Add flags: production JVM flag cheat sheet for Spring Boot on JDK 25/27

MaxRAMPercentage vs a hardcoded -Xmx, -XX:-UseContainerSupport, the ActiveProcessorCount lie, and which diagnostic flags are worth leaving on in prod, verified in real Docker containers on JDK 25 and JDK 27. Co-Authored-By: Claude Sonnet 5 <[email protected]>
2026-10-01 04:34:04 +00:00
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+import java.util.ArrayList;
+import java.util.List;
+
+/**
+ * Allocates a large number of a trivial fixed-shape object and a trivial
+ * array, then sleeps so `jcmd <pid> GC.class_histogram` can be run against
+ * it from outside the container. Used to measure the per-instance byte cost
+ * that -XX:+/-UseCompactObjectHeaders actually makes, rather than quoting
+ * the JEP's own number. See
+ * https://ankurm.com/jvm-flags-for-spring-boot-in-containers-jdk-25-27/
+ */
+public class HeaderSizeDemo {
+    static final class Pair { int a; int b; Pair(int a, int b) { this.a = a; this.b = b; } }
+
+    public static void main(String[] args) throws Exception {
+        int count = args.length > 0 ? Integer.parseInt(args[0]) : 2_000_000;
+        List<Pair> pairs = new ArrayList<>(count);
+        for (int i = 0; i < count; i++) {
+            pairs.add(new Pair(i, i));
+        }
+        System.out.println("allocated " + pairs.size() + " Pair instances, pid=" + ProcessHandle.current().pid());
+        System.gc();
+        Thread.sleep(120_000);
+        System.out.println(pairs.size());
+    }
+}