Kotlin : Singleton not thread-safe

Used class is a singleton. In a multithreaded environment, this may lead to a compromise of data.

Singleton exists in a single copy, its fields are available to all users. As a result, a user can access the data of another user.

The Servlet class is a singleton. Its single instance is used to process all requests, some of which may act almost simultaneously from different threads. As a result, data relating to one user after saving into the Servlet class field become available to another user.