Is Spring Batch's ItemWriter a singleton class?

I have a batch job with the following definitions:

<batch:job id="job">
    <batch:split id="main" task-executor="simpleAsyncTaskExecutor">
        <batch:flow>
            <batch:step id="getAccountDetails">
                <batch:tasklet ref="getAccountDetailsTasklet"/>
            </batch:step>
        </batch:flow>
        <batch:flow>
            <batch:step id="processAccounts">
                <batch:tasklet transaction-manager="transactionManager" task-executor="threadPoolTaskExecutor" throttle-limit="${processor.maxThreads}">
                    <batch:chunk reader="queueReader" writer="myCustomItemWriter" commit-interval="${processor.commitInterval}"/>
                </batch:tasklet>
            </batch:step>
        </batch:flow>
    </batch:split>
</batch:job>

myCustomItemWriter basically iterates over the list of accounts passed by the queueReader and submits them to the database.

The job is scalable to 100 threads running the block in parallel. In myCustomItemWriter's class, I have a private property that maintains the sum of specific BigDecimal properties for each account it handles. So if there are 10000 accounts, I will have 100 threads each handling 100 accounts. I want to capture the sum of this attribute for all these 10000 accounts.

Here's my question: Is the ItemWriter a singleton (so just one private property to maintain this amount is enough)? If not, should I define the counter as an AtomicReference bean and inject it into my writer so that the same instance of the property is injected in all 100 threads?