Last week I managed to get hot reloading working in KorGE. This is pretty exciting because it allows to write code and see it reflected visually almost immediately, something really appreciated in graphics development, and this will allow to iterate much faster and learn KorGE faster too. The first PoC was implemented super quickly, and I was really impressed how fast the PoC was implemented. Still I faced some issues, and I iterated a bit before getting something really responsive. This post explains the issues I faced and the ideas for the future to improve it further.

The first thing I did was to determine how to do reloading of classes, and where to plug it in the application, so it worked. One option was to do some ClassLoader magic, but then I saw a thing called JavaAgent that supports instrumenting classes. So I decided to create a Java Agent as a first try. The java agent is something you specify either attaching to a running JVM, or before it starts passing it as an argument to the vm. So I created a gradle module generating a jar, that I could pass later when running the sandbox module, and in the KorGE gradle plugin.

Initially I in the java agent I did that the agent reloaded a specific class every several seconds, and then executed a code in the application triggering a reload event. Then inside scene containers I subscribed to that event and forced reloading the same scene again. So ran the application with the agent, and then manually triggered s kotlin build. It worked pretty well!

With that working, I decided to run in a separate terminal the gradle command to build the JVM classes in continuous mode: ./gradlew -t or ./gradlew —continuous. Then I tried and effectively it refreshed after 10 seconds or so. That was too much time, and only working for a single class. The first thing I noticed is that in continuous mode, it took too much time for gradle to detect source code changes on my Mac machine, so it seems that the old problem of the JVM doing polling for file watching was still there. I investigated and know what? It seems they fixed it recently by using a separate code for that instead of the JVM implementation! I was using 7.4.2 that was the latest version at the time of writing, so I tried a 7.5-rc-2. And the fix was there! Know the changes were detected much quicker. Still it took a lot of time each run to configure all the modules, so it was still pretty slow. Also, the every-5-seconds class refresh was adding time for getting fast feedback.

So after solving this, it was time to convert the proof of concept into something more generic. So I changed the timer, into a proper file watching using the java default implementation, I passed the paths where the .class files were being generated to the agent via gradle, and I used a high refreshing rate. But still pooling, so it was better but still it was taking a couple of seconds to detect modified classes. Also, the gradle configuration in continuous mode was slow and took a few seconds each time it recompiled, though I expected final projects with fewer modules to run faster, so I ignored that for now.

One of the problems I faced with doing the watching is that some files were modified sometimes a couple of times, or that sometimes the file watcher notifier detected some files in one batch and others later. So I had to wait an arbitrary amount of time, like half a second to avoid refreshing twice or having to wait even more.

I was not satisfied with that. We were introducing latency to several steps, and I wanted to reduce the feedback time to the very minimum. To avoid that I thought that I could notify the app when the compilation finished and do the refreshing just by then. To simplify that, I decided to try UDP. Sending a packet with the time before compilation, so it could detect files changed in that time without file watching. Like pooling but at the right time and with s not that big amount of folders/files since it only included classes from the final module. And not requiring something super complex.

When I tried UDP I was getting a null pointer exception every time I tried inside the gradle daemon. I don’t really know the reason, maybe security, but was not able to get it working. So I used the standard JVM HTTP server and HTTP client, to create an HTTP server on the JVM agent, and to notify the server from the gradle build by doing an HTTP request. It worked. And times were better. Now the only main performance issue was configuration time in the continuous mode. I investigated and tried configuration cache. When I tried it I got several errors that prevented it from working (like using the project instance) after configuration time. After some adjustments I got it working! And after that the times were pretty good.

Now it works awesomely, though when applying the JetPack Compose plugin, compilation times are not that great. Maybe it doesn’t support incremental yet?. In any case, not a big issue, since it will eventually improve.

Now the problem is that the classes reloaded by the java agent, cannot add or remove new methods or kotlin properties, so it works nicely to do stuff in an existing method but beyond that you have to relaunch the app. So this approach will need some work.

I will have to investigate the class loader approach. My main idea is to create two levels of class loaders. One that load classes from the com.soywiz.korge package, and one in front loading everything else and try to close or recreate a new class loader while keeping KorGE classes running and alive. Not sure which problems or performance issues we will face, but this should allow to add new/remove methods and properties. So maybe a combination of both approaches could work: getting fast feedback when updating an existing method, and a slightly worse reloading performance when adding methods, but still reloading without relaunching.

And after that, we could investigate serializing the state to reuse it between refreshes. Maybe we can annotate the properties we want to persist to avoid crashes from persisting everything