Gradle - Correctly merge classes dir when using dev mode

We were returning an empty dir before which would trigger the use of the
resources dir instead.

Also simplify the logic as it was quite hard to follow and it's actually
quite simple.

Fixes #42860

devtools/gradle/gradle-application-plugin/src/main/java/io/quarkus/gradle/tasks/QuarkusDev.java

@@ -633,7 +633,7 @@ private void addLocalProject(ResolvedDependency project, GradleDevModeLauncher.B
             }
         }
         Path classesDir = classesDirs.isEmpty() ? null
-                : QuarkusGradleUtils.mergeClassesDirs(classesDirs, project.getWorkspaceModule().getBuildDir(), root, false);
+                : QuarkusGradleUtils.mergeClassesDirs(classesDirs, project.getWorkspaceModule().getBuildDir(), true, false);
         Path generatedSourcesPath = sources.getSourceDirs().isEmpty() ? null
                 : sources.getSourceDirs().iterator().next().getAptSourcesDir();
 

devtools/gradle/gradle-application-plugin/src/main/java/io/quarkus/gradle/tasks/QuarkusGradleUtils.java

@@ -7,7 +7,7 @@
 import java.nio.file.Path;
 import java.util.Collection;
 import java.util.HashSet;
-import java.util.Iterator;
+import java.util.List;
 import java.util.Set;
 
 import org.gradle.api.Project;
@@ -46,44 +46,35 @@ public static String getClassesDir(SourceSet sourceSet, File tmpDir, boolean pop
     }
 
     public static Path mergeClassesDirs(Collection<Path> classesDirs, File tmpDir, boolean populated, boolean test) {
-        Path classesDir = null;
-        final Iterator<Path> i = classesDirs.iterator();
-        int dirCount = 0;
-        while (i.hasNext()) {
-            final Path next = i.next();
-            if (!Files.exists(next)) {
-                continue;
+        List<Path> existingClassesDirs = classesDirs.stream().filter(p -> Files.exists(p)).toList();
+
+        if (existingClassesDirs.size() == 0) {
+            return null;
+        }
+
+        if (existingClassesDirs.size() == 1) {
+            return existingClassesDirs.get(0);
+        }
+
+        try {
+            Path mergedClassesDir = tmpDir.toPath().resolve("quarkus-app-classes" + (test ? "-test" : ""));
+
+            if (!populated) {
+                return mergedClassesDir;
             }
-            try {
-                switch (dirCount++) {
-                    case 0:
-                        classesDir = next;
-                        break;
-                    case 1:
-                        //there does not seem to be any sane way of dealing with multiple output dirs, as there does not seem
-                        //to be a way to map them. We will need to address this at some point, but for now we just stick them
-                        //all in a temp dir
-                        final Path tmpClassesDir = tmpDir.toPath().resolve("quarkus-app-classes" + (test ? "-test" : ""));
-                        if (!populated) {
-                            return tmpClassesDir;
-                        }
-                        if (Files.exists(tmpClassesDir)) {
-                            IoUtils.recursiveDelete(tmpClassesDir);
-                        }
-                        IoUtils.copy(classesDir, tmpClassesDir);
-                        classesDir = tmpClassesDir;
-                    default:
-                        IoUtils.copy(next, classesDir);
-
-                }
-            } catch (IOException e) {
-                throw new UncheckedIOException(ERROR_COLLECTING_PROJECT_CLASSES, e);
+
+            if (Files.exists(mergedClassesDir)) {
+                IoUtils.recursiveDelete(mergedClassesDir);
             }
+
+            for (Path classesDir : existingClassesDirs) {
+                IoUtils.copy(classesDir, mergedClassesDir);
+            }
+
+            return mergedClassesDir;
+        } catch (IOException e) {
+            throw new UncheckedIOException(ERROR_COLLECTING_PROJECT_CLASSES, e);
         }
-        if (classesDir == null) {
-            return null;
-        }
-        return classesDir;
     }
 
 }