JavaKit: check each classPath element exists before starting VM

Fixes #136.

Author: lhoward

Sources/JavaKit/JavaKitVM/JavaVirtualMachine.swift

@@ -12,6 +12,12 @@
 //
 //===----------------------------------------------------------------------===//
 
+#if canImport(FoundationEssentials)
+import FoundationEssentials
+#else
+import Foundation
+#endif
+
 typealias JavaVMPointer = UnsafeMutablePointer<JavaVM?>
 #if canImport(Android)
 typealias JNIEnvPointer = UnsafeMutablePointer<JNIEnv?>
@@ -59,6 +65,12 @@ public final class JavaVirtualMachine: @unchecked Sendable {
     // Construct the complete list of VM options.
     var allVMOptions: [String] = []
     if !classPath.isEmpty {
+      let fileManager = FileManager.default
+      for path in classPath {
+        if !fileManager.fileExists(atPath: path) {
+          throw JavaKitError.classPathNotFound(path)
+        }
+      }
       let colonSeparatedClassPath = classPath.joined(separator: ":")
       allVMOptions.append("-Djava.class.path=\(colonSeparatedClassPath)")
     }
@@ -283,4 +295,8 @@ extension JavaVirtualMachine {
       }
     }
   }
+
+  enum JavaKitError: Error {
+    case classPathNotFound(String)
+  }
 }