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/src/main/java/com/ankurm/flagsdemo/FlagsDemoApplication.java

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+package com.ankurm.flagsdemo;
+
+import org.springframework.boot.SpringApplication;
+import org.springframework.boot.autoconfigure.SpringBootApplication;
+
+/**
+ * Companion app for "JVM Flags for Spring Boot in Containers (JDK 25/27)" on ankurm.com.
+ * See docs/output/*.txt for the captured runs this post quotes from, and
+ * {@link JvmDiagnosticsController} for the live /internal/jvm-flags endpoint
+ * used to inspect what the JVM actually resolved its memory/GC flags to
+ * inside a running container. Delete that endpoint before shipping this to
+ * a real service — it exists for this demo only.
+ */
+@SpringBootApplication
+public class FlagsDemoApplication {
+    public static void main(String[] args) {
+        SpringApplication.run(FlagsDemoApplication.class, args);
+    }
+}

flags/src/main/java/com/ankurm/flagsdemo/JvmDiagnosticsController.java

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+package com.ankurm.flagsdemo;
+
+import com.sun.management.HotSpotDiagnosticMXBean;
+import com.sun.management.OperatingSystemMXBean;
+import org.springframework.web.bind.annotation.GetMapping;
+import org.springframework.web.bind.annotation.RestController;
+
+import java.lang.management.GarbageCollectorMXBean;
+import java.lang.management.ManagementFactory;
+import java.lang.management.MemoryMXBean;
+import java.lang.management.MemoryUsage;
+import java.util.LinkedHashMap;
+import java.util.List;
+import java.util.Map;
+
+/**
+ * Prints what the JVM actually resolved its container-relevant flags to, at
+ * runtime, inside whatever cgroup it is running under. This is the live
+ * version of what docs/output/*.txt captures from -XX:+PrintFlagsFinal; the
+ * two should agree, and the post at ankurm.com cross-checks both.
+ *
+ * <p>Explained in the post:
+ * https://ankurm.com/jvm-flags-for-spring-boot-in-containers-jdk-25-27/
+ *
+ * <p><strong>Delete this endpoint before shipping to a real service.</strong>
+ * It leaks heap sizing and cgroup limits to anyone who can reach it.
+ */
+@RestController
+public class JvmDiagnosticsController {
+
+    private static final List<String> FLAGS_OF_INTEREST = List.of(
+            "MaxRAMPercentage", "MinRAMPercentage", "InitialRAMPercentage",
+            "MaxRAM", "MaxHeapSize", "InitialHeapSize", "MinHeapSize",
+            "UseContainerSupport", "ActiveProcessorCount",
+            "UseG1GC", "UseParallelGC", "UseZGC", "UseSerialGC",
+            "UseCompactObjectHeaders", "ParallelGCThreads", "ConcGCThreads"
+    );
+
+    @GetMapping("/internal/jvm-flags")
+    public Map<String, Object> jvmFlags() {
+        Map<String, Object> out = new LinkedHashMap<>();
+
+        OperatingSystemMXBean os =
+                (OperatingSystemMXBean) ManagementFactory.getOperatingSystemMXBean();
+        Map<String, Object> hostView = new LinkedHashMap<>();
+        hostView.put("availableProcessors", Runtime.getRuntime().availableProcessors());
+        hostView.put("totalMemorySizeBytes", os.getTotalMemorySize());
+        hostView.put("freeMemorySizeBytes", os.getFreeMemorySize());
+        out.put("osMXBeanView", hostView);
+
+        MemoryMXBean mem = ManagementFactory.getMemoryMXBean();
+        MemoryUsage heap = mem.getHeapMemoryUsage();
+        Map<String, Object> heapView = new LinkedHashMap<>();
+        heapView.put("initBytes", heap.getInit());
+        heapView.put("usedBytes", heap.getUsed());
+        heapView.put("committedBytes", heap.getCommitted());
+        heapView.put("maxBytes", heap.getMax());
+        out.put("heap", heapView);
+
+        List<String> gcNames = ManagementFactory.getGarbageCollectorMXBeans()
+                .stream().map(GarbageCollectorMXBean::getName).toList();
+        out.put("garbageCollectors", gcNames);
+
+        HotSpotDiagnosticMXBean diag =
+                ManagementFactory.getPlatformMXBean(HotSpotDiagnosticMXBean.class);
+        Map<String, String> flagView = new LinkedHashMap<>();
+        for (String name : FLAGS_OF_INTEREST) {
+            try {
+                flagView.put(name, diag.getVMOption(name).getValue());
+            } catch (IllegalArgumentException notThisJvm) {
+                flagView.put(name, "N/A on this build");
+            }
+        }
+        out.put("resolvedFlags", flagView);
+
+        out.put("jvmInputArguments",
+                ManagementFactory.getRuntimeMXBean().getInputArguments());
+        out.put("javaVersion", System.getProperty("java.version"));
+        out.put("javaVmName", System.getProperty("java.vm.name"));
+
+        return out;
+    }
+}

flags/src/main/resources/application.yml

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+server:
+  port: 8080
+
+management:
+  endpoints:
+    web:
+      exposure:
+        include: health,info
+
+spring:
+  application:
+    name: flags-demo
+
+logging:
+  level:
+    root: INFO