为何Spring单例Bean在字段与方法参数@Autowire时实例不同？

调用GET /test接口（对应方法BeanScopeTestController#checkTheSameInstance）时，输出结果为method equals: false; instance equals: false，这是为何？

Spring中@Bean的默认作用域是Singleton，原本期望类字段singletonBean1和方法参数singletonBean2从Spring容器中获取同一个实例，但实际注入的是SingletonBean的不同实例。是Spring MVC中方法参数上的@Autowire工作机制不同导致作用域被覆盖，还是另有原因？

预期是字段和方法参数注入的是同一个SingletonBean实例。

相关代码 #

BeanScopeTestController.java #

import lombok.extern.slf4j.Slf4j;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RestController;

@RestController
@RequestMapping("/test")
@Slf4j
class BeanScopeTestController {

    private final SingletonBean singletonBean1;

    @Autowired
    BeanScopeTestController(SingletonBean singletonBean) {
        this.singletonBean1 = singletonBean;
    }
    
    @GetMapping
    String checkTheSameInstance(@Autowired SingletonBean singletonBean2) {
        log.info(singletonBean1.toString());
        log.info(singletonBean2.toString());
        boolean methodEquals = singletonBean1.equals(singletonBean2);
        boolean instanceEquals = singletonBean1 == singletonBean2;
        return String.format("method equals: %s; instance equals: %s", methodEquals, instanceEquals);
    }

}

SingletonBean.java #

import lombok.EqualsAndHashCode;
import lombok.ToString;
import lombok.extern.slf4j.Slf4j;
import org.springframework.stereotype.Component;

@Slf4j
@Component
@ToString
@EqualsAndHashCode
class SingletonBean {

    private static int idProvider = 0;
    private final int id;

    public SingletonBean() {
        this.id = idProvider++;
    }
    
}

SpringDemoApplication.java #

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

@SpringBootApplication
public class SpringDemoApplication {

    public static void main(String[] args) {
        SpringApplication.run(SpringDemoApplication.class, args);
    }

}

核心问题在于Bean类的访问修饰符：

• SingletonBean和BeanScopeTestController都使用了默认的包访问权限（未添加public修饰符）。
• Spring的组件扫描机制默认只会识别并托管public修饰的类作为Bean。对于非public的类，即使添加了@Component或@RestController注解，Spring容器无法将其纳入单例管理逻辑，每次注入时都会通过反射创建一个新的实例。
• 这和Spring MVC中方法参数的@Autowired机制无关，本质是目标Bean没有被Spring容器正确管理为单例。

给SingletonBean和BeanScopeTestController添加public修饰符，让Spring组件扫描能正确识别并将它们作为单例Bean存入容器：

修改后的SingletonBean.java：

import lombok.EqualsAndHashCode;
import lombok.ToString;
import lombok.extern.slf4j.Slf4j;
import org.springframework.stereotype.Component;

@Slf4j
@Component
@ToString
@EqualsAndHashCode
public class SingletonBean { // 添加public修饰符

    private static int idProvider = 0;
    private final int id;

    public SingletonBean() {
        this.id = idProvider++;
    }
    
}

修改后的BeanScopeTestController.java：

import lombok.extern.slf4j.Slf4j;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RestController;

@RestController
@RequestMapping("/test")
@Slf4j
public class BeanScopeTestController { // 添加public修饰符

    private final SingletonBean singletonBean1;

    @Autowired
    BeanScopeTestController(SingletonBean singletonBean) {
        this.singletonBean1 = singletonBean;
    }
    
    @GetMapping
    String checkTheSameInstance(@Autowired SingletonBean singletonBean2) {
        log.info(singletonBean1.toString());
        log.info(singletonBean2.toString());
        boolean methodEquals = singletonBean1.equals(singletonBean2);
        boolean instanceEquals = singletonBean1 == singletonBean2;
        return String.format("method equals: %s; instance equals: %s", methodEquals, instanceEquals);
    }

}

修改后再次调用接口，会得到预期结果：method equals: true; instance equals: true。

内容的提问来源于stack exchange，提问作者Jasiu