问题
If I want to keep a global counter (e.g. to count number of incoming requests across multiple threads), then the best way to do in java would be to use a volatile int. Assuming, clojure is being used is there a better (better throughput) way to do?
回答1:
I would do this with an atom in Clojure:
(def counter (atom 0N))
;; increment the counter
(swap! counter inc)
;; read the counter
@counter
=> 1
This is totally thread-safe, and surprisingly high performance. Also, since it uses Clojure's abitrary-precision numeric handling, it isn't vulnerable to integer overflows in the way that a volatile int can be.....
回答2:
Define a global counter as an agent
(def counter (agent 0))
To increase the value contained in the agent you send a function (in this case inc) to the agent:
(send counter inc)
To read the current value you can use deref or the @ reader macro:
@counter ;; same as (deref counter)
Agents are only one of several available reference types. You can read more about these things on the Clojure website:
- High-level overview
- Software transactional memory with refs
- Asynchronous agents
- Atoms
来源：https://stackoverflow.com/questions/7461562/synchronized-counter-in-clojure