Spring MVC RestController scope

Question

I have the following Spring controller:

package hello;

import java.util.concurrent.atomic.AtomicLong;

import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RequestParam;
import org.springframework.web.bind.annotation.RestController;

@RestController
public class TestController {
    private final AtomicLong counter = new AtomicLong();

    @RequestMapping("/test")
    public String test() {
        long val = counter.incrementAndGet();
        return String.valueOf(val);
    }
}

Each time I access the REST API, it returns an incremented value. I am just learning Java and I am wondering why it does not always return 1 as a new instance of AtomicLong must have been created each time the request comes.

@chrylis: I am originally from .net background and just had a comparison with it. — Babu James
– Babu James, Commented Oct 15, 2015 at 7:34
In general we try to keep the Controller (and the Service) layers stateless. This is okay just for a learning purpose but note this kind of way is difficult to manage in a multi-tenant architecture (each node / controller has a state). that's why in resilient architecture, the state is delegated in a database or a shared file system (e.g. AWS S3). — рüффп
– рüффп, Commented Apr 3 at 5:44

Tunaki · Accepted Answer · 2015-10-15 07:32:12Z

17

No, the TestController bean is actually a singleton. @RestController annotation declares a Spring @Component whose scope is by default SINGLETON. This is documented in the @Scope annotation:

Defaults to an empty string ("") which implies SCOPE_SINGLETON.

This means that it will be the same instance of TestController that will handle every requests. Since counter is an instance variable, it will be same for every request.

edited Oct 15, 2015 at 7:32

answered Oct 15, 2015 at 7:29

Tunaki

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2 Comments

Babu James Over a year ago

Is it a good practice to keep it singleton or set the scope to so-called prototype?

Tunaki Over a year ago

@BabuJames For a Controller, I would say it is better to keep it a singleton. Controllers are not typically stateful so it makes sense to make them singleton.

Gaël J · Accepted Answer · 2015-10-15 07:29:46Z

4

A @RestController is not created for each request, it remains the same for every request. So your counter keeps its value and is incremented each time.

answered Oct 15, 2015 at 7:29

Gaël J

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