Demystifying Spring Task Executor

corePoolSize vs. maxPoolSize vs. queueCapacity

@Bean

  @Primary

  public TaskExecutor taskExecutor() {

    ThreadPoolTaskExecutor executor = new ThreadPoolTaskExecutor();

    executor.setCorePoolSize(3);

    executor.setMaxPoolSize(6);

    executor.setQueueCapacity(4);

    executor.setWaitForTasksToCompleteOnShutdown(false);

    executor.setThreadNamePrefix("my-async");

    executor.initialize();

    return executor;

  }

Core pool size

In a nut shell, Core pool size is the happy path.
When a new task is submitted and fewer than Core Pool Size threads are running, a new thread is created to handle the request, even if other threads are idle. If there are greater than Core Pool Size but fewer than Max Pool Size threads running, a new thread is created only if no threads are idle.

Max pool size

It defines the upper limit of threads that can ever be created! Once limit is reached(no thread is available), no more tasks are accepted.

Queue capacity

Defines the number of tasks to queue when all core pool are filled. Threads will be scalable to maximum pool size when queue is full.

Explain to me like I'm 5 years old

A simple and efficient way to get this is by considering yourself being in a bank.

You ever stood in a line even though there is a window available?
Why would the bank keep a customer in a queue if there are available cash-window? That's exactly the case with core pool size. If there are un-used threads, new tasks are directly allocated to them.

One the available window(core pool size) is full, customers are asked to wait in the queue. This is where the queue-capacity comes in picture! New tasks are keep on queueing until no more tasks can be queued.

What if the wait-lounge is full of customers? Bank can allocate/open new cash-windows as they see spike in number of customers(tasks). They had 3 more in reserve.
Here comes the max pool size in context.

Once these windows(threads) are all occupied, Bank can no longer server any new customers(tasks). Agree? And henceforth, new tasks are not accepted!