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[Hello-NativeAOTFromJNI] Add NativeAOT sample
Instead of a managed app creating a Java VM and calling into Java, do the opposite: have a Java app call into Native Library built using NativeAOT, using `[UnmanagedCallersOnly]` to provide the entry points.
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‎samples/Hello-NativeAOTFromJNI/Hello-NativeAOTFromJNI.csproj‎

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<Project Sdk="Microsoft.NET.Sdk">
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<PropertyGroup>
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<TargetFramework>net8.0</TargetFramework>
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<RootNamespace>Hello_NativeAOTFromJNI</RootNamespace>
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<ImplicitUsings>enable</ImplicitUsings>
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<Nullable>enable</Nullable>
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<PublishAot>true</PublishAot>
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<AllowUnsafeBlocks>true</AllowUnsafeBlocks>
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<NativeLib>Shared</NativeLib>
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</PropertyGroup>
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<ItemGroup>
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<ProjectReference Include="..\..\src\Java.Interop\Java.Interop.csproj"
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AdditionalProperties="Standalone=True"
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/>
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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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</ItemGroup>
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<ItemGroup>
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<Content Include="$(OutputPath)hello-from-java.jar" CopyToPublishDirectory="PreserveNewest" />
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</ItemGroup>
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<ItemGroup>
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<HelloNativeAOTFromJNIJar Include="$(MSBuildThisFileDirectory)java\**\*.java" />
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</ItemGroup>
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<Import Project="Hello-NativeAOTFromJNI.targets" />
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</Project>

‎samples/Hello-NativeAOTFromJNI/Hello-NativeAOTFromJNI.targets‎

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<Project>
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<Target Name="BuildNativeAOTFromJNIJar"
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BeforeTargets="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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<_Source Include="@(HelloNativeAOTFromJNIJar->Replace('%5c', '/'))" />
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</ItemGroup>
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<WriteLinesToFile
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File="$(IntermediateOutputPath)_java_sources.txt"
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Lines="@(_Source)"
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Overwrite="True"
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/>
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<ItemGroup>
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<_JavacOpt Include="$(_JavacSourceOptions)" />
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<_JavacOpt Include="-d &quot;$(IntermediateOutputPath)h-classes&quot; " />
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<_JavacOpt Include="-classpath &quot;$(OutputPath)java-interop.jar&quot; " />
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<_JavacOpt Include="&quot;@$(IntermediateOutputPath)_java_sources.txt&quot;" />
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<_JavacOpt Include="-h &quot;$(IntermediateOutputPath)h-classes&quot; " />
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</ItemGroup>
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<Exec Command="&quot;$(JavaCPath)&quot; @(_JavacOpt, ' ')" />
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<Delete Files="$(IntermediateOutputPath)_java_sources.txt" />
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<Exec Command="&quot;$(JarPath)&quot; cf &quot;$(OutputPath)hello-from-java.jar&quot; -C &quot;$(IntermediateOutputPath)h-classes&quot; ." />
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</Target>
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<Target Name="_NativeLibRequiresLibPrefix"
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AfterTargets="Publish">
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<Copy
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Condition=" '$(OS)' != 'Windows_NT' "
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SourceFiles="$(PublishDir)$(AssemblyName).dylib"
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DestinationFiles="$(PublishDir)lib$(AssemblyName).dylib"
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/>
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<Copy SourceFiles="$(OutputPath)hello-from-java.jar" DestinationFolder="$(PublishDir)" />
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</Target>
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<Target Name="RunJavaSample">
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<ItemGroup>
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<_Classpath Include="hello-from-java.jar" />
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<_Classpath Include="java-interop.jar" />
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</ItemGroup>
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<PropertyGroup>
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<_CPSep Condition=" '$(OS)' == 'Windows_NT' ">;</_CPSep>
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<_CPSep Condition=" '$(_CPSep)' == '' ">:</_CPSep>
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<_CP>@(_Classpath, '$(_CPSep)')</_CP>
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</PropertyGroup>
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<Exec
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Command="&quot;$(JavaPath)&quot; -classpath &quot;$(_CP)&quot; com/microsoft/hello_from_jni/App"
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WorkingDirectory="$(PublishDir)"
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/>
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</Target>
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</Project>

‎samples/Hello-NativeAOTFromJNI/JNIEnvInit.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 JNIEnvInit
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{
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// static JniRuntime? runtime;
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[UnmanagedCallersOnly (EntryPoint="JNI_OnLoad")]
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static int JNI_OnLoad (IntPtr vm, IntPtr reserved)
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{
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try {
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// runtime = new JniRuntime (null);
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// return runtime.JniVersion;
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return (int) JniVersion.v1_2;
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}
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catch (Exception e) {
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Console.Error.WriteLine ($"JNI_OnLoad: {e}");
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return 0;
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}
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}
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[UnmanagedCallersOnly (EntryPoint="JNI_OnUnload")]
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static void JNI_Onload (IntPtr vm, IntPtr reserved)
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{
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// runtime?.Dispose ();
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}
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// symbol name from `$(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 void sayHello (IntPtr jnienv, IntPtr klass)
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{
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Console.WriteLine ($"Hello from .NET NativeAOT!");
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}
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}

‎samples/Hello-NativeAOTFromJNI/README.md‎

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# Hello From JNI
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[JNI][0] supports *two* modes of operation:
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1. Native code creates the JVM, e.g. via [`JNI_CreateJavaVM()`][1], or
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2. The JVM already exists, and calls [`JNI_OnLoad()`][2] when loading a native library.
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Java.Interop samples and unit tests rely on the first approach.
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.NET Android / neé Xamarin.Android is the second approach.
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Bring an example of the latter into a Java.Interop sample, using [NativeAOT][3].
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## Building
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Building a native library with NativeAOT requires a Release configuration build.
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For in-repo use, that means that xamarin/Java.Interop itself needs to be built in
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Release configuration:
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```sh
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% dotnet build -c Release -t:Prepare
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% dotnet build -c Release
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```
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Once Java.Interop itself is built, you can build the sample:
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```sh
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% dotnet build -c Release
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% dotnet publish -r osx-x64
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```
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The resulting native library contains the desired symbols:
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```sh
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% nm bin/Release/osx-x64/publish/Hello-NativeAOTFromJNI.dylib | grep ' S '
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00000000000cb710 S _JNI_OnLoad
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00000000000cb820 S _JNI_OnUnload
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00000000000cb840 S _Java_com_microsoft_hello_1from_1jni_NativeAOTInit_sayHello
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```
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Use the `RunJavaSample` target to run Java, which will run
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`System.loadLibrary("Hello-NativeAOTFromJNI")`, which will cause the
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NativeAOT-generated `libHello-NativeAOTFromJNI.dylib` to be run:
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```sh
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% dotnet build -c Release -r osx-x64 -t:RunJavaSample -v m --nologo --no-restore
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Hello from Java!
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Hello from .NET NativeAOT!
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Build succeeded.
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0 Warning(s)
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0 Error(s)
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Time Elapsed 00:00:00.83
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```
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[0]: https://docs.oracle.com/javase/8/docs/technotes/guides/jni/spec/jniTOC.html
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[1]: https://docs.oracle.com/javase/8/docs/technotes/guides/jni/spec/invocation.html#creating_the_vm
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[2]: https://docs.oracle.com/javase/8/docs/technotes/guides/jni/spec/invocation.html#JNJI_OnLoad
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[3]: https://github.com/dotnet/samples/blob/main/core/nativeaot/NativeLibrary/README.md

‎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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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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NativeAOTInit.sayHello();
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}
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}

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

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package com.microsoft.hello_from_jni;
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class NativeAOTInit {
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static {
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System.loadLibrary("Hello-NativeAOTFromJNI");
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}
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public static native void sayHello();
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}

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