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JSignal

Hotswap

One of JSignal’s main features is it’s built in support for hotswap development, as in, certain code changes can be applied to the user interface without having to restart a running application. This feature is supported via Espresso (recommended) and Hotswap Agent on a DCEVM patched JVM. Both of these JVMs provide the unique ability to apply complex classfile changes at runtime while also having a plugin API for customizing reload behavior. When hotswap is enabled for JSignal, it creates and maintains a runtime tree of instantiated component objects. After a class reload occurs, it will reactivley rerun the render function of any component that class is instatiated in while also invalidating some internal layout and paint caches.

Setup

Intellij Gradle Settings

To set up hotswap in Intellij Idea with Gradle, change the following:

  • set Build, Execution, Deployment > Build Tools > Gradle > Build and run using & Run tests using to IntelliJ IDEA
  • disable Build, Execution, Deployment > Debugger > HotSwap > Build project before reloading classes

Application Code

The application code also needs to be changed in order to add hotswap component instrumentation. The easiest way to do this is by changing the app start code to look like this: UiThread.start(() -> UiUtil.provideHotswapInstrumentation(() -> { ... }));. The library also provides a UiUtil.conditionallyProvideHotswapInstrumentation which will check only enable hotswap if the system variable jsignal.hotswap is set to true.

Also known as Java on Truffle, this is an implementation of Java using GraalVM’s truffle language framework. It also supports advanced hotswap capabilities, and a has a nice plugin API. Instructions for setting this up in Intellij Idea CE are below:

  • Download and use the GraalVM Espresso JVM
  • If using Intellij and Gradle, set Build, Execution, Deployment > Build Tools > Gradle > Gradle JVM to a normal JDK like OpenJDK, as it will not function correctly on Espresso
  • Add the following VM command line arguments:
-truffle -XX:+IgnoreUnrecognizedVMOptions

Hotswap Agent

This method of hotwap support uses the DCEVM JVM patch and Hotswap Agent, which is fundamentally a framework built around Java’s instrumentation API. Instructions for setting this up in Intellij Idea CE are below:

-XX:+AllowEnhancedClassRedefinition -XX:HotswapAgent=external -javaagent:<path/to/jar>

Limitations

One of the most apparent limitations is the loss of local state for children of reloaded components. While the signal based reactive model and incremental node tree in JSignal has a number of benefits, this is one of the unfortunate downsides. By contrast, frameworks that use some sort of diffing algorithm generally do not suffer from this issue.