Lookup native thread ID for Java thread from within Java

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

public class NativeThreadIDLookup {
    public static void main(String[] args) throws Exception {
        for (int i = 0; i < 20; i++) {
            Thread t = new Thread() {
                public void run() {
                    while (true) {
                        try {
                            Thread.sleep(10000);
                        } catch (InterruptedException ex) {
                        }
                    }
                };
            };
            t.setName("Target-" + i);
            t.start();
        }
        Thread.sleep(500);
        Thread[] ts = new Thread[Thread.activeCount() * 2];
        Thread.enumerate(ts);
        Field f = Thread.class.getDeclaredField("eetop");
        f.setAccessible(true);
        Field uf = Unsafe.class.getDeclaredField("theUnsafe");
        uf.setAccessible(true);
        Unsafe u = (Unsafe) uf.get(null);
        for (int i = 5; i < ts.length; i++) {
            if (ts[i] != null) {
                long pid = (Long) f.get(ts[i]);

                // osthread pointer is 4 slots before stack size and one slot after a 0.
                // nid should be findable if you know at least one value :)

                // Java 1.7.0_10 x64
                // int osthread = u.getInt(pid+4*41);
                // int nid = u.getInt(osthread+4*5);

                // Java 1.7.0_40 x86
                // Java 1.7.0_72 x86
                // int osthread = u.getInt(pid+4*43);
                // int nid = u.getInt(osthread+4*7);

                // Java 1.7.0_72 x64
                Object osthread = unpack(u, u.getInt(pid + 4 * 68));
                int nid = u.getInt(osthread, 4L * 11);

                System.out.println(nid + "/0x" + Integer.toHexString(nid) + "\t" + Long.toHexString(pid) + "\t" + ts[i].getId() + "\t" + ts[i].getName());
            }
        }
    }

    private static Object unpack(Unsafe u, int addr) throws Exception {
        Unpacker uu = new Unpacker();
        u.putInt(uu, u.objectFieldOffset(Unpacker.class.getDeclaredField("value")), addr);
        return uu.value;
    }

    private static class Unpacker {
        public Object value;
    }
}