Monday, 11 October 2010

Allowing full access to objects in rules

Some months back one of the developers at JBoss was trying to use Byteman to test some code and he was rather puzzled that his rule was not compiling correctly. It turned out that his rule action included an assignment to a private field. Up until now Byteman has only allowed rule actions to change public state or call public methods. Rule conditions and actions have no special privileges as far as encapsulation is concerned.

As an example, given this class
class Regulator
private float throttle;
public void adjustThrottle(float amount);
the following rule fails
CLASS SteamEngine
METHOD adjustPressure(float)
AT CALL Regulator.adjustThrottle
IF ($0.getRegulator().throttle + $1 > 0.95)
DO traceln("Illegal setting for throttle"),
THROW new BoilerRupture("too much pressure")
Class Regulator allows clients to increment or decrement the throttle setting but it does not enable access to the current value, presumably because it handles any illegal settings itself. However, when testing or validating the application it is useful to be able to identify erroneous situations such as the one above where a client is making an invalid request. Regulator may handle the error but it is still important to know where errors arise.

Unfortunately, the rule does not typecheck correctly. Byteman complains that the condition is trying to access an unknown field called throttle. The same issue applies with method invocations. If a method is private then any attempt to call it inside a rule condition or action leads to an unknown method error during type checking. Well, the answer is obvious isn't it. Why not just let rules access whatever state they want?

I have modified the trunk release of Byteman to do just this. It is working and there don't appear to be any problems with it. Of course, enabling this feature raises a whole load more opportunities for you to shoot yourself in the foot when using Byteman. In particular, if you use Byteman to modify JVM static and instance data or call private JVM methods you can easily break your Java runtime into shiny little pieces. But then again, this feature makes it possible to do a whole load more tracing and verification of your application's behaviour and to engineeer a lot more unexpected situations during testing. Caveat emptor, as the Romans used to say.

Assuming no issues arise during testing over the next few weeks this feature should be included in the upcoming 1.3.1 Byteman release. If you want an early peek then check out and build the trunk code and give it a try.