Fix thread safety in Json.mapper() (refs #4672)

Author: Victor Uria Valle

modules/swagger-core/src/main/java/io/swagger/v3/core/util/Json.java

@@ -6,13 +6,12 @@
 
 public class Json {
 
-    private static ObjectMapper mapper;
+    private static final class ObjectMapperHolder {
+        private static final ObjectMapper MAPPER = ObjectMapperFactory.createJson();
+    }
 
     public static ObjectMapper mapper() {
-        if (mapper == null) {
-            mapper = ObjectMapperFactory.createJson();
-        }
-        return mapper;
+        return ObjectMapperHolder.MAPPER;
     }
 
     public static ObjectWriter pretty() {