feat(jdk12): add demo for JEP 344

src/main/java/org/javademos/init/Java12DemoLoader.java

@@ -11,6 +11,7 @@
 import org.javademos.java12.jep334.ConstantsAPIDemo;
 import org.javademos.java12.jep340.OneAArch64Port;
 import org.javademos.java12.jep341.DefaultCDSArchives;
+import org.javademos.java12.jep344.AbortableMixedCollectionsG1;
 
 /**
  * Loads demos for Java 12.
@@ -25,5 +26,6 @@ public void loadDemos(Map<Integer, IDemo> demos) {
         demos.put(334, new ConstantsAPIDemo());
         demos.put(340, new OneAArch64Port());
         demos.put(341, new DefaultCDSArchives());
+        demos.put(344, new AbortableMixedCollectionsG1());
     }
 }

src/main/java/org/javademos/java12/jep344/AbortableMixedCollectionsG1.java

@@ -0,0 +1,48 @@
+package org.javademos.java12.jep344;
+
+import java.lang.management.ManagementFactory;
+
+import org.javademos.commons.IDemo;
+
+/// Demo for JDK 12 feature JEP 344 - Abortable Mixed Collections for G1.
+///
+/// JEP history:
+/// - JDK 12: [JEP 344 - Abortable Mixed Collections for G1](https://openjdk.org/jeps/344)
+///
+/// Further reading:
+/// - [JDK-8190269 - JEP 344: Abortable Mixed Collections for G1](https://bugs.openjdk.org/browse/JDK-8190269)
+/// - [Garbage-First (G1) Garbage Collector tuning guide](https://docs.oracle.com/en/java/javase/21/gctuning/garbage-first-g1-garbage-collector1.html)
+///
+/// @author alois.seckar@gmail.com
+public class AbortableMixedCollectionsG1 implements IDemo {
+    @Override
+    public void demo() {
+        info(344);
+
+        // This JEP is an internal improvement of the G1 garbage collector (default since JDK 9),
+        // so there is no new API to call from application code.
+
+        // G1 tries to meet a user-supplied pause time target (`-XX:MaxGCPauseMillis`, 200 ms by default).
+        // Before each pause it uses heuristics to pick a "collection set" - the heap regions to collect.
+        // Once started, all live objects in that set had to be evacuated without stopping.
+        // If heuristics worked with "stale" data (e.g. application behavior changed), the set
+        // could be too large and the pause exceeded the target. This mostly happened during
+        // "mixed" collections, which collect young regions plus some old regions.
+
+        // Since JDK 12, when G1 detects it repeatedly picks too much work, it splits
+        // the collection set into two parts:
+        // - mandatory - must be collected at once (young regions, possibly some old regions)
+        // - optional - only old regions (e.g. ~20 % of the predicted set)
+        // After the mandatory part is done, G1 processes the optional part in smaller
+        // increments (down to a single region) and may stop ("abort") whenever the remaining
+        // pause time runs out. Space reclamation still progresses region by region.
+        // As predictions become accurate again, the optional part shrinks until G1
+        // fully relies on its heuristics once more.
+
+        // The mechanism is automatic and needs no flags; it only applies when G1 is in use.
+        // Collector details are visible via `-Xlog:gc+phases=debug`.
+        System.out.println("Active garbage collectors:");
+        ManagementFactory.getGarbageCollectorMXBeans()
+                .forEach(gc -> System.out.println("- " + gc.getName()));
+    }
+}

src/main/resources/JDK12Info.json

@@ -46,5 +46,13 @@
     "dscr": "The JDK build now generates a default class data-sharing (CDS) archive on 64-bit platforms, improving out-of-the-box startup time.",
     "link": false,
     "code": true
+  },
+  {
+    "jep": 344,
+    "jdk": 12,
+    "name": "JEP 344 - Abortable Mixed Collections for G1",
+    "dscr": "G1 can split mixed collections into mandatory and optional parts and abort the optional part to better meet the pause time target.",
+    "link": false,
+    "code": true
   }
 ]