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
parent e83c9949d8
commit 2f88734f5d
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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;
+    }
+}