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

…14501)

When using Vite, DevModeInitializer blocks dev-server startup until a
VaadinServlet is deployed because it needs to get the servlet path to use.
If the container lazily loads servlets, Vite will not start until the first
HTTP request for the Vaadin servlet is received.
This change sets load-on-startup feature for automatically deployed Vaadin
servlet, to ensure that the servlet and Vite are loaded on the startup of the
Web application

Part of #14479

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);