import java.lang.reflect.Field;

/**
 * Where does the first field of an object start? Code that hard-codes "12 bytes of header" gets this wrong
 * on JDK 27. Run with:  java --sun-misc-unsafe-memory-access=allow Offsets.java
 * (The flag silences the JDK 24+ warning about sun.misc.Unsafe; nothing here reads or writes memory.)
 */
public class Offsets {

    static class P {
        long id;
        int priceCents;
        int stock;
        boolean active;
        Object sku;
    }

    @SuppressWarnings("removal")
    public static void main(String[] args) throws Exception {
        Field f = sun.misc.Unsafe.class.getDeclaredField("theUnsafe");
        f.setAccessible(true);
        sun.misc.Unsafe u = (sun.misc.Unsafe) f.get(null);
        System.out.println("java " + System.getProperty("java.version")
                + "  compact headers: " + java.lang.management.ManagementFactory
                .getPlatformMXBean(com.sun.management.HotSpotDiagnosticMXBean.class)
                .getVMOption("UseCompactObjectHeaders").getValue());
        long first = Long.MAX_VALUE;
        for (Field x : P.class.getDeclaredFields()) {
            long off = u.objectFieldOffset(x);
            first = Math.min(first, off);
            System.out.printf("  %-11s offset %2d%n", x.getName(), off);
        }
        System.out.println("  first field starts at byte " + first + "  (so the header is at most " + first + " bytes)");
        System.out.println("  a hard-coded 12-byte header would be " + (first == 12 ? "right" : "WRONG") + " here");
    }
}