如何测试我对 ConcurrentHashMap 的使用是线程安全的？

Question

I have written the code below and want to test its concurrency. Even though I have used ConcurrentHashMap , I am not confident that the code is threadsafe because get and update/replace are used on different lines. Can anyone suggest how to test this?

ConcurrentHashMap<String, int> somemap = new ConcurrentHashMap<>(); 
boolean some_method() {
    somemap.putifAbsent("somekey",value); 
    final int val = somemap.get("somekey");
    if(val < total_count) {
        val++;
    }
    somemap.replace("somekey", val);
}

Answer 1

Can anyone suggest how to test this?

Testing for thread safety is not reliable. The reason is, The specification for a programming language or library may promise that if your multi-threaded program obeys certain rules, then you can expect it to behave in a certain way, but you will practically never find a promise that if your program breaks the rules then it will fail to meet your expectations.

I personally have seen a software update go through weeks of unit testing and integration testing, and then be shipped out into the field, where it ran for half a year before a piece of "thread unsafe" code caused it to fail at a customer site.

The only way to be sure that your multi-threaded program will work, is to prove that (A) it says what you think it says, and (B) it obeys the rules.

Your example obeys the rules, but I don't think it does what you think it does. It has a race condition. Namely, two threads both could call get("somekey") at the same time, and both get the same value. Then they both could compute the same new value, and they both could put(...) it back. Two calls to some_method() , but the end result is that the value only goes up by one.