Add a new validation

Author: mcruzdev

server/deployment/src/main/java/io/quarkiverse/openapi/server/generator/deployment/codegen/ApicurioOpenApiServerCodegen.java

@@ -58,10 +58,7 @@ public boolean shouldRun(Path sourceDir, Config config) {
     public boolean trigger(CodeGenContext context) throws CodeGenException {
         final Path openApiDir = getInputBaseDir(context.inputDir(), context.config());
 
-        if (!Files.exists(openApiDir)) {
-            throw new CodeGenException(
-                    "The OpenAPI input base directory does not exist, please, create the directory on " + context.inputDir());
-        }
+        validateOpenApiDir(context, openApiDir);
 
         final Path outDir = context.outDir();
         final ApicurioCodegenWrapper apicurioCodegenWrapper = new ApicurioCodegenWrapper(
@@ -99,6 +96,19 @@ public boolean trigger(CodeGenContext context) throws CodeGenException {
         return true;
     }
 
+    private static void validateOpenApiDir(CodeGenContext context, Path openApiDir) throws CodeGenException {
+        if (!Files.exists(openApiDir)) {
+            throw new CodeGenException(
+                    "The OpenAPI input base directory does not exist, please, create the directory on " + context.inputDir());
+        }
+
+        if (Files.isDirectory(openApiDir)) {
+            throw new CodeGenException(
+                    "The OpenAPI input base directory is not a directory, please, create the directory on " + context.inputDir()
+            );
+        }
+    }
+
     private File convertToJSON(Path yamlPath) throws CodeGenException {
         try {
             ObjectMapper yamlReader = new ObjectMapper(new YAMLFactory());