Linux: support cross-compile native library for ARM64 on x86_64 Linux (issue #899)

.github/workflows/natives.yml

@@ -36,6 +36,10 @@ jobs:
               if: matrix.os == 'ubuntu'
               run: sudo apt install libxt-dev
 
+            - name: install g++-aarch64-linux-gnu
+              if: matrix.os == 'ubuntu'
+              run: sudo apt install g++-aarch64-linux-gnu
+
             - name: Setup Java 11
               uses: actions/setup-java@v4
               with:

flatlaf-natives/flatlaf-natives-linux/README.md

@@ -5,12 +5,15 @@ This sub-project contains the source code for the FlatLaf Linux native library.
 
 The native library can be built only on Linux and requires a C++ compiler.
 
+The native library is available for following CPU architectures: `x86_64` (or
+`amd64`) and `arm64` (or `aarch64`).
+
 To be able to build FlatLaf on any platform, and without C++ compiler, the
-pre-built native library is checked into Git at
+pre-built native libraries are checked into Git at
 [flatlaf-core/src/main/resources/com/formdev/flatlaf/natives/](https://github.com/JFormDesigner/FlatLaf/tree/main/flatlaf-core/src/main/resources/com/formdev/flatlaf/natives).
 
-The native library was built on a GitHub server with the help of GitHub Actions.
+The native libraries were built on a GitHub server with the help of GitHub
-See:
+Actions. See:
 [Native Libraries](https://github.com/JFormDesigner/FlatLaf/actions/workflows/natives.yml)
 workflow. Then the produced Artifacts ZIP was downloaded and the native library
 checked into Git.
@@ -18,19 +21,27 @@ checked into Git.
 
 ## Development
 
-To build the library on Linux, some packages needs to be installed.
+To build the library on Linux, some packages needs to be installed:
+
+- `build-essential` - GCC and development tools
+- `libxt-dev` - X11 toolkit development headers
+- `g++-aarch64-linux-gnu` - GNU C++ compiler for the arm64 architecture (only on
+  x86_64 Linux for cross-compiling for arm64 architecture)
 
 
 ### Ubuntu
 
-`build-essential` contains GCC and development tools. `libxt-dev` contains the
-X11 toolkit development headers.
-
 ~~~
 sudo apt update
 sudo apt install build-essential libxt-dev
 ~~~
 
+Only on x86_64 Linux for cross-compiling for arm64 architecture:
+
+~~~
+sudo apt install g++-aarch64-linux-gnu
+~~~
+
 
 ### CentOS
 

flatlaf-natives/flatlaf-natives-linux/build.gradle.kts

@@ -45,8 +45,11 @@ tasks {
 
 		if( org.gradle.internal.os.OperatingSystem.current().isLinux ) {
 			val osArch = System.getProperty( "os.arch" )
-			if( osArch == "amd64" || osArch == "x86-64" )
+			if( osArch == "amd64" ) {
 				dependsOn( "linkReleaseX86-64" )
+				if( file( "/usr/bin/aarch64-linux-gnu-gcc" ).exists() )
+					dependsOn( "linkCrossAarch64" )
+			}
 			if( osArch == "aarch64" )
 				dependsOn( "linkReleaseAarch64" )
 		}
@@ -97,4 +100,67 @@ tasks {
 			}
 		}
 	}
+
+	if( org.gradle.internal.os.OperatingSystem.current().isLinux &&
+		System.getProperty( "os.arch" ) == "amd64" &&
+		file( "/usr/bin/aarch64-linux-gnu-gcc" ).exists() )
+	{
+		register<Exec>( "compileCrossAarch64Cpp" ) {
+			val include = layout.projectDirectory.dir( "src/main/headers" )
+			val src = layout.projectDirectory.dir( "src/main/cpp" )
+			workingDir = file( layout.buildDirectory.dir( "obj/main/release/aarch64-cross" ) )
+
+			doFirst {
+				workingDir.mkdirs()
+			}
+
+			commandLine = listOf(
+				"aarch64-linux-gnu-gcc",
+				"-c",
+				"-fPIC",
+				"-fvisibility=hidden",
+				"-O3",
+				"-I", "${javaHome}/include",
+				"-I", "${javaHome}/include/linux",
+				"-I", "$include",
+
+				"$src/ApiVersion.cpp",
+				"$src/X11WmUtils.cpp",
+			)
+		}
+
+		register<Exec>( "linkCrossAarch64" ) {
+			dependsOn( "compileCrossAarch64Cpp" )
+
+			val nativesDir = project( ":flatlaf-core" ).projectDir.resolve( "src/main/resources/com/formdev/flatlaf/natives" )
+			val libraryName = "libflatlaf-linux-arm64.so"
+			val outDir = file( layout.buildDirectory.dir( "lib/main/release/aarch64-cross" ) )
+			val objDir = file( layout.buildDirectory.dir( "obj/main/release/aarch64-cross" ) )
+
+			doFirst {
+				outDir.mkdirs()
+			}
+
+			commandLine = listOf(
+				"aarch64-linux-gnu-gcc",
+				"-shared",
+				"-Wl,-soname,$libraryName",
+				"-o", "$outDir/$libraryName",
+
+				"$objDir/ApiVersion.o",
+				"$objDir/X11WmUtils.o",
+
+				"-L${layout.projectDirectory}/lib/aarch64",
+				"-ljawt",
+			)
+
+			doLast {
+				// copy shared library to flatlaf-core resources
+				copy {
+					from( "$outDir/$libraryName" )
+					into( nativesDir )
+				}
+			}
+		}
+	}
 }

flatlaf-natives/flatlaf-natives-linux/lib/README.md

@@ -0,0 +1,4 @@
+Contains libraries used to compile FlatLaf native libraries.
+
+- `aarch64/libjawt.so` is `<jdk>/lib/libjawt.so` from Eclipse Temurin 11.0.25+9 aarch64,
+  which is required to cross build aarch64/arm64 .so on x86_64

flatlaf-natives/flatlaf-natives-windows/README.md

@@ -8,8 +8,8 @@ The native library can be built only on Windows and requires a C++ compiler.
 Tested with Microsoft Visual C++ (MSVC) 2019 and 2022 (comes with Visual Studio
 2019 and 2022).
 
-The native library is available for folloging CPU architectures: `x86_64` (or
+The native library is available for following CPU architectures: `x86_64` (or
-`amd64`), `x86` and `arm64`.
+`amd64`), `x86` and `arm64` (or `aarch64`).
 
 To be able to build FlatLaf on any platform, and without C++ compiler, the
 pre-built DLLs are checked into Git at