Benchmarks

Criterion-based benchmarks measuring FFI call overhead for the generated Java bindings. Uses the same uniffi-fixture-benchmarks fixture and benchmark structure as upstream uniffi-rs, making results directly comparable with Kotlin, Python, and Swift.

Prerequisites

Rust toolchain and JDK 21+ - all provided by nix develop.

Running

cargo bench                                  # full suite
cargo bench -- call-only                     # filter by name
cargo bench -- --save-baseline before        # save a Criterion baseline
cargo bench -- --load-baseline before        # compare against a saved baseline

How It Works

cargo bench runs benches/benchmarks.rs which:

1. Builds the uniffi-fixture-benchmarks cdylib (opt-level 3)
2. Generates Java bindings and packages them into a jar
3. Compiles benches/bindings/RunBenchmarks.java
4. Runs the Java process, which calls Benchmarks.runBenchmarks("java", callback)

Criterion runs inside the Rust fixture library. The Java side implements TestCallbackInterface and provides timing for function-call benchmarks via runTest() (using System.nanoTime()). Callback benchmarks are timed directly by Criterion on the Rust side.

Benchmark Groups

• function-calls - Java calling Rust functions across 12 type categories
• callbacks - Rust calling Java callback methods across the same 12 categories

Results

HTML reports are written to target/criterion/. Raw data persists across runs for regression detection.

Comparison: Java FFM vs JNA and upstream languages

April 1, 2026 on an M4 Max 2024 MacBook Pro, 36GB memory. Java FFM uses JDK 25 with the Foreign Function & Memory API. Java JNA uses JDK 21. Kotlin, Python, and Swift results are from upstream uniffi-rs main.

Function Calls (foreign code calling Rust)

Test Case	Java FFM	Java JNA	Kotlin	Python	Swift
call-only	4.3 ns	1.83 us	2.89 us	792 ns	165 ns
primitives	4.5 ns	1.62 us	3.27 us	1.39 us	191 ns
strings	324 ns	11.46 us	15.08 us	9.06 us	929 ns
large-strings	3.69 us	15.63 us	21.69 us	12.33 us	1.28 us
records	371 ns	11.62 us	15.29 us	18.66 us	2.54 us
enums	334 ns	14.47 us	15.41 us	14.22 us	2.04 us
vecs	581 ns	13.14 us	15.36 us	18.02 us	4.55 us
hash-maps	638 ns	12.91 us	15.48 us	23.15 us	6.39 us
interfaces	530 ns *	6.07 us	105.60 us	4.21 us	409 ns
trait-interfaces	532 ns *	11.98 us	150.70 us	4.30 us	477 ns
nested-data	1.90 us	12.56 us	15.64 us	77.09 us	20.86 us
errors	682 ns	4.51 us	6.44 us	3.20 us	646 ns

Callbacks (Rust calling foreign code)

Test Case	Java FFM	Java JNA	Kotlin	Python	Swift
call-only	55 ns	1.98 us	2.76 us	492 ns	121 ns
primitives	59 ns	2.66 us	3.40 us	804 ns	166 ns
strings	348 ns	13.76 us	15.69 us	8.17 us	811 ns
large-strings	3.84 us	17.47 us	20.55 us	10.77 us	1.60 us
records	449 ns	11.55 us	15.95 us	14.26 us	2.86 us
enums	406 ns	11.17 us	14.93 us	11.04 us	2.47 us
vecs	593 ns	11.11 us	15.02 us	17.95 us	5.01 us
hash-maps	723 ns	12.36 us	15.72 us	21.39 us	7.16 us
interfaces	964 ns *	7.44 us	408.65 us	4.19 us	448 ns
trait-interfaces	790 ns *	7.49 us	361.69 us	4.33 us	552 ns
nested-data	1.93 us	19.77 us	16.29 us	67.20 us	24.54 us
errors	638 ns	6.37 us	7.98 us	4.35 us	574 ns

* Interface benchmarks have high variance due to GC pauses; min values are ~250-470ns.