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2 + 2 = 5 (See related posts)

Here is some very exciting Java code for printing 5.

   1  
   2  import java.security.*;
   3  import java.lang.reflect.*;
   4  import java.util.*;
   5  
   6  public class Test
   7  {
   8      public static void main(String[] args) throws Exception
   9      {   
  10          AccessController.doPrivileged( new PrivilegedAction<Object>(){ public Object run(){try {
  11          Field field = Class.forName("java.lang.Integer$IntegerCache").getDeclaredFields()[0];
  12          field.setAccessible(true);
  13  
  14          Integer[] cache = (Integer[])field.get(null);
  15              
  16          cache[130] = 3;  
  17              
  18          Integer foo = 2;
  19   
  20          System.out.println(foo + 2); 
  21              
  22          return null;} catch(Exception e) { throw new RuntimeException(e); } } }); 
  23      }
  24  
  25  
  26  }
  27
to java reflection evil by DRMacIver on Wed Mar 14 18:01:11 -0400 2007

Comments on this post

gmiller posts on Mar 18, 2007 at 01:52
Yet another entry for the "How to write unmaintainable code" document.
llucifer posts on Mar 30, 2007 at 03:44
Nice one! Where does the number 130 come from? Please provide more information on the "internal".
DRMacIver posts on Mar 31, 2007 at 14:02
How this works is as follows:

Integer maintains an internal cache of Integer objects for values from -128 to 127. This is stored on the inner class IntegerClass (this is what the Class.forName("java.lang.Integer$IntegerCache") line fetches) as an array. When you do Integer.valueOf(int) it will fetch an object from this cache rather than construct a new one if it is within the appropriate range. Because the cache starts at -128, 130 is the index of the entry for 2.

Autoboxing then takes care of the rest. When you declare Integer foo = 2 this gets autoboxed - i.e. converted to Integer.valueOf(2), which is now 3.

When we do 2 + 2 the Integer object is unboxed to an int rather than the other way round, so foo + 2 becomes foo.intValue() + 2, which is 3 + 2.

The rest of the code is just the neccessary hackery in order to beat the Java permissions system into submission.

I didn't come up with this, but unfortunately I've lost the original article where I found this hack in. The above is my attempt at recreating it.



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