Are multiple requests handled by single thread in Spring Boot until certain requests threshold is reached?

Question

In my Spring Boot 2.1.6 project (based on Tomcat) I have a rest controller. I added a default constructor to it which prints something. I thought in Tomcat-based servers each request is handled in separate thread. So I expected each request to trigger a new controller object and as a result new print from the constructor. However, I made a test of sending 30 requests to the rest controller and I only see that the print was made once. So as far as I understand the rest controller handles all those requests in one thread.

My question is whether indeed multiple requests are handled in a single thread or maybe there's certain request threshold upon which another thread will be opened? I'm using default Spring Boot configuration perhaps this is controlled somewhere in the config?

This is the code for my controller:

@RestController
public class TrackingEventController {

    public TrackingEventController() {
        System.out.println("from TrackingEventController");
    }

    @RequestMapping(method=GET, path=trackingEventPath)
    public ResponseEntity<Object> handleTrackingEvent(
            @RequestParam(name = Routes.event) String event,
            @RequestParam(name = Routes.pubId) String pubId,
            @RequestParam(name = Routes.advId) String advId) {

        return new ResponseEntity<>(null, new HttpHeaders(), HttpStatus.OK);
    }
}

Answer 1

You're mixing two orthogonal concepts:

1. a thread
2. a controller instance

A single thread could create and/or use one, or several controller instances.

Multiple threads could also create and/or use one, or several controller instances.

The two are unrelated.

And how it actually works is

• Spring beans are singletons by default, so Spring creates a single instance of your controller
• A servlet container uses a pool of threads.
• Every time a request comes in, a thread is chosen from the pool, and this thread handles the request. If the request is mapped to your controller, then the appropriate method of the unique controller instance is executed by this thread.

If you want to know which thread is handling the current request, add this to your controller method:

System.out.println(Thread.currentThread().getName());

Answer 2

Spring boot Tomcat thread pool default size is 200. You can make out that different threads server different requests. Put a debug point on some REST endpoint, and call it multiple times from Postman etc. From debugger, check the thread name. s.b.