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 523 in a real container: what does the JVM pick when nobody asks for a collector?
+# Runs GcReport in Docker with 1 CPU and 2 CPUs, on the JDK 26 and JDK 27 images.
+#   ./scripts/g1-default.sh            -> docs/output/01-g1-default-1cpu-vs-2cpu.txt
+#                                         docs/output/02-g1-default-memory.txt
+set -euo pipefail
+source "$(dirname "$0")/env.sh"
+BUILD="$ROOT/build/g1"; rm -rf "$BUILD"; mkdir -p "$BUILD"
+"$JDK27/bin/javac" --release 26 -d "$BUILD" "$ROOT"/g1-container/src/*.java   # one class file set runs on both JDKs
+
+run() { # image label docker-args... -- java-args...
+  local img="$1" label="$2"; shift 2
+  echo "\$ docker run --rm $* $img java -cp /app GcReport"
+  docker run --rm "$@" -v "$BUILD:/app:ro" "$img" java -cp /app GcReport
+  echo
+}
+
+{
+  echo "# JEP 523: which collector does the JVM pick when you do not choose one?"
+  echo "# Same class files, same vendor (Amazon Corretto), same 2 GB memory limit. Only the JDK version and the CPU limit change."
+  echo
+  for cpus in 1 2; do
+    for img in "$IMG26" "$IMG27"; do
+      echo "=== $img  --cpus=$cpus --memory=2g"
+      run "$img" x --cpus=$cpus --memory=2g
+    done
+  done
+} > "$OUT/01-g1-default-1cpu-vs-2cpu.txt" 2>&1
+
+{
+  echo "# The other half of the old rule: memory. The old heuristic wanted at least 2 CPUs AND about 1792 MB."
+  echo "# With 2 CPUs the CPU half of the rule is satisfied, so only the memory limit differs between these runs."
+  echo
+  for mem in 1g 2g; do
+    for img in "$IMG26" "$IMG27"; do
+      echo "=== $img  --cpus=2 --memory=$mem"
+      run "$img" x --cpus=2 --memory=$mem
+    done
+  done
+} > "$OUT/02-g1-default-memory.txt" 2>&1
+
+{
+  echo "# The opt-out still works on 27: an explicit choice always wins over the JVM's."
+  echo
+  echo "=== $IMG27  --cpus=1 --memory=2g   -XX:+UseSerialGC"
+  echo "\$ docker run --rm --cpus=1 --memory=2g $IMG27 java -XX:+UseSerialGC -cp /app GcReport"
+  docker run --rm --cpus=1 --memory=2g -v "$BUILD:/app:ro" "$IMG27" java -XX:+UseSerialGC -cp /app GcReport
+} > "$OUT/03-g1-default-optout.txt" 2>&1
+echo "wrote 01-, 02-, 03- transcripts to $OUT"