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]>

flags/tools/HeaderSizeDemo.java

@@ -0,0 +1,26 @@
+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());
+    }
+}

flags/tools/MemoryHog.java

@@ -0,0 +1,37 @@
+import java.util.ArrayList;
+import java.util.List;
+
+/**
+ * Deliberately allocates and retains heap in fixed-size chunks until it is
+ * killed or finishes, printing progress as it goes. Used to demonstrate what
+ * actually happens when a container's memory limit, the JVM's heap flags,
+ * and real allocation pressure disagree with each other. See the post at
+ * https://ankurm.com/jvm-flags-for-spring-boot-in-containers-jdk-25-27/
+ * and docs/output/05-oom-*.txt / 06-oom-*.txt for the captured runs.
+ */
+public class MemoryHog {
+    public static void main(String[] args) throws Exception {
+        int chunkMb = args.length > 0 ? Integer.parseInt(args[0]) : 16;
+        int sleepMs = args.length > 1 ? Integer.parseInt(args[1]) : 150;
+        List<byte[]> held = new ArrayList<>();
+        long totalMb = 0;
+        System.out.println("MemoryHog starting: chunk=" + chunkMb + "MB sleep=" + sleepMs + "ms");
+        System.out.flush();
+        while (true) {
+            byte[] chunk = new byte[chunkMb * 1024 * 1024];
+            for (int i = 0; i < chunk.length; i += 4096) {
+                chunk[i] = 1; // touch each page so it is actually resident, not just reserved
+            }
+            held.add(chunk);
+            totalMb += chunkMb;
+            Runtime rt = Runtime.getRuntime();
+            System.out.printf("held=%dMB heapUsed=%dMB heapMax=%dMB freeHeap=%dMB%n",
+                    totalMb,
+                    (rt.totalMemory() - rt.freeMemory()) / (1024 * 1024),
+                    rt.maxMemory() / (1024 * 1024),
+                    rt.freeMemory() / (1024 * 1024));
+            System.out.flush();
+            Thread.sleep(sleepMs);
+        }
+    }
+}