<p>What could the best strategy for writing validation layer for mid-enterprise level business application built on Spring 2.5</p>

<p>I know that Spring provides facility where we can implement Validator interface and write validation logic in validate method. But this will be restricted to only web requests coming through spring controller.</p>

<p>I would like to develop the validation framework which can be utilized during web-services calls.</p>

<p>In other words, the framework can remain and be called independently without the need of implementing Validator interface and then too it can be automatically integrated into Spring MVC flow.</p>

<p>Hope you get my point.</p>

The Spring Validation framework can be used outside of Spring MVC. What WebServices Stack are you using? If you are using Spring-WS (Spring's Web Services stack) they have special instructions on how to set up the validator here:
http://static.springframework.org/spring-ws/sites/1.5/reference/html/server.html#d0e2313
If you are using some other stack, it is probably easier to implement something for that stack (or find one) that will use Spring's validation framework.