What causes a ClassNotFoundException when calling Android Java code from Unity?

The Java class name passed to AndroidJavaClass is wrong, the containing library was not included in the Gradle build, or R8/ProGuard minification stripped the class from the release build.

How do I catch this when it only happens for some players?

Capture it automatically from the player's device with the full stack trace, the device and OS, the build, and a breadcrumb trail of what they did. That turns a vague complaint into a specific, reproducible issue you can fix without owning the hardware it happens on.

How to Fix “AndroidJavaException: java.lang.ClassNotFoundException” in Unity

Quick answer: Verify the fully qualified class name, ensure the AAR/plugin is in the build, and add a ProGuard keep rule so minification does not strip classes you access only via reflection.

Calls into Android Java that work in a debug build and explode in release are the signature of minification stripping. This error names the missing class — the fix depends on why it is missing. Here is the sequence.

How to fix it

1. Check the fully qualified name

The string in new AndroidJavaClass("com.example.sdk.Bridge") must match package + class exactly. Inner classes use $: com.example.Outer$Inner.

2. Confirm the library ships in the build

The .aar or .jar must be under Assets/Plugins/Android (or referenced via a Gradle dependency) and enabled for the Android platform in the importer.

3. Add keep rules for release

Unity invokes Java by reflection, so R8 sees the class as unused. Enable a custom ProGuard file and add -keep class com.example.sdk.** { *; } for every reflected class.

4. Test the release path before shipping

Build with minification exactly as your release pipeline does and exercise every native call — this class of crash never appears in development builds.

Catching the ones you can't reproduce

The hardest version of this to fix is the one you can't reproduce — it only happens on a player's hardware, OS, driver, or save state, under conditions that simply aren't present on your machine. A report that says “it crashed” or “it froze” gives you nothing to act on, so the bug survives release after release while quietly costing you players.

Automatic error capture closes that gap. Each failure arrives with its full stack trace, the device and OS, the build number, and a breadcrumb trail of what the player did right before it broke, so even a failure you have never seen becomes a specific, reproducible issue. Fold identical failures into one signature ranked by how many players each hits, and your worklist sorts itself worst-first instead of arriving as a stream of vague complaints.

This is where a tool like Bugnet earns its place. Its SDK captures every Unity error automatically with the full stack trace plus device, OS, memory, build, and game-state context, folds duplicates into one grouped issue with an occurrence count, and ties each to the build it first appeared on — so you fix the problem that hurts the most players first and confirm it is gone when its signature disappears from the next release.

Reproduce it once with full context and the fix writes itself. The hunt is the expensive part.