Introduce a SwiftPM plugin to compile Java sources

Makefile

@@ -63,8 +63,8 @@ all:
 $(BUILD_DIR)/debug/libJavaKit.$(LIB_SUFFIX) $(BUILD_DIR)/debug/Java2Swift:
 	swift build
 
-javakit-run: $(BUILD_DIR)/debug/libJavaKit.$(LIB_SUFFIX) $(BUILD_DIR)/debug/libExampleSwiftLibrary.$(LIB_SUFFIX)
-	./gradlew Samples:JavaKitSampleApp:run
+javakit-run:
+	cd Samples/JavaKitSampleApp && swift build && java -cp .build/plugins/outputs/javakitsampleapp/JavaKitExample/destination/JavaCompilerPlugin/Java -Djava.library.path=.build/debug com.example.swift.JavaKitSampleMain
 
 Java2Swift: $(BUILD_DIR)/debug/Java2Swift
 

Package.swift

@@ -86,6 +86,14 @@ let package = Package(
       targets: ["Java2SwiftTool"]
     ),
 
+    // ==== Plugin for building Java code
+    .plugin(
+        name: "JavaCompilerPlugin",
+        targets: [
+            "JavaCompilerPlugin"
+        ]
+    ),
+
     // ==== jextract-swift (extract Java accessors from Swift interface files)
 
     .executable(
@@ -107,11 +115,6 @@ let package = Package(
 
     // ==== Examples
 
-    .library(
-      name: "JavaKitExample",
-      type: .dynamic,
-      targets: ["JavaKitExample"]
-    ),
     .library(
       name: "ExampleSwiftLibrary",
       type: .dynamic,
@@ -197,14 +200,12 @@ let package = Package(
         .linkedLibrary("jvm"),
       ]
     ),
-    .target(
-      name: "JavaKitExample",
-      dependencies: ["JavaKit"],
-      swiftSettings: [
-        .swiftLanguageMode(.v5),
-        .unsafeFlags(["-I\(javaIncludePath)", "-I\(javaPlatformIncludePath)"])
-      ]
+
+    .plugin(
+        name: "JavaCompilerPlugin",
+        capability: .buildTool()
     ),
+
     .target(
       name: "ExampleSwiftLibrary",
       dependencies: [],

Plugins/JavaCompilerPlugin/JavaCompilerPlugin.swift

@@ -0,0 +1,90 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2024 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
+//
+//===----------------------------------------------------------------------===//
+
+import Foundation
+import PackagePlugin
+
+@main
+struct JavaCompilerBuildToolPlugin: BuildToolPlugin {
+  func createBuildCommands(context: PluginContext, target: Target) throws -> [Command] {
+    guard let sourceModule = target.sourceModule else { return [] }
+
+    // Collect all of the Java source files within this target's sources.
+    let javaFiles = sourceModule.sourceFiles.map { $0.url }.filter {
+      $0.pathExtension == "java"
+    }
+    if javaFiles.isEmpty {
+      return []
+    }
+
+    // Note: Target doesn't have a directoryURL counterpart to directory,
+    // so we cannot eliminate this deprecation warning.
+    let sourceDir = target.directory.string
+
+    // The class files themselves will be generated into the build directory
+    // for this target.
+    let classFiles = javaFiles.map { sourceFileURL in
+      let sourceFilePath = sourceFileURL.path
+      guard sourceFilePath.starts(with: sourceDir) else {
+        fatalError("Could not get relative path for source file \(sourceFilePath)")
+      }
+
+      return URL(
+        filePath: context.pluginWorkDirectoryURL.path
+      ).appending(path: "Java")
+       .appending(path: String(sourceFilePath.dropFirst(sourceDir.count)))
+       .deletingPathExtension()
+       .appendingPathExtension("class")
+    }
+
+    let javaHome = URL(filePath: findJavaHome())
+    let javaClassFileURL = context.pluginWorkDirectoryURL
+      .appending(path: "Java")
+    return [
+      .buildCommand(
+        displayName: "Compiling \(javaFiles.count) Java files for target \(sourceModule.name) to \(javaClassFileURL)",
+        executable: javaHome
+          .appending(path: "bin")
+          .appending(path: "javac"),
+        arguments: javaFiles.map { $0.path(percentEncoded: false) } + [
+          "-d", javaClassFileURL.path()
+        ],
+        inputFiles: javaFiles,
+        outputFiles: classFiles
+      )
+    ]
+  }
+}
+
+// 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.")
+}

README.md

@@ -95,7 +95,9 @@ Sample apps are located in the `Samples/` directory, and they showcase full "rou
 To run a simple app showcasing a Swift process calling into a Java library you can run: 
 
 ```bash
-./gradlew Samples:JavaKitSampleApp:run
+cd Samples/JavaKitSampleApp
+swift build
+java -cp .build/plugins/outputs/javakitsampleapp/JavaKitExample/destination/JavaCompilerPlugin/Java -Djava.library.path=.build/debug com.example.swift.JavaKitSampleMain
 ```
 
 #### jextract (Java -> Swift)

Samples/JavaKitSampleApp/Package.swift

@@ -0,0 +1,43 @@
+// swift-tools-version: 6.0
+// The swift-tools-version declares the minimum version of Swift required to build this package.
+
+import CompilerPluginSupport
+import PackageDescription
+
+let package = Package(
+  name: "JavaKitSampleApp",
+  platforms: [
+    .macOS(.v13),
+    .iOS(.v13),
+    .tvOS(.v13),
+    .watchOS(.v6),
+    .macCatalyst(.v13),
+  ],
+
+  products: [
+    .library(
+      name: "JavaKitExample",
+      type: .dynamic,
+      targets: ["JavaKitExample"]
+    ),
+  ],
+
+  dependencies: [
+    .package(name: "swift-java", path: "../../")
+  ],
+
+  targets: [
+    .target(
+      name: "JavaKitExample",
+      dependencies: [
+        .product(name: "JavaKit", package: "swift-java")
+      ],
+      swiftSettings: [
+        .swiftLanguageMode(.v5)
+      ],
+      plugins: [
+        .plugin(name: "JavaCompilerPlugin", package: "swift-java")
+      ]
+    ),
+  ]
+)

Samples/JavaKitSampleApp/Sources/JavaKitExample/JavaKitExample.swift

@@ -0,0 +1,120 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2024 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
+//
+//===----------------------------------------------------------------------===//
+
+import JavaKit
+import JavaRuntime
+
+enum SwiftWrappedError: Error {
+  case message(String)
+}
+
+@JavaClass("com.example.swift.HelloSwift")
+struct HelloSwift {
+  @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 {
+    print("Hello from Swift!")
+    let answer = self.sayHelloBack(i + j)
+    print("Swift got back \(answer) from Java")
+
+    print("We expect the above value to be the initial value, \(self.javaClass.initialValue)")
+
+    print("Updating Java field value to something different")
+    self.value = 2.71828
+
+    let newAnswer = self.sayHelloBack(17)
+    print("Swift got back updated \(newAnswer) from Java")
+
+    let newHello = HelloSwift(environment: javaEnvironment)
+    print("Swift created a new Java instance with the value \(newHello.value)")
+
+    let name = newHello.name
+    print("Hello to \(name)")
+    newHello.greet("Swift 👋🏽 How's it going")
+
+    self.name = "a 🗑️-collected language"
+    _ = self.sayHelloBack(42)
+
+    let strings = doublesToStrings(doubles: [3.14159, 2.71828])
+    print("Converting doubles to strings: \(strings)")
+
+    // Try downcasting
+    if let helloSub = self.as(HelloSubclass.self) {
+      print("Hello from the subclass!")
+      helloSub.greetMe()
+
+      assert(helloSub.super.value == 2.71828)
+    } else {
+      fatalError("Expected subclass here")
+    }
+
+    // Check "is" behavior
+    assert(newHello.is(HelloSwift.self))
+    assert(!newHello.is(HelloSubclass.self))
+
+    // Create a new instance.
+    let helloSubFromSwift = HelloSubclass(greeting: "Hello from Swift", environment: javaEnvironment)
+    helloSubFromSwift.greetMe()
+
+    do {
+      try throwMessage(message: "I am an error")
+    } catch {
+      print("Caught Java error: \(error)")
+    }
+
+    return i * j
+  }
+
+  @ImplementsJava
+  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)
+}