Exceptions - how to access data in stack frames?

Question

Presently when an exception is thrown, the stack trace with line numbers can be printed.
e.printStackTrace().
I feel that it is equally important to be able to print out the state ie. the data at each step of the stack frame.
By data, I mean local and member variables, method parameters. This could be very useful.
Question: is the data in the stack frame (including frames below the current one) accessible from a catch block?
thanks,
Anil Philip
Kansas City, MO
Software Engineer,
Sprint PCS

Answer 1

Hello Anil,
You have my permission to post my reply at the forums. And you are correct that the approach I mentioned (which many developers follow) works only with the current stack frame.
All the best.
Jeff
-----Original Message-----
From: Jeff Friesen [mailto:[email protected]]
Sent: Tuesday, March 04, 2003 1:42 PM
To: Philip, Anil [PCS]
Cc: Jennifer Orr
Subject: Reply to your question
Hello Anil,
Being able to obtain current state values from within a catch clause is helpful in figuring out what a program was doing just
prior to an exception. (When catch clauses are left empty, there's obviously no need to obtain those values.) To obtain the
current state values (found in local variables, method parameters, and member -- field -- variables), developers typically do
the following:
1) Subclass an appropriate exception class (such as Exception for checked exceptions or RuntimeException for unchecked
exceptions).
2) Within the subclass, declare private field variables to hold appropriate state values.
3) Within the subclass, declare some combination of a constructor and setter methods to initialize those field variables to
current state values.
4) Within the subclass, declare getter methods to return those state values.
5) Just before throwing an exception, create an object from the exception subclass and populate that object (via constructor
and setter methods) with current state values found in local variables, method parameters, and fields.
6) From within the appropriate catch clause, call the getter methods to obtain the state values.
What I've just stated is commonly done by developers. Why doesn't Sun provide an automatic means for obtaining all these
values? In other words, why doesn't Sun do much of the above work for you? I personally believe that performance has a lot to
do with the answer. When you think about the potentially infinite number of local variable, method parameter, and field
combinations, an automated mechanism to accomplish this task would be time-consuming -- like garbage collection. For the
many catch clauses that don't require access to state values, the time needed to retrieve such values wouldn't be justified. (I'm
sure better reasons than the performance reason I've given can be stated: I can't think of any other reasons, at the moment.)
In closing, my suggestion is to design appropriate exception classes that capture just the amount of state needed to figure
out why an exception has occurred and properly recover from that exception.
Jeff
P.S.
As to your "is the data in the stack frame (including frames below the current one) accessible from a catch block," my answer
is that it's accessible if you've stored those values in an exception object prior to throwing the exception. Otherwise, I
don't believe it is accessible -- at least not from StackTraceElement or the Reflection API.