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From Oliver Deakin <oliver.dea...@googlemail.com>
Subject Re: [classlib] Looking up exception messages
Date Thu, 08 Oct 2009 16:02:58 GMT
Tim Ellison wrote:
> On 08/Oct/2009 14:21, Oliver Deakin wrote:
>> Tim Ellison wrote:
>>> Part of the patch for HARMONY-6346 includes some improvements to the way
>>> we handle localized exception messages.  That prompted me to write down
>>> a few thoughts...
>>> I don't really like the way we do I18N of exception messages.  We create
>>> message string like this
>>>   throw new IOException(Messages.getString("archive.1E"));
>> One way to handle this could be to use an exception manager class.
>> Rather than carrying out the lookup on the error message at the time the
>> exception is thrown, instead associate the exception object with the
>> error message key using a exception manager class. Then in the
>> getMessage(), getLocalizedMessage() etc. methods the exception will call
>> into the exception manager to get it's message (and can cache it locally
>> in case of future lookups) if one exists.
>> So instead of:
>> throw new IOException(Messages.getString("archive.1E"));
>> you might have (here create() returns the exception passed to it as the
>> first parameter):
>> throw ExceptionManager.create(new IOException(), "archive.1E");
>> Once the exception is caught and it's message string queried, it will
>> call ExceptionManager.getMessage(this) to see if it has an associated
>> message available. This way the error message lookup will only happen if
>> the message is actually queried. The references in the ExceptionManager
>> could be soft so that if the exception is not caught or is discarded,
>> then the ExceptionManager does not unnecesarily hang onto it's error
>> message.
> Yep, so to replay what you just said, the 'guts' of the ExceptionManager
> would be a WeakHashMap<Throwable, Object[]> of 'pending' exceptions that
> may be asked for their formatted message.
> The Object[] would contain the message id and arguments that only get
> looked up and formatted if asked.

Sounds about right to me.

>>> The string "archive.1E" is a key into the resource files loaded by the
>>> Messages class based on the current locale.
>>> 1) I have no concrete proof, but I'm prepared to bet that 90% of the
>>> exceptions thrown never get asked for their message string.  Yet we are
>>> setting it eagerly - mainly because there is no mechanism on exceptions
>>> to answer the message lazily.
>>> Simple messages may not be too bad, but we also have formatted messages
>>> where we replace token in the message with actual parameters.  A
>>> particular bad example requires formatting more argument strings, e.g.
>>>   throw new IllegalArgumentException(
>>>       Msg.getString("K031e", new String[] { "query", uri.toString() }));
>>> in this case we format the URI and format the message string, and
>>> probably never use the answer.
>> For this example, the create() method could be made to accept variable
>> arguments. It's first argument would be the exception object, the second
>> would be the message identifier, and any subsequent arguments would be
>> the values to be inserted into tokens in the message string. This would
>> mean that in the above example the uri.toString() would still be
>> evaluated at the time the exception was created, but the lookup and
>> formatting of the error string would only be done when the actual
>> exception message was queried. (I havn't looked deeply at this case so
>> it may be more complex then that).
> ...and if you wanted to be sneaky, you could change the message id to be
> the actual string for `you favorite locale` which would mean there was
> no loading of message resource bundles if you happened to be running in
> the most-favored-locale, e.g.
> throw ExceptionManager.create(new IOException(), "Stream is closed");

True, although I'm not entirely convinced this is a good idea for a 
couple of reasons:
 - Changing all our message files to have "Full English message"="Full 
translated message" might make them quite cluttered and hard to read.
 - Some messages may contain the '=' character (for example, archive.15) 
which would mess up the parsing of message files.

> All this is dependent upon the premise that messages are not used often,
> I need to check that first.

Indeed - this might be overkill if it only has a small impact.


> Regards,
> Tim
>>> 2) The use of ResourceBundles for handling the strings seems like
>>> overkill too.
>>> Each time we need a string, we load up the entire set of error messages
>>> for that module into a Properties (i.e. hash table) and hold onto them
>>> forever.
>>> The HARMONY-6346 patch makes the resource file soft referenced so that
>>> may help, but I can't help thinking there is a simpler mechanism for
>>> loading the one or two strings that we need rather than the whole lot +
>>> data structure each time.
>>> Regards,
>>> Tim

Oliver Deakin
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