As a Java EE developer, I sometimes envy how fast it’s possible to see the result of a code change in a running application with interpreted languages like PHP or JavaScript. With Java, it’s always necessary to rebuild the source code in a bytecode, which can be then safely updated only by restarting the whole application. And all developers know that restoring the desired state of the application after a fresh restart takes time and is tedious.

Many developers know that JRebel can help a lot with updating the code on the fly. There’s been a lot of effort put into it to support all sorts of code and resource changes and refresh them with virtually any Java framework used by the application. But the downside is that it’s pretty expensive for a casual developer, doing just some hacking on his/her own or working on a non-commercial project. I have some experience with JRebel and I liked it a lot, but I was using it on a commercial project where I didn’t pay for the license. A while ago I’ve come across an opensource alternative called HotswapAgent, which has worked very well for me for my personal Java EE projects. I’m going to write up how I got it running in my IDE and my Java EE server of choice – Payara Server. (more…)