HotSpot's FieldsAllocationStyle=2 doesn't work as expected?

A.java

class A {
  Object a1;
  int a2;
}

class B extends A {
  Object b1;
  int b2;
  float b3;
}

class C extends B {
  Object c1;
  float c2;
  int c3;
}

class D extends C {
  Object d1;
  int d2;
}

doesn't work on 6u25 or 6u26 Windows x86

$ export JAVA_OPTS='-XX:FieldsAllocationStyle=2'
$ groovysh
Groovy Shell (1.7.6, JVM: 1.6.0_26)
Type 'help' or '\h' for help.
-------------------------------------------------------------------------------
groovy:000> ObjectLayouts.printInstanceFieldOffsets(D)
Instance field offsets for class D:
  +12:  [A] a1 : class java.lang.Object
  +8:   [A] a2 : int
  +24:  [B] b1 : class java.lang.Object
  +16:  [B] b2 : int
  +20:  [B] b3 : float
  +36:  [C] c1 : class java.lang.Object
  +28:  [C] c2 : float
  +32:  [C] c3 : int
  +44:  [D] d1 : class java.lang.Object
  +40:  [D] d2 : int
===> null
groovy:000> import java.lang.management.ManagementFactory
===> [import java.lang.management.ManagementFactory]
groovy:000> import com.sun.management.HotSpotDiagnosticMXBean
===> [import java.lang.management.ManagementFactory, import com.sun.management.HotSpotDiagnosticMXBean]
groovy:000> HOTSPOT_BEAN_NAME = 'com.sun.management:type=HotSpotDiagnostic'
===> com.sun.management:type=HotSpotDiagnostic
groovy:000> server = ManagementFactory.platformMBeanServer
===> com.sun.jmx.mbeanserver.JmxMBeanServer@8ddb93
groovy:000> bean = ManagementFactory.newPlatformMXBeanProxy(server, HOTSPOT_BEAN_NAME, HotSpotDiagnosticMXBean)
===> MXBeanProxy(com.sun.jmx.mbeanserver.JmxMBeanServer@8ddb93[com.sun.management:type=HotSpotDiagnostic])
groovy:000> bean.getVMOption('FieldsAllocationStyle')
===> VM option: FieldsAllocationStyle value: 2  origin: VM_CREATION (read-write)
groovy:000>

doesn't work on 6u25 Windows x86

$ export JAVA_OPTS='-XX:FieldsAllocationStyle=2'
$ groovysh
Groovy Shell (1.7.6, JVM: 1.6.0_26)
Type 'help' or '\h' for help.
-------------------------------------------------------------------------------
groovy:000> ObjectLayouts.printInstanceFieldOffsets(D)
Instance field offsets for class D:
  +12:  [A] a1 : class java.lang.Object
  +8:   [A] a2 : int
  +24:  [B] b1 : class java.lang.Object
  +16:  [B] b2 : int
  +20:  [B] b3 : float
  +36:  [C] c1 : class java.lang.Object
  +28:  [C] c2 : float
  +32:  [C] c3 : int
  +44:  [D] d1 : class java.lang.Object
  +40:  [D] d2 : int
===> null
groovy:000> import java.lang.management.ManagementFactory
===> [import java.lang.management.ManagementFactory]
groovy:000> import com.sun.management.HotSpotDiagnosticMXBean
===> [import java.lang.management.ManagementFactory, import com.sun.management.HotSpotDiagnosticMXBean]
groovy:000> HOTSPOT_BEAN_NAME = 'com.sun.management:type=HotSpotDiagnostic'
===> com.sun.management:type=HotSpotDiagnostic
groovy:000> server = ManagementFactory.platformMBeanServer
===> com.sun.jmx.mbeanserver.JmxMBeanServer@8ddb93
groovy:000> bean = ManagementFactory.newPlatformMXBeanProxy(server, HOTSPOT_BEAN_NAME, HotSpotDiagnosticMXBean)
===> MXBeanProxy(com.sun.jmx.mbeanserver.JmxMBeanServer@8ddb93[com.sun.management:type=HotSpotDiagnostic])
groovy:000> bean.getVMOption('FieldsAllocationStyle')
===> VM option: FieldsAllocationStyle value: 2  origin: VM_CREATION (read-write)
groovy:000>

Fields.java

import java.lang.reflect.Field;
import java.lang.reflect.Modifier;

public class Fields {
    public static boolean isStatic(Field f) {
        return Modifier.isStatic(f.getModifiers());
    }
}

Notes.md

I thought FieldsAllocationStyle=2 was supposed to alternate between FieldsAllocationStyle=0 and 1 in an inheritance chain. But that doesn't seem to be the case in this example.

I was expecting the output to be something like this:

Instance field offsets for class D:
  +12:  [A] a1 : class java.lang.Object
  +8:   [A] a2 : int
  +16:  [B] b1 : class java.lang.Object
  +20:  [B] b2 : int
  +24:  [B] b3 : float
  +36:  [C] c1 : class java.lang.Object
  +28:  [C] c2 : float
  +32:  [C] c3 : int
  +40:  [D] d1 : class java.lang.Object
  +44:  [D] d2 : int

The changeset that introduced this new feature was made in April, 2010. http://hg.openjdk.java.net/jdk6/jdk6/hotspot/rev/b9d85fcdf743

Update: It works on 6u25 Linux x64, but not on 6u25/6u26 Windows x86.

ObjectLayouts.java

import java.lang.reflect.Field;
import sun.misc.Unsafe;

public class ObjectLayouts {
    public static void printStaticFieldOffsets(Class<?> clazz)
            throws Exception {
        if (clazz == null) {
            System.err.println("got null pointer, skipping");
            return;
        }
        
        System.out.printf("Static field offsets for class %s:\n", clazz.getName());
        printStaticFieldOffsets0(clazz);
    }
    
    private static void printStaticFieldOffsets0(Class<?> clazz)
            throws Exception {
        if (clazz == null) return;
        
        printStaticFieldOffsets0(clazz.getSuperclass());
        
        Unsafe unsafe = getUnsafe();
        for (Field f : clazz.getDeclaredFields()) {
            if (Fields.isStatic(f)) {
                long offset = unsafe.staticFieldOffset(f);
                System.out.printf("  +%d:\t[%s] %s : %s\n",
                    offset, clazz.getName(), f.getName(), f.getType());
            }
        }
    }
    
    public static void printInstanceFieldOffsets(Class<?> clazz)
            throws Exception {
        if (clazz == null) {
            System.err.println("got null pointer, skipping");
            return;
        }
        
        System.out.printf("Instance field offsets for class %s:\n", clazz.getName());
        printInstanceFieldOffsets0(clazz);
    }
    
    private static void printInstanceFieldOffsets0(Class<?> clazz)
            throws Exception {
        if (clazz == null) return;
        
        printInstanceFieldOffsets0(clazz.getSuperclass());
        
        Unsafe unsafe = getUnsafe();
        for (Field f : clazz.getDeclaredFields()) {
            if (!Fields.isStatic(f)) {
                long offset = unsafe.objectFieldOffset(f);
                System.out.printf("  +%d:\t[%s] %s : %s\n",
                    offset, clazz.getName(), f.getName(), f.getType());
            }
        }
    }
    
    private static Unsafe getUnsafe() throws Exception {
        Class<?> unsafeClass = Unsafe.class;
        for (Field f : unsafeClass.getDeclaredFields()) {
            if ("theUnsafe".equals(f.getName())) {
                f.setAccessible(true);
                return (Unsafe) f.get(null);
            }
        }
        return null;
    }
}

worked on 6u25 Linux x64

[sajia@v036122 ~]$ export JAVA_OPTS='-XX:FieldsAllocationStyle=2'
[sajia@v036122 ~]$ groovysh
Groovy Shell (1.7.7, JVM: 1.6.0_25)
Type 'help' or '\h' for help.
----------------------------------------------------------------------------------------------------------------------------
groovy:000> import java.lang.management.ManagementFactory
===> [import java.lang.management.ManagementFactory]
groovy:000> import com.sun.management.HotSpotDiagnosticMXBean
===> [import java.lang.management.ManagementFactory, import com.sun.management.HotSpotDiagnosticMXBean]
groovy:000> HOTSPOT_BEAN_NAME = "com.sun.management:type=HotSpotDiagnostic"
===> com.sun.management:type=HotSpotDiagnostic
groovy:000> server = ManagementFactory.platformMBeanServer
===> com.sun.jmx.mbeanserver.JmxMBeanServer@460ab1b4
groovy:000> bean = ManagementFactory.newPlatformMXBeanProxy(server, HOTSPOT_BEAN_NAME, HotSpotDiagnosticMXBean)
===> MXBeanProxy(com.sun.jmx.mbeanserver.JmxMBeanServer@460ab1b4[com.sun.management:type=HotSpotDiagnostic])
groovy:000> bean.getVMOption('FieldsAllocationStyle')
===> VM option: FieldsAllocationStyle value: 2  origin: VM_CREATION (read-write)
groovy:000> quit
[sajia@v036122 experiment]$ cd experiment/test_object_layout/
[sajia@v036122 test_object_layout]$ ls
A.class  A.java  B.class  C.class  Fields.class  Fields.java  ObjectLayouts.class  ObjectLayouts.java
[sajia@v036122 test_object_layout]$ groovysh
Groovy Shell (1.7.7, JVM: 1.6.0_25)
Type 'help' or '\h' for help.
----------------------------------------------------------------------------------------------------------------------------
groovy:000> ObjectLayouts.printInstanceFieldOffsets(C)
Instance field offsets for class C:
  +16:  [A] aObj : class java.lang.Object
  +12:  [A] aInt : int
  +20:  [B] bObj : class java.lang.Object
  +24:  [B] bInt : int
  +28:  [B] bFloat : float
  +36:  [C] cObj : class java.lang.Object
  +32:  [C] cInt : int
===> null
groovy:000> quit