SimpleNaiveBayesDocumentClassifier applies a documented 'field^boost' (e.g. 'title^10') only on its first call: it writes the stripped field name back into the caller's textFieldNames array, so every later classification silently returns the unboosted result (released since 6.0.0)

Finding LFV-382Area: classification / document classifiers (SimpleNaiveBayesDocumentClassifier field boosts)Priority: mediumImpact: silent wrong resultsNovelty: new

Finding page for LFV-382

AI-assisted finding, confirmed by automated checks, not yet triaged by a human Lucene committer. This site files nothing upstream.

What it shows

You build a SimpleNaiveBayesDocumentClassifier with textFieldNames {"title^10", "body"}, as its constructor javadoc allows, and classify documents with it one after another, expecting the title boost on every call. The first call applies the boost but also overwrites "title^10" with "title" in textFieldNames, which is the caller's own array kept by reference, so from the second call on every boost is 1: classifying the same document twice gives fruit (0.99994) and then vehicle (0.55373), exactly the unboosted classifier's answer, and on the upstream test's own fixture ('title^100') the class flips from batman to the video-game class. No error is raised, and a second classifier built from the same String[] is unboosted even on its first call.

The test shows that SimpleNaiveBayesDocumentClassifier built with the documented field boost "title^10" applies the boost only on its first call; the same document classified a second time gets the unboosted result.

Expected vs actual

Expected: both assignClass calls return the boosted result fruit 0.9989943793309103; actual: the second returns vehicle 0.7933884297520658 (the unboosted answer).

Files

Run it

# Lucene checkout at the tested commit, built with: ./gradlew --no-daemon assemble   (JDK 25)
LUCENE=/path/to/lucene; V=12.0.0-SNAPSHOT
CP=$LUCENE/lucene/core/build/libs/lucene-core-$V.jar
CP=$CP:$LUCENE/lucene/codecs/build/libs/lucene-codecs-$V.jar
CP=$CP:$LUCENE/lucene/test-framework/build/libs/lucene-test-framework-$V.jar
CP=$CP:$LUCENE/lucene/analysis/common/build/libs/lucene-analysis-common-$V.jar
CP=$CP:$LUCENE/lucene/classification/build/libs/lucene-classification-$V.jar
CP=$CP:$LUCENE/lucene/grouping/build/libs/lucene-grouping-$V.jar
CP=$CP:$LUCENE/lucene/queries/build/libs/lucene-queries-$V.jar
CP=$CP:randomizedtesting-runner-2.9.2.jar:junit-4.13.2.jar:hamcrest-3.0.jar   # test-framework dependencies (Maven Central)
CP=$CP:junit-platform-console-standalone-6.1.3.jar   # JUnit Platform console launcher (Maven Central)

javac -d classes -cp "$CP" $(find src -name '*.java')

# run 1: assertions on, seed DEADBEEF
java -ea -esa --add-modules jdk.incubator.vector --enable-native-access=ALL-UNNAMED -Dtests.seed=DEADBEEF \
     -cp "classes:$CP" org.junit.platform.console.ConsoleLauncher \
     execute --select-class org.apache.lucene.classification.document.TestSimpleNaiveBayesDocumentClassifierBoostReuse

Gradle alternative: copy the test into the matching module's src/test/ tree of the checkout and run ./gradlew -p lucene/<module> test --tests <class> -Ptests.seed=<seed> (add -Ptests.asserts=false for a run without assertions).

Formal models and scripts this finding's part of the hunt used: classification

Feedback

Open an issue in lucene-ai-bug-hunt. Please do not open issues or pull requests in apache/lucene about this finding unless a Lucene committer agrees it should be filed.

Snapshot of 2026-11-04. AI-assisted; not yet triaged by a human Lucene committer. Feedback: open an issue in lucene-ai-bug-hunt.