static long dataSize(O o) {
    return dataSize(o, new IdentityHashMap);
  }
  
  static long dataSize(Object o, IdentityHashMap seen) {
    if (o == null || seen.containsKey(o)) return 0;
    seen.put(o, Boolean.TRUE);
    long size = 12; // HotSpot header size
    Class c = o.getClass();
    if (c.isArray()) {
      size += 4; // array size field
      int n = Array.getLength(o);
      size += n * dataSize_arrayElementSize(c.getComponentType());
      if (!c.getComponentType().isPrimitive())
        for (int i = 0; i < n; i++)
          size += dataSize(Array.get(o, i), seen);
    } else
      while (c != null) {
        Field[] fields = c.getDeclaredFields();
        for (Field field : fields) {
          if ((field.getModifiers() & Modifier.STATIC) != 0)
            continue;
          if (field.getType().isPrimitive())
            size += dataSize_primitiveSize(field.getType());
          else {
            size += 4; // pointer
            Object value = null;
            try {
              field.setAccessible(true);
              value = field.get(o);
            } catch (IllegalAccessException e) {
              throw new RuntimeException(e);
            }
            size += dataSize(value, seen);
          }
        }
        c = c.getSuperclass();
      }
    return size;
  }
  
  static long dataSize_primitiveSize(Class type) {
    if (type == boolean.class) return 4;
    if (type == int.class) return 4;
    if (type == long.class) return 8;
    if (type == float.class) return 4;
    if (type == short.class) return 4;
    if (type == char.class) return 4;
    if (type == byte.class) return 4;
    if (type == double.class) return 8;
    throw fail("woot? " + type);
  }
  
  static long dataSize_arrayElementSize(Class type) {
    if (type == boolean.class) return 1;
    if (type == int.class) return 4;
    if (type == long.class) return 8;
    if (type == float.class) return 4;
    if (type == short.class) return 2;
    if (type == char.class) return 2;
    if (type == byte.class) return 1;
    if (type == double.class) return 8;
    ret 4; // pointers
  }

Began life as a copy of #1000524

Took information from http://www.slideshare.net/cnbailey/memory-efficient-java

This should be correct for 32bit VMs.