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From Luc Maisonobe <Luc.Maison...@free.fr>
Subject Re: [all] exceptions and localization
Date Sun, 04 Feb 2007 17:30:13 GMT
Phil Steitz wrote :
>
> The problem with that is that generic handlers that display messages would
> not work. I like the idea of carrying the data along, though and exposing
> it, as well as providing translate (see
> http://svn.apache.org/viewvc/jakarta/commons/proper/math/trunk/src/mantissa/src/org/spaceroots/mantissa/MantissaException.java?view=markup

>
>
> )
> and getMessage(Locale) methods.  That gives client apps the most
> flexibility.  What do you think, Luc?

Trying to synthetize all use cases, here is what I think is expected by 
everyone who has expressed views :

 1) allow application level code to contribute to/configure  message 
building
 2) allow low layers to build messages with variable parts
 3) allow localized messages
 4) expose message parts (key and parameters)
 5) allow handlers using only no arguments getMessage() to work as usual
 6) use a mechanism robust enough to work even in case of code changes
     and missing localization data (of course in degraded, i.e. non 
localized
    mode)

The following proposal is an attempt to address all these points.

Two main classes would be used: the MathException base class and a 
MessageFactory base class. MathException would delegate to 
MessageFactory the task to build the message from its String key and 
Object[] parameters (point 2). Both the key, parameters and message 
would be available with appropriate getters (point 4 and 5). A static 
method in MathException would allow upper layers to prepend a customized 
MessageFactory instance (point 1) in front of already registered 
factories following a chain of responsibility. The initial chain would 
contain three different MessageFactory instances :
  - a LocalizedMessageFactory using the key to retrieve a pattern from
    a ResourceBundle for the default Locale (Locale.geDefault()) and
    MessageFormat to build the message (point 3)
 - a NonLocalizedMessageFactory using the key directly as a pattern
   for MessageFormat
- a RawMessageFormat simply concatenating everything with toString

The chain of responsibility is intended to provide robustness (point 6): 
the first message factory providing a non null message is used. If a 
factory fails, the next one is used. The last factory cannot fail.

A user who simply wants to get messages localized for a Locale which is 
not the default one would therefore only need to do :

  MathException.prependMessageFactory(new 
LocalizedMessageFactory(myLocale));

This would work even if its locale is not supported as its factory would 
always fail but the following ones would be triggered as needed.

I think an appropriate policy would be to always use US-english patterns 
as keys as it would allow to get meaningful messages even if everything 
else fails, and as it would help the translators tasks.

Does this seems sensible ?

Luc


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