Make a workaround for Spring boot run as jar (#8808)

* Make a workaround for Spring boot run as jar

Fixes #8705

* Add a unit test for implemented workaround.

* Add forgotten resource

* Improve variable name and add comments

(cherry picked from commit 6f1d6a5)

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

@@ -28,20 +28,21 @@
 import java.io.InputStreamReader;
 import java.io.Serializable;
 import java.io.UncheckedIOException;
+import java.net.MalformedURLException;
 import java.net.URL;
 import java.net.URLConnection;
 import java.nio.charset.StandardCharsets;
 import java.util.ArrayList;
 import java.util.List;
 import java.util.stream.Collectors;
 
-import com.vaadin.flow.server.frontend.FrontendUtils;
+import org.slf4j.LoggerFactory;
+
 import com.vaadin.flow.server.startup.ApplicationRouteRegistry;
 
 import elemental.json.Json;
 import elemental.json.JsonArray;
 import elemental.json.JsonObject;
-import org.slf4j.LoggerFactory;
 
 /**
  * Registry for PWA data.
@@ -59,6 +60,8 @@
  * @since 1.2
  */
 public class PwaRegistry implements Serializable {
+
+    private static final String META_INF_RESOURCES = "/META-INF/resources";
     private static final String HEADLESS_PROPERTY = "java.awt.headless";
     private static final String APPLE_STARTUP_IMAGE = "apple-touch-startup-image";
     private static final String APPLE_IMAGE_MEDIA = "(device-width: %dpx) and (device-height: %dpx) "
@@ -96,18 +99,21 @@ public PwaRegistry(PWA pwa, ServletContext servletContext)
 
         // Build pwa elements only if they are enabled
         if (pwaConfiguration.isEnabled()) {
-            URL logo = servletContext
-                    .getResource(pwaConfiguration.relIconPath());
-            URL offlinePage = servletContext
-                    .getResource(pwaConfiguration.relOfflinePath());
+            URL logo = getResourceUrl(servletContext,
+                    pwaConfiguration.relIconPath());
+
+            URL offlinePage = getResourceUrl(servletContext,
+                    pwaConfiguration.relOfflinePath());
             // Load base logo from servlet context if available
             // fall back to local image if unavailable
             BufferedImage baseImage = getBaseImage(logo);
 
             if (baseImage == null) {
-                LoggerFactory.getLogger(PwaRegistry.class).error("Image is not found or can't be loaded: " + logo);
+                LoggerFactory.getLogger(PwaRegistry.class).error(
+                        "Image is not found or can't be loaded: " + logo);
             } else {
-                // Pick top-left pixel as fill color if needed for image resizing
+                // Pick top-left pixel as fill color if needed for image
+                // resizing
                 int bgColor = baseImage.getRGB(0, 0);
 
                 // initialize icons
@@ -127,6 +133,19 @@ public PwaRegistry(PWA pwa, ServletContext servletContext)
         }
     }
 
+    private URL getResourceUrl(ServletContext context, String path)
+            throws MalformedURLException {
+        URL resourceUrl = context.getResource(path);
+        if (resourceUrl == null) {
+            // this is a workaround specific for Spring default static resources
+            // location: see #8705
+            String cpPath = path.startsWith("/") ? META_INF_RESOURCES + path
+                    : META_INF_RESOURCES + "/" + path;
+            resourceUrl = PwaRegistry.class.getResource(cpPath);
+        }
+        return resourceUrl;
+    }
+
     private List<PwaIcon> initializeIcons(BufferedImage baseImage,
             int bgColor) {
         for (PwaIcon icon : getIconTemplates(pwaConfiguration.getIconPath())) {

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

@@ -0,0 +1,53 @@
+/*
+ * Copyright 2000-2020 Vaadin Ltd.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License"); you may not
+ * use this file except in compliance with the License. You may obtain a copy of
+ * the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+ * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+ * License for the specific language governing permissions and limitations under
+ * the License.
+ */
+package com.vaadin.flow.server;
+
+import javax.servlet.ServletContext;
+
+import java.io.ByteArrayOutputStream;
+import java.io.IOException;
+import java.util.List;
+
+import org.junit.Assert;
+import org.junit.Test;
+import org.mockito.Mockito;
+
+@PWA(name = "foo", shortName = "bar")
+public class PwaRegistryTest {
+
+    @Test
+    public void pwaIconIsGeneratedBasedOnClasspathIcon_servletContextHasNoResources()
+            throws IOException {
+        ServletContext context = Mockito.mock(ServletContext.class);
+        // PWA annotation has default value for "iconPath" but servlet context
+        // has no resource for that path, in that case the ClassPath URL will be
+        // checked which is "META-INF/resources/icons/icon.png" (this path
+        // available is in the test resources folder). The icon in this path
+        // differs from the default icon and set of icons will be generated
+        // based on it
+        PwaRegistry registry = new PwaRegistry(
+                PwaRegistryTest.class.getAnnotation(PWA.class), context);
+        List<PwaIcon> icons = registry.getIcons();
+        // This icon has width 32 and it's generated based on a custom icon (see
+        // above)
+        PwaIcon pwaIcon = icons.stream().filter(icon -> icon.getWidth() == 32)
+                .findFirst().get();
+        ByteArrayOutputStream stream = new ByteArrayOutputStream();
+        pwaIcon.write(stream);
+        // the default image has 47 on the position 36
+        Assert.assertEquals(26, stream.toByteArray()[36]);
+    }
+}