wip towards making jars with dylibs

This allows us to include dylibs in the jar and load them as resources.

This allows us to:

```
-> % java --enable-native-access=ALL-UNNAMED -cp
../../SwiftKit/build/classes/java/main:./build/libs/swift-and-java-jar-sample-lib-1.0-SNAPSHOT.jar
-Djava.library.path=/usr/lib/swift/ Example.java
[swift][MySwiftLibrary/MySwiftLibrary.swift:27](helloWorld())
helloWorld()
```

and we can even include the swiftCore libs in the jar to make it really
fat and include all swift dependencies. I think we should maybe make
that an option in the inevitable gradle plugin we'll need to do here.

Author: ktoso

Plugins/JExtractSwiftPlugin/JExtractSwiftPlugin.swift

@@ -23,6 +23,16 @@ struct JExtractSwiftBuildToolPlugin: BuildToolPlugin {
     // Note: Target doesn't have a directoryURL counterpart to directory,
     // so we cannot eliminate this deprecation warning.
     let sourceDir = target.directory.string
+    
+    let t = target.dependencies.first!
+    switch (t) {
+    case .target(let t):
+      t.sourceModule
+    case .product(let p):
+      p.sourceModules
+    @unknown default:
+      fatalError("Unknown target dependency type: \(t)")
+    }
 
     let toolURL = try context.tool(named: "JExtractSwiftTool").url
     let configuration = try readConfiguration(sourceDir: "\(sourceDir)")

Samples/SwiftAndJavaJarSampleLib/Example.java

@@ -0,0 +1,23 @@
+//===----------------------------------------------------------------------===//
+//
+// 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 com.example.swift.MySwiftLibrary;
+
+public class Example {
+
+    public static void main(String[] args) {
+        MySwiftLibrary.helloWorld();
+    }
+
+}

Samples/SwiftAndJavaJarSampleLib/Package.swift

@@ -0,0 +1,77 @@
+// swift-tools-version: 6.0
+// The swift-tools-version declares the minimum version of Swift required to build this package.
+
+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: "SwiftAndJavaJarSampleLib",
+  platforms: [
+    .macOS(.v10_15)
+  ],
+  products: [
+    .library(
+      name: "MySwiftLibrary",
+      type: .dynamic,
+      targets: ["MySwiftLibrary"]
+    ),
+
+  ],
+  dependencies: [
+    .package(name: "swift-java", path: "../../"),
+  ],
+  targets: [
+    .target(
+      name: "MySwiftLibrary",
+      dependencies: [
+        .product(name: "SwiftKitSwift", package: "swift-java"),
+      ],
+      exclude: [
+        "swift-java.config",
+      ],
+      swiftSettings: [
+        .swiftLanguageMode(.v5),
+        .unsafeFlags(["-I\(javaIncludePath)", "-I\(javaPlatformIncludePath)"])
+      ],
+      plugins: [
+        .plugin(name: "JExtractSwiftPlugin", package: "swift-java"),
+      ]
+    ),
+  ]
+)

Samples/SwiftAndJavaJarSampleLib/Sources/MySwiftLibrary/MySwiftLibrary.swift

@@ -0,0 +1,101 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// This is a "plain Swift" file containing various types of declarations,
+// that is exported to Java by using the `jextract-swift` tool.
+//
+// No annotations are necessary on the Swift side to perform the export.
+
+#if os(Linux)
+import Glibc
+#else
+import Darwin.C
+#endif
+
+public func helloWorld() {
+  p("\(#function)")
+}
+
+public func globalTakeInt(i: Int) {
+  p("i:\(i)")
+}
+
+public func globalTakeIntInt(i: Int, j: Int) {
+  p("i:\(i), j:\(j)")
+}
+
+public func globalCallMeRunnable(run: () -> ()) {
+  run()
+}
+
+public class MySwiftClass {
+
+  public var len: Int
+  public var cap: Int
+
+  public init(len: Int, cap: Int) {
+    self.len = len
+    self.cap = cap
+
+    p("\(MySwiftClass.self).len = \(self.len)")
+    p("\(MySwiftClass.self).cap = \(self.cap)")
+    let addr = unsafeBitCast(self, to: UInt64.self)
+    p("initializer done, self = 0x\(String(addr, radix: 16, uppercase: true))")
+  }
+
+  deinit {
+    let addr = unsafeBitCast(self, to: UInt64.self)
+    p("Deinit, self = 0x\(String(addr, radix: 16, uppercase: true))")
+  }
+
+  public var counter: Int32 = 0
+
+  public func voidMethod() {
+    p("")
+  }
+
+  public func takeIntMethod(i: Int) {
+    p("i:\(i)")
+  }
+
+  public func echoIntMethod(i: Int) -> Int {
+    p("i:\(i)")
+    return i
+  }
+
+  public func makeIntMethod() -> Int {
+    p("make int -> 12")
+    return 12
+  }
+
+  public func makeRandomIntMethod() -> Int {
+    return Int.random(in: 1..<256)
+  }
+}
+
+// ==== Internal helpers
+
+private func p(_ msg: String, file: String = #fileID, line: UInt = #line, function: String = #function) {
+  print("[swift][\(file):\(line)](\(function)) \(msg)")
+  fflush(stdout)
+}
+
+#if os(Linux)
+// FIXME: why do we need this workaround?
+@_silgen_name("_objc_autoreleaseReturnValue")
+public func _objc_autoreleaseReturnValue(a: Any) {}
+
+@_silgen_name("objc_autoreleaseReturnValue")
+public func objc_autoreleaseReturnValue(a: Any) {}
+#endif

Samples/SwiftAndJavaJarSampleLib/Sources/MySwiftLibrary/swift-java.config

@@ -0,0 +1,3 @@
+{
+  "javaPackage": "com.example.swift"
+}