SONARJAVA-4647: Fix cache misses for default-package and cache-hit files

- scanFile now always writes to cache (empty string for default package),
  so unchanged files without a package declaration are correctly skipped
  via scanWithoutParsing on subsequent runs
- scanWithoutParsing now calls copyFromPrevious to propagate the cache
  entry to the write cache, preventing it from disappearing after a hit
- Empty package name is excluded from badPackageNames in both paths

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: asya-vorobeva

java-checks/src/main/java/org/sonar/java/checks/naming/BadPackageNameCheck.java

@@ -48,22 +48,27 @@ public class BadPackageNameCheck implements JavaFileScanner, EndOfAnalysis {
 
   @Override
   public boolean scanWithoutParsing(InputFileScannerContext context) {
-    var bytes = context.getCacheContext().getReadCache().readBytes(CACHE_KEY_PREFIX + context.getInputFile().key());
+    var cacheKey = CACHE_KEY_PREFIX + context.getInputFile().key();
+    var bytes = context.getCacheContext().getReadCache().readBytes(cacheKey);
     if (bytes == null) {
       return false;
     }
-    handlePackageName(new String(bytes, StandardCharsets.UTF_8));
+    context.getCacheContext().getWriteCache().copyFromPrevious(cacheKey);
+    String name = new String(bytes, StandardCharsets.UTF_8);
+    if (!name.isEmpty()) {
+      handlePackageName(name);
+    }
     return true;
   }
 
   @Override
   public void scanFile(JavaFileScannerContext context) {
     var packageDeclaration = context.getTree().packageDeclaration();
-    if (packageDeclaration != null) {
-      String name = PackageUtils.packageName(packageDeclaration, ".");
-      if (context.getCacheContext().isCacheEnabled()) {
-        context.getCacheContext().getWriteCache().write(CACHE_KEY_PREFIX + context.getInputFile().key(), name.getBytes(StandardCharsets.UTF_8));
-      }
+    String name = packageDeclaration != null ? PackageUtils.packageName(packageDeclaration, ".") : "";
+    if (context.getCacheContext().isCacheEnabled()) {
+      context.getCacheContext().getWriteCache().write(CACHE_KEY_PREFIX + context.getInputFile().key(), name.getBytes(StandardCharsets.UTF_8));
+    }
+    if (!name.isEmpty()) {
       handlePackageName(name);
     }
   }

java-checks/src/test/java/org/sonar/java/checks/naming/BadPackageNameCheckTest.java

@@ -24,10 +24,12 @@
 import org.sonar.java.checks.verifier.internal.InternalReadCache;
 import org.sonar.java.checks.verifier.internal.InternalWriteCache;
 
+import static org.assertj.core.api.Assertions.assertThat;
 import static org.mockito.ArgumentMatchers.any;
 import static org.mockito.Mockito.spy;
 import static org.mockito.Mockito.times;
 import static org.mockito.Mockito.verify;
+import static org.sonar.java.checks.verifier.TestUtils.mainCodeSourcesPath;
 
 class BadPackageNameCheckTest {
 
@@ -99,6 +101,31 @@ void caching() {
 
     verify(check, times(0)).scanFile(any());
     verify(check, times(1)).scanWithoutParsing(any());
+    assertThat(writeCache2.getData()).containsExactlyInAnyOrderEntriesOf(writeCache.getData());
+  }
+
+  @Test
+  void caching_default_package() {
+    String defaultPackageFile = mainCodeSourcesPath("DefaultPackage.java");
+
+    CheckVerifier.newVerifier()
+      .onFile(defaultPackageFile)
+      .withCheck(new BadPackageNameCheck())
+      .withCache(readCache, writeCache)
+      .verifyNoIssues();
+
+    var check = spy(new BadPackageNameCheck());
+    var populatedReadCache = new InternalReadCache().putAll(writeCache);
+    var writeCache2 = new InternalWriteCache().bind(populatedReadCache);
+    CheckVerifier.newVerifier()
+      .withCache(populatedReadCache, writeCache2)
+      .addFiles(InputFile.Status.SAME, defaultPackageFile)
+      .withCheck(check)
+      .verifyNoIssues();
+
+    verify(check, times(0)).scanFile(any());
+    verify(check, times(1)).scanWithoutParsing(any());
+    assertThat(writeCache2.getData()).containsExactlyInAnyOrderEntriesOf(writeCache.getData());
   }
 
   @Test