Chuck Norris throws exceptions that can't be caught.
If you catch ChuckNorrisException, you'll probably die.
This is an implementation of uncatchable exception for Java.
public class Example {
public static void main(String[] args) {
try {
throw new ChuckNorrisException();
} catch(ChuckNorrisException thr) {
System.err.println("I've caught "+thr.getClass().getSimpleName());
} finally {
System.out.println("Finalization.");
}
}
}For any other exception, the result of this program would be obvious. However, with ChuckNorrisException we get roundhouse-kicked:
Finalization.
Exception in thread "main" com.zyxist.chuck.ChuckNorrisException: I'm uncatchable!
at com.zyxist.Example.main(Example.java:8)More examples can be found in the source code!
There are several features of Java compiler and JVM used to make this work:
- there may exist multiple classes with the same name, but they must be loaded by different classloaders,
- bytecode generation allows us generating an evil class and predenting we are "nice",
- certain methods, such as
getMessage()andprintStackTrace()can be overriden, - it is possible to alter the stacktrace of the exception with
setStacktrace()method, thus hiding our manipulations, - shutdown hook is our last line of defense.
Basically, the constructor of the exception uses Javassist to make a copy of another class, RoundhouseKick, changes its name to ChuckNorrisException and generates a classloader for it. After instantiating that class, it is something completely different from JVM perspective and our catch no longer matches it. With bytecode manipulation, we can also easily avoid catch (Exception). We can't avoid being caught by catch (Throwable), but with the rest of the tricks, we can escape it without problems, and still hide from being easily detected.
More explanation (in Polish) can be found at my blog: www.zyxist.com.
The code is in public domain. But please, don't use it in production.