add invoice file download to InvoiceChain

src/main/java/de/focus_shift/lexoffice/java/sdk/RequestContext.java

@@ -10,6 +10,7 @@
 import org.springframework.http.ResponseEntity;
 import org.springframework.http.client.ClientHttpRequestFactory;
 import org.springframework.http.client.HttpComponentsClientHttpRequestFactory;
+import org.springframework.http.converter.ByteArrayHttpMessageConverter;
 import org.springframework.http.converter.json.JacksonJsonHttpMessageConverter;
 import org.springframework.web.client.RestTemplate;
 import tools.jackson.databind.json.JsonMapper;
@@ -32,7 +33,7 @@ public class RequestContext {
                 .build();
         this.requestFactory = new HttpComponentsClientHttpRequestFactory(httpClient);
         restTemplate = new RestTemplate(requestFactory);
-        restTemplate.setMessageConverters(List.of(new JacksonJsonHttpMessageConverter(getJsonMapper())));
+        restTemplate.setMessageConverters(List.of(new JacksonJsonHttpMessageConverter(getJsonMapper()), new ByteArrayHttpMessageConverter()));
     }
 
 
@@ -71,6 +72,25 @@ public synchronized <R, E> R execute(RestUriBuilder uriBuilder, HttpMethod metho
         return response.getBody();
     }
 
+    public synchronized byte[] downloadFile(RestUriBuilder uriBuilder) {
+
+        HttpHeaders headers = new HttpHeaders();
+        headers.setAccept(List.of(MediaType.APPLICATION_PDF, MediaType.ALL));
+        headers.add("Authorization", "Bearer " + apiBuilder.getApiToken());
+
+        checkThrottlePeriod();
+
+        HttpEntity<Void> requestEntity = new HttpEntity<>(headers);
+        ResponseEntity<byte[]> response = restTemplate.exchange(uriBuilder.build(), HttpMethod.GET, requestEntity, byte[].class);
+        lastCall = System.currentTimeMillis();
+        if (apiBuilder.throttleProviderPresent()) {
+            apiBuilder.getThrottleProvider()
+                    .apiCalled();
+        }
+
+        return response.getBody();
+    }
+
     public synchronized void delete(RestUriBuilder uriBuilder) {
 
         HttpHeaders headers = new HttpHeaders();

src/main/java/de/focus_shift/lexoffice/java/sdk/chain/InvoiceChain.java

@@ -16,6 +16,10 @@ public Invoice get(String id) {
         return new Get(context).get(id);
     }
 
+    public byte[] downloadFile(String id) {
+        return new DownloadFile(context).download(id);
+    }
+
     public Create create() {
         return new Create(context);
     }
@@ -35,6 +39,19 @@ public Invoice get(String id) {
         }
     }
 
+    protected static class DownloadFile extends ExecutableRequestChain {
+
+        public DownloadFile(RequestContext context) {
+            super(context, "/invoices");
+        }
+
+        @SneakyThrows
+        public byte[] download(String id) {
+            getUriBuilder().appendPath("/" + id + "/file");
+            return getContext().downloadFile(getUriBuilder());
+        }
+    }
+
     public static class Create extends ExecutableRequestChain {
         private static final ParameterizedTypeReference<Invoice> TYPE_REFERENCE = new ParameterizedTypeReference<Invoice>() {
         };

src/test/java/de/focus_shift/lexoffice/java/sdk/chain/InvoiceChainIntegrationTest.java

@@ -166,6 +166,26 @@ void createInvoice() {
                     true)));
   }
 
+  @Test
+  void downloadInvoiceFile() {
+    String invoiceId = "e9066f04-8cc7-4616-93f8-ac9571ec5e11";
+    byte[] pdfContent = "%PDF-1.4 fake pdf content".getBytes();
+
+    stubFor(
+        get(urlPathEqualTo("/v1/invoices/" + invoiceId + "/file"))
+            .willReturn(
+                aResponse()
+                    .withHeader("Content-Type", "application/pdf")
+                    .withHeader(
+                        "Content-Disposition",
+                        "attachment; filename=\"RE1019.pdf\"")
+                    .withBody(pdfContent)));
+
+    byte[] result = lexofficeApi.invoice().downloadFile(invoiceId);
+
+    assertThat(result).isEqualTo(pdfContent);
+  }
+
   @Test
   void createInvoiceFinalized() {
     stubFor(