Add RegisterNatives overload for raw function pointers

src/Java.Interop/GlobalSuppressions.cs

@@ -67,6 +67,7 @@
 [assembly: SuppressMessage ("Performance", "CA1813:Avoid unsealed attributes", Justification = "Can't break public API", Scope = "type", Target = "~T:Java.Interop.JniValueMarshalerAttribute")]
 
 [assembly: SuppressMessage ("Performance", "CA1815:Override equals and operator equals on value types", Justification = "<Pending>", Scope = "type", Target = "~T:Java.Interop.JniNativeMethodRegistration")]
+[assembly: SuppressMessage ("Performance", "CA1815:Override equals and operator equals on value types", Justification = "<Pending>", Scope = "type", Target = "~T:Java.Interop.JniNativeMethod")]
 [assembly: SuppressMessage ("Performance", "CA1815:Override equals and operator equals on value types", Justification = "<Pending>", Scope = "type", Target = "~T:Java.Interop.JniNativeMethodRegistrationArguments")]
 [assembly: SuppressMessage ("Performance", "CA1815:Override equals and operator equals on value types", Justification = "<Pending>", Scope = "type", Target = "~T:Java.Interop.JniTransition")]
 

src/Java.Interop/Java.Interop/JniEnvironment.Types.cs

@@ -276,6 +276,29 @@ public static void RegisterNatives (JniObjectReference type, JniNativeMethodRegi
 				}
 			}
 
+			/// <summary>
+			/// Registers JNI native methods using blittable <see cref="JniNativeMethod"/> structs
+			/// with raw function pointers and UTF-8 name/signature pointers.
+			/// Calls the JNI RegisterNatives function directly without delegate marshaling.
+			/// </summary>
+			public static unsafe void RegisterNatives (JniObjectReference type, ReadOnlySpan<JniNativeMethod> methods)
+			{
+				IntPtr env = JniEnvironment.EnvironmentPointer;
+				fixed (JniNativeMethod* methodsPtr = methods) {
+#if FEATURE_JNIENVIRONMENT_JI_FUNCTION_POINTERS
+					var registerNatives = (delegate* unmanaged<IntPtr, IntPtr, JniNativeMethod*, int, int>)
+						(void*) (*((JNIEnv**)env))->RegisterNatives;
+#else
+					var registerNatives = (delegate* unmanaged<IntPtr, IntPtr, JniNativeMethod*, int, int>)
+						JniEnvironment.CurrentInfo.Invoker.env.RegisterNatives;
+#endif
+					int r = registerNatives (env, type.Handle, methodsPtr, methods.Length);
+					if (r != 0) {
+						throw new InvalidOperationException ($"Could not register native methods for class '{GetJniTypeNameFromClass (type)}'; JNIEnv::RegisterNatives() returned {r}.");
+					}
+				}
+			}
+
 			public static void UnregisterNatives (JniObjectReference type)
 			{
 				int r   = _UnregisterNatives (type);

src/Java.Interop/Java.Interop/JniNativeMethodRegistration.cs

@@ -1,6 +1,7 @@
 #nullable enable
 
 using System;
+using System.Runtime.InteropServices;
 using System.Threading;
 
 using Java.Interop;
@@ -20,4 +21,23 @@ public JniNativeMethodRegistration (string name, string signature, Delegate mars
 			Marshaler   = marshaler ?? throw new ArgumentNullException (nameof (marshaler));
 		}
 	}
+
+	/// <summary>
+	/// Blittable JNI native method registration for use with raw function pointers.
+	/// Layout matches JNI's <c>JNINativeMethod</c> struct exactly.
+	/// </summary>
+	[StructLayout (LayoutKind.Sequential)]
+	public unsafe struct JniNativeMethod
+	{
+		byte* name;
+		byte* signature;
+		IntPtr functionPointer;
+
+		public JniNativeMethod (byte* name, byte* signature, IntPtr functionPointer)
+		{
+			this.name = name;
+			this.signature = signature;
+			this.functionPointer = functionPointer;
+		}
+	}
 }

src/Java.Interop/PublicAPI.Unshipped.txt

@@ -12,3 +12,7 @@ Java.Interop.JniRuntime.JniValueManager.GetPeer(Java.Interop.JniObjectReference
 Java.Interop.JniTypeSignatureAttribute.InvokerType.get -> System.Type?
 Java.Interop.JniTypeSignatureAttribute.InvokerType.set -> void
 Java.Interop.IJavaPeerable.JniObjectReferenceControlBlock.get -> nint
+Java.Interop.JniNativeMethod
+Java.Interop.JniNativeMethod.JniNativeMethod() -> void
+Java.Interop.JniNativeMethod.JniNativeMethod(byte* name, byte* signature, nint functionPointer) -> void
+static Java.Interop.JniEnvironment.Types.RegisterNatives(Java.Interop.JniObjectReference type, System.ReadOnlySpan<Java.Interop.JniNativeMethod> methods) -> void

tests/Java.Interop-Tests/Java.Interop-Tests.csproj

@@ -53,6 +53,7 @@
     <JavaInteropTestJar Include="$(MSBuildThisFileDirectory)java\net\dot\jni\test\RenameClassDerived.java" />
     <JavaInteropTestJar Include="$(MSBuildThisFileDirectory)java\net\dot\jni\test\AndroidInterface.java" />
     <JavaInteropTestJar Include="$(MSBuildThisFileDirectory)java\net\dot\jni\test\DesugarAndroidInterface$_CC.java" />
+    <JavaInteropTestJar Include="$(MSBuildThisFileDirectory)java\net\dot\jni\test\RegisterNativesTestType.java" />
     <JavaInteropTestJar Include="$(MSBuildThisFileDirectory)java\net\dot\jni\test\SelfRegistration.java" />
     <JavaInteropTestJar Include="$(MSBuildThisFileDirectory)java\net\dot\jni\test\TestType.java" />
   </ItemGroup>

tests/Java.Interop-Tests/Java.Interop/JniTypeTest.cs

@@ -1,4 +1,5 @@
 using System;
+using System.Runtime.InteropServices;
 
 using Java.Interop;
 
@@ -182,6 +183,37 @@ public void RegisterNativeMethods ()
 				Assert.AreEqual (JniObjectReferenceType.Global, TestType_class.PeerReference.Type);
 			}
 		}
+
+		[Test]
+		public unsafe void RegisterNativeMethods_JniNativeMethod ()
+		{
+			using (var nativeClass = new JniType ("net/dot/jni/test/RegisterNativesTestType")) {
+				Span<JniNativeMethod> methods = stackalloc JniNativeMethod [1];
+				fixed (byte* namePtr = "add"u8)
+				fixed (byte* sigPtr = "(II)I"u8) {
+					methods [0] = new JniNativeMethod (namePtr, sigPtr,
+						(IntPtr) (delegate* unmanaged<IntPtr, IntPtr, int, int, int>) &NativeAdd);
+					JniEnvironment.Types.RegisterNatives (nativeClass.PeerReference, methods);
+				}
+
+				// Call the native method from Java to verify registration worked
+				var ctor = JniEnvironment.InstanceMethods.GetMethodID (nativeClass.PeerReference, "<init>", "()V");
+				var obj = JniEnvironment.Object.NewObject (nativeClass.PeerReference, ctor);
+				try {
+					var addMethod = JniEnvironment.InstanceMethods.GetMethodID (nativeClass.PeerReference, "add", "(II)I");
+					var args = stackalloc JniArgumentValue [2];
+					args [0] = new JniArgumentValue (3);
+					args [1] = new JniArgumentValue (4);
+					int result = JniEnvironment.InstanceMethods.CallIntMethod (obj, addMethod, args);
+					Assert.AreEqual (7, result);
+				} finally {
+					JniObjectReference.Dispose (ref obj);
+				}
+			}
+		}
+
+		[UnmanagedCallersOnly]
+		static int NativeAdd (IntPtr jnienv, IntPtr self, int a, int b) => a + b;
 	}
 }
 

tests/Java.Interop-Tests/java/net/dot/jni/test/RegisterNativesTestType.java

@@ -0,0 +1,5 @@
+package net.dot.jni.test;
+
+public class RegisterNativesTestType {
+	public native int add (int a, int b);
+}