Normalize headers in components (#22672)

* normalize headers in components

* update comment

Author: wing328

modules/openapi-generator/src/main/java/org/openapitools/codegen/OpenAPINormalizer.java

@@ -356,6 +356,7 @@ protected void normalize() {
         normalizeComponentsSecuritySchemes();
         normalizeComponentsSchemas();
         normalizeComponentsResponses();
+        normalizeComponentsHeaders();
     }
 
     /**
@@ -558,8 +559,21 @@ protected void normalizeHeaders(Map<String, Header> headers) {
 
         for (String headerKey : headers.keySet()) {
             Header h = headers.get(headerKey);
-            Schema updatedHeader = normalizeSchema(h.getSchema(), new HashSet<>());
-            h.setSchema(updatedHeader);
+            if (h.getSchema() != null) { // not a $ref header
+                // example of header class
+                //    description: null
+                //    required: null
+                //    deprecated: null
+                //    style: null
+                //    explode: null
+                //    schema: null
+                //    examples: null
+                //    example: null
+                //    content: null
+                //    $ref: #/components/headers/Location
+                Schema updatedHeader = normalizeSchema(h.getSchema(), new HashSet<>());
+                h.setSchema(updatedHeader);
+            }
         }
     }
 
@@ -638,6 +652,18 @@ protected void normalizeComponentsResponses() {
         }
     }
 
+    /**
+     * Normalizes schemas in component's headers.
+     */
+    protected void normalizeComponentsHeaders() {
+        Map<String, Header> headers = openAPI.getComponents().getHeaders();
+        if (headers == null) {
+            return;
+        }
+
+        normalizeHeaders(headers);
+    }
+
     /**
      * Auto fix a self referencing schema using any type to replace the self-referencing sub-item.
      *