Hi Ceki,
On 03.02.2009, at 17:54, Ceki Gulcu wrote:
Hi Maarteen,
Maarten Bosteels wrote:
Hello,
I think moving r.lock(); outside of the try block WILL help,
because the original exception will no longer be hidden.
Even though I hate being a smart ass and I also don't like to say
something like "See, I said so", I have to do just that.
This is exactly what I meant when I wrote
"It's generally a very bad idea to save a bit of standard code just
because a possible problem isn't imaginable at the moment. I'm not
even sure if the code without try {} finally {} will execute faster or
if there won't be any difference at all. "
in http://jira.qos.ch/browse/LBCORE-67 and is also the prime reason
why I submitted the ticket in the first place.
The documentation of the Lock interface at http://java.sun.com/javase/6/docs/api/java/util/concurrent/locks/Lock.html
clearly states:
<quote>
With this increased flexibility comes additional responsibility. The
absence of block-structured locking removes the automatic release of
locks that occurs with synchronized methods and statements. In most
cases, the following idiom should be used:
Lock l = ...;
l.lock();
try {
// access the resource protected by this lock
} finally {
l.unlock();
}
</quote>
The "should" is only there because there might be an obscure case
where a different pattern would probably be helpful - and this isn't
such a case.
Moving r.lock() outside the try block MAY help because the finally
block which throws a related but distinct exception will no longer be
invoked. The second exception effectively hides the first
exception. However, this is true if r.lock() throws an exception
*visible* in logback code. I tend to think, for reasons explained in
my previous post, that somewhere within r.lock() call there is a
RuntimeException thrown but which is absorbed somewhere within the
r.lock() invocation. So, there is a good chance (but less than 100%)
that displacing the r.lock() call to outside the try block will *not*
reveal anything new.
The documentation of the lock() method at http://java.sun.com/javase/6/docs/api/java/util/concurrent/locks/Lock.html#lock()
states:
<quote>
Implementation Considerations
A Lock implementation may be able to detect erroneous use of the lock,
such as an invocation that would cause deadlock, and may throw an
(unchecked) exception in such circumstances. The circumstances and the
exception type must be documented by that Lock implementation.
</quote>
Furthermore, the documentation of ReentrantReadWriteLock (the one used
in AppenderAttachableImpl) at http://java.sun.com/javase/6/docs/api/java/util/concurrent/locks/ReentrantReadWriteLock.html
does actually document a case where exactly such a scenario happens:
<quote>
Implementation Notes
This lock supports a maximum of 65535 recursive write locks and 65535
read locks. Attempts to exceed these limits result in Error throws
from locking methods.
</quote>
While this most likely isn't the case here it certainly illustrates
one thing:
Just stick to the pattern.
Having said that, if for one reason or another, the r.lock() call
throws an exception, which it certainly does, even if it is not
visible, there is arguably no point in calling r.unlock(). Calling
r.lock() outside the try block MAY reveal the original exception
without having a downside.
It will.
On the other hand, if calling r.unlock has
a positive side-effect even if r.lock() fails, then moving r.lock()
outside the try block may not be such a good idea after all.
It won't have a positive effect because it would be - in fact - an
undocumented behavior and that is *never* something to desire.
lock() does only throw an unchecked exception if it couldn't acquire
the lock (for one reason or another) while unlock() throws the
exception mentioned in the subject if the lock is not held (see http://java.sun.com/javase/6/docs/api/java/util/concurrent/locks/ReentrantReadWriteLock.WriteLock.html#unlock()
).
To summarize, although not guaranteed, there is a good chance that
moving the r.lock() call outside the try block will reveal new
information. However, this change may be problematic in certain cases,
which at this time we can only imagine but not pinpoint to. In
Rumsfeld-speak, moving r.lock has a known but uncertain positive
effect and uncertain and unknown negative effects.
Everything is perfectly documented and defined. This is mainly caused
by the fact that java.util.concurrent was created by Doug Lea and this
guy really knows what he's doing. His book on concurrency is quite
excellent.
Beside that, I dislike Rumsfeld ;)
Alternatively, we could invoke r.unlock in a safe manner, that is
within a try/catch block (embedded within finally). This would not
obfuscate the first exception and would ensure that r.unlock is always
invoked.
I am not sure splitting hairs as demonstrated above gets us any closer
to a solution. Zoltan, do you need any specific help from my side?
I fail to see the hairsplitting in Maartens message and I agree with
him.
Instead of invoking unlock and circumventing the built in security
measures using try{...}catch(Throwable){} we should just use the Lock
as defined in the API.
Sorry for being nosy,
Joern.
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