fix: .interoperabilityMode(.Cxx) build issue

.github/workflows/pull_request.yml

@@ -176,6 +176,70 @@ jobs:
       - name: Swift Test
         run: "swift test"
 
+  # Test that projects depending on swift-java can build with C++ interoperability enabled.
+  # This is important because users may enable -cxx-interoperability-mode=default in their
+  # own projects, and swift-java's public API must be compatible with that mode.
+  # See: https://github.com/swiftlang/swift-java/issues/391
+  verify-samples-cxx-interop:
+    name: Sample ${{ matrix.sample_app }} (C++ Interop) (${{ matrix.os_version }} swift:${{ matrix.swift_version }} jdk:${{matrix.jdk_vendor}})
+    runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        swift_version: ['6.1.3', '6.2', 'nightly']
+        os_version: ['jammy']
+        jdk_vendor: ['corretto']
+        sample_app: [
+          'JavaDependencySampleApp',
+          'JavaKitSampleApp',
+          'JavaProbablyPrime',
+          'JavaSieve',
+          'SwiftAndJavaJarSampleLib',
+          'SwiftJavaExtractFFMSampleApp',
+          'SwiftJavaExtractJNISampleApp',
+        ]
+    container:
+      image: ${{ (contains(matrix.swift_version, 'nightly') && 'swiftlang/swift') || 'swift' }}:${{ matrix.swift_version }}-${{ matrix.os_version }}
+    env:
+      CXX_INTEROP: '1'
+    steps:
+      - uses: actions/checkout@v4
+      - name: Prepare CI Environment
+        uses: ./.github/actions/prepare_env
+      - name: "Verify sample with C++ Interop: ${{ matrix.sample_app }}"
+        run: .github/scripts/validate_sample.sh Samples/${{ matrix.sample_app }}
+
+  verify-samples-cxx-interop-macos:
+    name: Sample ${{ matrix.sample_app }} (C++ Interop) (${{ matrix.os_version }} swift:${{ matrix.swift_version }} jdk:${{matrix.jdk_vendor}})
+    runs-on: [self-hosted, macos, sequoia, ARM64]
+    strategy:
+      fail-fast: false
+      matrix:
+        swift_version: ['6.2']
+        os_version: ['macos']
+        jdk_vendor: ['corretto']
+        sample_app: [
+          'JavaDependencySampleApp',
+          'JavaKitSampleApp',
+          'JavaProbablyPrime',
+          'JavaSieve',
+          'SwiftAndJavaJarSampleLib',
+          'SwiftJavaExtractFFMSampleApp',
+          'SwiftJavaExtractJNISampleApp',
+        ]
+    env:
+      CXX_INTEROP: '1'
+    steps:
+      - uses: actions/checkout@v4
+      - name: Prepare CI Environment
+        uses: ./.github/actions/prepare_env
+      - name: Install Swiftly
+        run: ./.github/scripts/install_swiftly.sh
+        env:
+          SWIFT_VERSION: "${{ matrix.swift_version }}"
+      - name: "Verify sample with C++ Interop: ${{ matrix.sample_app }}"
+        run: .github/scripts/validate_sample.sh Samples/${{ matrix.sample_app }}
+
   build-swift-android:
     name: Sample SwiftJavaExtractJNISampleApp (Android) (${{ matrix.os_version }} swift:${{ matrix.swift_version }} jdk:${{matrix.jdk_vendor}} android:${{matrix.sdk_triple}})
     runs-on: ubuntu-latest

Samples/JavaDependencySampleApp/Package.swift

@@ -40,6 +40,12 @@ let javaIncludePath = "\(javaHome)/include"
   #error("Currently only macOS and Linux platforms are supported, this may change in the future.")
 #endif
 
+// Support C++ interoperability mode via CXX_INTEROP environment variable.
+// This is used to test that swift-java's public API is compatible with projects
+// that enable C++ interoperability mode.
+// See: https://github.com/swiftlang/swift-java/issues/391
+let cxxInteropEnabled = ProcessInfo.processInfo.environment["CXX_INTEROP"] == "1"
+
 let package = Package(
   name: "JavaDependencySampleApp",
   platforms: [
@@ -73,6 +79,7 @@ let package = Package(
       swiftSettings: [
         .unsafeFlags(["-I\(javaIncludePath)", "-I\(javaPlatformIncludePath)"]),
         .swiftLanguageMode(.v5),
+        .interoperabilityMode(.Cxx, .when(platforms: cxxInteropEnabled ? [.macOS, .linux] : [])),
       ],
       plugins: [
         .plugin(name: "SwiftJavaPlugin", package: "swift-java"),
@@ -92,6 +99,7 @@ let package = Package(
       swiftSettings: [
         .unsafeFlags(["-I\(javaIncludePath)", "-I\(javaPlatformIncludePath)"]),
         .swiftLanguageMode(.v5),
+        .interoperabilityMode(.Cxx, .when(platforms: cxxInteropEnabled ? [.macOS, .linux] : [])),
       ],
       plugins: [
 //        .plugin(name: "SwiftJavaBootstrapJavaPlugin", package: "swift-java"),

Samples/JavaKitSampleApp/Package.swift

@@ -39,6 +39,12 @@ let javaIncludePath = "\(javaHome)/include"
   let javaPlatformIncludePath = "\(javaIncludePath)/win32)"
 #endif
 
+// Support C++ interoperability mode via CXX_INTEROP environment variable.
+// This is used to test that swift-java's public API is compatible with projects
+// that enable C++ interoperability mode.
+// See: https://github.com/swiftlang/swift-java/issues/391
+let cxxInteropEnabled = ProcessInfo.processInfo.environment["CXX_INTEROP"] == "1"
+
 let package = Package(
   name: "JavaKitSampleApp",
   platforms: [
@@ -70,7 +76,8 @@ let package = Package(
       ],
       swiftSettings: [
         .swiftLanguageMode(.v5),
-        .unsafeFlags(["-I\(javaIncludePath)", "-I\(javaPlatformIncludePath)"])
+        .unsafeFlags(["-I\(javaIncludePath)", "-I\(javaPlatformIncludePath)"]),
+        .interoperabilityMode(.Cxx, .when(platforms: cxxInteropEnabled ? [.macOS, .linux] : [])),
       ],
       plugins: [
         .plugin(name: "JavaCompilerPlugin", package: "swift-java"),

Samples/JavaProbablyPrime/Package.swift

@@ -4,6 +4,14 @@
 import CompilerPluginSupport
 import PackageDescription
 
+import class Foundation.ProcessInfo
+
+// Support C++ interoperability mode via CXX_INTEROP environment variable.
+// This is used to test that swift-java's public API is compatible with projects
+// that enable C++ interoperability mode.
+// See: https://github.com/swiftlang/swift-java/issues/391
+let cxxInteropEnabled = ProcessInfo.processInfo.environment["CXX_INTEROP"] == "1"
+
 let package = Package(
   name: "JavaProbablyPrime",
   platforms: [
@@ -34,7 +42,8 @@ let package = Package(
         .product(name: "ArgumentParser", package: "swift-argument-parser"),
       ],
       swiftSettings: [
-        .swiftLanguageMode(.v5)
+        .swiftLanguageMode(.v5),
+        .interoperabilityMode(.Cxx, .when(platforms: cxxInteropEnabled ? [.macOS, .linux] : [])),
       ],
       plugins: [
         .plugin(name: "SwiftJavaPlugin", package: "swift-java"),

Samples/JavaSieve/Package.swift

@@ -39,6 +39,12 @@ let javaIncludePath = "\(javaHome)/include"
   #error("Currently only macOS and Linux platforms are supported, this may change in the future.")
 #endif
 
+// Support C++ interoperability mode via CXX_INTEROP environment variable.
+// This is used to test that swift-java's public API is compatible with projects
+// that enable C++ interoperability mode.
+// See: https://github.com/swiftlang/swift-java/issues/391
+let cxxInteropEnabled = ProcessInfo.processInfo.environment["CXX_INTEROP"] == "1"
+
 let package = Package(
     name: "JavaSieve",
     platforms: [
@@ -59,7 +65,8 @@ let package = Package(
             ],
             exclude: ["swift-java.config"],
             swiftSettings: [
-              .unsafeFlags(["-I\(javaIncludePath)", "-I\(javaPlatformIncludePath)"])
+              .unsafeFlags(["-I\(javaIncludePath)", "-I\(javaPlatformIncludePath)"]),
+              .interoperabilityMode(.Cxx, .when(platforms: cxxInteropEnabled ? [.macOS, .linux] : [])),
             ],
             plugins: [
               .plugin(name: "SwiftJavaPlugin", package: "swift-java"),
@@ -77,7 +84,8 @@ let package = Package(
             ],
             exclude: ["swift-java.config"],
             swiftSettings: [
-              .unsafeFlags(["-I\(javaIncludePath)", "-I\(javaPlatformIncludePath)"])
+              .unsafeFlags(["-I\(javaIncludePath)", "-I\(javaPlatformIncludePath)"]),
+              .interoperabilityMode(.Cxx, .when(platforms: cxxInteropEnabled ? [.macOS, .linux] : [])),
             ],
             plugins: [
               .plugin(name: "SwiftJavaPlugin", package: "swift-java"),

Samples/SwiftAndJavaJarSampleLib/Package.swift

@@ -40,6 +40,12 @@ let javaIncludePath = "\(javaHome)/include"
   #error("Currently only macOS and Linux platforms are supported, this may change in the future.")
 #endif
 
+// Support C++ interoperability mode via CXX_INTEROP environment variable.
+// This is used to test that swift-java's public API is compatible with projects
+// that enable C++ interoperability mode.
+// See: https://github.com/swiftlang/swift-java/issues/391
+let cxxInteropEnabled = ProcessInfo.processInfo.environment["CXX_INTEROP"] == "1"
+
 let package = Package(
   name: "SwiftAndJavaJarSampleLib",
   platforms: [
@@ -70,7 +76,8 @@ let package = Package(
       ],
       swiftSettings: [
         .swiftLanguageMode(.v5),
-        .unsafeFlags(["-I\(javaIncludePath)", "-I\(javaPlatformIncludePath)"])
+        .unsafeFlags(["-I\(javaIncludePath)", "-I\(javaPlatformIncludePath)"]),
+        .interoperabilityMode(.Cxx, .when(platforms: cxxInteropEnabled ? [.macOS, .linux] : [])),
       ],
       plugins: [
         .plugin(name: "JExtractSwiftPlugin", package: "swift-java"),

Samples/SwiftJavaExtractFFMSampleApp/Package.swift

@@ -40,6 +40,12 @@ let javaIncludePath = "\(javaHome)/include"
   #error("Currently only macOS and Linux platforms are supported, this may change in the future.")
 #endif
 
+// Support C++ interoperability mode via CXX_INTEROP environment variable.
+// This is used to test that swift-java's public API is compatible with projects
+// that enable C++ interoperability mode.
+// See: https://github.com/swiftlang/swift-java/issues/391
+let cxxInteropEnabled = ProcessInfo.processInfo.environment["CXX_INTEROP"] == "1"
+
 let package = Package(
   name: "SwiftJavaExtractFFMSampleApp",
   platforms: [
@@ -72,7 +78,8 @@ let package = Package(
       ],
       swiftSettings: [
         .swiftLanguageMode(.v5),
-        .unsafeFlags(["-I\(javaIncludePath)", "-I\(javaPlatformIncludePath)"])
+        .unsafeFlags(["-I\(javaIncludePath)", "-I\(javaPlatformIncludePath)"]),
+        .interoperabilityMode(.Cxx, .when(platforms: cxxInteropEnabled ? [.macOS, .linux] : [])),
       ],
       plugins: [
         .plugin(name: "JExtractSwiftPlugin", package: "swift-java"),

Samples/SwiftJavaExtractJNISampleApp/Package.swift

@@ -40,6 +40,12 @@ let javaIncludePath = "\(javaHome)/include"
   #error("Currently only macOS and Linux platforms are supported, this may change in the future.")
 #endif
 
+// Support C++ interoperability mode via CXX_INTEROP environment variable.
+// This is used to test that swift-java's public API is compatible with projects
+// that enable C++ interoperability mode.
+// See: https://github.com/swiftlang/swift-java/issues/391
+let cxxInteropEnabled = ProcessInfo.processInfo.environment["CXX_INTEROP"] == "1"
+
 let package = Package(
   name: "JExtractJNISampleApp",
   platforms: [
@@ -70,6 +76,7 @@ let package = Package(
       swiftSettings: [
         .swiftLanguageMode(.v5),
         .unsafeFlags(["-I\(javaIncludePath)", "-I\(javaPlatformIncludePath)"]),
+        .interoperabilityMode(.Cxx, .when(platforms: cxxInteropEnabled ? [.macOS, .linux] : [])),
       ],
       plugins: [
         .plugin(name: "JExtractSwiftPlugin", package: "swift-java")

Sources/CSwiftJavaJNI/include/CSwiftJavaJNI.h

@@ -17,4 +17,52 @@
 
 #include <jni.h>
 
+// Provide C-compatible type aliases for JNI types.
+// When Swift modules have C++ interoperability enabled, jni.h is parsed
+// in C++ mode where JNIEnv is defined as a struct (JNIEnv_). However,
+// SwiftJava modules are compiled without C++ interop, causing JNIEnv
+// to be a pointer type. This mismatch causes type errors.
+//
+// These typedefs provide consistent C-style pointer types that work
+// regardless of the C++ interoperability mode.
+// See: https://github.com/swiftlang/swift-java/issues/391
+#ifdef __cplusplus
+// Android NDK uses JNINativeInterface instead of JNINativeInterface_
+#ifdef __ANDROID__
+typedef const JNINativeInterface *CJNIEnv;
+#else
+typedef const JNINativeInterface_ *CJNIEnv;
+#endif
+typedef _jobject *Cjobject;
+typedef _jclass *Cjclass;
+typedef _jstring *Cjstring;
+typedef _jarray *Cjarray;
+typedef _jobjectArray *CjobjectArray;
+typedef _jbooleanArray *CjbooleanArray;
+typedef _jbyteArray *CjbyteArray;
+typedef _jcharArray *CjcharArray;
+typedef _jshortArray *CjshortArray;
+typedef _jintArray *CjintArray;
+typedef _jlongArray *CjlongArray;
+typedef _jfloatArray *CjfloatArray;
+typedef _jdoubleArray *CjdoubleArray;
+typedef _jthrowable *Cjthrowable;
+#else
+typedef JNIEnv CJNIEnv;
+typedef jobject Cjobject;
+typedef jclass Cjclass;
+typedef jstring Cjstring;
+typedef jarray Cjarray;
+typedef jobjectArray CjobjectArray;
+typedef jbooleanArray CjbooleanArray;
+typedef jbyteArray CjbyteArray;
+typedef jcharArray CjcharArray;
+typedef jshortArray CjshortArray;
+typedef jintArray CjintArray;
+typedef jlongArray CjlongArray;
+typedef jfloatArray CjfloatArray;
+typedef jdoubleArray CjdoubleArray;
+typedef jthrowable Cjthrowable;
+#endif
+
 #endif /* CSwiftJavaJNI_h */

Sources/JExtractSwiftLib/JNI/JNISwift2JavaGenerator+SwiftThunkPrinting.swift

@@ -367,8 +367,16 @@ extension JNISwift2JavaGenerator {
     func innerBody(in printer: inout CodePrinter) -> String {
       let loweredResult = nativeSignature.result.conversion.render(&printer, result)
 
-      if !decl.functionSignature.result.type.isVoid {
-        return "return \(loweredResult)"
+      // For async functions, loweredResult is empty as they handle return internally
+      // via CompletableFuture. Only apply unsafeBitCast for non-void, non-async functions.
+      if !decl.functionSignature.result.type.isVoid && !loweredResult.isEmpty {
+        let resultType = nativeSignature.result.javaType.jniTypeName
+        // Use unsafeBitCast for C++ interoperability compatibility.
+        // SwiftJava uses JNI types (jstring, jobject, etc.) internally,
+        // but generated thunks use C-compatible types (Cjstring, Cjobject, etc.)
+        // to ensure compatibility with modules that have C++ interop enabled.
+        // See: https://github.com/swiftlang/swift-java/issues/391
+        return "return unsafeBitCast(\(loweredResult), to: \(resultType).self)"
       } else {
         return loweredResult
       }
@@ -454,7 +462,9 @@ extension JNISwift2JavaGenerator {
 
     let thunkParameters =
       [
-        "environment: UnsafeMutablePointer<JNIEnv?>!",
+        // Use CJNIEnv for C++ interoperability compatibility.
+        // See: https://github.com/swiftlang/swift-java/issues/391
+        "environment: UnsafeMutablePointer<CJNIEnv?>!",
         "thisClass: jclass"
       ] + translatedParameters
     let thunkReturnType = resultType != .void ? " -> \(resultType.jniTypeName)" : ""

Sources/JExtractSwiftLib/JavaParameter.swift

@@ -47,7 +47,8 @@ struct JavaParameter {
     var jniTypeName: String {
       switch self {
       case .concrete(let type): type.jniTypeName
-      case .generic: "jobject?"
+      // Use C-compatible type for C++ interoperability
+      case .generic: "Cjobject?"
       }
     }
 

Sources/JavaTypes/JavaType+JNI.swift

@@ -14,6 +14,8 @@
 
 extension JavaType {
   /// Map this Java type to the appropriate JNI type name.
+  /// Uses C-compatible types (Cjobject, Cjstring, etc.) for C++ interoperability.
+  /// See: https://github.com/swiftlang/swift-java/issues/391
   package var jniTypeName: String {
     switch self {
     case .boolean: "jboolean"
@@ -25,19 +27,19 @@ extension JavaType {
     case .int: "jint"
     case .long: "jlong"
     case .void: "void"
-    case .array(.boolean): "jbooleanArray?"
-    case .array(.byte): "jbyteArray?"
-    case .array(.char): "jcharArray?"
-    case .array(.short): "jshortArray?"
-    case .array(.int): "jintArray?"
-    case .array(.long): "jlongArray?"
-    case .array(.float): "jfloatArray?"
-    case .array(.double): "jdoubleArray?"
-    case .array: "jobjectArray?"
-    case .class(package: "java.lang", name: "String", _): "jstring?"
-    case .class(package: "java.lang", name: "Class", _): "jclass?"
-    case .class(package: "java.lang", name: "Throwable", _): "jthrowable?"
-    case .class: "jobject?"
+    case .array(.boolean): "CjbooleanArray?"
+    case .array(.byte): "CjbyteArray?"
+    case .array(.char): "CjcharArray?"
+    case .array(.short): "CjshortArray?"
+    case .array(.int): "CjintArray?"
+    case .array(.long): "CjlongArray?"
+    case .array(.float): "CjfloatArray?"
+    case .array(.double): "CjdoubleArray?"
+    case .array: "CjobjectArray?"
+    case .class(package: "java.lang", name: "String", _): "Cjstring?"
+    case .class(package: "java.lang", name: "Class", _): "Cjclass?"
+    case .class(package: "java.lang", name: "Throwable", _): "Cjthrowable?"
+    case .class: "Cjobject?"
     }
   }
 

Sources/SwiftJava/JVM/JavaVirtualMachine.swift

@@ -19,8 +19,10 @@ import Foundation
 #endif
 
 public typealias JavaVMPointer = UnsafeMutablePointer<JavaVM?>
+// Use CJNIEnv for C++ interoperability compatibility.
+// See: https://github.com/swiftlang/swift-java/issues/391
 #if canImport(Android)
-typealias JNIEnvPointer = UnsafeMutablePointer<JNIEnv?>
+typealias JNIEnvPointer = UnsafeMutablePointer<CJNIEnv?>
 #else
 typealias JNIEnvPointer = UnsafeMutableRawPointer
 #endif