Basically looks at your program like a fishnet.  It makes sure that
the stack looks the same no matter how you arrive at any node.
It simulates what the stack would look like for each piece of code,
and makes sure it is consistent with the operands.

It is fairly easy to imagine a verifier for a perfectly linear
program.  You just simulate the stack types (not values).

When you add the fishnet complexity, each linear piece starts out with
the stack in a given state.

--
Canadian Mind Products, Roedy Green.
Coaching, problem solving, economical contract programming.
See http://mindprod.com/jgloss/jgloss.html for The Java Glossary.

Errors?

Believe it or not, it actually works by magic!

But seriously, the Java Virtual Machine (JVM) needs to implement the
Java Language Specification (commonly abbreviated JLS). This typically
includes a class verifier to check that byte code is correct, and a
class loader to check security policy constraints (implicit or
explcit).

Or did I misunderstand your question?

-Hans

For details, look at
http://www.securingjava.com/

The whole book is online.
JK