Spring：为什么注入 bean 的公共字段仅在依赖 bean 中为 null？

如何解决Spring：为什么注入 bean 的公共字段仅在依赖 bean 中为 null？

我的服务 bean 在构造函数中设置了两个公共字段：

@Service
public class MyService {
    private static final Logger log = LoggerFactory.getLogger(MyService.class);
    
    public Boolean nativeHome;
    public String url;
    
    @Autowired
    public MyService(
            @Value("${homepage.native}") Boolean nativeHome,@Value("${homepage.url}") String url) {
        this.nativeHome = nativeHome;
        this.url = url;
        log.debug(this.nativeHome + ""); // not null
        log.debug(this.url); // not null
    }
}

这个 bean 通过它的构造函数被注入到控制器组件中：

@RestController
public class MyController {
    private static final Logger log = LoggerFactory.getLogger(MyController.class);
    
    private MyService myService;

    @Autowired
    public MyController(MyService myService) {
        this.myService = myService;
        log.debug(this.myService.nativeHome + ""); // null
        log.debug(this.myService.url); // null 
    }
}

Spring bootstrap 日志以及记录器调试消息显示这两个字段在服务 bean 中设置正确，但它们在控制器 bean 中是 null：

15:47:44.397 [main] DEBUG c.r.spring.service.MyService.<init>(40) - false
15:47:44.397 [main] DEBUG c.r.spring.service.MyService.<init>(41) - http://...
[...]
15:47:46.160 [main] DEBUG c.r.s.controller.MyController.<init>(34) - null
15:47:46.160 [main] DEBUG c.r.s.controller.MyController.<init>(35) - null

如果我将这两个字段声明为 private 并在控制器中使用 getter，那么这两个字段不再是 null。为什么会这样？

为了完整性，我将提供完整的项目设置，即使它可能与问题无关。

---

Spring 版本：5.2.8.RELEASESpring 的根 applicationContext.xml

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns:context="http://www.springframework.org/schema/context"
    xmlns:aop="http://www.springframework.org/schema/aop"
    xsi:schemaLocation="http://www.springframework.org/schema/beans
        http://www.springframework.org/schema/beans/spring-beans.xsd
        http://www.springframework.org/schema/context
        http://www.springframework.org/schema/context/spring-context.xsd
        http://www.springframework.org/schema/aop
        http://www.springframework.org/schema/aop/spring-aop.xsd">

    <context:annotation-config />
    
    <aop:aspectj-autoproxy />
    
    <context:component-scan base-package="c.r.spring">
        <context:exclude-filter type="regex" expression="c\.r\.spring\.controller.*"/>
    </context:component-scan>
    
    <import resource="classpath:spring/spring-jdbc.xml" />
    
</beans>

Web 应用程序上下文在 dispatcher-servlet.xml 中配置：

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns:mvc="http://www.springframework.org/schema/mvc"
    xmlns:context="http://www.springframework.org/schema/context"
    xsi:schemaLocation="http://www.springframework.org/schema/mvc
                http://www.springframework.org/schema/mvc/spring-mvc.xsd
                http://www.springframework.org/schema/beans
                http://www.springframework.org/schema/beans/spring-beans.xsd
                http://www.springframework.org/schema/context
                http://www.springframework.org/schema/context/spring-context.xsd">

    <mvc:annotation-driven>
        <mvc:path-matching trailing-slash="true" />
    </mvc:annotation-driven>

    <mvc:default-servlet-handler />

    <mvc:cors>
        <mvc:mapping path="/**"
            allowed-origins="http://devel.r.com:3000"
            allowed-methods="GET,POST,PUT,DELETE,OPTIONS"
            allowed-headers="Authorization,Content-Type"
            exposed-headers="Access-Control-Allow-Origin"
            allow-credentials="true" />
    </mvc:cors>

    <context:component-scan base-package="c.r.spring.controller" />
    
    <bean
        class="org.springframework.web.servlet.view.InternalResourceViewResolver">
        <property name="viewClass" value="org.springframework.web.servlet.view.JstlView" />
        <property name="prefix" value="/WEB-INF/spring/views/" />
        <property name="suffix" value=".jsp" />
    </bean>

</beans>

以及 web.xml 的相关部分：

<?xml version="1.0" encoding="ISO-8859-1"?>
<web-app xmlns="http://java.sun.com/xml/ns/javaee"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd"
    version="3.0">
    
    <listener>
        <listener-class>org.springframework.web.context.ContextLoaderListener</listener-class>
    </listener>
    <context-param>
        <param-name>contextConfigLocation</param-name>
        <param-value>/WEB-INF/spring/applicationContext.xml</param-value>
    </context-param>

    <servlet>
        <servlet-name>dispatcher</servlet-name>
        <servlet-class>org.springframework.web.servlet.dispatcherServlet</servlet-class>
        <load-on-startup>1</load-on-startup>
    </servlet>
    <servlet-mapping>
        <servlet-name>dispatcher</servlet-name>
        <url-pattern>/</url-pattern>
    </servlet-mapping>

</web-app>