@Async — how does it work? What is the default executor?

@Async works through an AOP proxy: when you call an @Async method through the bean's proxy, the proxy submits the method to a TaskExecutor and returns to the caller immediately, while the actual body runs on a pool thread. It's the same proxy mechanism as @Transactional — the annotation is meaningless until @EnableAsync registers the AsyncAnnotationBeanPostProcessor that wraps matching beans.

What happens on a call

@Service
public class MailService {
    @Async
    public void send(String to) {            // returns void
        // runs on a TaskExecutor thread, not the caller's
        smtp.deliver(to);
    }
}

The caller's thread is freed the instant send() is invoked; the actual SMTP work happens elsewhere. If the method returns a Future/CompletableFuture, the proxy hands back a placeholder the caller can poll or chain later.

The default executor in Spring Boot 3

This is the precise part interviewers probe. Plain Spring (no Boot) historically defaulted to a SimpleAsyncTaskExecutor, which creates a new thread per task — fine for demos, dangerous in production. Spring Boot 3 auto-configures a ThreadPoolTaskExecutor bean named applicationTaskExecutor (also exposed as taskExecutor), and @Async picks it up automatically. Defaults: core pool 8, unbounded queue, max effectively unbounded — sensible but worth tuning.

spring.task.execution.pool.core-size=16
spring.task.execution.pool.max-size=64
spring.task.execution.pool.queue-capacity=1000

If spring.threads.virtual.enabled=true (Boot 3.2+, JDK 21), the async executor switches to a virtual-thread executor instead — each task gets its own virtual thread.

Choosing a specific executor

Name a bean and reference it: @Async("reportExecutor"). Define your own by implementing AsyncConfigurer (which also lets you set an AsyncUncaughtExceptionHandler for void methods, whose exceptions are otherwise swallowed).