Register JNI natives via blittable JniNativeMethod

[simonrozsival]

Summary

Register JNI native methods by marshalling into the blittable JniNativeMethod struct and calling the existing RegisterNatives(JniObjectReference, ReadOnlySpan<JniNativeMethod>) overload, instead of invoking the JNI RegisterNatives function pointer with a non-blittable managed array (JniNativeMethodRegistration[]).

This avoids relying on the runtime to marshal an array of a non-blittable struct across a delegate* unmanaged<> call — a path that is currently miscompiled by crossgen2 under composite ReadyToRun + PGO and corrupts the registered method names.

Refs dotnet/android#11633

Background: what breaks

The default (non-trimmable / llvm-ir typemap) registration funnels through:

// JniEnvironment.Types
public static void RegisterNatives (JniObjectReference type, JniNativeMethodRegistration [] methods, int numMethods)
    => _RegisterNatives (type, methods, numMethods);   // generated invoker:
// delegate* unmanaged<IntPtr, jobject, JniNativeMethodRegistration[], int, int> RegisterNatives

JniNativeMethodRegistration is non-blittable (string Name; string Signature; Delegate Marshaler). Passing JniNativeMethodRegistration[] through the delegate* unmanaged<> requires the runtime to marshal the array element-by-element.

ManagedPeer..cctor, AndroidTypeManager, ManagedTypeManager, and every JniType.RegisterNativeMethods caller funnel through this single method (_RegisterNatives has exactly one caller), so fixing it here fixes all of them.

Root cause (a crossgen2 / runtime regression)

dotnet/runtime #126911 ("Move built-in array marshalling to managed", 2026-05-01) moved array-of-struct marshalling from native C++ into the managed generic System.StubHelpers.StructureMarshaler<T> : IArrayElementMarshaler<T, StructureMarshaler<T>>. Its element converter is an intrinsic with a blittable-only fallback body:

// src/coreclr/System.Private.CoreLib/src/System/StubHelpers.cs
// "Non-blittable structs should have a custom IL body generated with the marshaling logic."
[Intrinsic]
private static void ConvertToUnmanagedCore (ref T managed, byte* unmanaged, ref CleanupWorkListElement? cleanupWorkList)
    => SpanHelpers.Memmove (ref *unmanaged, ref Unsafe.As<T,byte>(ref managed), (nuint)sizeof(T));

For non-blittable T, the VM generates a real per-field marshalling body at JIT time (StructMarshalStubs::TryGenerateStructMarshallingMethod, dllimport.cpp). crossgen2 has no equivalent, so when PGO/MIBC marks the marshaller hot and crossgen2 precompiles the shared canonical (__Canon) instantiation, it emits the literal Memmove fallback — a raw blit of the managed object references into the native struct.

Disassembly of the precompiled canonical converter from a composite-R2R + MIBC image:

StructureMarshaler`1<__Canon>.ConvertToUnmanagedCore:
    ldr  x0, [x1]     ; first 8 bytes of the managed struct = the Name string REFERENCE
    str  x0, [x2]     ; stored straight into the native JNINativeMethod.name  ← no string→char* marshalling
    ret

So JNI receives a managed string object pointer where it expects a UTF-8 char*, the method name is garbage, and registration fails with NoSuchMethodError during startup (e.g. MauiApplication, net.dot.jni.ManagedPeer).

This only manifests when crossgen2 precompiles the marshaller (MIBC marks it hot). Without a profile the JIT compiles it and registration is correct — which is why the same app works without a startup MIBC.

Why this is the right fix

• It eliminates the non-blittable array marshalling at this call site entirely. The blittable RegisterNatives(ReadOnlySpan<JniNativeMethod>) path marshals names/signatures to UTF-8 ourselves (Marshal.StringToCoTaskMemUTF8) and passes a blittable JniNativeMethod* — there is no StructureMarshaler<T> involved, so it is correct regardless of the crossgen2 bug.
• It matches the trimmable type-map path, which already used the blittable overload and was therefore never affected.
• It is the single registration chokepoint, so it fixes ManagedPeer, AndroidTypeManager, and ManagedTypeManager together.

We do not want to marshal arrays of non-blittable types across delegate* unmanaged<> / P/Invoke boundaries given this runtime limitation. A scan of the shipped runtime path (Java.Interop + Mono.Android) shows RegisterNatives was the only such site: it is the only invoker function pointer with an array parameter, the only non-blittable-array native call; all JValue[] call sites already pin (fixed (JValue* …)) and pass a blittable pointer.

Changes

• Rework JniEnvironment.Types.RegisterNatives(JniObjectReference, JniNativeMethodRegistration[], int) to marshal Name/Signature to unmanaged UTF-8 and a function pointer, then dispatch to the blittable RegisterNatives(JniObjectReference, ReadOnlySpan<JniNativeMethod>) overload (no more _RegisterNatives / non-blittable delegate* unmanaged<> call).
• Preserve JNI error behavior in the blittable overload: it now observes and rethrows (clearing) any pending Java exception from JNIEnv::RegisterNatives() — e.g. NoSuchMethodError — and guards type.IsValid, matching the generated _RegisterNatives wrapper it replaces. This benefits both the array-based path and the trimmable type-map path (which previously could leave a pending exception in the JNIEnv).
• Add regression tests for the JniNativeMethodRegistration[] path to validate correct UTF-8 marshaling and function pointer conversion, covering both stack-allocated (≤32 methods) and heap-allocated (>32 methods) code paths.

Implementation notes

• UTF-8 name/signature buffers are allocated with Marshal.StringToCoTaskMemUTF8 and freed in a finally block with Marshal.ZeroFreeCoTaskMemUTF8, guarding against IntPtr.Zero to prevent crashes with uninitialized entries. GC.KeepAlive (methods) keeps the marshaler delegates alive across the native call.
• Null Marshaler delegates are explicitly checked with an actionable error message that includes the array index and method signature, rather than relying on the generic ArgumentNullException from Marshal.GetFunctionPointerForDelegate.
• Marshal.GetFunctionPointerForDelegate is called inline within the already-RequiresDynamicCode-annotated method. This JniNativeMethodRegistration[] path runs only on JIT-capable runtimes (MonoVM/CoreCLR); NativeAOT registers through the trimmable type map with statically-compiled function pointers and never reaches it.

Verification

Reproduced and fixed in an isolated net11.0 console app (no Java.Interop/Android types): a non-blittable struct[] passed through a delegate* unmanaged<> to a real native function corrupts only under composite R2R + MIBC (when the call site is hot); the blittable equivalent is correct under all configurations (JIT, plain R2R, composite R2R, composite R2R + MIBC).

Tracking

• Underlying runtime regression introduced by Move built-in array marshalling to managed and make olevariant.cpp Windows-only runtime#126911; durable fix belongs in crossgen2 (expand the marshalling intrinsic for non-blittable __Canon, or defer it to the JIT). This PR is the Java.Interop-side fix and is correct independent of the runtime change.

[Copilot]

Pull request overview

This PR updates Java.Interop’s JNI native-method registration to avoid passing a non-blittable JniNativeMethodRegistration[] through a delegate* unmanaged<> call path, instead marshalling into blittable JniNativeMethod entries and using the existing RegisterNatives(JniObjectReference, ReadOnlySpan<JniNativeMethod>) overload. It also improves AOT analyzer signaling for APIs that rely on dynamic code and clarifies test suppressions.

Changes:

• Reworked JniEnvironment.Types.RegisterNatives(JniObjectReference, JniNativeMethodRegistration[], int) to marshal names/signatures to unmanaged UTF-8 and dispatch via the blittable overload.
• Added pending-Java-exception surfacing/clearing to the blittable RegisterNatives overload and added RequiresDynamicCode/IL3050 suppressions where appropriate.
• Updated tests and ManagedPeer to suppress IL3050 where they intentionally exercise non-AOT-compatible registration APIs.

Show a summary per file

File	Description
tests/Java.Interop-Tests/Java.Interop/JniTypeTest.cs	Adds IL3050 suppressions for tests that intentionally call non-AOT-compatible native registration APIs.
src/Java.Interop/Java.Interop/ManagedPeer.cs	Suppresses IL3050 on ManagedPeer static ctor due to non-AOT-compatible native registration usage.
src/Java.Interop/Java.Interop/JniType.cs	Marks RegisterNativeMethods(JniNativeMethodRegistration[]) as RequiresDynamicCode for clearer AOT analyzer behavior.
src/Java.Interop/Java.Interop/JniEnvironment.Types.cs	Implements marshalling to blittable JniNativeMethod and improves exception handling for RegisterNatives.

Copilot's findings

• Files reviewed: 4/4 changed files
• Comments generated: 2

[jonathanpeppers]

/review

[github-actions]

✅ Java.Interop PR Reviewer completed successfully!

[github-actions]

🤖 ⚠️ Needs Changes

Thorough, well-documented fix. The root-cause analysis (crossgen2 emitting the blittable Memmove fallback of the StructureMarshaler<__Canon> intrinsic when it precompiles the hot, non-blittable array marshaller for a delegate* unmanaged<> call) is convincing, and rerouting through the blittable RegisterNatives (ReadOnlySpan<JniNativeMethod>) overload is the right call. I independently verified:

• ✅ The hand-rolled marshalling is correct: StringToCoTaskMemUTF8 ↔ ZeroFreeCoTaskMemUTF8 are correctly paired, unmanagedStrings.Clear () + the finally correctly free partially-allocated buffers if an allocation fails mid-loop, and GC.KeepAlive (methods) is correctly placed after the native call to keep the marshaler delegates rooted across it (long-term rooting stays with JniType.methods / ManagedPeer._members, unchanged).
• ✅ [RequiresDynamicCode] coverage is complete — ManagedPeer, both tests, and ReflectionJniTypeManager (class-level annotation) are all covered, so no new IL3050 escapes the build.
• ✅ GetExceptionForLastThrowable () does call ExceptionClear (), so the new "surface and clear" behavior matches the comment and is a genuine improvement for the trimmable type-map path too.
• ✅ CI (dotnet.java-interop) is green; mergeable_state: blocked reflects review requirements, not a failing check.

Main ask: this fixes a method-name corruption bug (refs dotnet/android#11633) but adds no test that exercises the reworked JniNativeMethodRegistration[] marshalling loop directly — the existing tests either pass zero methods or hit the already-correct blittable overload. A targeted regression test would lock in the fix (details inline).

Findings: 1 ⚠️ warning (missing regression test), 1 💡 suggestion (null-Marshaler behavioral change). No correctness or safety defects found in the new code.

Generated by Java.Interop PR Reviewer for issue #1474 · 585.1 AIC · ⌖ 47.3 AIC · ⊞ 34.6K
_{Comment /review to run again}

[simonrozsival]

@copilot address code review

addressed