http://blog.sforce.com/sforce/2010/05/url-rewriting-for-your-customizing-your-site-urls.html
 
 
global class ApexClassName implements Site.UrlRewriter {
 
  global PageReference mapRequestUrl(PageReference externalUrl) {//}
 
   global List<PageReference> generateUrlFor(List<PageReference> myForcedotcomUrls) {//...}
 
}
 
Note that a developer may not need all of the methods and may choose to return a null. 
 
Here is the implementation of our the global Apex class that maps the URLs from our example: 
 
global class myRewriter implements Site.UrlRewriter {
 
 //Variables to represent the friendly URLs for pages
 
 String DIRECTORY = '/friendly/';
 
 //Variables to represent my custom Visualforce pages that display page information
 
 String VISUALFORCE_PAGE = '/page?pageid=';
 
 // The first global method for mapping external URL to an internal one
 
 global PageReference mapRequestUrl(PageReference myFriendlyUrl){
 
    String url = myFriendlyUrl.getUrl();
 
    if(url.startsWith(DIRECTORY)){
 
       String name = url.substring(DIRECTORY.length(),url.length());
 
       //Select the ID of the page that matches the name from the URL
 
       Page__c site_page = [select id from Page__c where name =:name LIMIT 1];
 
       //Construct a new page reference in the form of my Visualforce page
 
       return new PageReference(VISUALFORCE_PAGE + site_page.id);
 
    }
 
    //If the URL isn't in the form of a cmasforce page, continue with the request
 
    return null;
 
  }
 
  // The second global method for mapping internal Ids to URLs
 
  global List<PageReference> generateUrlFor(List<PageReference> mySalesforceUrls){
 
    //A list of pages to return after all the links have been evaluated
 
    List<PageReference> myFriendlyUrls = new
 
    List<PageReference>();
 
    for(PageReference mySalesforceUrl : mySalesforceUrls){
 
      //Get the URL of the page
 
      String url = mySalesforceUrl.getUrl();
 
      //If this looks like a page that needs to be mapped, transform it
 
      if(url.startsWith(VISUALFORCE_PAGE)){
 
        //Extract the ID from the query parameter
 
        String id= url.substring(VISUALFORCE_PAGE.length(), url.length());
 
        //Query for the name of the cmsforce page to put in the URL
 
        Page__c site_page2 = [select name from Page__c where id =:id LIMIT 1];
 
        //Construct the new URL
 
        myFriendlyUrls.add(new PageReference(DIRECTORY+ site_page2.name));
 
     }  
 
     else {
 
       //If this doesn't start like an cmsforce page, don't do any transformations
 
       myFriendlyUrls.add(mySalesforceUrl);
 
     }
 
  }
 
  //Return the full list of pages
 
  return myFriendlyUrls;
 
  }
 
}