Add total item count header to paged responses.

src/main/java/cz/cvut/kbss/study/rest/handler/HateoasPagingListener.java

@@ -34,6 +34,7 @@ public void onApplicationEvent(PaginatedResultRetrievedEvent event) {
         if (header.hasLinks()) {
             event.getResponse().addHeader(HttpHeaders.LINK, header.toString());
         }
+        event.getResponse().addHeader(Constants.X_TOTAL_COUNT_HEADER, Long.toString(page.getTotalElements()));
     }
 
     private String generateNextPageLink(Page<?> page, UriComponentsBuilder uriBuilder) {

src/main/java/cz/cvut/kbss/study/util/Constants.java

@@ -52,4 +52,10 @@ private Constants() {
      * Name of the request parameter specifying sorting.
      */
     public static final String SORT_PARAM = "sort";
+
+    /**
+     * Represents the X-Total-Count HTTP header used to convey the total number of items in paged or otherwise
+     * restricted response.
+     */
+    public static final String X_TOTAL_COUNT_HEADER = "X-Total-Count";
 }

src/test/java/cz/cvut/kbss/study/rest/handler/HateoasPagingListenerTest.java

@@ -22,6 +22,7 @@
 
 import static org.hamcrest.MatcherAssert.assertThat;
 import static org.hamcrest.Matchers.containsString;
+import static org.junit.jupiter.api.Assertions.assertEquals;
 import static org.junit.jupiter.api.Assertions.assertNotNull;
 import static org.junit.jupiter.api.Assertions.assertNull;
 
@@ -182,4 +183,13 @@ public void generatesFirstAndLastLinksForOnlyPage() {
         assertThat(lastLink, containsString(pageSize(size)));
     }
 
+    @Test
+    public void generatesTotalCountHeader() {
+        final int size = records.size();
+        final Page<PatientRecordDto> page = new PageImpl<>(records, PageRequest.of(0, size / 2), records.size());
+        listener.onApplicationEvent(event(page));
+        final String totalCountHeader = responseMock.getHeader(Constants.X_TOTAL_COUNT_HEADER);
+        assertNotNull(totalCountHeader);
+        assertEquals(size, Integer.parseInt(totalCountHeader));
+    }
 }