JavaKit: Rework @ImplementsJava to be more like @implements language feature

Package.swift

@@ -262,7 +262,8 @@ let package = Package(
       ],
       swiftSettings: [
         .swiftLanguageMode(.v5),
-        .enableUpcomingFeature("BareSlashRegexLiterals")
+        .enableUpcomingFeature("BareSlashRegexLiterals"),
+        .unsafeFlags(["-I\(javaIncludePath)", "-I\(javaPlatformIncludePath)"]),
       ]
     ),
 
@@ -281,7 +282,8 @@ let package = Package(
 
       swiftSettings: [
         .swiftLanguageMode(.v5),
-        .enableUpcomingFeature("BareSlashRegexLiterals")
+        .unsafeFlags(["-I\(javaIncludePath)", "-I\(javaPlatformIncludePath)"]),
+        .enableUpcomingFeature("BareSlashRegexLiterals"),
       ]
     ),
 
@@ -296,7 +298,8 @@ let package = Package(
         "JavaTypes",
       ],
       swiftSettings: [
-        .swiftLanguageMode(.v5)
+        .swiftLanguageMode(.v5),
+        .unsafeFlags(["-I\(javaIncludePath)", "-I\(javaPlatformIncludePath)"]),
       ]
     ),
 
@@ -314,7 +317,8 @@ let package = Package(
       name: "JavaKitTests",
       dependencies: ["JavaKit", "JavaKitNetwork"],
       swiftSettings: [
-        .swiftLanguageMode(.v5)
+        .swiftLanguageMode(.v5),
+        .unsafeFlags(["-I\(javaIncludePath)", "-I\(javaPlatformIncludePath)"])
       ]
     ),
 
@@ -341,7 +345,8 @@ let package = Package(
       name: "Java2SwiftTests",
       dependencies: ["Java2SwiftLib"],
       swiftSettings: [
-        .swiftLanguageMode(.v5)
+        .swiftLanguageMode(.v5),
+        .unsafeFlags(["-I\(javaIncludePath)", "-I\(javaPlatformIncludePath)"])
       ]
     ),
 

Plugins/Java2SwiftPlugin/Java2SwiftPlugin.swift

@@ -93,12 +93,44 @@ struct Java2SwiftBuildToolPlugin: BuildToolPlugin {
       outputDirectory.appending(path: "\(swiftName).swift")
     }
 
+    // Find the Java .class files generated from prior plugins.
+    let compiledClassFiles = sourceModule.pluginGeneratedResources.filter { url in
+      url.pathExtension == "class"
+    }
+
+    if let firstClassFile = compiledClassFiles.first {
+      // Keep stripping off parts of the path until we hit the "Java" part.
+      // That's where the class path starts.
+      var classpath = firstClassFile
+      while classpath.lastPathComponent != "Java" {
+        classpath.deleteLastPathComponent()
+      }
+      arguments += [ "--classpath", classpath.path() ]
+
+      // For each of the class files, note that it can have Swift-native
+      // implementations. We figure this out based on the path.
+      for classFile in compiledClassFiles {
+        var classFile = classFile.deletingPathExtension()
+        var classNameComponents: [String] = []
+
+        while classFile.lastPathComponent != "Java" {
+          classNameComponents.append(classFile.lastPathComponent)
+          classFile.deleteLastPathComponent()
+        }
+
+        let className = classNameComponents
+          .reversed()
+          .joined(separator: ".")
+        arguments += [ "--swift-native-implementation", className]
+      }
+    }
+
     return [
       .buildCommand(
         displayName: "Wrapping \(config.classes.count) Java classes target \(sourceModule.name) in Swift",
         executable: try context.tool(named: "Java2Swift").url,
         arguments: arguments,
-        inputFiles: [ configFile ],
+        inputFiles: [ configFile ] + compiledClassFiles,
         outputFiles: outputSwiftFiles
       )
     ]

Samples/JavaKitSampleApp/Package.swift

@@ -4,6 +4,42 @@
 import CompilerPluginSupport
 import PackageDescription
 
+import class Foundation.FileManager
+import class Foundation.ProcessInfo
+
+// Note: the JAVA_HOME environment variable must be set to point to where
+// Java is installed, e.g.,
+//   Library/Java/JavaVirtualMachines/openjdk-21.jdk/Contents/Home.
+func findJavaHome() -> String {
+  if let home = ProcessInfo.processInfo.environment["JAVA_HOME"] {
+    return home
+  }
+
+  // This is a workaround for envs (some IDEs) which have trouble with
+  // picking up env variables during the build process
+  let path = "\(FileManager.default.homeDirectoryForCurrentUser.path()).java_home"
+  if let home = try? String(contentsOfFile: path, encoding: .utf8) {
+    if let lastChar = home.last, lastChar.isNewline {
+      return String(home.dropLast())
+    }
+
+    return home
+  }
+
+  fatalError("Please set the JAVA_HOME environment variable to point to where Java is installed.")
+}
+let javaHome = findJavaHome()
+
+let javaIncludePath = "\(javaHome)/include"
+#if os(Linux)
+  let javaPlatformIncludePath = "\(javaIncludePath)/linux"
+#elseif os(macOS)
+  let javaPlatformIncludePath = "\(javaIncludePath)/darwin"
+#else
+  // TODO: Handle windows as well
+  #error("Currently only macOS and Linux platforms are supported, this may change in the future.")
+#endif
+
 let package = Package(
   name: "JavaKitSampleApp",
   platforms: [
@@ -34,11 +70,12 @@ let package = Package(
         .product(name: "JavaKitJar", package: "swift-java"),
       ],
       swiftSettings: [
-        .swiftLanguageMode(.v5)
+        .swiftLanguageMode(.v5),
+        .unsafeFlags(["-I\(javaIncludePath)", "-I\(javaPlatformIncludePath)"])
       ],
       plugins: [
+        .plugin(name: "JavaCompilerPlugin", package: "swift-java"),
         .plugin(name: "Java2SwiftPlugin", package: "swift-java"),
-        .plugin(name: "JavaCompilerPlugin", package: "swift-java")
       ]
     ),
   ]

Samples/JavaKitSampleApp/Sources/JavaKitExample/Java2Swift.config

@@ -1,5 +1,8 @@
 {
   "classes" : {
-    "java.util.ArrayList" : "ArrayList"
+    "java.util.ArrayList" : "ArrayList",
+    "com.example.swift.HelloSwift" : "HelloSwift",
+    "com.example.swift.HelloSubclass" : "HelloSubclass",
+    "com.example.swift.JavaKitSampleMain" : "JavaKitSampleMain"
   }
 }

Samples/JavaKitSampleApp/Sources/JavaKitExample/JavaKitExample.swift

@@ -18,31 +18,10 @@ enum SwiftWrappedError: Error {
   case message(String)
 }
 
-@JavaClass("com.example.swift.HelloSwift")
-struct HelloSwift {
+@JavaImplementation("com.example.swift.HelloSwift")
+extension HelloSwift: HelloSwiftNativeMethods {
   @JavaMethod
-  init(environment: JNIEnvironment)
-
-  @JavaMethod
-  func sayHelloBack(_ i: Int32) -> Double
-
-  @JavaMethod
-  func greet(_ name: String)
-
-  @JavaMethod
-  func doublesToStrings(doubles: [Double]) -> [String]
-
-  @JavaMethod
-  func throwMessage(message: String) throws
-
-  @JavaField
-  var value: Double
-
-  @JavaField
-  var name: String
-
-  @ImplementsJava
-  func sayHello(i: Int32, _ j: Int32) -> Int32 {
+  func sayHello(_ i: Int32, _ j: Int32) -> Int32 {
     print("Hello from Swift!")
     let answer = self.sayHelloBack(i + j)
     print("Swift got back \(answer) from Java")
@@ -65,7 +44,7 @@ struct HelloSwift {
     self.name = "a 🗑️-collected language"
     _ = self.sayHelloBack(42)
 
-    let strings = doublesToStrings(doubles: [3.14159, 2.71828])
+    let strings = doublesToStrings([3.14159, 2.71828])
     print("Converting doubles to strings: \(strings)")
 
     // Try downcasting
@@ -83,42 +62,24 @@ struct HelloSwift {
     assert(!newHello.is(HelloSubclass.self))
 
     // Create a new instance.
-    let helloSubFromSwift = HelloSubclass(greeting: "Hello from Swift", environment: javaEnvironment)
+    let helloSubFromSwift = HelloSubclass("Hello from Swift", environment: javaEnvironment)
     helloSubFromSwift.greetMe()
 
     do {
-      try throwMessage(message: "I am an error")
+      try throwMessage("I am an error")
     } catch {
       print("Caught Java error: \(error)")
     }
 
     return i * j
   }
 
-  @ImplementsJava
-  func throwMessageFromSwift(message: String) throws -> String {
+  @JavaMethod
+  func throwMessageFromSwift(_ message: String) throws -> String {
     throw SwiftWrappedError.message(message)
   }
 }
 
-extension JavaClass<HelloSwift> {
-  @JavaStaticField
-  var initialValue: Double
-}
-
-@JavaClass("com.example.swift.HelloSubclass", extends: HelloSwift.self)
-struct HelloSubclass {
-  @JavaField
-  var greeting: String
-
-  @JavaMethod
-  func greetMe()
-
-  @JavaMethod
-  init(greeting: String, environment: JNIEnvironment)
-}
-
-
 func removeLast(arrayList: ArrayList<JavaClass<HelloSwift>>) {
   if let lastObject = arrayList.getLast() {
     _ = arrayList.remove(lastObject)

Samples/JavaKitSampleApp/Sources/JavaKitExample/com/example/swift/HelloSubclass.java

@@ -21,7 +21,7 @@ public HelloSubclass(String greeting) {
         this.greeting = greeting;
     }
 
-    private void greetMe() {
+    public void greetMe() {
         super.greet(greeting);
     }
 }

Samples/JavaKitSampleApp/Sources/JavaKitExample/com/example/swift/HelloSwift.java

@@ -15,9 +15,9 @@
 package com.example.swift;
 
 public class HelloSwift {
-    private double value;
-    private static double initialValue = 3.14159;
-    private String name = "Java";
+    public double value;
+    public static double initialValue = 3.14159;
+    public String name = "Java";
 
     static {
         System.loadLibrary("JavaKitExample");
@@ -31,7 +31,7 @@ public HelloSwift() {
     public native String throwMessageFromSwift(String message) throws Exception;
 
     // To be called back by the native code
-    private double sayHelloBack(int i) {
+    public double sayHelloBack(int i) {
         System.out.println("And hello back from " + name + "! You passed me " + i);
         return value;
     }
@@ -40,7 +40,7 @@ public void greet(String name) {
         System.out.println("Salutations, " + name);
     }
 
-    String[] doublesToStrings(double[] doubles) {
+    public String[] doublesToStrings(double[] doubles) {
         int size = doubles.length;
         String[] strings = new String[size];
 

Samples/JavaKitSampleApp/Sources/JavaKitExample/com/example/swift/JavaKitSampleMain.java

@@ -19,7 +19,6 @@
  * For the Swift implementation refer to
  */
 public class JavaKitSampleMain {
-
     public static void main(String[] args) {
         int result = new HelloSubclass("Swift").sayHello(17, 25);
         System.out.println("sayHello(17, 25) = " + result);

Sources/Java2Swift/JavaToSwift.swift

@@ -48,6 +48,11 @@ struct JavaToSwift: ParsableCommand {
   )
   var classpath: [String] = []
 
+  @Option(
+    help: "The names of Java classes whose declared native methods will be implemented in Swift."
+  )
+  var swiftNativeImplementation: [String] = []
+
   @Option(name: .shortAndLong, help: "The directory in which to output the generated Swift files or the Java2Swift configuration file.")
   var outputDirectory: String? = nil
 
@@ -181,6 +186,10 @@ struct JavaToSwift: ParsableCommand {
       environment: environment
     )
 
+    // Keep track of all of the Java classes that will have
+    // Swift-native implementations.
+    translator.swiftNativeImplementations = Set(swiftNativeImplementation)
+
     // Note all of the dependent configurations.
     for (swiftModuleName, dependentConfig) in dependentConfigs {
       translator.addConfiguration(