import hudson.Extension;
import hudson.model.Descriptor;
import hudson.security.SecurityRealm;
import hudson.util.FormValidation;
import org.kohsuke.stapler.DataBoundConstructor;
import org.kohsuke.stapler.QueryParameter;
public final class XSecurityRealm extends SecurityRealm {
    public final String v;
    @DataBoundConstructor public XSecurityRealm(String v) {
        this.v = v;
    }
    @Override public SecurityComponents createSecurityComponents() {
        throw new UnsupportedOperationException();
    }
    @Extension public static final class DescriptorImpl extends Descriptor<SecurityRealm> {
        @Override public String getDisplayName() {
            return "X";
        }
        public FormValidation doCheckV(@QueryParameter String v) {
            return FormValidation.error(new IllegalStateException("Processing " + v), "Could not process " + v);
        }
    }
}

<?xml version="1.0" encoding="UTF-8"?>
<?jelly escape-by-default='true'?>
<j:jelly xmlns:j="jelly:core" xmlns:f="/lib/form">
    <f:entry field="v" title="Some value">
        <f:textbox/>
    </f:entry>
</j:jelly>