servlet-hello-world subscription error 405 method no allowed

[docwarems]

I'm looking für sample code for subscriptions without Spring (Boot). The "servlet-hello-world" example should be one. While the "graphql" Servlet works, the "subscriptions" endpoint gives a 405 method not allowed from the introspection query.

The "GraphQL Playground for Chrome" console displays this error:

Same error with GraphiQL Playground