Add a SwiftPM build plugin to generate Swift wrappers for Java classes

Package.swift

@@ -56,6 +56,11 @@ let package = Package(
       targets: ["JavaKit"]
     ),
 
+    .library(
+      name: "JavaRuntime",
+      targets: [ "JavaRuntime"]
+    ),
+
     .library(
       name: "JavaKitJar",
       targets: ["JavaKitReflection"]
@@ -83,7 +88,7 @@ let package = Package(
 
     .executable(
       name: "Java2Swift",
-      targets: ["Java2SwiftTool"]
+      targets: ["Java2Swift"]
     ),
 
     // ==== Plugin for building Java code
@@ -94,6 +99,14 @@ let package = Package(
         ]
     ),
 
+    // ==== Plugin for wrapping Java classes in Swift
+    .plugin(
+        name: "Java2SwiftPlugin",
+        targets: [
+            "Java2SwiftPlugin"
+        ]
+    ),
+
     // ==== jextract-swift (extract Java accessors from Swift interface files)
 
     .executable(
@@ -206,6 +219,14 @@ let package = Package(
         capability: .buildTool()
     ),
 
+    .plugin(
+        name: "Java2SwiftPlugin",
+        capability: .buildTool(),
+        dependencies: [
+          "Java2Swift"
+        ]
+    ),
+
     .target(
       name: "ExampleSwiftLibrary",
       dependencies: [],
@@ -251,7 +272,7 @@ let package = Package(
     ),
 
     .executableTarget(
-      name: "Java2SwiftTool",
+      name: "Java2Swift",
       dependencies: [
         .product(name: "SwiftBasicFormat", package: "swift-syntax"),
         .product(name: "SwiftSyntax", package: "swift-syntax"),

Plugins/Java2SwiftPlugin/Configuration.swift

@@ -0,0 +1,26 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+/// Configuration for the Java2Swift translation tool, provided on a per-target
+/// basis.
+struct Configuration: Codable {
+  /// The Java class path that should be passed along to the Java2Swift tool.
+  var classPath: String? = nil
+
+  /// The Java classes that should be translated to Swift. The keys are
+  /// canonical Java class names (e.g., java.util.Vector) and the values are
+  /// the corresponding Swift names (e.g., JavaVector). If the value is `nil`,
+  /// then the Java class name will be used for the Swift name, too.
+  var classes: [String: String?] = [:]
+}

Plugins/Java2SwiftPlugin/Java2SwiftPlugin.swift

@@ -0,0 +1,154 @@
+//===----------------------------------------------------------------------===//
+//
+// 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 Java2SwiftBuildToolPlugin: BuildToolPlugin {
+  func createBuildCommands(context: PluginContext, target: Target) throws -> [Command] {
+    guard let sourceModule = target.sourceModule else { return [] }
+
+    // Note: Target doesn't have a directoryURL counterpart to directory,
+    // so we cannot eliminate this deprecation warning.
+    let sourceDir = target.directory.string
+
+    // Read a configuration file JavaKit.config from the target that provides
+    // information needed to call Java2Swift.
+    let configFile = URL(filePath: sourceDir)
+      .appending(path: "Java2Swift.config")
+    let configData = try Data(contentsOf: configFile)
+    let config = try JSONDecoder().decode(Configuration.self, from: configData)
+
+    /// Find the manifest files from other Java2Swift executions in any targets
+    /// this target depends on.
+    var manifestFiles: [URL] = []
+    func searchForManifestFiles(in target: any Target) {
+      let dependencyURL = URL(filePath: target.directory.string)
+
+      // Look for a checked-in manifest file.
+      let generatedManifestURL = dependencyURL
+        .appending(path: "generated")
+        .appending(path: "\(target.name).swift2java")
+      let generatedManifestString = generatedManifestURL
+        .path(percentEncoded: false)
+
+      if FileManager.default.fileExists(atPath: generatedManifestString) {
+        manifestFiles.append(generatedManifestURL)
+      }
+
+      // TODO: Look for a manifest file that was built by the plugin itself.
+    }
+
+    // Process direct dependencies of this target.
+    for dependency in target.dependencies {
+      switch dependency {
+      case .target(let target):
+        searchForManifestFiles(in: target)
+
+      case .product(let product):
+        for target in product.targets {
+          searchForManifestFiles(in: target)
+        }
+
+      @unknown default:
+        break
+      }
+    }
+
+    // Process indirect target dependencies.
+    for dependency in target.recursiveTargetDependencies {
+      searchForManifestFiles(in: dependency)
+    }
+
+    /// Determine the list of Java classes that will be translated into Swift,
+    /// along with the names of the corresponding Swift types. This will be
+    /// passed along to the Java2Swift tool.
+    let classes = config.classes.map { (javaClassName, swiftName) in
+      (javaClassName, swiftName ?? javaClassName.defaultSwiftNameForJavaClass)
+    }.sorted { (lhs, rhs) in
+      lhs.0 < rhs.0
+    }
+
+    let outputDirectory = context.pluginWorkDirectoryURL
+      .appending(path: "generated")
+
+    var arguments: [String] = [
+      "--module-name", sourceModule.name,
+      "--output-directory", outputDirectory.path(percentEncoded: false),
+    ]
+    if let classPath = config.classPath {
+      arguments += ["-cp", classPath]
+    }
+    arguments += manifestFiles.flatMap { manifestFile in
+      [ "--manifests", manifestFile.path(percentEncoded: false) ]
+    }
+    arguments += classes.map { (javaClassName, swiftName) in
+      "\(javaClassName)=\(swiftName)"
+    }
+
+    /// Determine the set of Swift files that will be emitted by the Java2Swift
+    /// tool.
+    let outputSwiftFiles = classes.map { (javaClassName, swiftName) in
+      outputDirectory.appending(path: "\(swiftName).swift")
+    } + [
+      outputDirectory.appending(path: "\(sourceModule.name).swift2java")
+    ]
+
+    return [
+      .buildCommand(
+        displayName: "Wrapping \(classes.count) Java classes target \(sourceModule.name) in Swift",
+        executable: try context.tool(named: "Java2Swift").url,
+        arguments: arguments,
+        inputFiles: [ configFile ],
+        outputFiles: outputSwiftFiles
+      )
+    ]
+  }
+}
+
+// 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.")
+}
+
+extension String {
+  /// For a String that's of the form java.util.Vector, return the "Vector"
+  /// part.
+  fileprivate var defaultSwiftNameForJavaClass: String {
+    if let dotLoc = lastIndex(of: ".") {
+      let afterDot = index(after: dotLoc)
+      return String(self[afterDot...])
+    }
+
+    return self
+  }
+}

Samples/JavaKitSampleApp/Package.swift

@@ -30,14 +30,16 @@ let package = Package(
     .target(
       name: "JavaKitExample",
       dependencies: [
-        .product(name: "JavaKit", package: "swift-java")
+        .product(name: "JavaKit", package: "swift-java"),
+        .product(name: "JavaKitJar", package: "swift-java"),
       ],
       swiftSettings: [
         .swiftLanguageMode(.v5)
       ],
       plugins: [
+        .plugin(name: "Java2SwiftPlugin", package: "swift-java"),
         .plugin(name: "JavaCompilerPlugin", package: "swift-java")
       ]
     ),
   ]
-)
+)

Samples/JavaKitSampleApp/Sources/JavaKitExample/Java2Swift.config

@@ -0,0 +1,5 @@
+{
+  "classes" : {
+    "java.util.Vector" : null
+  }
+}

Samples/JavaKitSampleApp/Sources/JavaKitExample/JavaKitExample.swift

@@ -13,7 +13,6 @@
 //===----------------------------------------------------------------------===//
 
 import JavaKit
-import JavaRuntime
 
 enum SwiftWrappedError: Error {
   case message(String)
@@ -118,3 +117,8 @@ struct HelloSubclass {
   @JavaMethod
   init(greeting: String, environment: JNIEnvironment)
 }
+
+
+func returnNilVector() -> Vector<JavaClass<HelloSwift>>? {
+  nil
+}