Use of ActionErrors in struts

• Struts uses the ActionErrors instance returned by validate() to display the error messages on the redisplayed form.
• ActionErrors (note the plural) is very much like a HashMap, and it is used to store error messages, indexed by error keys. The error message is not a String, but an instance of ActionMessage.
• ActionMessages are added to an ActionErrors instance using the add() function:
• public void add(String key, ActionMessage message)
• eg. new ActionMessage("reg.error.password.mismatch")
• For one thing, it’s obvious that we’re not passing in the actual error message! Instead, we’re passing in a key to an error message, stored in a properties file accessible to Struts.
• Properties files are just text files, which contain key/value pairs. In this case, the key is reg.error.password.mismatch, and the value might be The passwords you keyed in don't match!.
• # Properties file for Registration webapp
• # Error messages: reg.error.userid.missing=The user id is missing.
• reg.error.userid.exists = The user id exists. Choose another.
• reg.error.userid.bad = Use only alphanumerics for the userid.
• reg.error.password.mismatch = The passwords you keyed in don't match! reg.error.password.long = The password is too long!
• reg.error.password.short = The password is too short!
• The string argument of the ActionMessage constructor must be a key on a properties file.
• This might seem restrictive, but there’s a very good reason for this. This is how Struts makes it easy to localize an application.
• For example, if you wanted a German version of the Registration webapp, all you’d have to do would be hand over this properties file (registration.properties) for translation. You’d save the translated version as registration_de.properties, and bundle it with your webapp.
• German users would see the German version of the Registration webapp and everyone else would see the default English version. Localizing a webapp was never so easy!