Java 27 and 26: runnable demos and captured output for every JEP, plus version lanes

Co-Authored-By: Claude Sonnet 5 <[email protected]> Claude-Session: https://claude.ai/code/session_01B38FGKKam5SCGgwgduVAh3
2026-09-21 15:08:50 +00:00
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+#!/usr/bin/env bash
+# JEP 534: object header size on JDK 26 (12 bytes) vs JDK 27 (8 bytes), measured with JOL 0.17.
+#   ./scripts/object-headers.sh  -> docs/output/10-headers-jdk26.txt, 11-headers-jdk27.txt,
+#                                   12-headers-jdk27-opt-out.txt, 13-jol-record-failure.txt,
+#                                   14-jol-record-workaround.txt
+set -euo pipefail
+source "$(dirname "$0")/env.sh"
+
+LIB="$ROOT/.lib"; mkdir -p "$LIB"
+JOL="$LIB/jol-core-0.17.jar"
+if [ ! -f "$JOL" ]; then
+  curl -sfL -o "$JOL" https://repo1.maven.org/maven2/org/openjdk/jol/jol-core/0.17/jol-core-0.17.jar
+fi
+echo "4c98e9e61b3f189241057cb21b4331d1093e5b85  $JOL" | sha1sum -c - >/dev/null   # Maven Central's own .sha1
+
+BUILD="$ROOT/build/headers"; rm -rf "$BUILD"; mkdir -p "$BUILD"
+"$JDK27/bin/javac" --release 26 -cp "$JOL" -d "$BUILD" "$ROOT"/object-headers/src/*.java
+
+# JOL wants to attach to itself (for Instrumentation) and calls sun.misc.Unsafe, which JDK 24+ warns about.
+COMMON=(-Djdk.attach.allowAttachSelf=true -XX:+EnableDynamicAgentLoading --sun-misc-unsafe-memory-access=allow -cp "$BUILD:$JOL")
+
+# The Serviceability Agent line JOL 0.17 prints on JDK 27 is JOL failing to read a flag that no longer exists
+# (UseCompressedClassPointers is obsolete in 27); it is kept in the transcript because it is real.
+"$JDK26/bin/java" "${COMMON[@]}" HeaderDemo                                     > "$OUT/10-headers-jdk26.txt" 2>&1
+"$JDK27/bin/java" "${COMMON[@]}" HeaderDemo                                     > "$OUT/11-headers-jdk27.txt" 2>&1
+"$JDK27/bin/java" -XX:-UseCompactObjectHeaders "${COMMON[@]}" HeaderDemo        > "$OUT/12-headers-jdk27-opt-out.txt" 2>&1
+
+# JOL 0.17 cannot lay out a record without a workaround. Capture the failure and the fix.
+"$JDK27/bin/java" "${COMMON[@]}" RecordProbe                                    > "$OUT/13-jol-record-failure.txt" 2>&1 || true
+"$JDK27/bin/java" -Djol.magicFieldOffset=true "${COMMON[@]}" RecordProbe         > "$OUT/14-jol-record-workaround.txt" 2>&1
+
+# One compact table of instance sizes, extracted from the three transcripts (not retyped).
+{
+  echo "# Instance sizes in bytes, extracted from 10-, 11- and 12- transcripts"
+  for f in 10-headers-jdk26 11-headers-jdk27 12-headers-jdk27-opt-out; do
+    echo "--- $f"
+    awk '/^=== /{t=$0} /^Instance size/{print t " -> " $0} /^1,000,000 /{print}' "$OUT/$f.txt"
+  done
+} > "$OUT/15-headers-summary.txt"
+cat "$OUT/15-headers-summary.txt"