to-%s.csv".formatted(from, to);
      return ResponseEntity.ok()
              .contentType(new MediaType("text", "csv", StandardCharsets.UTF_8))
              .header(HttpHeaders.CONTENT_DISPOSITION, "attachment; filename=\"" + filename + "\"")
              .cacheControl(CacheControl.noCache())
              .body(body);
  }
45 
  private static String escape(String value) {
      if (value == null) return "";
      if (value.contains(",") || value.contains("\"") || value.contains("\n")) {
          return '"' + value.replace("\"", "\"\"") + '"';
      }
      return value;
  }
53}

01 / 01

STEP 01

Walkthrough

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Three takeaways

1Returning a StreamingResponseBody lets you write to the response as data arrives instead of materializing a whole payload.
2Pairing a JPA Stream inside a try-with-resources keeps the result set open only for the duration of the write.
3Setting Content-Disposition and a specific media type turns a plain endpoint into a browser-triggered file download.

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