Why Spring Boot Controllers Are Singleton by Default and How to Avoid Thread Safety Issues

Spring MVC controllers are singleton by default, so defining non‑static member variables can cause data logic confusion and thread‑safety problems.

Below is a simple demonstration.

package com.riemann.springbootdemo.controller;

import org.springframework.context.annotation.Scope;
import org.springframework.stereotype.Controller;
import org.springframework.web.bind.annotation.RequestMapping;

/**
 * @author riemann
 * @date 2019/07/29 22:56
 */
@Controller
public class ScopeTestController {

    private int num = 0;

    @RequestMapping("/testScope")
    public void testScope() {
        System.out.println(++num);
    }

    @RequestMapping("/testScope2")
    public void testScope2() {
        System.out.println(++num);
    }
}

When accessing http://localhost:8080/testScope the output is 1; then accessing http://localhost:8080/testScope2 yields 2. The different values show the controller is not thread‑safe.

Adding @Scope("prototype") to make the controller prototype:

package com.riemann.springbootdemo.controller;

import org.springframework.context.annotation.Scope;
import org.springframework.stereotype.Controller;
import org.springframework.web.bind.annotation.RequestMapping;

/**
 * @author riemann
 * @date 2019/07/29 22:56
 */
@Controller
@Scope("prototype")
public class ScopeTestController {

    private int num = 0;

    @RequestMapping("/testScope")
    public void testScope() {
        System.out.println(++num);
    }

    @RequestMapping("/testScope2")
    public void testScope2() {
        System.out.println(++num);
    }
}

Now both URLs return 1, confirming each request gets a new instance.

Conclusion: singleton controllers are unsafe because their fields are shared across requests.

Solution

Do not define member variables in a controller.

If a non‑static field is necessary, annotate the controller with @Scope("prototype") to use prototype scope.

Alternatively, use ThreadLocal variables inside the controller.

Additional Information

Spring bean scopes include:

singleton : a single instance per Spring container (eagerly created unless lazy‑init is used).

prototype : a new instance each time getBean is called; Spring does not manage the lifecycle after creation.

request : a new instance for each HTTP request in a web application.

session : a new instance for each HTTP session.

global session : a global web scope similar to the servlet application context.