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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+import com.sun.management.HotSpotDiagnosticMXBean;
+import java.lang.management.GarbageCollectorMXBean;
+import java.lang.management.ManagementFactory;
+import java.util.stream.Collectors;
+
+/**
+ * Prints which garbage collector the JVM picked when nobody asked for one, and why.
+ *
+ * <p>The "why" is the interesting part: HotSpot records the ORIGIN of every flag value.
+ * {@code ERGONOMIC} means the JVM chose it by looking at the machine; {@code DEFAULT} means
+ * it is simply the built-in value; {@code COMMAND_LINE} means somebody typed it.
+ *
+ * <p>Explained in docs/02-g1-default.md. Run by scripts/g1-default.sh.
+ */
+public class GcReport {
+
+    public static void main(String[] args) {
+        Runtime rt = Runtime.getRuntime();
+        System.out.println("java.version      = " + System.getProperty("java.version"));
+        System.out.println("availableCpus     = " + rt.availableProcessors());
+        System.out.println("maxHeapMB         = " + rt.maxMemory() / (1024 * 1024));
+        System.out.println("collector beans   = " + ManagementFactory.getGarbageCollectorMXBeans().stream()
+                .map(GarbageCollectorMXBean::getName).collect(Collectors.joining(", ")));
+
+        HotSpotDiagnosticMXBean hs = ManagementFactory.getPlatformMXBean(HotSpotDiagnosticMXBean.class);
+        for (String flag : new String[] {"UseG1GC", "UseSerialGC", "UseParallelGC"}) {
+            var opt = hs.getVMOption(flag);
+            System.out.printf("%-17s = %-5s (origin %s)%n", flag, opt.getValue(), opt.getOrigin());
+        }
+    }
+}