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"Full(er)" sample
What `Hello-NativeAOTFromJNI` previously did was quite minimal: 1. Use `[UnmanagedCallersOnly]` to provide a `JNI_OnLoad()` method which 2. Initialized the Java.Interop runtime, allowing 3. An `[UnmanagedCallersOnly]`-provided `Java_…` method which is called from Java. All quite low level, all miles away from .NET Android. Expand the sample to: 1. Contain a `Java.Lang.Object` subclass, which contains a `[JavaCallable]` method. 2. Call `jcw-gen` to generate Java Callable Wrappers for (1), containing the `[JavaCallable]` method. 3. Call `jnimarshalmethod-gen` to generate marshal methods for (1), as NativeAOT doesn't support System.Reflection.Emit. 4. Instantiate (1) *from Java*, and invoke the `[JavaCallable]` method. *Now* we're (kinda) getting something that looks like .NET Android. But first, we need to make that *work*: Update `Java.Interop.Tools.JavaCallableWrappers` so that it will emit `native` method declarations for `[JavaCallable]` methods, not just method overrides and `[Export]` methods. Update `Java.Interop.Tools.Expressions` so that the `_JniMarshal_*` delegate types have `[UnmanagedFunctionPointer(CallingConvention.Winapi)]`, as this is what allows NativeAOT to emit appropriate "stubs"; see also da9f188. Update `Java.Interop.Tools.Expressions.ExpressionAssemblyBuilder` to no longer attempt to "remove and fixup" `System.Private.CoreLib`. So long as `ExpressionAssemblyBuilder` output is *only* used in "completed" apps (not distributed in NuGet packages in some "intermediate" form), referencing `System.Private.CoreLib` is "fine". Additionally, trying to remove `System.Private.CoreLib` broke things when adding `[UnmanagedFunctionPointer]`, as `CallingConvention` could not be resolved, resulting in `jnimarshalmethod-gen` erroring out with: error JM4006: jnimarshalmethod-gen: Unable to process assembly '/Volumes/Xamarin-Work/src/xamarin/Java.Interop/samples/Hello-NativeAOTFromJNI/bin/Debug/Hello-NativeAOTFromJNI.dll' Failed to resolve System.Runtime.InteropServices.CallingConvention Mono.Cecil.ResolutionException: Failed to resolve System.Runtime.InteropServices.CallingConvention at Mono.Cecil.Mixin.CheckedResolve(TypeReference self) at Mono.Cecil.SignatureWriter.WriteCustomAttributeEnumValue(TypeReference enum_type, Object value) … (This is because `CallingConvention` is in `System.Runtime.InteropServices.dll`, which isn't referenced.) We could "fix" this by explicitly adding a reference to `System.Runtime.InteropServices.dll`, but this is just one of an unknown number of corner cases. Give up for now. Update `jnimarshalmethod-gen` assembly location probing: it was given the *full assembly name* of `Java.Base`: # jonp: resolving assembly: Java.Base, Version=7.0.0.0, Culture=neutral, PublicKeyToken=null …and failing to find `Java.Base.dll`, because it was looking for `Java.Base, Version=7.0.0.0, Culture=neutral, PublicKeyToken=null.dll`. Oops. Use `AssemblyName` to parse the string and extract out the assembly name., so that `Java.Base.dll` is probed for and found. With all that…it still fails: % (cd bin/Release/osx-x64/publish ; java -cp hello-from-java.jar:java-interop.jar com/microsoft/hello_from_jni/App) Hello from Java! C# init() Hello from .NET NativeAOT! String returned to Java: Hello from .NET NativeAOT! Exception in thread "main" com.xamarin.java_interop.internal.JavaProxyThrowable: System.IO.FileNotFoundException: Could not resolve assembly 'Hello-NativeAOTFromJNI'. at System.Reflection.TypeNameParser.ResolveAssembly(String) + 0x97 at System.Reflection.TypeNameParser.GetType(String, ReadOnlySpan`1, String) + 0x32 at System.Reflection.TypeNameParser.NamespaceTypeName.ResolveType(TypeNameParser&, String) + 0x17 at System.Reflection.TypeNameParser.GetType(String, Func`2, Func`4, Boolean, Boolean, Boolean, String) + 0x99 at Java.Interop.ManagedPeer.RegisterNativeMembers(IntPtr jnienv, IntPtr klass, IntPtr n_nativeClass, IntPtr n_assemblyQualifiedName, IntPtr n_methods) + 0x103 at com.xamarin.java_interop.ManagedPeer.registerNativeMembers(Native Method) at example.ManagedType.<clinit>(ManagedType.java:15) at com.microsoft.hello_from_jni.App.main(App.java:13) `App.main()` has `new example.ManagedType()`, which hits the `ManagedType` static constructor of: public /* partial */ class ManagedType extends java.lang.Object implements com.xamarin.java_interop.GCUserPeerable { /** @hide */ public static final String __md_methods; static { __md_methods = "n_GetString:()Ljava/lang/String;:__export__\n" + ""; com.xamarin.java_interop.ManagedPeer.registerNativeMembers (ManagedType.class, "Example.ManagedType, Hello-NativeAOTFromJNI", __md_methods); } } The `ManagedPeer.registerNativeMembers()` call is what is needed to register the native `ManagedPeer.getString()` method, so that it can be called. This is good. (Though `__md_methods` containing *anything* is not desired, but that's a different problem.) `ManagedPeer.RegisterNativeMembers()` is given the assembly-qualified name `Example.ManagedType, Hello-NativeAOTFromJNI`, and tries to: Type.GetType ("Example.ManagedType, Hello-NativeAOTFromJNI", throwOnError: true); …which then proceeds to throw, because in NativeAOT *there are no assemblies*, and thus `Type.GetType()` *cannot work*. Oops. Thus, the only way to make something remotely like .NET Android infrastructure work is to *require* the use of `Java_…` native method names and `[UnmanagedCallersOnly]` on marshal methods. (In .NET Android parlance, the experimental `$(AndroidEnableMarshalMethods)`=True is required.)
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‎samples/Hello-NativeAOTFromJNI/App.cs‎

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using System.Runtime.InteropServices;
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using Java.Interop;
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namespace Hello_NativeAOTFromJNI;
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static class App {
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// symbol name from `$(IntermediateOutputPath)/h-classes/com_microsoft_hello_from_jni_NativeAOTInit.h`
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[UnmanagedCallersOnly (EntryPoint="Java_com_microsoft_hello_1from_1jni_App_sayHello")]
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static IntPtr sayHello (IntPtr jnienv, IntPtr klass)
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{
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var s = $"Hello from .NET NativeAOT!";
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Console.WriteLine (s);
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var h = JniEnvironment.Strings.NewString (s);
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var r = JniEnvironment.References.NewReturnToJniRef (h);
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JniObjectReference.Dispose (ref h);
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return r;
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}
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}

‎samples/Hello-NativeAOTFromJNI/Hello-NativeAOTFromJNI.csproj‎

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<ProjectReference Include="..\..\src\Java.Runtime.Environment\Java.Runtime.Environment.csproj"
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AdditionalProperties="Standalone=True"
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/>
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<ProjectReference Include="..\..\src\Java.Base\Java.Base.csproj" />
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<ProjectReference Include="..\..\src\Java.Interop.Export\Java.Interop.Export.csproj" />
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</ItemGroup>
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<ItemGroup>

‎samples/Hello-NativeAOTFromJNI/Hello-NativeAOTFromJNI.targets‎

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<Project>
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<PropertyGroup>
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<DotnetToolPath>$(DOTNET_HOST_PATH)</DotnetToolPath>
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<UtilityOutputFullPath>$(MSBuildThisFileDirectory)..\..\bin\Debug-net7.0</UtilityOutputFullPath>
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</PropertyGroup>
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<Target Name="_CreateJavaCallableWrappers"
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Condition=" '$(TargetPath)' != '' "
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BeforeTargets="BuildNativeAOTFromJNIJar"
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Inputs="$(TargetPath)"
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Outputs="$(IntermediateOutputPath)java\.stamp">
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<RemoveDir Directories="$(IntermediateOutputPath)java" />
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<MakeDir Directories="$(IntermediateOutputPath)java" />
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<ItemGroup>
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<!-- I can't find a good way to trim the trailing `\`, so append with `.` so we can sanely quote for $(_Libpath) -->
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<_RefAsmDirs Include="@(ReferencePathWithRefAssemblies->'%(RootDir)%(Directory).'->Distinct())" />
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</ItemGroup>
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<PropertyGroup>
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<_JcwGen>"$(UtilityOutputFullPath)/jcw-gen.dll"</_JcwGen>
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<_Target>--codegen-target JavaInterop1</_Target>
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<_Output>-o "$(IntermediateOutputPath)/java"</_Output>
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<_Libpath>@(_RefAsmDirs->'-L "%(Identity)"', ' ')</_Libpath>
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</PropertyGroup>
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<Exec Command="$(DotnetToolPath) $(_JcwGen) &quot;$(TargetPath)&quot; $(_Target) $(_Output) $(_Libpath)" />
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<Touch Files="$(IntermediateOutputPath)java\.stamp" AlwaysCreate="True" />
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</Target>
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<Target Name="_AddMarshalMethods"
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Condition=" '$(TargetPath)' != '' "
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Inputs="$(TargetPath)"
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Outputs="$(IntermediateOutputPath).added-marshal-methods"
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AfterTargets="_CreateJavaCallableWrappers">
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<ItemGroup>
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<!-- I can't find a good way to trim the trailing `\`, so append with `.` so we can sanely quote for $(_Libpath) -->
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<_RefAsmDirs Include="@(ReferencePathWithRefAssemblies->'%(RootDir)%(Directory).'->Distinct())" />
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</ItemGroup>
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<PropertyGroup>
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<_JnimarshalmethodGen>"$(UtilityOutputFullPath)/jnimarshalmethod-gen.dll"</_JnimarshalmethodGen>
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<_Verbosity>-v -v --keeptemp</_Verbosity>
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<_Libpath>-L "$(TargetDir)" @(_RefAsmDirs->'-L "%(Identity)"', ' ')</_Libpath>
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<!-- <_Output>-o "$(IntermediateOutputPath)/jonp"</_Output> -->
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</PropertyGroup>
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<Exec Command="$(DotnetToolPath) $(_JnimarshalmethodGen) &quot;$(TargetPath)&quot; $(_Verbosity) $(_Libpath)" />
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<Touch Files="$(IntermediateOutputPath).added-marshal-methods" AlwaysCreate="True" />
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</Target>
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<Target Name="BuildNativeAOTFromJNIJar"
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BeforeTargets="Build"
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AfterTargets="Build"
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Inputs="@(HelloNativeAOTFromJNIJar)"
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Outputs="$(OutputPath)hello-from-java.jar">
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<MakeDir Directories="$(IntermediateOutputPath)h-classes" />
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<ItemGroup>
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<_JcwSource Include="$(IntermediateOutputPath)java\**\*.java" />
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</ItemGroup>
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<ItemGroup>
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<_Source Include="@(_JcwSource->Replace('%5c', '/'))" />
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<_Source Include="@(HelloNativeAOTFromJNIJar->Replace('%5c', '/'))" />
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</ItemGroup>
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<WriteLinesToFile

‎samples/Hello-NativeAOTFromJNI/JNIEnvInit.cs‎ renamed to ‎samples/Hello-NativeAOTFromJNI/JavaInteropRuntime.cs‎

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namespace Hello_NativeAOTFromJNI;
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static class JNIEnvInit
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static class JavaInteropRuntime
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{
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static JniRuntime? runtime;
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runtime?.Dispose ();
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}
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// symbol name from `$(IntermediateOutputPath)/h-classes/com_microsoft_hello_from_jni_NativeAOTInit.h`
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[UnmanagedCallersOnly (EntryPoint="Java_com_microsoft_hello_1from_1jni_NativeAOTInit_sayHello")]
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static IntPtr sayHello (IntPtr jnienv, IntPtr klass)
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[UnmanagedCallersOnly (EntryPoint="Java_com_microsoft_java_1interop_JavaInteropRuntime_init")]
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static void init ()
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{
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var s = $"Hello from .NET NativeAOT!";
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Console.WriteLine (s);
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var h = JniEnvironment.Strings.NewString (s);
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var r = JniEnvironment.References.NewReturnToJniRef (h);
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JniObjectReference.Dispose (ref h);
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return r;
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Console.Error.WriteLine ($"C# init()");
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}
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}

‎samples/Hello-NativeAOTFromJNI/ManagedType.cs‎

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namespace Example;
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using Java.Interop;
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[JniTypeSignature ("example/ManagedType")]
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class ManagedType : Java.Lang.Object {
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public ManagedType ()
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{
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}
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[JavaCallable ("getString")]
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public Java.Lang.String GetString ()
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{
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return new Java.Lang.String ("Hello from C#, via Java.Interop!");
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}
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}

‎samples/Hello-NativeAOTFromJNI/java/com/microsoft/hello_from_jni/App.java‎

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package com.microsoft.hello_from_jni;
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import com.microsoft.java_interop.JavaInteropRuntime;
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import example.ManagedType;
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class App {
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public static void main(String[] args) {
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System.out.println("Hello from Java!");
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String s = NativeAOTInit.sayHello();
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JavaInteropRuntime.init();
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String s = sayHello();
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System.out.println("String returned to Java: " + s);
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ManagedType mt = new ManagedType();
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System.out.println("mt.getString()=" + mt.getString());
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}
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static native String sayHello();
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}

‎samples/Hello-NativeAOTFromJNI/java/com/microsoft/hello_from_jni/NativeAOTInit.java‎

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This file was deleted.

‎samples/Hello-NativeAOTFromJNI/java/com/microsoft/java_interop/JavaInteropRuntime.java‎

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package com.microsoft.java_interop;
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public class JavaInteropRuntime {
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static {
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System.loadLibrary("Hello-NativeAOTFromJNI");
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}
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private JavaInteropRuntime() {
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}
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public static native void init();
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}

‎src/Java.Interop.Tools.Expressions/Java.Interop.Tools.Expressions/ExpressionAssemblyBuilder.cs‎

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using System.Collections.Generic;
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using System.Diagnostics;
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using System.Linq.Expressions;
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using System.Runtime.InteropServices;
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using System.Text;
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using Java.Interop;
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);
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delegateDef.BaseType = DeclaringAssemblyDefinition.MainModule.ImportReference (typeof (MulticastDelegate));
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var ufpCtor = typeof (UnmanagedFunctionPointerAttribute).GetConstructor (new[]{typeof (CallingConvention)});
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var ufpCtorRef = DeclaringAssemblyDefinition.MainModule.ImportReference (ufpCtor);
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var ufpAttr = new CustomAttribute (ufpCtorRef);
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ufpAttr.ConstructorArguments.Add (
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new CustomAttributeArgument (
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DeclaringAssemblyDefinition.MainModule.ImportReference (typeof (CallingConvention)),
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CallingConvention.Winapi));
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delegateDef.CustomAttributes.Add (ufpAttr);
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var delegateCtor = new MethodDefinition (
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name: ".ctor",
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attributes: MethodAttributes.Public | MethodAttributes.HideBySig | MethodAttributes.SpecialName | MethodAttributes.RTSpecialName,
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var module = DeclaringAssemblyDefinition.MainModule;
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var c = new MemoryStream ();
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DeclaringAssemblyDefinition.Write (c);
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DeclaringAssemblyDefinition.Write (path);
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c.Position = 0;
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if (KeepTemporaryFiles) {
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c.Position = 0;
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}
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#if false
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// `Failed to resolve System.Runtime.InteropServices.CallingConvention`
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// because `System.Runtime.InteropServices` is not referenced.
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Logger (TraceLevel.Verbose, $"# jonp: ---");
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var rp = new ReaderParameters {
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module.AssemblyReferences.Remove (selfRef);
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}
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newAsm.Write (path);
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#endif // false
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}
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static AssemblyNameReference GetSystemRuntimeReference ()

‎src/Java.Interop.Tools.JavaCallableWrappers/Java.Interop.Tools.JavaCallableWrappers/JavaCallableWrapperGenerator.cs‎

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var baseRegisteredMethod = GetBaseRegisteredMethod (minfo);
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if (baseRegisteredMethod != null)
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AddMethod (baseRegisteredMethod, minfo);
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else if (minfo.AnyCustomAttributes (typeof(ExportFieldAttribute))) {
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else if (minfo.AnyCustomAttributes ("Java.Interop.JavaCallableAttribute")) {
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AddMethod (null, minfo);
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HasExport = true;
170+
} else if (minfo.AnyCustomAttributes (typeof(ExportFieldAttribute))) {
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AddMethod (null, minfo);
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HasExport = true;
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} else if (minfo.AnyCustomAttributes (typeof (ExportAttribute))) {
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return new ExportAttribute (name) {ThrownNames = thrown, SuperArgumentsString = superArgs};
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}
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418+
ExportAttribute ToExportAttributeFromJavaCallableAttribute (CustomAttribute attr, IMemberDefinition declaringMember)
419+
{
420+
var name = attr.ConstructorArguments.Count > 0 ? (string) attr.ConstructorArguments [0].Value : declaringMember.Name;
421+
return new ExportAttribute (name);
422+
}
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internal static ExportFieldAttribute ToExportFieldAttribute (CustomAttribute attr)
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{
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return new ExportFieldAttribute ((string) attr.ConstructorArguments [0].Value);
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IEnumerable<ExportAttribute> GetExportAttributes (IMemberDefinition p)
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{
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return GetAttributes<ExportAttribute> (p, a => ToExportAttribute (a, p));
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return GetAttributes<ExportAttribute> (p, a => ToExportAttribute (a, p))
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.Concat (GetAttributes<ExportAttribute> (p, "Java.Interop.JavaCallableAttribute", a => ToExportAttributeFromJavaCallableAttribute (a, p)));
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}
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static IEnumerable<ExportFieldAttribute> GetExportFieldAttributes (Mono.Cecil.ICustomAttributeProvider p)
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static IEnumerable<TAttribute> GetAttributes<TAttribute> (Mono.Cecil.ICustomAttributeProvider p, Func<CustomAttribute, TAttribute?> selector)
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where TAttribute : class
460470
{
461-
return p.GetCustomAttributes (typeof (TAttribute))
471+
return GetAttributes (p, typeof (TAttribute).FullName, selector);
472+
}
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static IEnumerable<TAttribute> GetAttributes<TAttribute> (Mono.Cecil.ICustomAttributeProvider p, string attributeName, Func<CustomAttribute, TAttribute?> selector)
475+
where TAttribute : class
476+
{
477+
return p.GetCustomAttributes (attributeName)
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.Select (selector)
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.Where (v => v != null)
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.Select (v => v!);

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