Merge pull request #9526 from alimate/BAEL-4116

BAEL-4116: Objects Memory Layout in Java

core-java-modules/core-java-jvm/src/test/java/com/baeldung/memlayout/MemoryLayoutUnitTest.java

@@ -0,0 +1,108 @@
+package com.baeldung.memlayout;
+
+import org.junit.Test;
+import org.openjdk.jol.info.ClassLayout;
+import org.openjdk.jol.vm.VM;
+import sun.misc.Contended;
+
+public class MemoryLayoutUnitTest {
+
+    private volatile Object consumer;
+
+    @Test
+    public void printingTheVMDetails() {
+        System.out.println(VM.current().details());
+    }
+
+    @Test
+    public void simpleMemoryLayout() {
+        System.out.println(ClassLayout.parseClass(SimpleInt.class).toPrintable());
+    }
+
+    @Test
+    public void identityHashCodeMemoryLayout() {
+        SimpleInt instance = new SimpleInt();
+        System.out.println(ClassLayout.parseInstance(instance).toPrintable());
+
+        System.out.println("The identity hash code is " + System.identityHashCode(instance));
+        System.out.println(ClassLayout.parseInstance(instance).toPrintable());
+    }
+
+    @Test
+    public void alignmentMemoryLayout() {
+        System.out.println(ClassLayout.parseClass(SimpleLong.class).toPrintable());
+    }
+
+    @Test
+    public void fieldPackingMemoryLayout() {
+        System.out.println(ClassLayout.parseClass(FieldsArrangement.class).toPrintable());
+    }
+
+    @Test
+    public void monitorLockMemoryLayout() {
+        Lock lock = new Lock();
+        System.out.println(ClassLayout.parseInstance(lock).toPrintable());
+
+        synchronized (lock) {
+            System.out.println(ClassLayout.parseInstance(lock).toPrintable());
+        }
+    }
+
+    @Test
+    public void ageAndTenuringMemoryLayout() {
+        Object instance = new Object();
+        long lastAddr = VM.current().addressOf(instance);
+        ClassLayout layout = ClassLayout.parseInstance(instance);
+
+        for (int i = 0; i < 10_000; i++) {
+            long currentAddr = VM.current().addressOf(instance);
+            if (currentAddr != lastAddr) {
+                System.out.println(layout.toPrintable());
+            }
+
+            for (int j = 0; j < 10_000; j++) {
+                consumer = new Object();
+            }
+
+            lastAddr = currentAddr;
+        }
+    }
+
+    @Test
+    public void contendedMemoryLayout() {
+        System.out.println(ClassLayout.parseClass(Isolated.class).toPrintable());
+    }
+
+    @Test
+    public void arrayMemoryLayout() {
+        boolean[] booleans = new boolean[3];
+        System.out.println(ClassLayout.parseInstance(booleans).toPrintable());
+    }
+
+    private static class SimpleInt {
+        private int state;
+    }
+
+    private static class SimpleLong {
+        private long state;
+    }
+
+    private static class FieldsArrangement {
+        private boolean first;
+        private char second;
+        private double third;
+        private int forth;
+        private boolean fifth;
+    }
+
+    private static class Lock {}
+
+    private static class Isolated {
+
+        @Contended
+        private int i;
+
+        @Contended
+        private long l;
+    }
+}