默认情况下，Servlet、Controller、Service、Dao 什么的，在 SpringMVC 中都是单例，spring mvc declaring all beans singleton 有说：
Because Spring beans are typically stateless, you can safely call them from multiple threads. That’s how your application works: there is only one instance of every controller, service, DAO, etc. But your servlet container (through Spring) calls these beans from multiple threads - and it’s completely thread safe.
In fact in plain servlets the situation is the same - there is only instance of each servlet and it can be accessed by infinite number of threads. As long as this servlet is stateless or properly synchronized.
Do not confuse Spring with stateless session beans in ejb that are pooled and each client gets its own instance from the pool. In fact that’s a bit dumb - since the beans are stateless by the definition, there is no point in pooling them and preventing concurrent access…
request1 -> thread1 -> xxxControllerInstance1.process()
request2 -> thread2 -> xxxControllerInstance1.process()
request3 -> thread3 -> xxxControllerInstance1.process()