Inside the new JNI layer in Expo Modules 2.0

Kolibri is a lightweight C++20 JNI library created for Expo Modules 2.0 on Android to replace fbjni and reduce overhead in communication among Kotlin, C++, and JavaScript. While JNI is required for Android’s Java-native boundary, its safety mechanisms and wrapper libraries can add significant costs through repeated environment lookups, exception checks, reference management, and object construction. Kolibri combines optional safety features with low-level control, allowing developers to retain managed references and exception handling where useful while omitting them in performance-critical paths. Its Kotlin compiler plugin passes C++ pointers directly to native methods for hybrid objects, while lifecycle scopes release resources associated with a destroyed JavaScript runtime and help prevent use-after-free errors. Rather than constructing Java objects through many JNI calls, Kolibri transfers JavaScript data to Kotlin through a shared direct binary buffer, reducing boundary-crossing work. Benchmarks reported by Expo show Kolibri matching raw JNI performance for C++-to-Java calls, making Kotlin-to-C++ hybrid-object calls 28 to 51 times faster than fbjni, and transferring JavaScript objects to Kotlin 3 to 5 times faster than JNI-based object construction, though it introduces an additional library and is intentionally specialized rather than general-purpose.

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