feat(jep379): Implement Shenandoah GC demo and update JDK15 metadata

src/main/java/org/javademos/init/Java15.java

@@ -9,6 +9,7 @@
 import org.javademos.java15.jep372.NashornRemovalDemo;
 import org.javademos.java15.jep373.DatagramSocketDemo15;
 import org.javademos.java15.jep375.InstanceofPatternMatchingSecondPreview;
+import org.javademos.java15.jep379.ShenandoahJEP379;
 import org.javademos.java15.jep381.SolarisSparcRemovalDemo;
 import org.javademos.java15.jep383.ForeignMemoryAccessDemo;
 import org.javademos.java15.jep384.RecordsSecondPreviewDemo;
@@ -34,12 +35,15 @@ public static ArrayList<IDemo> getDemos() {
         java15DemoPool.add(new DatagramSocketDemo15());
         // JEP 375
         java15DemoPool.add(new InstanceofPatternMatchingSecondPreview());
+        // JEP 379: Shenandoah: A Low-Pause-Time Garbage Collector (Production)
+        java15DemoPool.add(new ShenandoahJEP379()); 
         // JEP 381
         java15DemoPool.add(new SolarisSparcRemovalDemo());
         // JEP 383
         java15DemoPool.add(new ForeignMemoryAccessDemo());
         // JEP 384
         java15DemoPool.add(new RecordsSecondPreviewDemo());
+
 
         return java15DemoPool;
     }

src/main/java/org/javademos/java15/jep379/ShenandoahJEP379.java

@@ -0,0 +1,89 @@
+package org.javademos.java15.jep379;
+
+import org.javademos.commons.IDemo;
+import java.util.ArrayList;
+import java.util.List;
+
+/**
+ * ## ☕ JEP 379: Shenandoah: A Low-Pause-Time Garbage Collector (Production)
+ *
+ * This JEP finalized the Shenandoah garbage collector, making it a production feature
+ * in JDK 15. This meant the `-XX:+UnlockExperimentalVMOptions` flag was no longer
+ * required to use it.
+ *
+ * Shenandoah is a **latency-focused** GC that performs most of its work, including
+ * heap compaction, concurrently with the application threads.
+ *
+ * ### Key Benefits:
+ * - **Low Pause Times:** GC pause times are consistently very short (often < 10ms).
+ * - **Heap Size Independence:** Pause times are largely independent of the heap size.
+ * This makes it ideal for applications with very large heaps (hundreds of GBs).
+ *
+ * ### History:
+ * - **JEP 189 (JDK 12):** Shenandoah was first introduced as an *Experimental* feature.
+ * - **JEP 379 (JDK 15):** Promoted Shenandoah to a *Production* feature.
+ *
+ * ### How to Run the Demo:
+ * Since this feature is configured via JVM arguments, the demo primarily demonstrates
+ * how to force some GC activity and prints the required flags.
+ *
+ * To run the main application with Shenandoah, you only need:
+ * `java -XX:+UseShenandoahGC -Xms... -Xmx... -jar your-app.jar`
+ *
+ * @see <a href="https://openjdk.org/jeps/379">JEP 379</a>
+ * @see <a href="https://openjdk.org/jeps/189">JEP 189 (Initial Introduction)</a>
+ */
+public class ShenandoahJEP379 implements IDemo {
+
+    private static final int ALLOC_SIZE = 100_000_000; // Allocate 100MB per step
+    private static final int GC_CYCLES = 5;            // Number of cycles to run
+
+    @Override
+    public void demo() {
+        info(379);
+
+        System.out.println("Shenandoah GC is primarily configured with JVM flags, not code.");
+        System.out.println("--------------------------------------------------------------------------------------------------");
+        System.out.println("To use this collector (on a Java 15+ compatible JDK):");
+        System.out.println("JVM Flags: -XX:+UseShenandoahGC -Xms4G -Xmx4G -Xlog:gc*=info:file=tmp/shenandoah_gc.log");
+        System.out.println("  - NOTE: -Xms and -Xmx are recommended to be the same size.");
+        System.out.println("--------------------------------------------------------------------------------------------------");
+
+        try {
+            System.out.println("Simulating a memory-stressing workload to trigger GC activity...");
+
+            // Allocate objects to force GC cycles
+            List<byte[]> allocationList = new ArrayList<>();
+            for (int i = 0; i < GC_CYCLES; i++) {
+                System.out.printf("Cycle %d: Allocating %d bytes (%.2fMB)...%n", 
+                                  i + 1, ALLOC_SIZE, ALLOC_SIZE / 1024.0 / 1024.0);
+
+                // Keep some objects alive (e.g., in a list)
+                allocationList.add(new byte[ALLOC_SIZE]);
+
+                // Allow some time for concurrent GC to run
+                Thread.sleep(100); 
+            }
+
+            System.out.println("\nWorkload finished. Note: Shenandoah attempts to run concurrently.");
+            System.out.println("The short pauses are only visible when running with GC logging enabled.");
+            System.out.printf("Final allocated size kept alive: %.2fMB%n", 
+                              allocationList.size() * ALLOC_SIZE / 1024.0 / 1024.0);
+
+            // Important: Call System.gc() to try and trigger a full GC. 
+            // The logs will show if Shenandoah runs a concurrent cycle or a Stop-The-World (STW) full GC.
+            System.out.println("\nCalling System.gc()... (Shenandoah typically runs a concurrent cycle)");
+            System.gc();
+
+            // Clear the list to allow all memory to be reclaimed
+            allocationList.clear(); 
+            System.out.println("Cleared allocation list.");
+
+        } catch (InterruptedException e) {
+            Thread.currentThread().interrupt();
+        } catch (OutOfMemoryError e) {
+            System.err.println("!!! ERROR: Demo ran out of memory. Try increasing -Xmx size in your VM options. !!!");
+        }
+
+    }
+}

src/main/resources/JDK15Info.json

@@ -47,6 +47,14 @@
     "link": true,
     "code": false
   },
+  {
+    "jep": 379,
+    "jdk": 15,
+    "name": "JEP 379 - Shenandoah: A Low-Pause-Time Garbage Collector",
+    "dscr": "Promoted Shenandoah from an experimental to a production feature.",
+    "link": false,
+    "code": true
+  },
   {
     "jep": 381,
     "jdk": 15,