[jextract] add support for throwing functions in JNI mode (#277)

Author: madsodgaard

Sources/JExtractSwiftLib/FFM/CDeclLowering/FFMSwift2JavaGenerator+FunctionLowering.swift

@@ -526,7 +526,8 @@ public struct LoweredFunctionSignature: Equatable {
     SwiftFunctionSignature(
       selfParameter: nil,
       parameters: allLoweredParameters,
-      result: SwiftResult(convention: .direct, type: result.cdeclResultType)
+      result: SwiftResult(convention: .direct, type: result.cdeclResultType),
+      effectSpecifiers: []
     )
   }
 }

Sources/JExtractSwiftLib/ImportedDecls.swift

@@ -98,6 +98,10 @@ public final class ImportedFunc: ImportedDecl, CustomStringConvertible {
     return prefix + context + self.name
   }
 
+  var isThrowing: Bool {
+    self.functionSignature.effectSpecifiers.contains(.throws)
+  }
+
   init(
     module: String,
     swiftDecl: any DeclSyntaxProtocol,

Sources/JExtractSwiftLib/JNI/JNISwift2JavaGenerator.swift

@@ -124,7 +124,6 @@ extension JNISwift2JavaGenerator {
       "thisClass: jclass"
     ] + translatedParameters.map { "\($0.0): \($0.1.jniTypeName)"}
     let swiftReturnType = decl.functionSignature.result.type
-
     let thunkReturnType = !swiftReturnType.isVoid ? " -> \(swiftReturnType.javaType.jniTypeName)" : ""
 
     printer.printBraceBlock(
@@ -137,17 +136,32 @@ extension JNISwift2JavaGenerator {
         let label = originalParam.argumentLabel.map { "\($0): "} ?? ""
         return "\(label)\(originalParam.type)(fromJNI: \(translatedParam.0), in: environment!)"
       }
-      let functionDowncall = "\(swiftModuleName).\(decl.name)(\(downcallParameters.joined(separator: ", ")))"
+      let tryClause: String = decl.isThrowing ? "try " : ""
+      let functionDowncall = "\(tryClause)\(swiftModuleName).\(decl.name)(\(downcallParameters.joined(separator: ", ")))"
 
-      if swiftReturnType.isVoid {
-        printer.print(functionDowncall)
+      let innerBody = if swiftReturnType.isVoid {
+        functionDowncall
       } else {
+        """
+        let result = \(functionDowncall)
+        return result.getJNIValue(in: environment)")
+        """
+      }
+
+      if decl.isThrowing {
+        let dummyReturn = !swiftReturnType.isVoid ? "return \(swiftReturnType).jniPlaceholderValue" : ""
         printer.print(
           """
-          let result = \(functionDowncall)
-          return result.getJNIValue(in: environment)")
+          do {
+            \(innerBody)
+          } catch {
+            environment.throwAsException(error)
+            \(dummyReturn)
+          }
           """
         )
+      } else {
+        printer.print(innerBody)
       }
     }
   }
@@ -197,6 +211,7 @@ extension JNISwift2JavaGenerator {
     let params = decl.functionSignature.parameters.enumerated().map { idx, param in
       "\(param.type.javaType) \(param.parameterName ?? "arg\(idx))")"
     }
+    let throwsClause = decl.isThrowing ? " throws Exception" : ""
 
     printer.print(
       """
@@ -208,7 +223,7 @@ extension JNISwift2JavaGenerator {
       */
       """
     )
-    printer.print("public static native \(returnType) \(decl.name)(\(params.joined(separator: ", ")));")
+    printer.print("public static native \(returnType) \(decl.name)(\(params.joined(separator: ", ")))\(throwsClause);")
   }
 }
 

Sources/JExtractSwiftLib/SwiftTypes/SwiftEffectSpecifier.swift

@@ -0,0 +1,17 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2025 Apple Inc. and the Swift.org project authors
+// Licensed under Apache License v2.0
+//
+// See LICENSE.txt for license information
+// See CONTRIBUTORS.txt for the list of Swift.org project authors
+//
+// SPDX-License-Identifier: Apache-2.0
+//
+//===----------------------------------------------------------------------===//
+
+enum SwiftEffectSpecifier: Equatable {
+  case `throws`
+}

Sources/JExtractSwiftLib/SwiftTypes/SwiftFunctionSignature.swift

@@ -21,11 +21,18 @@ public struct SwiftFunctionSignature: Equatable {
   var selfParameter: SwiftSelfParameter?
   var parameters: [SwiftParameter]
   var result: SwiftResult
+  var effectSpecifiers: [SwiftEffectSpecifier]
 
-  init(selfParameter: SwiftSelfParameter? = nil, parameters: [SwiftParameter], result: SwiftResult) {
+  init(
+    selfParameter: SwiftSelfParameter? = nil,
+    parameters: [SwiftParameter],
+    result: SwiftResult,
+    effectSpecifiers: [SwiftEffectSpecifier]
+  ) {
     self.selfParameter = selfParameter
     self.parameters = parameters
     self.result = result
+    self.effectSpecifiers = effectSpecifiers
   }
 }
 
@@ -68,13 +75,16 @@ extension SwiftFunctionSignature {
       throw SwiftFunctionTranslationError.generic(generics)
     }
 
+    let (parameters, effectSpecifiers) = try Self.translateFunctionSignature(
+      node.signature,
+      symbolTable: symbolTable
+    )
+
     self.init(
       selfParameter: .initializer(enclosingType),
-      parameters: try Self.translateFunctionSignature(
-        node.signature,
-        symbolTable: symbolTable
-      ),
-      result: SwiftResult(convention: .direct, type: enclosingType)
+      parameters: parameters,
+      result: SwiftResult(convention: .direct, type: enclosingType),
+      effectSpecifiers: effectSpecifiers
     )
   }
 
@@ -120,7 +130,7 @@ extension SwiftFunctionSignature {
     }
 
     // Translate the parameters.
-    let parameters = try Self.translateFunctionSignature(
+    let (parameters, effectSpecifiers) = try Self.translateFunctionSignature(
       node.signature,
       symbolTable: symbolTable
     )
@@ -136,26 +146,28 @@ extension SwiftFunctionSignature {
       result = .void
     }
 
-    self.init(selfParameter: selfParameter, parameters: parameters, result: result)
+    self.init(selfParameter: selfParameter, parameters: parameters, result: result, effectSpecifiers: effectSpecifiers)
   }
 
   /// Translate the function signature, returning the list of translated
-  /// parameters.
+  /// parameters and effect specifiers.
   static func translateFunctionSignature(
     _ signature: FunctionSignatureSyntax,
     symbolTable: SwiftSymbolTable
-  ) throws -> [SwiftParameter] {
-    // FIXME: Prohibit effects for now.
-    if let throwsClause = signature.effectSpecifiers?.throwsClause {
-      throw SwiftFunctionTranslationError.throws(throwsClause)
+  ) throws -> ([SwiftParameter], [SwiftEffectSpecifier]) {
+    var effectSpecifiers = [SwiftEffectSpecifier]()
+    if signature.effectSpecifiers?.throwsClause != nil {
+      effectSpecifiers.append(.throws)
     }
     if let asyncSpecifier = signature.effectSpecifiers?.asyncSpecifier {
       throw SwiftFunctionTranslationError.async(asyncSpecifier)
     }
 
-    return try signature.parameterClause.parameters.map { param in
+    let parameters = try signature.parameterClause.parameters.map { param in
       try SwiftParameter(param, symbolTable: symbolTable)
     }
+
+    return (parameters, effectSpecifiers)
   }
 
   init(_ varNode: VariableDeclSyntax, isSet: Bool, enclosingType: SwiftType?, symbolTable: SwiftSymbolTable) throws {
@@ -195,6 +207,22 @@ extension SwiftFunctionSignature {
     }
     let valueType = try SwiftType(varTypeNode, symbolTable: symbolTable)
 
+    var effectSpecifiers: [SwiftEffectSpecifier]? = nil
+    switch binding.accessorBlock?.accessors {
+    case .getter(let getter):
+      if let getter = getter.as(AccessorDeclSyntax.self) {
+        effectSpecifiers = Self.effectSpecifiers(from: getter)
+      }
+    case .accessors(let accessors):
+      if let getter = accessors.first(where: { $0.accessorSpecifier.tokenKind == .keyword(.get) }) {
+        effectSpecifiers = Self.effectSpecifiers(from: getter)
+      }
+    default:
+      break
+    }
+
+    self.effectSpecifiers = effectSpecifiers ?? []
+
     if isSet {
       self.parameters = [SwiftParameter(convention: .byValue, parameterName: "newValue", type: valueType)]
       self.result = .void
@@ -203,6 +231,14 @@ extension SwiftFunctionSignature {
       self.result = .init(convention: .direct, type: valueType)
     }
   }
+
+  private static func effectSpecifiers(from decl: AccessorDeclSyntax) -> [SwiftEffectSpecifier] {
+    var effectSpecifiers = [SwiftEffectSpecifier]()
+    if decl.effectSpecifiers?.throwsClause != nil {
+      effectSpecifiers.append(.throws)
+    }
+    return effectSpecifiers
+  }
 }
 
 extension VariableDeclSyntax {

Tests/JExtractSwiftTests/JNI/JNIModuleTests.swift

@@ -27,6 +27,11 @@ struct JNIModuleTests {
     public func copy(_ string: String) -> String
   """
 
+  let globalMethodThrowing = """
+    public func methodA() throws
+    public func methodB() throws -> Int64
+  """
+
   @Test
   func generatesModuleJavaClass() throws {
     let input = "public func helloWorld()"
@@ -150,4 +155,67 @@ struct JNIModuleTests {
       ]
     )
   }
+
+  @Test
+  func globalMethodThrowing_javaBindings() throws {
+    try assertOutput(
+      input: globalMethodThrowing,
+      .jni,
+      .java,
+      expectedChunks: [
+        """
+        /**
+          * Downcall to Swift:
+          * {@snippet lang=swift :
+          * public func methodA() throws
+          * }
+          */
+        public static native void methodA() throws Exception;
+        """,
+        """
+        /**
+          * Downcall to Swift:
+          * {@snippet lang=swift :
+          * public func methodB() throws -> Int64
+          * }
+          */
+        public static native long methodB() throws Exception;
+        """,
+      ]
+    )
+  }
+
+  @Test
+  func globalMethodThrowing_swiftThunks() throws {
+    try assertOutput(
+      input: globalMethodThrowing,
+      .jni,
+      .swift,
+      detectChunkByInitialLines: 1,
+      expectedChunks: [
+          """
+          @_cdecl("Java_com_example_swift_SwiftModule_methodA")
+          func swiftjava_SwiftModule_methodA(environment: UnsafeMutablePointer<JNIEnv?>!, thisClass: jclass) {
+            do { 
+              try SwiftModule.methodA()
+            } catch {
+              environment.throwAsException(error)
+            }
+          }
+          """,
+          """
+          @_cdecl("Java_com_example_swift_SwiftModule_methodB")
+          func swiftjava_SwiftModule_methodB(environment: UnsafeMutablePointer<JNIEnv?>!, thisClass: jclass) -> jlong {
+            do { 
+              let result = try SwiftModule.methodB()
+              return result.getJNIValue(in: environment)
+            } catch {
+              environment.throwAsException(error)
+              return Int64.jniPlaceholderValue
+            }
+          }
+          """,
+      ]
+    )
+  }
 }