Re: Catching NPEs

John Ersatznom <j.ersatz@nowhere.invalid>
Fri, 29 Dec 2006 11:30:22 -0500
Arne Vajh??j wrote:

It is usually consider bad practice to present
a stack trace to the end user.

If the application has state in memory that needs to be
persisted it is a very good idea to save the data
before exiting.

If it is a server type application it would be a good idea
to either just terminate one thread and let the other threads
continue serving their clients or at least restart the entire
server app.

I think that in most real world applications you would catch
that kind of exceptions.

I have developed an app along these lines. There's a FooThread class and
a FooThreadFactory, constructors and other methods, and stuff like:

public void run () {
    try {
        some stuff
    } catch (RuntimeException e) {
        factory.notifyThreadDeath(this, e);
        throw e;
    } catch (OutOfMemoryError e) {
        factory.notifyThreadDeath(this, e);
        throw e;
    } catch (Error e) {
        factory.notifyThreadDeath(this, e);
        throw e;

Unexpected exceptions are caught and rethrown. Anything other than OOME
results in the thread factory being notified via a
notifyThreadDeath(FooThread, Throwable) method. This logs the stack
trace and decrements a thread count. Elsewhere the factory has a
periodically-called method to interrupt or start threads as need be,
based on a desired number (depending on the number of requests
occurring) and the actual number. (It also keeps a collection of the
actual thread objects, so it can pick some at random to interrupt. The
stuff in the try block involves taking a request off a blocking queue
and handling it in an endless while loop. There's a try that catches
InterruptedExceptions and breaks the while loop, also calling
factory.notifyThreadDeath(this, null).

Upshot: stack traces are logged so things can be debugged. Request
servicing continues, and doesn't even diminish in capacity since the
thread that dies is recreated immediately unless it's no longer needed
anyway. The code that loops the factory's thread-management method
periodically just does so and sleeps, and doesn't do anything that's at
all exception prone itself (it doesn't new anything, for instance,
except thread objects when necessary). OOME causes it to save some data
(mostly caches) and shut down gracefully. (I note that even though the
total megabytage of live objects should remain bounded, it does
eventually slow down and then catch OOME if it runs a long time. Heap
fragmentation?) Restarting the process if it dies somehow is handled
externally. Since it's a network using app, most exceptions are I/O
exceptions that get handled closer to the source, usually by sending the
client an error message and moving on to the next request; they tend to
indicate a problem with connectivity or the client having sent a bad or
malformed request that includes a bogus URL, parse error, or similarly.
Still, the odd wackiness trickles up from time to time. I've had library
code randomly bubble up NPEs and various IndexOutOfBoundsException
flavours, for example, on rare occasions. Lately (since the deployment
environment switched to the Java 6 beta runtime environment) there've
been NPEs coming out of Sun code we can't explain (mainly from one
particular PriorityQueue deep in the bowels of the app -- I found an "in
progress, bug" bug on Sun's site about this one, where someone had it
occur with a comparator not consistent with the objects' equals method,
which is the case in my code also. A quick check of this newsgroup turns
up a complaint of something similar again.) The app recovers from all of
these insults with maybe one in ten thousand requests ending up
incompletely processed. The transaction gets rolled back, but
transactions are pretty much independent of each other, and client
requests that are well-formed but get a "temporary service error"
message are resubmitted by the client after a backoff delay anyway.

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