Demo: library catalog with jextract JNI callbacks, official Swift Android SDK

Demo/Package.swift

@@ -16,6 +16,14 @@ let package = Package(
     dependencies: [
         .package(
             path: "../"
+        ),
+        .package(
+            url: "https://github.com/swiftlang/swift-java.git",
+            branch: "main"
+        ),
+        .package(
+            url: "https://github.com/swift-android-sdk/swift-android-native.git",
+            from: "2.1.0"
         )
     ],
     targets: [
@@ -25,12 +33,43 @@ let package = Package(
                 .product(
                     name: "AndroidKit",
                     package: "Android"
-                )
+                ),
+                "CatalogBridge"
             ],
             path: "./app/src/main/swift",
             swiftSettings: [
               .swiftLanguageMode(.v5)
             ]
+        ),
+        .target(
+            name: "CatalogBridge",
+            dependencies: [
+                .product(
+                    name: "SwiftJava",
+                    package: "swift-java"
+                ),
+                .product(
+                    name: "AndroidKit",
+                    package: "Android"
+                ),
+                .product(
+                    name: "AndroidContext",
+                    package: "swift-android-native"
+                )
+            ],
+            path: "./app/src/main/swift-bridge/CatalogBridge",
+            exclude: [
+              "swift-java.config"
+            ],
+            swiftSettings: [
+              .swiftLanguageMode(.v5)
+            ],
+            plugins: [
+              .plugin(
+                name: "JExtractSwiftPlugin",
+                package: "swift-java"
+              )
+            ]
         )
     ]
 )

Demo/app/build.gradle.kts

@@ -4,6 +4,93 @@ plugins {
     alias(libs.plugins.kotlin.compose)
 }
 
+// ---------------------------------------------------------------------------
+// swift-java (jextract, JNI mode) integration
+//
+// The Swift package (SwiftAndroidApp + CatalogBridge) is cross-compiled to
+// native `.so`s with the official Swift Android SDK; the `JExtractSwiftPlugin`
+// emits the Java JNI bindings as a side effect of `swift build`. Gradle then
+// adds the generated Java + SwiftKitCore runtime sources to the source set and
+// stages every required `.so` into `jniLibs`.
+// ---------------------------------------------------------------------------
+
+val swiftPackageRoot: File = rootProject.projectDir
+val androidTriple = "aarch64-unknown-linux-android28"
+val swiftAbi = "arm64-v8a"
+val userHome: String = System.getProperty("user.home")
+
+// Configuration the Swift package is cross-compiled with, independent of the
+// Android build type. Override with `-PswiftBuildConfig=release`.
+val swiftBuildConfig: String = (findProperty("swiftBuildConfig") as String?) ?: "debug"
+
+// Locates both the toolchain and the matching Android SDK artifactbundle.
+val swiftToolchainVersion: String = (findProperty("swiftToolchainVersion") as String?) ?: "6.3.3"
+
+// The Android SDK's prebuilt Swift modules require the matching swift.org
+// toolchain. Override with `-PswiftBin=/path/to/swift` if installed elsewhere.
+val swiftBin: String = (findProperty("swiftBin") as String?)
+    ?: "$userHome/Library/Developer/Toolchains/swift-$swiftToolchainVersion-RELEASE.xctoolchain/usr/bin/swift"
+
+// swift-java's `enableJavaCallbacks` feature runs its own internal Gradle
+// sub-build to compile the generated Java callback interfaces, requiring a
+// modern JDK — independent of whatever JDK runs *this* Gradle build.
+// Override with `-PcallbacksJdkHome=/path/to/jdk`.
+val callbacksJdkHome: String = (findProperty("callbacksJdkHome") as String?)
+    ?: "/opt/homebrew/opt/openjdk@25"
+
+// SwiftPM plugin output convention: `outputs/<package-dir-lowercased>/<TargetName>/...`.
+val generatedJavaDir = File(swiftPackageRoot, ".build/plugins/outputs/demo/CatalogBridge/destination/JExtractSwiftPlugin/src/generated/java")
+val swiftKitCoreDir = File(swiftPackageRoot, ".build/checkouts/swift-java/SwiftKitCore/src/main/java")
+val swiftBuildDir = File(swiftPackageRoot, ".build/$androidTriple/$swiftBuildConfig")
+val swiftAndroidRuntimeDir = File(
+    (findProperty("swiftAndroidRuntimeDir") as String?)
+        ?: "$userHome/Library/org.swift.swiftpm/swift-sdks/swift-${swiftToolchainVersion}-RELEASE_android.artifactbundle/swift-android/swift-resources/usr/lib/swift-aarch64/android"
+)
+
+// Supplies libc++_shared.so, which every Swift Android binary links against.
+val ndkRoot = File(
+    (findProperty("ndkRoot") as String?)
+        ?: System.getenv("ANDROID_NDK_HOME")
+        ?: System.getenv("ANDROID_NDK_ROOT")
+        ?: "$userHome/Library/Android/sdk/ndk/27.2.12479018"
+)
+
+// Cross-compile the Swift package + generate the JNI Java bindings.
+val jextract = tasks.register<Exec>("jextract") {
+    workingDir = swiftPackageRoot
+    environment("JAVA_HOME", callbacksJdkHome)
+    commandLine(
+        swiftBin, "build",
+        "--swift-sdk", androidTriple,
+        "-c", swiftBuildConfig,
+        "--disable-sandbox"
+    )
+    outputs.dir(generatedJavaDir)
+    outputs.file(File(swiftBuildDir, "libSwiftAndroidApp.so"))
+    // `swift build` is incremental itself; let it decide what is stale.
+    outputs.upToDateWhen { false }
+}
+
+// Stage the cross-compiled libraries, the swift-java runtime, the Swift
+// Android runtime and libc++_shared into jniLibs so they end up in the APK.
+val stageJniLibs = tasks.register<Copy>("stageJniLibs") {
+    dependsOn(jextract)
+    into(layout.projectDirectory.dir("src/main/jniLibs/$swiftAbi"))
+    from(swiftBuildDir) {
+        // Every dynamic library product built by the package graph — anything
+        // less leaves a dangling `dlopen` at runtime.
+        include("*.so")
+    }
+    from(swiftAndroidRuntimeDir) {
+        include("*.so")
+        // Test-only runtime libraries are not needed by the app.
+        exclude("*Testing*", "libXCTest.so")
+    }
+    from(File(ndkRoot, "toolchains/llvm/prebuilt/darwin-x86_64/sysroot/usr/lib/aarch64-linux-android")) {
+        include("libc++_shared.so")
+    }
+}
+
 android {
     namespace = "com.pureswift.swiftandroid"
     compileSdk = 35
@@ -16,7 +103,7 @@ android {
         versionName = "1.0"
         ndk {
             //noinspection ChromeOsAbiSupport
-            abiFilters += listOf("arm64-v8a")
+            abiFilters += listOf(swiftAbi)
         }
         testInstrumentationRunner = "androidx.test.runner.AndroidJUnitRunner"
     }
@@ -40,6 +127,18 @@ android {
     buildFeatures {
         compose = true
     }
+
+    // Generated JNI bindings + the SwiftKitCore Java runtime (consumed as
+    // source, since swift-java is not published to Maven).
+    sourceSets["main"].java.srcDir(generatedJavaDir)
+    sourceSets["main"].java.srcDir(swiftKitCoreDir)
+    // These two annotations pull in `jdk.jfr` (Java Flight Recorder), which is
+    // unavailable on Android; the JNI bindings don't reference them.
+    sourceSets["main"].java.filter.exclude(
+        "org/swift/swiftkit/core/annotations/ThreadSafe.java",
+        "org/swift/swiftkit/core/annotations/Unsigned.java"
+    )
+
     packaging {
         resources {
             excludes += listOf("/META-INF/{AL2.0,LGPL2.1}")
@@ -54,16 +153,16 @@ android {
     }
 }
 
-// Compile native Swift code for the demo app with `skip android build`.
-val buildSwift by tasks.registering(Exec::class) {
-    group = "build"
-    description = "Build native Swift sources for Android"
-    workingDir(rootProject.projectDir)
-    commandLine("bash", "build-swift.sh")
+// Ensure Swift is built + bindings generated + libs staged before Java or
+// Kotlin compiles (Kotlin also consumes the generated Java bindings).
+tasks.withType<JavaCompile>().configureEach {
+    dependsOn(jextract)
+}
+tasks.withType<org.jetbrains.kotlin.gradle.tasks.KotlinCompile>().configureEach {
+    dependsOn(jextract)
 }
-
 tasks.named("preBuild") {
-    dependsOn(buildSwift)
+    dependsOn(stageJniLibs)
 }
 
 dependencies {

Demo/app/src/main/AndroidManifest.xml

@@ -5,6 +5,9 @@
     <uses-permission android:name="android.permission.BLUETOOTH" />
     <uses-permission android:name="android.permission.BLUETOOTH_ADMIN" />
     <uses-permission android:name="android.permission.ACCESS_COARSE_LOCATION" />
+    <uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />
+    <uses-permission android:name="android.permission.ACCESS_WIFI_STATE" />
+    <uses-permission android:name="android.permission.NFC" />
 
     <application
         android:name=".Application"

Demo/app/src/main/java/com/pureswift/swiftandroid/Application.kt

@@ -1,6 +1,8 @@
 package com.pureswift.swiftandroid
 
+import android.util.Log
 import com.example.swift.HelloSubclass
+import com.pureswift.swiftandroid.bridge.CatalogBridge
 
 class Application: android.app.Application() {
 
@@ -11,6 +13,17 @@ class Application: android.app.Application() {
     override fun onCreate() {
         super.onCreate()
         onCreateSwift()
+        testCatalogBridge()
+    }
+
+    /// Smoke test for the jextract JNI bridge: a Java->Swift downcall and a
+    /// Swift->Java callback (closure/Runnable block).
+    private fun testCatalogBridge() {
+        val count = CatalogBridge.catalogEntryCount()
+        Log.d("CatalogBridge", "catalogEntryCount() = $count")
+        CatalogBridge.runBlock {
+            Log.d("CatalogBridge", "runBlock callback invoked from Swift")
+        }
     }
 
     private external fun onCreateSwift()