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From "Ate Douma (JIRA)" <jetspeed-...@jakarta.apache.org>
Subject [jira] Created: (JS2-219) A new message definition and automatic translation facility: KeyedMessage.
Date Mon, 07 Mar 2005 13:56:52 GMT
A new message definition and automatic translation facility: KeyedMessage.
--------------------------------------------------------------------------

         Key: JS2-219
         URL: http://issues.apache.org/jira/browse/JS2-219
     Project: Jetspeed 2
        Type: Improvement
  Components: i18n and l10n  
    Versions: 2.0-dev/cvs    
    Reporter: Ate Douma
 Assigned to: Ate Douma 
     Fix For: 2.0-dev/cvs, 2.0-M2


I've been working on enhancements of the Login Portlet to provide better feedback to the user
about login errors.
Because the exceptions thrown by the Security layer currently use statically defined Exception
messages to distinguish the different conditions, checking which type of error occurred is
very cumbersome to do. Furthermore, in some situations the statically defined messages
are only used as templates (.e.g. a specific User/Role/Group name is postfixed) making it
even harder to compare the exceptions.

I've come up with a generic, transparent and very easy to use, i18n supporting, message definition
and automatic translation facility which also provides named constant definitions (phew, heavy
definition).

For this I created a new class org.apache.jetspeed.i18n.KeyedMessage.

As I've provided extensive documentation through javadoc I'll provide the class level description
below to explain its purpose and usage including a real ;-) example.

I've used this KeyedMessage to replace all SecurityException constant String messages with
a KeyedMessage variant. 
Furthermore, I've enhanced JetspeedException (and JetspeedRuntimeException) to recognize Exceptions
constructed using a KeyMessage, and provided a org.apache.jetspeed.exception.JetspeedExceptionMessages.properties
resource bundle (as well as a Dutch variant) containing SecurityException translations.

After I've committed these changes, the benefit of this new facility can be easily tested
as follows:
- login as admin
- switch the locale to Dutch using the Locale Portlet
- go to the UserBrowserPortlet
- select the guest user
- try to delete the guest user
- You will see a Dutch error message saying: "De gebruiker guest is beveiligd."
  (Previously, you would have been presented with: "The anonymous user is protected.")

I didn't have to change anything in the UserBrowserPorlet or UserDetailPorlet for this to
work.


A copy of the class level documentation of org.apache.jetspeed.i18n.KeyedMessage follows:
=========================================================================================
public class KeyedMessage extends java.lang.Object implements java.io.Serializable

KeyedMessage provides an automatically derived i18n message key based on its static instance
definition and can be used as comparable constant too.


Purpose
-------
With a KeyedMessage a named constant message (format) can be statically defined which automatically
translate themselves for a specific locale using an automatically derived ResourceBundle or
even a specified one.


Key derivation
--------------
Because KeyedMessages are created with a default message (format), even if no ResourceBundle
or its key is defined or can't be found, message translation is still possible.

A KeyedMessage automatically derives the ResourceBundle lookup key from its (statically defined)
instance field name using the following format:

  <containingClass.name>.<staticInstanceField.name>

The containingClass is derived at construction time by analyzing the StackTraceElements of
a thrown exception. This requires the instance to be defined as a public static field!

At first access, the key is resolved by inspecting the derived containingClass for the declared
field defining this instance.

If the KeyedMessage instance wasn't defined as public static field, the key can't be resolved
and message translation using a ResourceBundle won't be possible. Translation using the default
message will still work though. Furthermore, this instance can't be used as comparable named
constant as the equals(Object)method will always return false in this case.


Default ResourceBundle name derivation
--------------------------------------
When the key of a KeyedMessage is resolved, the default ResourceBundle name for message translation
is retrieved from the defined public static String field named "KEYED_MESSAGE_BUNDLE"defined
in its containingClass or one of its superClasses or interfaces.

If this field cannot be found, the fully qualified name of the containingClass is used.

ResourceBundle names are cached in a Map for each containingClass and only derived for the
first KeyedMessage defined in a containingClass.

Again: only resolved instances can use a ResourceBundle for message translation.


Default Locale lookup
---------------------
When a message is translated without a specified Locale, CurrentLocale.get()is used to determine
the default Locale for the current Thread.

In Jetspeed, the LocalizationValve initializes the CurrentLocale on each request. KeyedMessages
accessed within the context of an Jetspeed request therefore will always be translated using
the current user Locale with the getMessage()or toString()methods.
Default ResourceBundle lookup

If a message translation is done using the default ResourceBundle name the ResourceBundle
is retrieved using the ClassLoader of the containingClass. This means the bundle(s) must be
provided in the same context as from where the containingClass is loaded. Usually (and preferably),
this will be from the shared classpath of the webserver.


MessageFormat parameters
------------------------
MessageFormat patterns can also be used for a KeyedMessage.
With the create(Object[])method a specialized copy of a KeyedMessage instance can be created
containing the arguments to be used during message translation.

This new copy remains equals(Object)to its source and can still be used for named constant
comparison.

For simplified usage, three create(Object),create(Object, Object)and create(Object, Object,
Object)methods are provided which delegate to create(Object[])with their argument(s) transformed
into an Object array.


Extending KeyedMessage
----------------------
An statically defined KeyedMessage can be used as a "simple" named constant.
If additional metadata is required like some kind of status, level or type indication, the
KeyedMessage class can easily be extended by providing a specialized version of the create(KeyedMessage,
Object[])copy factory.


Usage
-----
KeyedMessage has been used to replace the hardcoded SecurityException String constants.
The ResourceBundle name used is defined by JetspeedException.KEYED_MESSAGE_BUNDLE which is
the superClass of SecurityException.

For a different ResourceBundle to be used for SecurityException messages a KEYED_MESSAGE_BUNDLE
field can be defined in SecurityException too, overriding the one in JetspeedException.

Example:

       public class JetspeedException extends Exception {
           public static final String KEYED_MESSAGE_BUNDLE = "org.apache.jetspeed.exception.JetspeedExceptionMessages";
           ...
    
           public String getMessage() {
                if ( keyedMessage != null ) {
                   return keyedMessage.getMessage(); // translated using current Locale and
default ResourceBundle
                }
                return super.getMessage();
           }
       }
    
       public class SecurityException extends JetspeedException {
           public static final KeyedMessage USER_DOES_NOT_EXIST = new KeyedMessage("The user
{0} does not exist.");
           ...
       }
    
       // resource file: org.apache.jetspeed.exception.JetspeedExceptionMessages_nl.properties
       org.apache.jetspeed.security.SecurityException.USER_DOES_NOT_EXIST = De gebruiker {0}
bestaat niet.
       ...
    
       public class UserManagerImpl implements UserManager {
           public User getUser(String username) throws SecurityException {
               ...
               if (null == userPrincipal) { 
                   throw new SecurityException(SecurityException.USER_DOES_NOT_EXIST.create(username));
               }
               ...
           }
           ...
       }
    
       // example get User
       try {
           User user = userManager.getUser(userName);
       } catch (SecurityException sex) {
           if ( SecurityException.USER_DOES_NOT_EXISTS.equals(sex.getKeyedMessage()) {
               // handle USER_DOES_NOT_EXISTS error
           }
       }

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