Improve default Java home location and add testing for JavaVirtualMachine.shared

.github/workflows/pull_request.yml

@@ -15,10 +15,24 @@ jobs:
       enable_windows_checks: true
       windows_swift_versions: '["nightly-main"]'
       enable_macos_checks: true
+      macos_pre_build_command: brew install openjdk
       enable_ios_checks: false
       enable_linux_checks: true
       linux_os_versions: '["amazonlinux2", "bookworm", "noble", "jammy", "rhel-ubi9"]'
       linux_exclude_swift_versions: "[{\"swift_version\": \"5.9\"}, {\"swift_version\": \"5.10\"}]"
+      linux_pre_build_command: |
+        if command -v apt-get >/dev/null 2>&1 ; then # bookworm, noble, jammy
+          if command -v sudo &> /dev/null && [ "$EUID" -ne 0 ]; then
+            sudo apt-get update -y
+            sudo apt-get install -y default-jdk
+          else
+            apt-get update -y
+            apt-get install -y default-jdk
+          fi
+        elif command -v yum >/dev/null 2>&1 ; then # amazonlinux2, rhel-ubi9
+          yum update -y
+          yum install -y java-devel
+        fi
       enable_android_sdk_build: true
       enable_android_sdk_checks: true
   soundness:

Sources/SwiftJavaJNICore/VirtualMachine/JavaVirtualMachine.swift

@@ -427,6 +427,42 @@ private func symbol<T>(_ handle: DylibType, _ name: String) -> T? {
 }
 #endif
 
+/// The Java installation folder for the system, found either with `JAVA_HOME` environment or by checking standard locations
+func systemJavaHome() -> String? {
+  // always defer to JAVA_HOME when it is set in the environment
+  if let javaHome = ProcessInfo.processInfo.environment["JAVA_HOME"] {
+    return javaHome
+  }
+
+  // if JAVA_HOME is unset, hunt around some standard locations
+  var defaultJavaPaths: [String] = []
+
+  #if os(macOS)
+  #if arch(x86_64)
+  defaultJavaPaths += ["/usr/local/opt/java"] // macOS Homebrew x86
+  #else
+  defaultJavaPaths += ["/opt/homebrew/opt/java"] // macOS Homebrew
+  #endif
+  defaultJavaPaths += ["/System/Library/Frameworks/JavaVM.framework/Home"] // macOS legacy path
+  #endif
+
+  #if os(Linux)
+  defaultJavaPaths += [
+    "/usr/lib/jvm/default-java", // Ubuntu/Debian
+    "/usr/lib/jvm/default", // Arch
+    "/usr/lib/jvm/java", // rhel-ubi9, amazonlinux2
+  ]
+  #endif
+
+  if let javaHome = defaultJavaPaths.first(where: {
+    FileManager.default.fileExists(atPath: $0)
+  }) {
+    return javaHome
+  }
+
+  return nil
+}
+
 /// Located the shared library that includes the `JNI_GetCreatedJavaVMs` and `JNI_CreateJavaVM` entry points to the `JNINativeInterface` function table
 private func loadLibJava() throws -> DylibType {
   #if os(Android)
@@ -437,20 +473,7 @@ private func loadLibJava() throws -> DylibType {
   }
   #endif
 
-  guard
-    let javaHome = ProcessInfo.processInfo.environment["JAVA_HOME"]
-      ?? {
-        // if JAVA_HOME is unset, look in some standard locations
-        [
-          "/opt/homebrew/opt/java", // macOS Homebrew
-          "/usr/local/opt/java",
-          "/usr/lib/jvm/default-java", // Ubuntu/Debian
-          "/usr/lib/jvm/default", // Arch
-        ].first(where: {
-          FileManager.default.fileExists(atPath: $0)
-        })
-      }()
-  else {
+  guard let javaHome = systemJavaHome() else {
     throw JavaVirtualMachine.VMError.javaHomeNotFound
   }
 

Tests/SwiftJavaJNICoreTests/JavaVirtualMachineTests.swift

@@ -0,0 +1,62 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2026 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 Testing
+
+@testable import SwiftJavaJNICore
+
+@Suite
+struct JavaVirtualMachineTests {
+
+  static var isSupportedPlatform: Bool {
+    #if os(Android)
+    // Android tests are not currently run within an .apk and so do not have any ambient JVM
+    return false
+    #else
+    // disable test when we cannot find a system Java
+    return systemJavaHome() != nil
+    #endif
+  }
+
+  @Test(.enabled(if: isSupportedPlatform))
+  func loadJVMAndCallMethod() throws {
+    // Load or create a shared JVM instance
+    let jvm = try JavaVirtualMachine.shared()
+
+    // Get the JNI environment for the current thread
+    let env = try jvm.environment()
+
+    // Find java.lang.System
+    let systemClass = env.interface.FindClass(env, "java/lang/System")
+    #expect(systemClass != nil, "Failed to find java.lang.System")
+
+    // Look up the static method currentTimeMillis()J
+    let methodID = env.interface.GetStaticMethodID(
+      env,
+      systemClass,
+      "currentTimeMillis",
+      "()J"
+    )
+    #expect(methodID != nil, "Failed to find System.currentTimeMillis")
+
+    // Call System.currentTimeMillis() — returns jlong
+    let timeMillis = env.interface.CallStaticLongMethodA(
+      env,
+      systemClass,
+      methodID,
+      nil
+    )
+    #expect(timeMillis > 0, "Expected a positive timestamp, got \(timeMillis)")
+  }
+}