So, we eventually fixed this. There were 2 bugs that worked together to
make this a mystery. I had to guess at what the problems were, and fix them
blind, since I could not see the Exceptions. I am curious about why I was
not able to see the Exceptions.
About this:
> Exceptions are ALWAYS visible, the only way they get lost is
> the try/catch blocks you added in your code which effectively
> swallow and ignore them.
But the Exceptions I dealt with were invisible. We have the Clojure app set
up to run via Supervisord and the conf has these lines:
stderr_logfile=/var/log/nlph_stderr.log
stdout_logfile=/var/log/nlph_stdout.log
So in 2 terminal windows I would:
cd /var/log
tail -f nlph_stdout.log
And the same for nlph_stderr.log.
So I was looking at the output in my terminal window. And I could see all
of our logging statements appear. And the code clearly got to the line
where the exception happens, but then the Exception never appeared in the
terminal. It was invisible. I assume the output of this:
System.out.println(e.getMessage());
would appear in one of the files that I was tailing. But it never did.
So I am wondering why the Exceptions were invisible? Why didn't the print
statements make it to the terminal?
On Saturday, October 31, 2015 at 6:56:23 AM UTC-4, Thomas Heller wrote:
>
>
>> What could we do to make the Exception visible, assuming there is one?
>>
>>
> Exceptions are ALWAYS visible, the only way they get lost is the try/catch
> blocks you added in your code which effectively swallow and ignore them.
> The JVM will not randomly lose an Exception, it is always code you write
> that decided to handle them in some way. If you do not know how to handle
> an exception do not catch it, printing its stacktrace and ignoring it is
> never a good idea.
>
> In your case since you say that the function never returns I'd put my
> guess on an infinite loop. There is no exception that gets lost, your code
> is just still running. If your JVM is running locally you can attach to it
> via jvisualvm [1] and get a quick overview of what the JVM is doing. If you
> look at the thread information you'll see if something is still running or
> dead-locked somehow.
>
> You can also use a debugger and step through the java code step by step.
>
> Also, consider coding against interfaces in java (eg. java.util.List and
> java.util.Map instead of java.util.Collection), it will save you tons of
> conversion calls.
>
> HTH
> /thomas
>
> [1] https://visualvm.java.net/
>
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