Introduce a SwiftPM plugin to compile Java sources

Add a SwiftPM build tool plugin that compiles all of the Java source files
that are part of a given SwiftPM target into class files. Use this within
the JavaKit example so we can build everything through SwiftPM.

Author: DougGregor

Makefile

@@ -58,13 +58,13 @@ all:
 	@echo "  javakit-run: Run the JavaKit example program that uses Java libraries from Swift."
 	@echo "  javakit-generate: Regenerate the Swift wrapper code for the various JavaKit libraries from Java. This only has to be done when changing the Java2Swift tool."
 	@echo "  jextract-run: Run the Java example code that uses the wrapped Swift library. NOTE: this requires development toolchain described in the README."
-	@echo "  jextract-generate: Generate Java wrapper code for the example Swift library allowing Swift to be called from Java. NOTE: this requires development toolchain described in the README."
+	@echo "  jextract-generate: Generate Java wrapper code for the example Swift library allowing Swift to be called from Java. NOTE: this requires development toolchain described ein the README."
 
 $(BUILD_DIR)/debug/libJavaKit.$(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
+	java -cp .build/plugins/outputs/swift-java/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(
@@ -197,14 +205,24 @@ let package = Package(
         .linkedLibrary("jvm"),
       ]
     ),
+
+    .plugin(
+        name: "JavaCompilerPlugin",
+        capability: .buildTool()
+    ),
+
     .target(
       name: "JavaKitExample",
       dependencies: ["JavaKit"],
       swiftSettings: [
         .swiftLanguageMode(.v5),
         .unsafeFlags(["-I\(javaIncludePath)", "-I\(javaPlatformIncludePath)"])
+      ],
+      plugins: [
+        .plugin(name: "JavaCompilerPlugin")
       ]
     ),
+
     .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.")
+}

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

@@ -0,0 +1,27 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+package com.example.swift;
+
+public class HelloSubclass extends HelloSwift {
+    private String greeting;
+
+    public HelloSubclass(String greeting) {
+        this.greeting = greeting;
+    }
+
+    private void greetMe() {
+        super.greet(greeting);
+    }
+}

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

@@ -0,0 +1,57 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+package com.example.swift;
+
+public class HelloSwift {
+    private double value;
+    private static double initialValue = 3.14159;
+    private String name = "Java";
+
+    static {
+        System.loadLibrary("JavaKitExample");
+    }
+
+    public HelloSwift() {
+        this.value = initialValue;
+    }
+
+    public native int sayHello(int x, int y);
+    public native String throwMessageFromSwift(String message) throws Exception;
+
+    // To be called back by the native code
+    private double sayHelloBack(int i) {
+        System.out.println("And hello back from " + name + "! You passed me " + i);
+        return value;
+    }
+
+    public void greet(String name) {
+        System.out.println("Salutations, " + name);
+    }
+
+    String[] doublesToStrings(double[] doubles) {
+        int size = doubles.length;
+        String[] strings = new String[size];
+
+        for(int i = 0; i < size; i++) {
+            strings[i] = "" + doubles[i];
+        }
+
+        return strings;
+    }
+
+    public void throwMessage(String message) throws Exception {
+        throw new Exception(message);
+    }
+}

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

@@ -0,0 +1,27 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+package com.example.swift;
+
+/**
+ * This sample shows off a {@link HelloSwift} type which is partially implemented in Swift.
+ * 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);
+    }
+}