Understanding Spring Controller Scope and Thread Safety

Spring MVC controllers are singleton by default; using non‑static member variables can cause data logic confusion because the singleton is not thread‑safe.

controller默认是单例的，不要使用非静态的成员变量，否则会发生数据逻辑混乱。正因为单例所以不是线程安全的。

Below is a simple test demonstrating this issue:

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);
    }
}

Visiting http://localhost:8080/testScope prints 1; then visiting http://localhost:8080/testScope2 prints 2, showing the singleton controller is not thread‑safe.

Solution: Use Prototype Scope

Adding @Scope("prototype") to the controller makes each request obtain a new instance:

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 /testScope and /testScope2 start with num = 0, each printing 1, confirming prototype scope eliminates the thread‑unsafe behavior.

单例是不安全的，会导致属性重复使用。

Recommendations

Do not define member variables in controllers.

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

Consider using ThreadLocal variables inside controllers when appropriate.

Supplementary Information: Spring Bean Scopes

Spring defines five main bean scopes:

singleton : one instance per Spring IoC container (eagerly instantiated unless lazy-init is used).

prototype : a new instance is created each time the bean is requested; Spring does not manage the full lifecycle after creation.

request : (web only) a new instance per HTTP request, with Spring still managing the bean thereafter.

session : (web only) a new instance per HTTP session.

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