fix: HMR for translation files

* Clears resource bundle cache on translation resource redeployment
* Fires a HMR event to the browser when translations are reloaded

Limitation: Only supports DefaultI18NHandler where the paths are known. If you have a custom I18N handler, you might need a custom HMR supporting HotswapListener.

For #20118
Requires vaadin/hilla#2795 to fully fix the issue

flow-server/src/main/java/com/vaadin/flow/hotswap/Hotswapper.java

@@ -26,6 +26,7 @@
 import java.util.List;
 import java.util.Objects;
 import java.util.Optional;
+import java.util.ResourceBundle;
 import java.util.Set;
 import java.util.concurrent.ConcurrentHashMap;
 import java.util.stream.Collectors;
@@ -51,6 +52,9 @@
 import com.vaadin.flow.server.VaadinService;
 import com.vaadin.flow.server.VaadinSession;
 
+import elemental.json.Json;
+import elemental.json.JsonObject;
+
 /**
  * Entry point for application classes hot reloads.
  * <p>
@@ -165,13 +169,32 @@ public void onHotswap(URI[] createdResources, URI[] modifiedResources,
                     "Hotswap resources change event ignored because VaadinService has been destroyed.");
             return;
         }
-        // no-op for the moment, just logging for debugging purpose
-        // entry point for future implementations, like reloading I18n provider
         if (LOGGER.isTraceEnabled()) {
             LOGGER.trace(
                     "Created resources: {}, modified resources: {}, deletedResources: {}.",
                     createdResources, modifiedResources, deletedResources);
         }
+
+        if (anyMatches(".*/vaadin-i18n/.*\\.properties", createdResources,
+                modifiedResources, deletedResources)) {
+            // Clear resource bundle cache so that translations (and other
+            // resources) are reloaded
+            ResourceBundle.clearCache();
+            // Trigger any potential Hilla translation updates
+            liveReload.sendHmrEvent("translations-update", Json.createObject());
+        }
+
+    }
+
+    private boolean anyMatches(String regexp, URI[]... resources) {
+        for (URI[] uris : resources) {
+            for (URI uri : uris) {
+                if (uri.toString().matches(regexp)) {
+                    return true;
+                }
+            }
+        }
+        return false;
     }
 
     private void onHotswapInternal(HashSet<Class<?>> classes,

flow-server/src/main/java/com/vaadin/flow/internal/BrowserLiveReload.java

@@ -17,9 +17,10 @@
 
 import org.atmosphere.cpr.AtmosphereResource;
 
-import com.vaadin.flow.component.UI;
 import com.vaadin.flow.server.communication.FragmentedMessageHolder;
 
+import elemental.json.JsonObject;
+
 /**
  * Provides a way to reload browser tabs via web socket connection passed as a
  * {@link AtmosphereResource}.
@@ -117,4 +118,14 @@ default void refresh(boolean refreshLayouts) {
      */
     void onMessage(AtmosphereResource resource, String msg);
 
+    /**
+     * Send a client side HMR event.
+     * 
+     * @param event
+     *            the event name
+     * @param eventData
+     *            the event data
+     */
+    void sendHmrEvent(String event, JsonObject eventData);
+
 }

vaadin-dev-server/package.json

@@ -15,7 +15,7 @@
     "@web/dev-server-esbuild": "^0.3.3",
     "prettier": "^2.8.4",
     "tslib": "^2.5.3",
-    "vite": "^4.1.4"
+    "vite": "^5.4.8"
   },
   "dependencies": {
     "construct-style-sheets-polyfill": "^3.1.0",

vaadin-dev-server/src/main/frontend/vaadin-dev-tools.ts

@@ -77,6 +77,10 @@ type DevToolsConf = {
   liveReloadPort: number;
   token?: string;
 };
+
+// @ts-ignore
+const hmrClient: any = import.meta.hot ? import.meta.hot.hmrClient : undefined;
+
 @customElement('vaadin-dev-tools')
 export class VaadinDevTools extends LitElement {
   unhandledMessages: ServerMessage[] = [];
@@ -711,12 +715,22 @@ export class VaadinDevTools extends LitElement {
   }
   handleFrontendMessage(message: ServerMessage) {
     if (message.command === 'featureFlags') {
-    } else if (handleLicenseMessage(message)) {
+    } else if (handleLicenseMessage(message) || this.handleHmrMessage(message)) {
     } else {
       this.unhandledMessages.push(message);
     }
   }
 
+  handleHmrMessage(message: ServerMessage): boolean {
+    if (message.command !== 'hmr') {
+      return false;
+    }
+    if (hmrClient) {
+      hmrClient.notifyListeners(message.data.event, message.data.eventData);
+    }
+    return true;
+  }
+
   getDedicatedWebSocketUrl(): string | undefined {
     function getAbsoluteUrl(relative: string) {
       // Use innerHTML to obtain an absolute URL

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

@@ -381,4 +381,15 @@ public void clearFragmentedMessage(AtmosphereResource resource) {
         resources.put(ref, new FragmentedMessage());
     }
 
+    @Override
+    public void sendHmrEvent(String event, JsonObject eventData) {
+        JsonObject msg = Json.createObject();
+        msg.put("command", "hmr");
+        JsonObject data = Json.createObject();
+        msg.put("data", data);
+        data.put("event", event);
+        data.put("eventData", eventData);
+        broadcast(msg);
+    }
+
 }

vaadin-dev-server/vite.config.js

@@ -2,8 +2,6 @@ import { fileURLToPath } from 'url';
 import { defineConfig } from 'vite';
 import typescript from '@rollup/plugin-typescript';
 
-const { execSync } = require('child_process');
-
 export default defineConfig({
   build: {
     // Write output to resources to include it in Maven package
@@ -27,5 +25,7 @@ export default defineConfig({
         /^@vaadin.*/,
       ]
     }
-  }
+  },
+  // Preserve import.meta.hot in the built file so it can be replaced in the application instead
+  define: { 'import.meta.hot': 'import.meta.hot' }
 });