Use [UnmanagedFunctionPointer]

Commit db84cc3 noted that the sample failed to run because
NativeAOT didn't know how to produce the required "stubs" to allow
a `System.Delegate` field to be marshaled.

Commit a32e244 was an attempt to "fix" this by introducing a
`JniBlittableNativeMethodRegistration` and using C#9 function pointers
to register `ManagedPeer` marshal methods instead of delegates.
While this works, this is an extensive change, and leaves various
"corner cases" in how things fit together, such as the
`JniNativeMethodRegistrationArguments` struct used as part of
`jnimarshalmethod-gen`-based method registration.

@lambdageek found an easier solution: place
`[UnmanagedFunctionPointer]` onto the delegate declaration.  This
appears to tell NativeAOT to generate the required stubs, with
significantly fewer changes.  This also could fit nicely into a future
`generator` change to place `[UnmanagedFunctionPointer]` on all the
`_JniMarshal_*` declarations (56955d9).

build-tools/jnienv-gen/Generator.cs

@@ -758,7 +758,7 @@ abstract class TypeInfo
 			{ "void*",                      new BuiltinTypeInfo ("void*",                   "IntPtr") },
 			{ "const jchar*",               new StringTypeInfo ("const jchar*") },
 			{ "const char*",                new StringTypeInfo ("const char*") },
-			{ "const JNINativeMethod*",     new BuiltinTypeInfo ("const JNINativeMethod*",  "IntPtr") },
+			{ "const JNINativeMethod*",     new BuiltinTypeInfo ("const JNINativeMethod*",  "JniNativeMethodRegistration []") },
 			{ "jobjectRefType",             new BuiltinTypeInfo ("jobjectRefType",          "JniObjectReferenceType") },
 			{ "jfieldID",                   new InstanceFieldTypeInfo ("jfieldID") },
 			{ "jstaticfieldID",             new StaticFieldTypeInfo ("jstaticfieldID") },

samples/Hello-NativeAOTFromJNI/README.md

@@ -56,6 +56,7 @@ Time Elapsed 00:00:00.83
 % (cd bin/Release/osx-x64/publish ; java -cp hello-from-java.jar:java-interop.jar com/microsoft/hello_from_jni/App)
 Hello from Java!
 Hello from .NET NativeAOT!
+String returned to Java: Hello from .NET NativeAOT!
 ```
 
 Note the use of `(cd …; java …)` so that `libHello-NativeAOTFromJNI.dylib` is

src/Java.Interop/Java.Interop.csproj

@@ -26,7 +26,7 @@
     <OutputPath>$(ToolOutputFullPath)</OutputPath>
     <DocumentationFile>$(ToolOutputFullPath)Java.Interop.xml</DocumentationFile>
     <JNIEnvGenPath>$(BuildToolOutputFullPath)</JNIEnvGenPath>
-    <LangVersion Condition=" '$(JIBuildingForNetCoreApp)' == 'True' ">11.0</LangVersion>
+    <LangVersion Condition=" '$(JIBuildingForNetCoreApp)' == 'True' ">9.0</LangVersion>
     <LangVersion Condition=" '$(LangVersion)' == '' ">8.0</LangVersion>
     <Version>$(JICoreLibVersion)</Version>
     <Standalone Condition=" '$(Standalone)' == '' ">true</Standalone>

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

@@ -250,9 +250,6 @@ public static void RegisterNatives (JniObjectReference type, JniNativeMethodRegi
 					throw new ArgumentOutOfRangeException (nameof (numMethods), numMethods,
 							$"`numMethods` must be between 0 and `methods.Length` ({methods?.Length ?? 0})!");
 				}
-				if (methods == null || numMethods == 0) {
-					return;
-				}
 #if DEBUG && NETCOREAPP
 				for (int i = 0; methods != null && i < numMethods; ++i) {
 					var m   = methods [i];
@@ -265,42 +262,8 @@ public static void RegisterNatives (JniObjectReference type, JniNativeMethodRegi
 				}
 #endif  // DEBUG && NETCOREAPP
 
-				Span<JniBlittableNativeMethodRegistration> blittableMethods
-					= stackalloc JniBlittableNativeMethodRegistration [numMethods];
-				Span<IntPtr> freeStrings
-					= stackalloc IntPtr [numMethods * 2];
-				try {
-					for (int i = 0; i < numMethods; ++i) {
-						var name    = methods [i].Name == null ? IntPtr.Zero : Marshal.StringToCoTaskMemUTF8 (methods [i].Name);
-						var sig     = methods [i].Signature == null ? IntPtr.Zero : Marshal.StringToCoTaskMemUTF8 (methods [i].Signature);
-
-						freeStrings [(i*2)+0] = name;
-						freeStrings [(i*2)+1] = sig;
-
-						blittableMethods [i] = new JniBlittableNativeMethodRegistration (
-								name,
-								sig,
-								Marshal.GetFunctionPointerForDelegate (methods [i].Marshaler)
-						);
-					}
-					RegisterNatives (type, blittableMethods);
-				}
-				finally {
-					for (int i = 0; i < freeStrings.Length; ++i) {
-						Marshal.ZeroFreeCoTaskMemUTF8 (freeStrings [i]);
-					}
-				}
-			}
+				int r   = _RegisterNatives (type, methods, numMethods);
 
-			public static unsafe void RegisterNatives (JniObjectReference type, ReadOnlySpan<JniBlittableNativeMethodRegistration> methods)
-			{
-				if (methods.Length == 0) {
-					return;
-				}
-				int r;
-				fixed (JniBlittableNativeMethodRegistration* pMethods = methods) {
-					r = _RegisterNatives (type, (IntPtr) pMethods, methods.Length);
-				}
 				if (r != 0) {
 					throw new InvalidOperationException (
 							string.Format ("Could not register native methods for class '{0}'; JNIEnv::RegisterNatives() returned {1}.", GetJniTypeNameFromClass (type), r));

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

@@ -20,70 +20,4 @@ public JniNativeMethodRegistration (string name, string signature, Delegate mars
 			Marshaler   = marshaler ?? throw new ArgumentNullException (nameof (marshaler));
 		}
 	}
-
-	public struct JniBlittableNativeMethodRegistration : IEquatable<JniBlittableNativeMethodRegistration> {
-
-		IntPtr name, signature, marshaler;
-
-		public JniBlittableNativeMethodRegistration (IntPtr name, IntPtr signature, IntPtr marshaler)
-		{
-			if (name == IntPtr.Zero)
-				throw new ArgumentNullException (nameof (name));
-			if (signature == IntPtr.Zero)
-				throw new ArgumentNullException (nameof (signature));
-			if (marshaler == IntPtr.Zero)
-				throw new ArgumentNullException (nameof (marshaler));
-
-			this.name       = name;
-			this.signature  = signature;
-			this.marshaler  = marshaler;
-		}
-
-		public JniBlittableNativeMethodRegistration (ReadOnlySpan<byte> name, ReadOnlySpan<byte> signature, IntPtr marshaler)
-		{
-			if (name.Length == 0)
-				throw new ArgumentException ("must not be empty", nameof (name));
-			if (signature.Length == 0)
-				throw new ArgumentException ("must not be empty", nameof (signature));
-			if (marshaler == IntPtr.Zero)
-				throw new ArgumentNullException (nameof (marshaler));
-
-			this.name       = FromReadOnlySpan (name);
-			this.signature  = FromReadOnlySpan (signature);
-			this.marshaler  = marshaler;
-		}
-
-		// Dodgy AF, but as the C# compiler guarantees no allocations for u8 strings,
-		// the "address" of `value` will never move, so this is "fine"…
-		// *so long as* it's *only* used for "string"u8 values.
-		// https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/proposals/csharp-11.0/utf8-string-literals#detailed-design
-		// > Since the literals would be allocated as global constants, the lifetime of the
-		// > resulting ReadOnlySpan<byte> would not prevent it from being returned or passed around to elsewhere. 
-		static unsafe IntPtr FromReadOnlySpan (ReadOnlySpan<byte> value)
-		{
-			fixed (byte* p = value)
-				return (IntPtr) p;
-		}
-
-		public override bool Equals (object? obj)
-		{
-			if (obj is JniBlittableNativeMethodRegistration other) {
-				return Equals (other);
-			}
-			return false;
-		}
-
-		public override int GetHashCode () =>
-			HashCode.Combine (name, signature, marshaler);
-
-		public bool Equals (JniBlittableNativeMethodRegistration other) =>
-			name == other.name &&
-			signature == other.signature &&
-			marshaler == other.marshaler;
-
-		public static bool operator == (JniBlittableNativeMethodRegistration lhs, JniBlittableNativeMethodRegistration rhs) =>
-			lhs.Equals (rhs);
-		public static bool operator != (JniBlittableNativeMethodRegistration lhs, JniBlittableNativeMethodRegistration rhs) =>
-			!lhs.Equals (rhs);
-	}
 }

src/Java.Interop/Java.Interop/JniType.cs

@@ -168,13 +168,6 @@ public void RegisterNativeMethods (params JniNativeMethodRegistration[] methods)
 			RegisterWithRuntime ();
 		}
 
-		public void RegisterNativeMethods (ReadOnlySpan<JniBlittableNativeMethodRegistration> methods)
-		{
-			AssertValid ();
-			JniEnvironment.Types.RegisterNatives (PeerReference, methods);
-			RegisterWithRuntime ();
-		}
-
 		public void UnregisterNativeMethods ()
 		{
 			AssertValid ();

src/Java.Interop/Java.Interop/ManagedPeer.cs

@@ -20,27 +20,18 @@ namespace Java.Interop {
 
 		static  readonly    JniPeerMembers  _members        = new JniPeerMembers (JniTypeName, typeof (ManagedPeer));
 
-		static unsafe ManagedPeer ()
+		static ManagedPeer ()
 		{
-			Span<JniBlittableNativeMethodRegistration> registrations = stackalloc JniBlittableNativeMethodRegistration [2];
-
-			delegate* unmanaged<IntPtr /* jnienv */, IntPtr /* klass */,
-					IntPtr /* n_self */, IntPtr /* n_assemblyQualifiedName */, IntPtr /* n_constructorSignature */, IntPtr /* n_constructorArguments */,
-					void> construct = &Construct;
-			registrations [0] = new JniBlittableNativeMethodRegistration (
-					"construct"u8,
-					"(Ljava/lang/Object;Ljava/lang/String;Ljava/lang/String;[Ljava/lang/Object;)V"u8,
-					(IntPtr) construct);
-
-			delegate* unmanaged<IntPtr /* jnienv */, IntPtr /* klass */,
-					IntPtr /* n_nativeClass */, IntPtr /* n_assemblyQualifiedName */, IntPtr /* n_methods */,
-					void> registerNativeMembers = &RegisterNativeMembers;
-			registrations [1] = new JniBlittableNativeMethodRegistration (
-					"registerNativeMembers"u8,
-					"(Ljava/lang/Class;Ljava/lang/String;Ljava/lang/String;)V"u8,
-					(IntPtr) registerNativeMembers);
-
-			_members.JniPeerType.RegisterNativeMethods (registrations);
+			_members.JniPeerType.RegisterNativeMethods (
+					new JniNativeMethodRegistration (
+						"construct",
+						ConstructSignature,
+						new ConstructMarshalMethod (Construct)),
+					new JniNativeMethodRegistration (
+						"registerNativeMembers",
+						RegisterNativeMembersSignature,
+						new RegisterMarshalMethod (RegisterNativeMembers))
+			);
 		}
 
 		ManagedPeer ()
@@ -57,8 +48,16 @@ public override JniPeerMembers JniPeerMembers {
 			get {return _members;}
 		}
 
+		const string ConstructSignature = "(Ljava/lang/Object;Ljava/lang/String;Ljava/lang/String;[Ljava/lang/Object;)V";
+
 		// TODO: Keep in sync with the code generated by ExportedMemberBuilder
-		[UnmanagedCallersOnly]
+		[UnmanagedFunctionPointer (CallingConvention.Winapi)]
+		delegate void ConstructMarshalMethod (IntPtr jnienv,
+				IntPtr klass,
+				IntPtr n_self,
+				IntPtr n_assemblyQualifiedName,
+				IntPtr n_constructorSignature,
+				IntPtr n_constructorArguments);
 		static void Construct (
 				IntPtr jnienv,
 				IntPtr klass,
@@ -178,7 +177,14 @@ static Type[] GetParameterTypes (string? signature)
 			return pvalues;
 		}
 
-		[UnmanagedCallersOnly]
+		const   string  RegisterNativeMembersSignature  = "(Ljava/lang/Class;Ljava/lang/String;Ljava/lang/String;)V";
+
+		[UnmanagedFunctionPointer (CallingConvention.Winapi)]
+		delegate void RegisterMarshalMethod (IntPtr jnienv,
+				IntPtr klass,
+				IntPtr n_nativeClass,
+				IntPtr n_assemblyQualifiedName,
+				IntPtr n_methods);
 		static unsafe void RegisterNativeMembers (
 				IntPtr jnienv,
 				IntPtr klass,

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

@@ -56,8 +56,7 @@ public unsafe void Dispose_Exceptions ()
 			Assert.Throws<ObjectDisposedException> (() => t.IsAssignableFrom (null));
 			Assert.Throws<ObjectDisposedException> (() => t.IsInstanceOfType (new JniObjectReference ()));
 			Assert.Throws<ObjectDisposedException> (() => t.RegisterWithRuntime ());
-			Assert.Throws<ObjectDisposedException> (() => t.RegisterNativeMethods ((JniNativeMethodRegistration[])null));
-			Assert.Throws<ObjectDisposedException> (() => t.RegisterNativeMethods (new ReadOnlySpan<JniBlittableNativeMethodRegistration> ()));
+			Assert.Throws<ObjectDisposedException> (() => t.RegisterNativeMethods (null));
 			Assert.Throws<ObjectDisposedException> (() => t.UnregisterNativeMethods ());
 
 			JniFieldInfo    jif = null;