SimpleNaiveBayesDocumentClassifier returns NaN for every class and null from assignClass when the document has a field value with no tokens (empty, stop words only or punctuation only), because each value's log-likelihood is divided by its token count (released since 6.0.0)

Finding LFV-380Area: classification / document classifiers (SimpleNaiveBayesDocumentClassifier.assignClass and getClasses)Priority: mediumImpact: silent wrong resultsNovelty: new

Finding page for LFV-380

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

What it shows

You train SimpleNaiveBayesDocumentClassifier on the fields title and body and classify the document {title: 'apple', body: ''} (or a body such as 'the of and' that the analyzer reduces to stop words), expecting a class with a finite score, as the DocumentClassifier.assignClass javadoc and the '0-1 range' normalisation comment imply and as the same document without the body value gets (fruit). Instead getClasses returns fruit=NaN and vehicle=NaN, and assignClass returns null. calculateLogLikelihood returns result / tokenizedText.length, which is 0.0 / 0 = NaN for a value with no tokens; assignNormClasses adds that NaN to every class, and neither the log-sum-exp normalisation nor the 'score > maxscore' test in assignClass can recover from it.

The test shows that SimpleNaiveBayesDocumentClassifier returns no class when one field value of the document has no tokens ({title: 'apple', body: ''}): the value's log-likelihood is 0.0 / 0 = NaN, which makes every class score NaN.

Expected vs actual

Expected a non-null class with a finite score; actual assignClass returns null and getClasses gives fruit=NaN and vehicle=NaN, with -ea and -da.

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.TestSimpleNaiveBayesDocumentClassifierEmptyField

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.