Using SiteMesh for page modification, not just decoration

Did you know that SiteMesh contains an HTML Parser that can be used for page/HTML modificiation - not just decoration? In a mailing list message from May 2007, Joe Walnes explains how this is possible. In this post, Alex "the_mindstorm" asks if it's possible to add onclick event handlers to <a href> tags that are external to the current site. Here's how:

First, extend SiteMesh's HTML parser and add a new tag rule for processing <a> tags.

public class MyParser extends HTMLPageParser {

    // override default rulest
    protected void addUserDefinedRules(State html, PageBuilder page) {
        super.addUserDefinedRules(html, page);

        // Add a rule.
        // Rules allow the parser to do custom things when encountering HTML tags.
        html.addRule(new LinkRewritingRule()); // see below
    }
}

class LinkRewritingRule extends BasicRule {

    public LinkRewritingRule() {
        super("a"); // Handles all <a> tags.
    }

    public void process(Tag tag) { // Called when parser encounters <a>
        CustomTag newTag = new CustomTag(tag); // Allows tag modification
       
        // modify href attribute
        boolean caseSensitive = false; // allow HREF or href or HrEf
        String oldHref = newTag.getAttributeValue("href", caseSensitive);        
        if (oldHref != null) {
            String newHref = // do something;
            newTag.setAttributeValue("href", caseSensitive, newHref);
        }
       
            // write the new tag bag to the page
            newTag.writeTo (currentBuffer());
        }
    }
}

Then edit sitemesh.xml and update the <page-parser> tag to point to your new parser class. Pretty easy eh? Read Joe's full post for more tips and tricks.