Start with org.owasp.esapi.waf.ESAPIWebApplicationFirewallFilter.
From the javadocs:
This is the main class for the ESAPI Web Application Firewall (WAF). It is a standard J2EE servlet filter that, in different methods, invokes the reading of the configuration file and handles the runtime processing and enforcing of the developer-specified rules. Ideally the filter should be configured to catch all requests (/*) in web.xml. If there are URL segments that need to be extremely fast and don't require any protection, the pattern may be modified with extreme caution.