Tuesday, 17 August 2010

Final is not final anymore in java 1.5

I read somewhere about this concept.. This is very interesting ..  

Playing with your sanity - Strings

Have a look at the following code:
public class MindWarp {
  public static void main(String[] args) {
      "Romeo, Romeo, wherefore art thou oh Romero?");
  private static final String OH_ROMEO =
    "Romeo, Romeo, wherefore art thou oh Romero?";
  private static final Warper warper = new Warper();
If we are told that the class Warper does not produce any visible output when you construct it, what is the output of this program? The most correct answer is, "you don't know, depends on what Warper does". Now THERE's a nice question for the Sun Certified Java Programmer Examination.
In my case, running "java MindWarp" produces the following output
C:> java MindWarp 
Stop this romance nonsense, or I'll be sick
And here is the code for Warper:
import java.lang.reflect.*;

public class Warper {
  private static Field stringValue;
  static {
    // String has a private char [] called "value"
    // if it does not, find the char [] and assign it to value
    try {
      stringValue = String.class.getDeclaredField("value");
    } catch(NoSuchFieldException ex) {
      // safety net in case we are running on a VM with a
      // different name for the char array.
      Field[] all = String.class.getDeclaredFields();
      for (int i=0; stringValue == null && i
        if (all[i].getType().equals(char[].class)) {
          stringValue = all[i];
    if (stringValue != null) {
      stringValue.setAccessible(true); // make field public
  public Warper() {
    try {
        "Romeo, Romeo, wherefore art thou oh Romero?",
        "Stop this romance nonsense, or I'll be sick".
      stringValue.set("hi there", "cheers !".toCharArray());
    } catch(IllegalAccessException ex) {} // shhh
How is this possible? How can String manipulation in a completely different part of the program affect our class MindWarp?
To understand that, we have to look under the hood of Java. In the language specification it says in §3.10.5:
"Each string literal is a reference (§4.3) to an instance (§4.3.1, §12.5) of class String (§4.3.3). String objects have a constant value. String literals-or, more generally, strings that are the values of constant expressions (§15.28)-are "interned" so as to share unique instances, using the method String.intern."
The usefulness of this is quite obvious, we will use less memory if we have two Strings which are equivalent pointing at the same object. We can also manually intern Strings by calling the intern() method.
The language spec goes a bit further:
  1. Literal strings within the same class (§8) in the same package (§7) represent references to the same String object (§4.3.1).
  2. Literal strings within different classes in the same package represent references to the same String object.
  3. Literal strings within different classes in different packages likewise represent references to the same String object.
  4. Strings computed by constant expressions (§15.28) are computed at compile time and then treated as if they were literals.
  5. Strings computed at run time are newly created and therefore distinct.
  6. The result of explicitly interning a computed string is the same string as any pre-existing literal string with the same contents.
This means that if a class in another package "fiddles" with an interned String, it can cause havoc in your program. Is this a good thing? (You don't need to answer ;-)
Consider this example
public class StringEquals {
public static void main(String[] args) {
  System.out.println("hi there".equals("cheers !"));
private static final String greeting = "hi there";
private static final Warper warper = new Warper();
Running this against the Warper produces a result of true, which is really weird, and in my opinion, quite mind-bending. Hey, you can SEE the values there right in front of you and they are clearly NOT equal!
BTW, for simplicity, the Strings in my examples are exactly the same length, but you can change the length quite easily as well.
Last example concerns the HashCode of String, which is now cached for performance reasons mentioned in "Java Idiom and Performance Guide", ISBN 0130142603. (Just for the record, I was never and am still not convinced that caching the String hash code in a wrapper object is a good idea, but caching it in String itself is almost acceptable, considering String literals.)
public class CachingHashcode {
  public static void main(String[] args) {
    java.util.Map map = new java.util.HashMap();
    map.put("hi there", "You found the value");
    new Warper();
    System.out.println(map.get("hi there"));
  private static final String greeting = "hi there";
The output under JDK 1.3 is:
You found the value
{cheers !=You found the value}
Under JDK 1.2 it is
{cheers !=You found the value}
This is because in the JDK 1.3 SUN is caching the hash code so if it once calculated, it doesn't get recalculated, so if the value field changes, the hashcode stays the same.
Imagine trying to debug this program where SOMEWHERE, one of your hackers has done a "workaround" by modifying a String literal. The thought scares me.
[Heinz: Author's note: the comment below on using "final" to solve this problem is not correct. Firstly, you cannot make arrays immutable, which is a design flaw in Java, so you could still change the content of the array even if the handle were final. Secondly, in JDK 1.5, you can set final fields using reflection. See Java 5 - "final" is not final anymore and for a similar contortion with autoboxing see Mangling Integers.]
There is of course a small keyword that would have stopped this problem, namely "final". I got into the habit a few months ago to make all my data members final where possible, and it has paid off more than once. Surprisingly, the char array in String is not final.
Consider the following example code:
public class Bla {
  private char[] c1 = "hello".toCharArray();
  private final char[] c2 = "bye".toCharArray();
  public String toString() {
    return c1 + ", " + c2;

import java.lang.reflect.*;

public class BlaTest {
  private static Field c1;
  private static Field c2;
  static {
    try {
      c1 = Bla.class.getDeclaredField("c1");
      c2 = Bla.class.getDeclaredField("c2");
    } catch(NoSuchFieldException ex) { }
  public static void main(String[] args) {
    Bla bla = new Bla();
    try {
      c1.set(bla, "mutatedc1".toCharArray());
      c2.set(bla, "mutatedc2".toCharArray());
    } catch(IllegalAccessException ex) {
When I run my program, I can quite happily change c1, but when I try to change c2 I get an exception. String has no reason for value to be non-final, so it should be final. If you have contacts at SUN, please forward them this newsletter and ask them to make value final. It might stop some nasty Java viruses from completely messing up the JVM.