How to determine the number of threads created within a thread pool?

In Java, the number of threads in a pool is determined at its creation, usually through constructor parameters or factory methods of the ThreadPoolExecutor class.

For example, when creating a fixed thread pool using Executors.newFixedThreadPool(int nThreads), a pool with a fixed number of threads nThreads is created.

If ThreadPoolExecutor is used, then the number of threads is controlled by the parameters:

• corePoolSize — the minimum number of threads that are always maintained
• maximumPoolSize — the maximum number of threads that can be created if needed

Example of creating a pool with a fixed number of threads:

ExecutorService executor = Executors.newFixedThreadPool(5); // 5 threads

Or using ThreadPoolExecutor:

ThreadPoolExecutor executor = new ThreadPoolExecutor(
    3, // corePoolSize
    10, // maximumPoolSize
    60, TimeUnit.SECONDS, // keep-alive time
    new LinkedBlockingQueue<>()
);

Thus, to determine the number of threads, you should look at the pool creation parameters, especially corePoolSize and maximumPoolSize.