In the absence of an Executor bean in the context, Spring Boot auto-configures a ThreadPoolTaskExecutor with sensible defaults that can be automatically associated to asynchronous task execution (@EnableAsync) and Spring MVC asynchronous request processing.

Tip

If you have defined a custom Executor in the context, regular task execution (i.e. @EnableAsync) will use it transparently but the Spring MVC support will not be configured as it requires an AsyncTaskExecutor implementation (named applicationTaskExecutor). Depending on your target arrangement, you could change your Executor into a ThreadPoolTaskExecutor or define both a ThreadPoolTaskExecutor and an AsyncConfigurer wrapping your custom Executor. The auto-configured TaskExecutorBuilder allows you to easily create instances that reproduce what the auto-configuration does by default.

The thread pool uses 8 core threads that can grow and shrink according to the load. Those default settings can be fine-tuned using the spring.task.execution namespace as shown in the following example:

spring.task.execution.pool.max-size=16
spring.task.execution.pool.queue-capacity=100
spring.task.execution.pool.keep-alive=10s

This changes the thread pool to use a bounded queue so that when the queue is full (100 tasks), the thread pool increases to maximum 16 threads. Shrinking of the pool is more aggressive as threads are reclaimed when they are idle for 10 seconds (rather than 60 seconds by default).

A ThreadPoolTaskScheduler can also be auto-configured if need to be associated to scheduled task execution (@EnableScheduling). The thread pool uses one thread by default and those settings can be fine-tuned using the spring.task.scheduling namespace.

Both a TaskExecutorBuilder bean and a TaskSchedulerBuilder bean are made available in the context if a custom executor or scheduler needs to be created.