The Mobile Developer

Enabling HDML/WML Support in Apache

In a previous column I talked about the complications that WAP and related technologies (like HDML) are going to add to your life as a website designer/developer. Short of going off and rebuilding your website using XML (see Cocoon, part of the Apache XML Project, for one way to do it), how are you going to add even minimal support for cellphones to your website? Let me show you a few ideas that work with the Apache web server.

The first thing to do, of course, is register the appropriate MIME types with your web server. The registration associates a given MIME type (used by the browser to identify the kind of content the web server is sending it) with one or more file extensions. For Apache, add these lines to your httpd.conf file (if you have root access to the web server) or to your .htaccess file:

AddType text/vnd.wap.wml .wml
AddType text/x-hdml .hdml

Notice how we've added support for both WML and HDML. Although HDML is being superceded by WML, in North America most of the browsing cellphones do not yet support WML, so whenever possible try to support both. Now you can try placing a few simple WML/HDML files on your website and you should be able to access them from a microbrowser.

What if you don't yet have any interesting WML/HDML content to put on your website but you want to let users know that it's coming, maybe by redirecting them to a precanned "under construction" message? Ideally you would do this no matter what URL is being accessed. This is actually fairly simple to do using Apache's mod_rewrite module. Using mod_rewrite you can add directives to your httpd.conf or .htaccess file that redirect requests based on the browser type and/or the list of MIME types the browser accepts. Say you want to redirect all requests from browsers that accept the text/x-hdml MIME type to a servlet. Try the following:

RewriteEngine on
RewriteBase / # only needed in .htaccess file

# First rule
RewriteCond %{REQUEST_FILENAME} ^/servlet
RewriteRule ^(.+) 		- [PT]

# Second rule
RewriteCond %{HTTP_ACCEPT} text/x-hdml
RewriteRule ^/(.*)		/servlet/HandleHDMLRequests$1 [PT]

The first rule matches all URLs that start with "/servlet" and passes them through unchanged. The next rule redirects all HDML requests to a servlet called HandleHDMLRequests. The original URL path is appended to the servlet request path and is thereby available within the servlet, making it easy for you to tailor responses using the same servlet.

If you don't want to redirect requests to a servlet (or some other dynamic mechanism -- Perl scripts would work just as well), you can return a precanned file instead:

RewriteEngine on
RewriteBase / # only needed in .htaccess file

# Pass .hdml requests through unchanged
RewriteRule ^(.*).hdml$ $1.hdml [L]

# All HDML requests go to index.hdml
RewriteCond %{HTTP_ACCEPT} text/x-hdml
RewriteRule ^(.*)$ /index.hdml [PT]

Now all requests for HDML content return /index.hdml.

Don't forget to add WML equivalents for any redirections you do, which I've left out to keep things brief. Put the WML rules ahead of the HDML rules so that browsers that support both will get WML. And before you do any of this, read the excellent URL Rewriting Guide for a good understanding of how Apache rewriting works.