Purchase order PDFs in Spring Boot

Why purchase orders become PDFs

A purchase order is the document a supplier's accounts team matches an invoice against, and it travels by email between two organisations that share no system. That is the difference between a document and a view of a database, and it is why procurement teams keep asking for this.

What a purchase order has to carry

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

• the PO number, which the supplier invoice must quote back
• the ordering entity and the ship-to address, which are often different
• the bill-to address and any cost centre or GL code
• each line with the agreed price, not the supplier list price
• the delivery date required, and the approval reference

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>.

// PurchaseOrderPdfController.java
package com.example.purchase_orders;

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 PurchaseOrderPdfController {

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

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

  @GetMapping(value = "/purchase-orders/{id}.pdf", produces = MediaType.APPLICATION_PDF_VALUE)
  public ResponseEntity<StreamingResponseBody> pdf(@PathVariable String id) throws Exception {
    Context context = new Context();
    context.setVariable("purchaseOrder", purchaseOrderService.find(id));
    String html = templates.process("purchase_order", 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 purchase order 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=\"purchase-order-" + id + ".pdf\"")
        .body(out -> upstream.body().transferTo(out));
  }
}

The CSS that makes a purchase order page correctly

The layout problem specific to this document is that delivery and billing addresses sit alongside a line item table, and both must stay on the first page where a supplier looks for them. These rules handle it.

/* Backgrounds are dropped unless printBackground is set on the
   request, which is the usual reason a document renders in plain
   black and white when it looked right in the browser. */
@page {
  size: A4;
  margin: 18mm 16mm;
}

table { break-inside: avoid; }
h2    { break-after: avoid; }

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 purchase order lives afterwards

Rendering is the short part. A PO is retained alongside the invoice it authorised, because the pair is what an auditor matches. In practice it lives in the procurement system for as long as the supplier relationship does, which is longer than most teams plan for.

Getting the document right

• Check the PO number, because an invoice quoting the wrong one will not be paid.
• Generation is triggered by an approval completing in a procurement workflow, so size the timeout for that path rather than for a health check.
• These are produced one at a time by a person who is waiting, so latency is felt directly.
• 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 purchase order in other stacks.