Classic Highlighter's SimpleSpanFragmenter.start() does not reset waitForPos, so a Highlighter reused for the next text (or for the same text again) keeps suppressing fragment boundaries after a phrase match that ended the previous text: one oversized fragment, then one-word fragments and a different best fragment, depending on what was highlighted before

Finding LFV-861Area: highlighter / classic Highlighter fragmenting (SimpleSpanFragmenter.start, state carried across texts)Priority: lowImpact: minorNovelty: new

Finding page for LFV-861

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

What it shows

You reuse one classic Highlighter with a QueryScorer and a SimpleSpanFragmenter for several texts (Highlighter keeps one fragmenter and calls its start(), documented as 'Initializes the Fragmenter', for every text) and expect each text to be cut into the same fragments as with a fresh Highlighter. Instead start() resets the position and fragment count but not waitForPos, the position the fragmenter is still waiting for, so when a phrase match ended the previous text no boundary is allowed in the next text until it reaches that old position: with the phrase 'search library' at the end of the first text and fragment size 20, the next text starts with one 70-character fragment 'Highlighting picks short fragments of text around each match so that a', then one-word fragments ', reader', ', of', ', the', where a fresh Highlighter gives 'Highlighting picks', ' short fragments of', ' text around each', and getBestFragment also differs. Hits and scores are unaffected; only the snippet layout changes, and it depends on what the same Highlighter highlighted before.

The test shows that a classic Highlighter with SimpleSpanFragmenter that is reused after a text ending in a phrase match cuts the next text differently from a fresh Highlighter, because SimpleSpanFragmenter.start() does not reset waitForPos.

Expected vs actual

Expected the fresh Highlighter's fragments [Highlighting picks, short fragments of, text around each, ...]; actual [Highlighting picks short fragments of text around each match so that a, reader, of, the, ...] (one 70-char fragment, then one-word ones), 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/highlighter/build/libs/lucene-highlighter-$V.jar
CP=$CP:$LUCENE/lucene/memory/build/libs/lucene-memory-$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.search.highlight.TestSimpleSpanFragmenterReuse

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).

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.