This PR is to use LongAdder or volatile long to replace AtomicLong in some scenarios.
In the statistical summation scenario, LongAdder(introduced by jdk1.8) has better performance than AtomicLong in high concurrency update scenario. And if we just want to keep get and set operation for variable to be atomic, just add volatile at the front of the variable is enough, use AtomicLong is a little heavy.
NOTE: LongAdder is usually preferable to AtomicLong when multiple threads update a common sum that is used for purposes such as collecting statistics, not for fine-grained synchronization control, such as auto-incremental ids.
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