Sorry if this has already come up and I have missed this...(this issue
came up in a different discussion related to Java extensions and I
thought I would check on this list for what stance is being taken by JSR
(Please note that this message is not about the "Unified Model" -- it is
about what the Java Memory Model should be.)
The Java Memory Model conceptually deals with the specification of
out-of-order execution and conditional execution.
How does this interact with the Java requirement for precise exception
11.3.1 Exceptions are Precise
Exceptions are precise: when the transfer of control takes place, all
effects of the statements executed and expressions evaluated before the
point from which the exception is thrown must appear to have taken
place. No expressions, statements, or parts thereof that occur after the
point from which the exception is thrown may appear to have been
evaluated. If optimized code has speculatively executed some of the
expressions or statements which follow the point at which the exception
occurs, such code must be prepared to hide this speculative execution
from the user-visible state of the program.
Clearly, some issues are not problematic. For instance, the JMM
ultimately describes simply the values returned by Memory to a read
request from a thread. For properly initialized programs, this should
not cause any exceptions in itself. Once this value is received at the
thread, processing of this value may cause an exception (e.g. null
pointer exception) which will be raised by the thread in its normal
However, a central question that is not so easy to answer (because it
interacts fundamentally with the concurrency/memory consistency model)
is: what heap state is any exception handler guaranteed to see? For
instance, is it guranteed that all instructions (including reads and
writes) issued by this thread that should have happened-before the
instruction that threw the exception actually executed? 11.3.1 quoted
above says "yes". Is this what makes sense?
Further, can a Thread i see the result of an "out of order" write that
may have been done by Thread j ... but it so happened that Thread j
threw an exception when processing an instruction that happens-before
that write? 11.3.1 says no. Is this what makes sense?
I quickly scanned the JSR 133 draft but there appears to be no
discussion of this issue.
This seems like a major omission to me and there should be an explicit
discussion. Even if the discussion just reaffirms the "precise
Also, there should be several test cases in the standard suite that
illustrate the issues with the interaction between the memory model and
precise exception generation.
JavaMemoryModel mailing list - http://www.cs.umd.edu/~pugh/java/memoryModel
This archive was generated by hypermail 2b29 : Thu Oct 13 2005 - 07:01:02 EDT