fix: Initialize VaadinServlet after Lookup is available

flow-server/src/main/java/com/vaadin/flow/server/VaadinServlet.java

@@ -16,6 +16,7 @@
 package com.vaadin.flow.server;
 
 import javax.servlet.ServletConfig;
+import javax.servlet.ServletContext;
 import javax.servlet.ServletException;
 import javax.servlet.http.HttpServlet;
 import javax.servlet.http.HttpServletRequest;
@@ -27,6 +28,7 @@
 import java.util.Properties;
 
 import com.vaadin.flow.component.UI;
+import com.vaadin.flow.di.Lookup;
 import com.vaadin.flow.function.DeploymentConfiguration;
 import com.vaadin.flow.internal.CurrentInstance;
 import com.vaadin.flow.server.HandlerHelper.RequestType;
@@ -64,23 +66,35 @@ public class VaadinServlet extends HttpServlet {
     @Override
     public void init(ServletConfig servletConfig) throws ServletException {
         CurrentInstance.clearAll();
-        super.init(servletConfig);
 
-        try {
-            servletService = createServletService();
-        } catch (ServiceException e) {
-            throw new ServletException("Could not initialize VaadinServlet", e);
+        if (getServletConfig() == null) {
+            super.init(servletConfig);
+        }
+
+        ServletContext servletContext = getServletConfig().getServletContext();
+        if (new VaadinServletContext(servletContext)
+                .getAttribute(Lookup.class) == null) {
+            return;
         }
 
-        // Sets current service as it is needed in static file server even
-        // though there are no request and response.
-        servletService.setCurrentInstances(null, null);
+        if (servletService == null) {
+            try {
+                servletService = createServletService();
+            } catch (ServiceException e) {
+                throw new ServletException("Could not initialize VaadinServlet",
+                        e);
+            }
 
-        staticFileHandler = createStaticFileHandler(servletService);
+            // Sets current service as it is needed in static file server even
+            // though there are no request and response.
+            servletService.setCurrentInstances(null, null);
 
-        servletInitialized();
-        CurrentInstance.clearAll();
+            staticFileHandler = createStaticFileHandler(servletService);
 
+            servletInitialized();
+        }
+
+        CurrentInstance.clearAll();
     }
 
     /**

flow-server/src/test/java/com/vaadin/flow/server/VaadinServiceTest.java

@@ -212,6 +212,8 @@ public void nullUrlIsSetToACriticalNotification() {
     public void serviceContainsStreamRequestHandler()
             throws ServiceException, ServletException {
         ServletConfig servletConfig = new MockServletConfig();
+        servletConfig.getServletContext().setAttribute(Lookup.class.getName(),
+                Mockito.mock(Lookup.class));
         VaadinServlet servlet = new VaadinServlet() {
             @Override
             protected DeploymentConfiguration createDeploymentConfiguration()

flow-server/src/test/java/com/vaadin/flow/server/VaadinServletTest.java

@@ -15,14 +15,20 @@
  */
 package com.vaadin.flow.server;
 
+import javax.servlet.ServletConfig;
 import javax.servlet.ServletContext;
+import javax.servlet.ServletException;
 import javax.servlet.http.HttpServletRequest;
 
+import java.util.concurrent.atomic.AtomicBoolean;
+
 import net.jcip.annotations.NotThreadSafe;
 import org.junit.Assert;
 import org.junit.Test;
 import org.mockito.Mockito;
 
+import com.vaadin.flow.di.Lookup;
+
 @NotThreadSafe
 public class VaadinServletTest {
 
@@ -64,6 +70,86 @@ public void testGetLastPathParameter() {
                 .getLastPathParameter("http://myhost.com/a;hello/;b=1,c=2/"));
     }
 
+    @Test
+    public void init_superInitCalledOnce() throws ServletException {
+        AtomicBoolean called = new AtomicBoolean();
+        VaadinServlet servlet = new VaadinServlet() {
+
+            @Override
+            public void init() throws ServletException {
+                Assert.assertFalse(called.get());
+                called.set(true);
+            }
+        };
+
+        ServletConfig config = mockConfig();
+        servlet.init(config);
+
+        Assert.assertTrue(called.get());
+
+        servlet.init(mockConfig());
+
+        Assert.assertSame(config, servlet.getServletConfig());
+    }
+
+    @Test
+    public void init_noLookup_servletIsNotInitialized()
+            throws ServletException {
+        AtomicBoolean called = new AtomicBoolean();
+        VaadinServlet servlet = new VaadinServlet() {
+
+            @Override
+            protected void servletInitialized() throws ServletException {
+                called.set(true);
+            }
+        };
+
+        ServletConfig config = mockConfig();
+        servlet.init(config);
+
+        Assert.assertFalse(called.get());
+    }
+
+    @Test
+    public void init_contextHasLookup_servletIsInitialized()
+            throws ServletException {
+        AtomicBoolean called = new AtomicBoolean();
+        VaadinServlet servlet = new VaadinServlet() {
+
+            @Override
+            protected VaadinServletService createServletService()
+                    throws ServletException, ServiceException {
+                return Mockito.mock(VaadinServletService.class);
+            }
+
+            @Override
+            protected StaticFileHandler createStaticFileHandler(
+                    VaadinServletService servletService) {
+                return Mockito.mock(StaticFileHandler.class);
+            }
+
+            @Override
+            protected void servletInitialized() throws ServletException {
+                called.set(true);
+            }
+        };
+
+        ServletConfig config = mockConfig();
+        ServletContext servletContext = config.getServletContext();
+        Mockito.when(servletContext.getAttribute(Lookup.class.getName()))
+                .thenReturn(Mockito.mock(Lookup.class));
+        servlet.init(config);
+
+        Assert.assertTrue(called.get());
+    }
+
+    private ServletConfig mockConfig() {
+        ServletConfig config = Mockito.mock(ServletConfig.class);
+        ServletContext context = Mockito.mock(ServletContext.class);
+        Mockito.when(config.getServletContext()).thenReturn(context);
+        return config;
+    }
+
     private HttpServletRequest createRequest(
             MockServletServiceSessionSetup mocks, String path) {
         HttpServletRequest httpServletRequest = Mockito