Java Hot Code Replacement

I finally started taking advantage of a JVM feature that’s been around for almost a decade: hot code replacement. HCR was introduced in 1.4 as part of the Java Platform Debugger Architecture (JPDA). It provides the ability for code to be updated seamlessly in a running Java program. This can really cut down on the development cycle because the program doesn’t need to be started over from the beginning for every little change.

This is not actually a new concept or development style for me. Updating live, running code is fundamentally how Lisp programs are developed. This is how I wrote my Emacs web server. I opened a socket to listen on 8080, which originally did nothing but accept connections, and built up a web server behind the socket without ever taking it down. It’s be unusual to develop Lisp any other way.

Eclipse has directly supported HCR for some years now. I had heard about it, but never investigated the issue until I saw Markus “Notch” Persson using it while coding his Ludum Dare entry (source) on a live-stream. It was really neat to see it in use on a graphical program.

If you know anything about my professional habits, you know I’m not a fan of giant IDEs like Eclipse. I hack Java with a combination of Ant and Emacs, with my own custom extensions. So any hotswap solution has to be built on that. Luckily, I found this great Ant extension: hotswap. It provides a hotswap task for performing HCR.

It relies on Ant to prepare replacements and determine which files are to be swapped. They provide an example target on their website demonstrating how to set it up. Personally, I think it’s a sloppy way to do it. It writes a timestamp to a string (the only Ant data structure available for this), redundantly performs compilation, then parses the time string to compare it to all of the files, picking out new ones.

There’s actually an modified selector exactly for this type of job. Here’s my solution (as seen in my sample-java-project),

<target name="hotswap" depends="compile">
  <taskdef name="hotswap" classname="dak.ant.taskdefs.Hotswap"/>
  <hotswap verbose="true" port="9000">
    <fileset dir="${build.classes.dir}" includes="**/*.class">

The modified filter is not timestamp based. It creates a file containing the hash of all of your class files. Trivial changes, such as to comments or whitespace, will not trigger for replacement. Because that’s taken care of without the timestamp business, no need to write out the compile task again. We can just call the original as a dependency of this target.

An easy way to try this out for yourself now is with the hotswap-demo branch of my chess engine. (This branch is special because it forces the GUI to redraw every second, causing changes to take effect immediately.) Check out that branch, run the program with the run target, then in change the colors of the board in paintComponent() — change LIGHT to Color.WHITE and DARK to Color.GRAY, for example. Then, without stopping the program, run the hotswap target. Ant will inject the new code into the running program and the board will change before your eyes.

I look forward to making good use of this in the future. Expect it to be a typical Ant target in all of my Java projects from now on.

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Chris Wellons