Java Resource Bundles

Locale data

• All locale-specific data has to be stored somewhere - in text files or in Java classes
• The "default" locale needs to store things like: names of months ("January", etc); month abbreviations ("Jan", etc); day names ("Monday", etc); am/pm strings; bc/ad eras; default number pattern; date/time patterns; ...
• Each locale will override some of this: e.g. the fr locale overrides the month names as "janvier" etc, ; the Canadian French locale fr_CA further overrides the currency symbol (from nothing to "$") and the date/time patterns
• Java itself uses 135 files for such locale information (in JAVA_SRC/sun/text/resources such as LocaleElements.java, LocaleElements_fr.java and LocaleElements_fr_CA.java
• The mechanism used to support this can also be used for application-specific data, such as strings, filenames and image files

ResourceBundle

• The root class used to handle this is the ResourceBundle class
• This is an abstract class and either
  • has to be subclassed by you (MyAppResource extends ResourceBundle); or
  • you use a PropertyResourceBundle; or
  • you subclass a ListResourceBundle

ResourceBundle criticisms

• In O/O terms, this is a very good example of a poorly designed class - it iterates over inheritance trees instead of leaving it to Java's inheritance mechanism, and makes assumptions about subclasses that parent classes should not do. Nevertheless, it works in a klunky sort of way
• A ResourceBundle base name is a name with all locale tags left off. e.g. MyApp_fr_FR_euro, MyApp_fr_FR, MyApp_fr and MyApp all have the same basename, MyApp
• ResourceBundle's are created by static methods that take the basename and add locale info, trying to find a class that matches
• The algorithm to find a resource bundle is well-meaning but cumbersome and a little confusing. It uses the locale given and the default locale. e.g. if you want to find a resource bundle for MyApp in the fr_CA locale and the default locale is en_US, it will try to load the following
  • MyApp_fr_CA.class
  • MyApp_fr_CA.properties This can only be handled by a properties subclass!
  • MyApp_fr.class
  • MyApp_fr.properties
  • MyApp_en_US.class
  • MyApp_en_US.properties
  • MyApp_en.class
  • MyApp_en.properties
  • MyApp.class
  • MyApp.properties
  • MyApp

PropertyResourceBundle

• This is the simplest way of handling locale-specific resources which can be handled by strings
• A property resource bundle is a text file of "key=value" pairs such as

  OKButtonLabel=Ok
  CancelButtonLabel=Cancel
  LogoFile=company_logo.gif
        

• These are stored in a file baseName_locale.properties
• The English version would be in file MyApp_en.properties

  
  OKButtonLabel=Ok
  CancelButtonLabel=Cancel
  LogoFile=company_logo.gif
        

• The Australian English version would be in file MyApp_en_AU.properties

  
  OKButtonLabel=She'll be right mate
  CancelButtonLabel=Get lost
        

  There is no need to repeat the LogoFile since it can be found by the klunky version of inheritance used by resource bundles

Getting resource bundles

• the static method ResourceBundle.getgetBundle(String baseName, Locale locale) (plus variations) can be used to get a bundle
• A resource value can be found by ResourceBundle.getObject(String key) or ResourceBundle.getString(String key)
• The set of all keys can be found by ResourceBundle.getKeys()

Resources of objects

• A properties file can only contain text strings, which can be names of files, but not objects
• If you want to specify arbitrary objects or lists of objects then you need to subclass ResourceBundle or ListResourceBundle
• See the "Java I18nN" book for this

Use

Run by java -Duser.language=en -Duser.country=AU UseBundle