fix: Support Theme annotation with only defaults (#22738)

Support having Theme annotation
with only default values in
production mode byte scanning.
Previously we always fell back
on Lumo so the not seen Theme
annotation didn't have an impact.

Fixes #22731

Author: caalador

flow-server/src/main/java/com/vaadin/flow/server/frontend/scanner/FrontendClassVisitor.java

@@ -284,6 +284,7 @@ public AnnotationVisitor visitAnnotation(String descriptor,
             return null;
         }
         if (annotationClassName.contains(Theme.class.getName())) {
+            classInfo.theme.themeAnnotationPresent = true;
             return themeVisitor;
         }
         if (annotationClassName.contains(CssImport.class.getName())) {

flow-server/src/main/java/com/vaadin/flow/server/frontend/scanner/FrontendDependencies.java

@@ -944,6 +944,9 @@ private boolean hasThemeInfo(ClassInfo classInfo) {
         if (theme.isNotheme()) {
             return true;
         }
+        if (theme.themeAnnotationPresent) {
+            return true;
+        }
 
         return false;
     }

flow-server/src/main/java/com/vaadin/flow/server/frontend/scanner/ThemeData.java

@@ -31,6 +31,7 @@ final class ThemeData implements Serializable {
     String variant = "";
     String themeName = "";
     boolean notheme;
+    boolean themeAnnotationPresent;
 
     ThemeData(String themeClass, String variant, String themeName) {
         this.themeClass = themeClass;
@@ -70,6 +71,7 @@ public boolean equals(Object other) {
         }
         ThemeData that = (ThemeData) other;
         return notheme == that.notheme
+                && themeAnnotationPresent == that.themeAnnotationPresent
                 && Objects.equals(themeClass, that.themeClass)
                 && Objects.equals(themeName, that.themeName);
     }
@@ -79,12 +81,14 @@ public int hashCode() {
         // We might need to add variant when we wanted to fail in the
         // case of same theme class with different variant, which was
         // right in v13
-        return Objects.hash(themeClass, notheme, themeName);
+        return Objects.hash(themeClass, notheme, themeName,
+                themeAnnotationPresent);
     }
 
     @Override
     public String toString() {
-        return " notheme: " + notheme + "\n themeClass:" + themeClass
+        return " notheme: " + notheme + "\n themeAnnotationPresent: "
+                + themeAnnotationPresent + "\n themeClass:" + themeClass
                 + "\n variant: " + variant + "\n themeName: " + themeName;
     }
 }

flow-server/src/test/java/com/vaadin/flow/server/frontend/scanner/FrontendDependenciesTest.java

@@ -211,6 +211,30 @@ public void onlyThemeVariantDefined_getsLumoAsTheme_preserveVariant() {
                 dependencies.getThemeDefinition().getVariant());
     }
 
+    @Test
+    public void defaultThemeAnnotation_getsLumoAsTheme() {
+        Mockito.when(classFinder.getSubTypesOf(AppShellConfigurator.class))
+                .thenReturn(
+                        Collections.singleton(DefaultThemeAnnotation.class));
+
+        FrontendDependencies dependencies = new FrontendDependencies(
+                classFinder, false, null, true);
+
+        AbstractTheme theme = dependencies.getTheme();
+        Assert.assertNotNull(
+                "Theme should be found for @Theme with default values", theme);
+
+        ThemeDefinition themeDefinition = dependencies.getThemeDefinition();
+        Assert.assertNotNull("ThemeDefinition should be filled",
+                themeDefinition);
+        Assert.assertEquals("Should default to Lumo theme", FakeLumo.class,
+                themeDefinition.getTheme());
+        Assert.assertEquals("Variant should be empty", "",
+                themeDefinition.getVariant());
+        Assert.assertEquals("Theme name should be empty", "",
+                themeDefinition.getName());
+    }
+
     @Test
     public void hasErrorParameterComponent_entryPointIsCollected() {
         Mockito.when(classFinder.getSubTypesOf(HasErrorParameter.class))
@@ -554,6 +578,10 @@ public static class FaultyThemeAnnotation implements AppShellConfigurator {
     public static class ThemeVariantOnly implements AppShellConfigurator {
     }
 
+    @Theme
+    public static class DefaultThemeAnnotation implements AppShellConfigurator {
+    }
+
     @JsModule("reference.js")
     @Tag("div")
     public static class Referenced extends Component {