Spring boot Async

I am not sure how does Spring boot async work? I know that webserver has its own thread pool and every request is handled by a thread from the pool. Now when we implement asynchronous web service with spring boot and configure let's say 10 threads, does that mean there is thread inside a thread? To clarify, the web server will assign a request to a thread. Now when this thread starts execution and calls a service/function marked with @Asynch, it can create multiple threads to handle the request. Please let me know if my understanding is correct.

Its not a thread inside another. @Async will make the request execute in a separate thread and the caller of the request will not wait for called method completion

To enable Async, you can do by adding,

@Configuration
@EnableAsync
public class AsyncExample {
   // Define a threadpool executor if needed,
   @Bean(name = "threadPoolTaskExecutor")
   public Executor threadPoolTaskExecutor() {
      return new ThreadPoolTaskExecutor();
   }
}

Then you can define the async method as below with the defined threadpool executor,

@Async("threadPoolTaskExecutor")
public void asyncMethodWithConfiguredExecutor() {
   // Method Definition
}

Using @Async will make it execute in a separate thread i.e. the caller will not wait for the completion of the called method.
And you can configure the used Executor

@Configuration
@EnableAsync
public class SpringAsyncConfig {
     
    @Bean(name = "fooExecutor")
    public Executor fooExecutor() {
        return new ThreadPoolTaskExecutor();
    }
}

And using @Async("fooExecutor")

You can make a configuration as following to control more the thread parameters such as the name of thread with prefix, the pool size,... in this way you ensure that you control the thread pool.

   @Configuration
    @EnableAsync
    public class AsyncConfiguration implements AsyncConfigurer {
    
        @Override
        public Executor getAsyncExecutor() {
            ThreadPoolTaskExecutor executor = new ThreadPoolTaskExecutor();
            executor.setCorePoolSize(5);
            executor.setMaxPoolSize(10);
            executor.setQueueCapacity(5000);
            executor.setThreadNamePrefix("MyThread-");
            executor.setWaitForTasksToCompleteOnShutdown(true);
            executor.initialize();
            return executor;
        }
    
        @Override
        public AsyncUncaughtExceptionHandler getAsyncUncaughtExceptionHandler() {
            return new SimpleAsyncUncaughtExceptionHandler();
        }
    }

And you put the @Async annotation on the code bean or the method that you want to make it asynchronous.

For more information about this subject you can refer to the scheduling documentation of spring here.

I hope you will find this helpful