Craig Pardey

I’ve recently started trialling JRebel at work. JRebel is a piece of software that hot-swaps your code so that changes made in your IDE are reflected in your application server without requiring a restart.

The benefits of this idea are fairly obvious. I no longer have to wait 1 – 2 minutes for JBoss to restart each time I correct a spelling mistake etc. The time saved by not restarting probably make the product worth the $US149 licensing fee.

Then there is the significant reduction in context switching. I no longer make a code change, restart JBoss, read the New York Times for 2 minutes, forget what I changed, then try to test it. Instead I make the change and test it straight away. This can only mean fewer bugs (and perhaps a less-informed developer but people assume that nerds live under rocks anyway, so that’s ok).

Once JRebel was installed my JBoss startup time went from 1:40 to 3:45, but I figured that would be OK because I would only have to do that once a day. JRebel did most of what I expected it to, and I was happily coding away in my new no-context-switching life until I changed a Tapestry binding and the code wasn’t hot-swapped even though I had the Tapestry plug-in enabled.

This forced a JBoss restart (or several, to be honest) and I had lost all the benefit of JRebel. Later I discovered that I needed to combine a few things to get JRebel to work nicely with Tapestry. Here’s what I had to do:

1. Enable the Tapestry 4 plugin (-Drebel.tapestry4_plugin=true)
2. Enable Tapestry reset service (-Dorg.apache.tapestry.enable-reset-service=true)

Now whenever I change a Tapestry file, I simply invoke the Tapestry reset service (http://localhost:8080/myApp/app?service=reset) and then refresh the page. Voila! The code gets hot-swapped.

I think this technique works because invoking Tapestry’s reset service clears its cache and forces it to reload the classes which have been updated by JRebel.