Is there a way to get a reference address?

Question

In Java, is there a way to get reference address, say

String s = "hello"

can I get the address of s itself , also, can I get the address of the object which reference refers to?

Answer 1

You can get the object index with Unsafe. Depending on how the JVM is using the memory (32-bit addresses, 32-bit index, 32-bit index with offset, 64-bit address) can affect how useful the object index is.

Here is a program which assumes you have 32-bit index in a 64-bit JVM.

import sun.misc.Unsafe;

import java.lang.reflect.Field;
import java.util.Arrays;
import java.util.Collections;

public class OrderOfObjectsAfterGCMain {
    static final Unsafe unsafe = getUnsafe();
    static final boolean is64bit = true; // auto detect if possible.

    public static void main(String... args) {
        Double[] ascending = new Double[16];
        for(int i=0;i<ascending.length;i++)
            ascending[i] = (double) i;

        Double[] descending = new Double[16];
        for(int i=descending.length-1; i>=0; i--)
            descending[i] = (double) i;

        Double[] shuffled = new Double[16];
        for(int i=0;i<shuffled.length;i++)
            shuffled[i] = (double) i;
        Collections.shuffle(Arrays.asList(shuffled));

        System.out.println("Before GC");
        printAddresses("ascending", ascending);
        printAddresses("descending", descending);
        printAddresses("shuffled", shuffled);

        System.gc();
        System.out.println("\nAfter GC");
        printAddresses("ascending", ascending);
        printAddresses("descending", descending);
        printAddresses("shuffled", shuffled);

        System.gc();
        System.out.println("\nAfter GC 2");
        printAddresses("ascending", ascending);
        printAddresses("descending", descending);
        printAddresses("shuffled", shuffled);
    }

    public static void printAddresses(String label, Object... objects) {
        System.out.print(label + ": 0x");
        long last = 0;
        int offset = unsafe.arrayBaseOffset(objects.getClass());
        int scale = unsafe.arrayIndexScale(objects.getClass());
        switch (scale) {
            case 4:
                long factor = is64bit ? 8 : 1;
                final long i1 = (unsafe.getInt(objects, offset) & 0xFFFFFFFFL) * factor;
                System.out.print(Long.toHexString(i1));
                last = i1;
                for (int i = 1; i < objects.length; i++) {
                    final long i2 = (unsafe.getInt(objects, offset + i * 4) & 0xFFFFFFFFL) * factor;
                    if (i2 > last)
                        System.out.print(", +" + Long.toHexString(i2 - last));
                    else
                        System.out.print(", -" + Long.toHexString( last - i2));
                    last = i2;
                }
                break;
                case 8:
                    throw new AssertionError("Not supported");
        }
        System.out.println();
    }

    private static Unsafe getUnsafe() {
        try {
            Field theUnsafe = Unsafe.class.getDeclaredField("theUnsafe");
            theUnsafe.setAccessible(true);
            return (Unsafe) theUnsafe.get(null);
        } catch (Exception e) {
            throw new AssertionError(e);
        }
    }
}

Running on Java 6 update 26 (64-bit with compressed oops) and Java 7. Note: addresses and relative addresses are in hex.

Before GC
ascending: 0x782322b20, +18, +18, +18, +18, +18, +18, +18, +18, +18, +18, +18, +18, +18, +18, +18
descending: 0x782322e58, -18, -18, -18, -18, -18, -18, -18, -18, -18, -18, -18, -18, -18, -18, -18
shuffled: 0x782322ec0, +78, -30, +90, -c0, +18, +90, +a8, -30, -d8, +f0, -30, -90, +60, -48, +60

After GC
ascending: 0x686811590, -18, -18, -18, -18, -18, -18, -18, -18, -18, -18, -18, -18, -18, -18, -18
descending: 0x686811410, -18, -18, -18, -18, -18, -18, -18, -18, -18, -18, -18, -18, -18, -18, -18
shuffled: 0x686811290, -18, -18, -18, -18, -18, -18, -18, -18, -18, -18, -18, -18, -18, -18, -18

OR sometimes

Before GC
ascending: 0x782322b20, +18, +18, +18, +18, +18, +18, +18, +18, +18, +18, +18, +18, +18, +18, +18
descending: 0x782322e58, -18, -18, -18, -18, -18, -18, -18, -18, -18, -18, -18, -18, -18, -18, -18
shuffled: 0x782323028, -168, +150, -d8, -30, +60, +18, +30, +30, +18, -108, +30, -48, +78, +78, -30

After GC
ascending: 0x6868143c8, +4db0, +7120, -bd90, +bda8, -bd90, +4d40, +18, +18, -12710, +18, +80, +18, +ffa8, +220, +6b40
descending: 0x68681d968, +18, +d0, +e0, -165d0, +a8, +fea8, +c110, -5230, -d658, +6bd0, +be10, +1b8, +75e0, -19f68, +19f80
shuffled: 0x686823938, -129d8, +129f0, -17860, +4e88, +19fe8, -1ee58, +18, +18, +bb00, +6a78, -d648, -4e18, +4e40, +133e0, -c770