Resource preloading

Improves application startup time

CheerpJ runs Java applications and applets directly in the browser (no local Java install needed). To do this, CheerpJ loads runtime components (JavaScript modules) while the app executes. By default, these modules are loaded on demand, only when the app needs them.

This “load as needed” approach keeps the initial download small (typically 10–20MB for many apps) and everything is cached by the browser for faster repeat visits. The tradeoff is that CheerpJ can’t know ahead of time which modules your specific app will need, so it loads them one after another at runtime.

To reduce startup time in production, CheerpJ lets you preload the modules your app commonly uses, so they can download in parallel as the app starts.

What resource preloading does

• Without preloading: CheerpJ fetches runtime resources sequentially as they become necessary.
• With preloading: CheerpJ starts downloading a pre-defined set of resources immediately, allowing parallel downloads and faster startup.

When to use it

• You’re deploying an app to production and want faster first load
• Your users often hit a cold cache (first visit, cleared cache, new device)
• You want to avoid runtime “stalls” caused by sequential resource fetching

Overview of the workflow

1. Profile which runtime resources your app actually uses using cjGetRuntimeResources
2. Pass those resources to cheerpjInit via preloadResources

Step 1: Profile used runtime resources

1. Run your application normally with CheerpJ.
2. Once the app has loaded, open your browser’s JavaScript console (often Ctrl+Shift+I).
3. Run:

cjGetRuntimeResources();

The output will look like:

{"/lts/file1.jar":[int, int, ...], "/lts/file2.jar":[int,int, ...]}

If you can’t see the full output in the console, use:

document.write(cjGetRuntimeResources());

The console may wrap the output in quotes ("). If it does, ignore the quotes.

Step 2: Enable preloading in your integration

Update your cheerpjInit call to include the preloadResources option.

Example:

cheerpjInit({
  preloadResources: {
    "/lts/file1.jar": [int, int, ...],
    "/lts/file2.jar": [int, int, ...]
  }
});

Note that profiling the resources with cjGetRuntimeResources and providing the result to preloadResources must be done in separate sessions. First, profile the application in one session to generate the list of resources. Then, in subsequent sessions, pass this list to cheerpjInit to enable preloading.

What changes after enabling preloading

When preloadResources is enabled, CheerpJ can download multiple resources in parallel with the program’s execution, which can significantly reduce startup time, especially on first load.