Packing slip PDFs in Spring Boot

Why packing slips become PDFs

A packing slip is printed and physically placed in a box. It exists to be paper. That is the difference between a document and a view of a database, and it is why warehouse and fulfilment teams keep asking for this.

What a packing slip has to carry

Before any of the code below matters, the template has to produce a document that is actually a packing slip. These are the fields that make it one, and the ones a reader or an auditor will look for first.

• the order reference, which the picker checks against the pick list
• each line with SKU, description and quantity shipped
• backordered lines shown explicitly rather than omitted
• the ship-to address as it will appear on the carton
• the carrier reference or tracking number where it exists

Turning your Thymeleaf template into a document

You already have the markup. TemplateEngine.process("invoice", context) gives you it as a string, which is the only input the render needs. Thymeleaf's @{...} link syntax resolves against the servlet context path. In a rendered document that context does not exist, so build absolute URLs or inline the CSS.

The Spring Boot implementation

java.net.http.HttpClient ships with the runtime, so this adds no dependency. The render happens in a controller returning StreamingResponseBody, and you return ResponseEntity<StreamingResponseBody>.

// PackingSlipPdfController.java
package com.example.packing_slips;

import java.net.URI;
import java.net.http.HttpClient;
import java.net.http.HttpRequest;
import java.net.http.HttpResponse;
import java.time.Duration;

@RestController
public class PackingSlipPdfController {

  private final HttpClient http = HttpClient.newHttpClient();
  private final TemplateEngine templates;

  @Value("${pdfpipe.key}")
  private String apiKey;

  @GetMapping(value = "/packing-slips/{id}.pdf", produces = MediaType.APPLICATION_PDF_VALUE)
  public ResponseEntity<StreamingResponseBody> pdf(@PathVariable String id) throws Exception {
    Context context = new Context();
    context.setVariable("packingSlip", packingSlipService.find(id));
    String html = templates.process("packing_slip", context);

    String body = new ObjectMapper().writeValueAsString(Map.of(
        "html", html,
        "options", Map.of("format", "A4", "printBackground", true)));

    HttpRequest request = HttpRequest.newBuilder(URI.create("https://api.pdfpipe.xyz/v1/pdf"))
        .header("Authorization", "Bearer " + apiKey)
        .header("Content-Type", "application/json")
        .timeout(Duration.ofSeconds(60))
        .POST(HttpRequest.BodyPublishers.ofString(body))
        .build();

    // ofInputStream, not ofByteArray. A long packing slip on the heap is how a
    // container sized for JSON payloads meets the garbage collector.
    HttpResponse<InputStream> upstream =
        http.send(request, HttpResponse.BodyHandlers.ofInputStream());

    return ResponseEntity.ok()
        .header("Content-Disposition", "attachment; filename=\"packing-slip-" + id + ".pdf\"")
        .body(out -> upstream.body().transferTo(out));
  }
}

The CSS that makes a packing slip page correctly

The layout problem specific to this document is that it must print correctly on the label and A5 stock warehouses actually use, not just on A4. These rules handle it.

/* Warehouses print on A5 and label stock, not A4. */
@page {
  size: A5;
  margin: 8mm;
}

.signature {
  break-inside: avoid;
  margin-top: auto;
}

What goes wrong in Spring Boot

RestTemplate returning byte[] materialises the whole document on the heap, and a servlet container sized for JSON will meet the garbage collector after a handful of concurrent report renders. Use the reactive client or the JDK client with ofInputStream.

What people try first

Most Spring Boot projects reach for Flying Saucer, which renders XHTML strictly and rejects markup a browser would accept, or iText, whose AGPL licence makes commercial use a paid decision. That works until it is running on more than one machine, at which point the browser becomes the thing you operate rather than the thing you use.

Where the packing slip lives afterwards

Rendering is the short part. The packing slip is thrown away when the box is opened. It has essentially no retention requirement, which is unusual on this list and worth knowing: the document does not need to be stored, only printed correctly the first time.

Getting the document right

• Check the order reference and quantities, which a picker checks against the physical goods.
• Generation is triggered by an order moving to fulfilment, so size the timeout for that path rather than for a health check.
• Volume arrives in bursts, so queue the render rather than doing it inside the request that triggered it.
• Backgrounds are painted by default here, so a design that uses colour needs nothing set. Only an explicit print_background of false turns them off.

Frequently asked

Related

Other Spring Boot documents, and the same packing slip in other stacks.