Improve parameter validation

[thboileau]

Initial ticket : http://restlet.tigris.org/issues/show_bug.cgi?id=221

[jlouvel]

From original ticket:

Nic Pottier sent a detailled and interesting feed-back to the list on 15 Dec 2006. We should review the API for potential improvements in the area of parameter extraction and validation.

Some pointers given by Thierry Boileau:

• Struts Validator" (based on Jakarta Commons Validation)
• Jakarta Commons Validation
• http://www.devx.com/Java/Article/29392/1954?pf=true

Other pointers:

• Hibernate Validator
• Seam Validation
• Consider merging ExtractInfo and Variable
• Consider alternative/complimentary solutions (Struts, Rails, Seam, WADL, etc.)

Some thoughts:

• current routing provides validation of the resource URI -> 404 Not Found
• the application should be able to easily restore the client view for correction if validation fails
• keep the design centered on the Resource class, target of the request, especially for request entity validation.
• the Handler class or a new Validator filter could enforce some additional validation rules, like the presence and format of some attributes, and allow custom error entities to be returned.

Interesting blog post saying that validation is preferable in the domain model than in the view as it makes it more reusable:

• Validation belongs in the domain model, not the MVC framework
• JSR 303: Bean Validation - InfoQ
• JSR 303: Bean Validation
• Grails - Validation" (using closures)
• Spring Validation Framework
• Domain Models and Validation - An Architectural Discussion

I have added a Route.validateAttribute() method to validate the presence of an attribute and the format of its value.

Paul J. Lucas has proposed a DTO class to handle validation: http://restlet.tigris.org/servlets/BrowseList?list=discuss&by=thread&from=671986