feat: Watch the CSS files in the project's public static resources folder (#22392)

Watches files in src/main/resources/META-INF/resources/ and other public static resources folders to support HMR for StyleSheet("foo.css").

---------

Co-authored-by: caalador <mikael.grankvist@vaadin.com>

Author: mshabarov

flow-test-generic/src/main/java/com/vaadin/flow/testutil/ClassesSerializableTest.java

@@ -92,6 +92,7 @@ protected Stream<String> getExcludedPatterns() {
                 "com\\.vaadin\\.base\\.devserver\\.OpenInCurrentIde.*",
                 "com\\.vaadin\\.base\\.devserver\\.RestartMonitor",
                 "com\\.vaadin\\.base\\.devserver\\.ThemeLiveUpdater",
+                "com\\.vaadin\\.base\\.devserver\\.PublicResourcesCssLiveUpdater",
                 "com\\.vaadin\\.base\\.devserver\\.editor..*",
                 "com\\.vaadin\\.base\\.devserver\\.themeeditor..*",
                 "com\\.vaadin\\.base\\.devserver\\.util\\.BrowserLauncher",

vaadin-dev-server/src/main/java/com/vaadin/base/devserver/DevModeHandlerManagerImpl.java

@@ -114,6 +114,7 @@ public void initDevModeHandler(Set<Class<?>> classes, VaadinContext context)
             ApplicationConfiguration config = ApplicationConfiguration
                     .get(context);
             startWatchingThemeFolder(context, config);
+            startWatchingPublicResourcesCss(context, config);
             watchExternalDependencies(context, config);
             setFullyStarted(true);
         }, executorService);
@@ -245,4 +246,35 @@ private static Logger getLogger() {
         return LoggerFactory.getLogger(DevModeHandlerManagerImpl.class);
     }
 
+    // package-private for testing
+    void startWatchingPublicResourcesCss(VaadinContext context,
+            ApplicationConfiguration config) {
+        final File projectFolder = config.getProjectFolder();
+        List.of("src/main/resources/META-INF/resources",
+                "src/main/resources/resources", "src/main/resources/static",
+                "src/main/resources/public").stream().map(path -> {
+                    File resourcesFolder = new File(projectFolder, path);
+                    if (resourcesFolder.exists()) {
+                        return resourcesFolder.getAbsolutePath();
+                    }
+                    return null;
+                }).filter(path -> path != null).forEach(path -> {
+                    try {
+                        File resourcesFolder = new File(path);
+                        if (!resourcesFolder.isDirectory()) {
+                            getLogger().debug(
+                                    "No public resources folder found at {}",
+                                    resourcesFolder);
+                            return;
+                        }
+                        registerWatcherShutdownCommand(
+                                new PublicResourcesCssLiveUpdater(
+                                        resourcesFolder, context));
+                    } catch (Exception e) {
+                        getLogger().error(
+                                "Failed to start live-reload for public CSS resources",
+                                e);
+                    }
+                });
+    }
 }

vaadin-dev-server/src/main/java/com/vaadin/base/devserver/PublicResourcesCssLiveUpdater.java

@@ -0,0 +1,103 @@
+/*
+ * Copyright 2000-2025 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.base.devserver;
+
+import java.io.Closeable;
+import java.io.File;
+import java.io.IOException;
+import java.nio.charset.StandardCharsets;
+import java.nio.file.Files;
+
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+import com.vaadin.flow.internal.BrowserLiveReload;
+import com.vaadin.flow.internal.BrowserLiveReloadAccessor;
+import com.vaadin.flow.server.VaadinContext;
+
+/**
+ * Watches given resourcesFolder for CSS file changes and performs hot CSS
+ * updates via BrowserLiveReload.
+ */
+class PublicResourcesCssLiveUpdater implements Closeable {
+
+    private final File resourcesFolder;
+    private final VaadinContext context;
+    private FileWatcher watcher;
+
+    PublicResourcesCssLiveUpdater(File resourcesFolder, VaadinContext context)
+            throws IOException {
+        this.resourcesFolder = resourcesFolder;
+        this.context = context;
+        initWatcher();
+    }
+
+    private void initWatcher() throws IOException {
+        if (resourcesFolder == null || !resourcesFolder.isDirectory()) {
+            getLogger().debug(
+                    "Public resources folder {} not found, skipping watcher",
+                    resourcesFolder);
+            return;
+        }
+        var liveReloadOpt = BrowserLiveReloadAccessor
+                .getLiveReloadFromContext(context);
+        if (liveReloadOpt.isEmpty()) {
+            getLogger().debug(
+                    "Browser live reload not available, skipping public resources watcher");
+            return;
+        }
+        BrowserLiveReload liveReload = liveReloadOpt.get();
+        watcher = new FileWatcher(changed -> {
+            try {
+                if (changed.isFile() && changed.getName().endsWith(".css")) {
+                    // Path to be used from the browser: "/" + relative path
+                    String rel = resourcesFolder.toPath()
+                            .relativize(changed.toPath()).toString()
+                            .replace(File.separatorChar, '/');
+                    String browserPath = "/" + rel;
+                    String contents = Files.readString(changed.toPath(),
+                            StandardCharsets.UTF_8);
+                    liveReload.update(browserPath, contents);
+                }
+            } catch (Exception e) {
+                getLogger().error(
+                        "Unable to perform hot update of public resource CSS "
+                                + changed,
+                        e);
+                try {
+                    liveReload.reload();
+                } catch (Exception ignore) {
+                    // no-op
+                }
+            }
+        }, resourcesFolder);
+        watcher.start();
+        getLogger().debug("Watching {} for public CSS changes",
+                resourcesFolder);
+    }
+
+    @Override
+    public void close() throws IOException {
+        if (watcher != null) {
+            watcher.stop();
+            watcher = null;
+        }
+    }
+
+    private Logger getLogger() {
+        return LoggerFactory.getLogger(getClass());
+    }
+}