fix: set load-on-startup for automatically registered Vaadin servlet

flow-server/src/main/java/com/vaadin/flow/server/startup/ServletDeployer.java

@@ -336,6 +336,7 @@ private VaadinServletCreation createServletIfNotExists(
 
         registration.setAsyncSupported(true);
         registration.addMapping(path);
+        registration.setLoadOnStartup(1);
         return VaadinServletCreation.SERVLET_CREATED;
     }
 

flow-server/src/test/java/com/vaadin/flow/server/startup/ServletDeployerTest.java

@@ -39,13 +39,15 @@
 import org.junit.Rule;
 import org.junit.Test;
 import org.junit.rules.TemporaryFolder;
+import org.mockito.ArgumentMatchers;
 import org.mockito.Mockito;
 
 public class ServletDeployerTest {
     private final ServletDeployer deployer = new ServletDeployer();
 
     private List<String> servletNames;
     private List<String> servletMappings;
+    private List<Integer> servletLoadOnStartup;
 
     private boolean disableAutomaticServletRegistration = false;
 
@@ -84,6 +86,7 @@ private static class ComponentWithRoute extends Component {
     public void clearCaptures() {
         servletNames = new ArrayList<>();
         servletMappings = new ArrayList<>();
+        servletLoadOnStartup = new ArrayList<>();
     }
 
     @Test
@@ -93,6 +96,7 @@ public void automaticallyRegisterTwoServletsWhenNoServletsPresent()
 
         assertMappingsCount(1, 1);
         assertMappingIsRegistered(ServletDeployer.class.getName(), "/*");
+        assertLoadOnStartupSet();
     }
 
     @Test
@@ -114,6 +118,7 @@ public void registeredNonVaadinServlets_vaadinServletsAreRegistered()
                         singletonList("/test/*"), Collections.emptyMap())));
 
         assertMappingsCount(1, 1);
+        assertLoadOnStartupSet();
     }
 
     @Test
@@ -126,6 +131,7 @@ public void frontendServletIsNotRegisteredWhenProductionModeIsActive()
 
         assertMappingsCount(1, 1);
         assertMappingIsRegistered(ServletDeployer.class.getName(), "/*");
+        assertLoadOnStartupSet();
     }
 
     @Test
@@ -135,6 +141,7 @@ public void frontendServletIsNotRegistered_whenMainServletIsRegistered()
 
         assertMappingsCount(1, 1);
         assertMappingIsRegistered(ServletDeployer.class.getName(), "/*");
+        assertLoadOnStartupSet();
     }
 
     @Test
@@ -156,6 +163,7 @@ public void servletIsNotRegisteredWhenAnotherHasTheSamePathMapping_frontendServl
 
         assertMappingsCount(1, 1);
         assertMappingIsRegistered(ServletDeployer.class.getName(), "/*");
+        assertLoadOnStartupSet();
     }
 
     private void assertMappingsCount(int numServlets, int numMappings) {
@@ -184,6 +192,21 @@ private void assertMappingIsRegistered(String servletName,
                 pathIndex, servletNameIndex);
     }
 
+    private void assertLoadOnStartupSet() {
+        assertEquals("Servlet loadOnStartup should be invoked only once", 1,
+                servletLoadOnStartup.size());
+        assertEquals(
+                String.format(
+                        "Expected servlet loadOnStartup to be '%d' but was '%d",
+                        1, servletLoadOnStartup.get(0)),
+                (Integer) 1, servletLoadOnStartup.get(0));
+    }
+
+    private void assertLoadOnStartupNotSet() {
+        assertTrue("Servlet loadOnStartup should not have been invoked ",
+                servletLoadOnStartup.isEmpty());
+    }
+
     @SuppressWarnings({ "unchecked", "rawtypes" })
     private ServletContextEvent getContextEvent(
             ServletRegistration... servletRegistrations) throws Exception {
@@ -196,6 +219,8 @@ private ServletContextEvent getContextEvent(
                     this.servletMappings.addAll(Arrays.asList(mappings));
                     return Collections.emptySet();
                 });
+        Mockito.doAnswer(i -> this.servletLoadOnStartup.add(i.getArgument(0)))
+                .when(dynamicMock).setLoadOnStartup(ArgumentMatchers.anyInt());
 
         ServletContext contextMock = Mockito.mock(ServletContext.class);