When multiple clients request a servlet, the engine initiates a thread for each client, and the member variables of the servlet class are shared by all threads.

Because the implementation of the servlet is a singleton, multithreading
That is, n clients request the same servlet, they request the same object, the member variables belong to the object, and therefore the member variables are shared.
Thus, in servlet programming, a stateless servlet (which is not a write property, so the variables are in Dopost or doget) is thread-safe, otherwise, it might not be thread-safe because it is shared to member variables.

When multiple clients request a servlet, the engine initiates a thread for each client, and the member variables of the servlet class are shared by all threads.