How does Java Spring Async manage threads?

Question

I want to write a Spring Boot application with non-blocking requests in mind. In general, my controller methods are single-line methods, which just call a service method and then return, like this:

@RestController
class myController {
    ...
    @GetMapping("/")
    public String getString(){
        return service.getString().get();
    }
}

Let's say I have a Service implementation which looks something like this:

@Service
class myService{
    ...
    @Async
    public CompleteableFuture<String> getString(){
        return "Hello World!";
    }
}

(Assume I have elsewhere included @EnableAsync, and assume the controller code is modified to handle CompleteableFutures by extracting the String value and returning it)

When my controller calls my service, does Java still reserve a worker thread to wait on the return value from the Service, or is the worker thread returned to the thread pool until the service completes its work?

Thanks.

Answer 1

When my controller calls my service, does Java still reserve a worker thread to wait on the return value from the Service

No, your controller does not wait, the method @Async method is called asynchronously.

or is the worker thread returned to the thread pool until the service completes its work?

By default when @Async method is called Spring uses SimpleAsyncTaskExecutor which starts a new thread for each invocation, so no thread pool is used. You can however provide your own Task Executor or use ThreadPoolTaskExecutor - details.