java - 即使我在 Spring 类中声明该字段，为什么 @Autowired 为 null？

Question

这个问题已经有答案了:

Why is my Spring @Autowired field null? (21 个回答)

已关闭 3 年前。

我知道有一个关于这个问题的问题和很多答案。但即使在 Spring 类中，仍然没有答案为什么 @Autowired 变为 null。这一定是一个错误吗？

这是一个示例，其中 stockNamesRepository为空且不为空。

@Data
@Service 
@Configurable // I have tried with @component as well...not working
public class StocksCrud {

    @Autowired
    private StockNamesRepository stockNamesRepository; // This is null

    private Crud<StockNames> crud; // Vaadin object

    private ArrayList<StockNames> crudData;

    @PostConstruct
    public void init() {
        // Fill the crud
        crudData = new ArrayList<StockNames>();
        for(StockNames stockName: stockNamesRepository.findAll()) {
            crudData.add(new StockNames(stockName.getId(), stockName.getStockName()));
        }

    }

}

这不为空

@Component
public class InitialStartUp implements ApplicationListener<ContextRefreshedEvent> {

    @Autowired
    private StockNamesRepository stockNamesRepository; // Not null

    @Override
    public void onApplicationEvent(ContextRefreshedEvent event) {
        System.out.println(stockNamesRepository == null); // FALSE
    }

}

有人能告诉我为什么 Spring Framework 有这些神秘的行为吗？

这是错误。上面的代码是最小的，以便于阅读。

java.lang.NullPointerException: null
    at se.danielmartensson.jweekstocks.vaadin.ui.crud.StocksCrud.saveStock(StocksCrud.java:83) ~[main/:na]
    at se.danielmartensson.jweekstocks.vaadin.ui.crud.StocksCrud.lambda$new$9b1b5227$1(StocksCrud.java:65) ~[main/:na]
    at com.vaadin.flow.component.crud.Crud.lambda$null$3(Crud.java:215) ~[vaadin-crud-flow-2.1.0.jar:na]
    at java.lang.Iterable.forEach(Iterable.java:75) ~[na:1.8.0_232]
    at com.vaadin.flow.component.crud.Crud.lambda$registerHandlers$9b1b5227$4(Crud.java:215) ~[vaadin-crud-flow-2.1.0.jar:na]
    at com.vaadin.flow.component.ComponentEventBus.fireEventForListener(ComponentEventBus.java:205) ~[flow-server-2.1.2.jar:2.1.2]
    at com.vaadin.flow.component.ComponentEventBus.handleDomEvent(ComponentEventBus.java:373) ~[flow-server-2.1.2.jar:2.1.2]
    at com.vaadin.flow.component.ComponentEventBus.lambda$addDomTrigger$dd1b7957$1(ComponentEventBus.java:264) ~[flow-server-2.1.2.jar:2.1.2]
    at com.vaadin.flow.internal.nodefeature.ElementListenerMap.lambda$fireEvent$2(ElementListenerMap.java:441) ~[flow-server-2.1.2.jar:2.1.2]
    at java.util.ArrayList.forEach(ArrayList.java:1257) ~[na:1.8.0_232]
    at com.vaadin.flow.internal.nodefeature.ElementListenerMap.fireEvent(ElementListenerMap.java:441) ~[flow-server-2.1.2.jar:2.1.2]
    at com.vaadin.flow.server.communication.rpc.EventRpcHandler.handleNode(EventRpcHandler.java:59) ~[flow-server-2.1.2.jar:2.1.2]
    at com.vaadin.flow.server.communication.rpc.AbstractRpcInvocationHandler.handle(AbstractRpcInvocationHandler.java:64) ~[flow-server-2.1.2.jar:2.1.2]
    at com.vaadin.flow.server.communication.ServerRpcHandler.handleInvocationData(ServerRpcHandler.java:400) [flow-server-2.1.2.jar:2.1.2]
    at com.vaadin.flow.server.communication.ServerRpcHandler.lambda$handleInvocations$1(ServerRpcHandler.java:381) [flow-server-2.1.2.jar:2.1.2]
    at java.util.ArrayList.forEach(ArrayList.java:1257) ~[na:1.8.0_232]
    at com.vaadin.flow.server.communication.ServerRpcHandler.handleInvocations(ServerRpcHandler.java:381) [flow-server-2.1.2.jar:2.1.2]
    at com.vaadin.flow.server.communication.ServerRpcHandler.handleRpc(ServerRpcHandler.java:316) [flow-server-2.1.2.jar:2.1.2]
    at com.vaadin.flow.server.communication.UidlRequestHandler.synchronizedHandleRequest(UidlRequestHandler.java:89) ~[flow-server-2.1.2.jar:2.1.2]
    at com.vaadin.flow.server.SynchronizedRequestHandler.handleRequest(SynchronizedRequestHandler.java:40) ~[flow-server-2.1.2.jar:2.1.2]
    at com.vaadin.flow.server.VaadinService.handleRequest(VaadinService.java:1540) ~[flow-server-2.1.2.jar:2.1.2]
    at com.vaadin.flow.server.VaadinServlet.service(VaadinServlet.java:247) ~[flow-server-2.1.2.jar:2.1.2]
    at com.vaadin.flow.spring.SpringServlet.service(SpringServlet.java:95) ~[vaadin-spring-12.1.2.jar:na]
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:741) ~[tomcat-embed-core-9.0.29.jar:9.0.29]
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:231) ~[tomcat-embed-core-9.0.29.jar:9.0.29]
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) ~[tomcat-embed-core-9.0.29.jar:9.0.29]
    at org.apache.catalina.core.ApplicationDispatcher.invoke(ApplicationDispatcher.java:712) ~[tomcat-embed-core-9.0.29.jar:9.0.29]
    at org.apache.catalina.core.ApplicationDispatcher.processRequest(ApplicationDispatcher.java:459) ~[tomcat-embed-core-9.0.29.jar:9.0.29]
    at org.apache.catalina.core.ApplicationDispatcher.doForward(ApplicationDispatcher.java:352) ~[tomcat-embed-core-9.0.29.jar:9.0.29]
    at org.apache.catalina.core.ApplicationDispatcher.forward(ApplicationDispatcher.java:312) ~[tomcat-embed-core-9.0.29.jar:9.0.29]
    at org.springframework.web.servlet.mvc.ServletForwardingController.handleRequestInternal(ServletForwardingController.java:141) ~[spring-webmvc-5.2.2.RELEASE.jar:5.2.2.RELEASE]
    at org.springframework.web.servlet.mvc.AbstractController.handleRequest(AbstractController.java:177) ~[spring-webmvc-5.2.2.RELEASE.jar:5.2.2.RELEASE]
    at org.springframework.web.servlet.mvc.SimpleControllerHandlerAdapter.handle(SimpleControllerHandlerAdapter.java:52) ~[spring-webmvc-5.2.2.RELEASE.jar:5.2.2.RELEASE]
    at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:1040) ~[spring-webmvc-5.2.2.RELEASE.jar:5.2.2.RELEASE]
    at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:943) ~[spring-webmvc-5.2.2.RELEASE.jar:5.2.2.RELEASE]
    at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:1006) ~[spring-webmvc-5.2.2.RELEASE.jar:5.2.2.RELEASE]
    at org.springframework.web.servlet.FrameworkServlet.doPost(FrameworkServlet.java:909) ~[spring-webmvc-5.2.2.RELEASE.jar:5.2.2.RELEASE]
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:660) ~[tomcat-embed-core-9.0.29.jar:9.0.29]
    at org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:883) ~[spring-webmvc-5.2.2.RELEASE.jar:5.2.2.RELEASE]
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:741) ~[tomcat-embed-core-9.0.29.jar:9.0.29]
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:231) ~[tomcat-embed-core-9.0.29.jar:9.0.29]
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) ~[tomcat-embed-core-9.0.29.jar:9.0.29]
    at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:53) ~[tomcat-embed-websocket-9.0.29.jar:9.0.29]
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) ~[tomcat-embed-core-9.0.29.jar:9.0.29]
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) ~[tomcat-embed-core-9.0.29.jar:9.0.29]
    at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:320) ~[spring-security-web-5.2.1.RELEASE.jar:5.2.1.RELEASE]
    at org.springframework.security.web.access.intercept.FilterSecurityInterceptor.invoke(FilterSecurityInterceptor.java:126) ~[spring-security-web-5.2.1.RELEASE.jar:5.2.1.RELEASE]
    at org.springframework.security.web.access.intercept.FilterSecurityInterceptor.doFilter(FilterSecurityInterceptor.java:90) ~[spring-security-web-5.2.1.RELEASE.jar:5.2.1.RELEASE]
    at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:334) ~[spring-security-web-5.2.1.RELEASE.jar:5.2.1.RELEASE]
    at org.springframework.security.web.access.ExceptionTranslationFilter.doFilter(ExceptionTranslationFilter.java:118) ~[spring-security-web-5.2.1.RELEASE.jar:5.2.1.RELEASE]
    at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:334) ~[spring-security-web-5.2.1.RELEASE.jar:5.2.1.RELEASE]
    at org.springframework.security.web.session.SessionManagementFilter.doFilter(SessionManagementFilter.java:137) ~[spring-security-web-5.2.1.RELEASE.jar:5.2.1.RELEASE]
    at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:334) ~[spring-security-web-5.2.1.RELEASE.jar:5.2.1.RELEASE]
    at org.springframework.security.web.authentication.AnonymousAuthenticationFilter.doFilter(AnonymousAuthenticationFilter.java:111) ~[spring-security-web-5.2.1.RELEASE.jar:5.2.1.RELEASE]
    at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:334) ~[spring-security-web-5.2.1.RELEASE.jar:5.2.1.RELEASE]
    at org.springframework.security.web.servletapi.SecurityContextHolderAwareRequestFilter.doFilter(SecurityContextHolderAwareRequestFilter.java:158) ~[spring-security-web-5.2.1.RELEASE.jar:5.2.1.RELEASE]
    at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:334) ~[spring-security-web-5.2.1.RELEASE.jar:5.2.1.RELEASE]
    at org.springframework.security.web.savedrequest.RequestCacheAwareFilter.doFilter(RequestCacheAwareFilter.java:63) ~[spring-security-web-5.2.1.RELEASE.jar:5.2.1.RELEASE]
    at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:334) ~[spring-security-web-5.2.1.RELEASE.jar:5.2.1.RELEASE]
    at org.springframework.security.web.authentication.AbstractAuthenticationProcessingFilter.doFilter(AbstractAuthenticationProcessingFilter.java:200) ~[spring-security-web-5.2.1.RELEASE.jar:5.2.1.RELEASE]
    at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:334) ~[spring-security-web-5.2.1.RELEASE.jar:5.2.1.RELEASE]
    at org.springframework.security.web.authentication.logout.LogoutFilter.doFilter(LogoutFilter.java:116) ~[spring-security-web-5.2.1.RELEASE.jar:5.2.1.RELEASE]
    at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:334) ~[spring-security-web-5.2.1.RELEASE.jar:5.2.1.RELEASE]
    at org.springframework.security.web.header.HeaderWriterFilter.doHeadersAfter(HeaderWriterFilter.java:92) ~[spring-security-web-5.2.1.RELEASE.jar:5.2.1.RELEASE]
    at org.springframework.security.web.header.HeaderWriterFilter.doFilterInternal(HeaderWriterFilter.java:77) ~[spring-security-web-5.2.1.RELEASE.jar:5.2.1.RELEASE]
    at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:119) ~[spring-web-5.2.2.RELEASE.jar:5.2.2.RELEASE]
    at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:334) ~[spring-security-web-5.2.1.RELEASE.jar:5.2.1.RELEASE]
    at org.springframework.security.web.context.SecurityContextPersistenceFilter.doFilter(SecurityContextPersistenceFilter.java:105) ~[spring-security-web-5.2.1.RELEASE.jar:5.2.1.RELEASE]
    at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:334) ~[spring-security-web-5.2.1.RELEASE.jar:5.2.1.RELEASE]
    at org.springframework.security.web.context.request.async.WebAsyncManagerIntegrationFilter.doFilterInternal(WebAsyncManagerIntegrationFilter.java:56) ~[spring-security-web-5.2.1.RELEASE.jar:5.2.1.RELEASE]
    at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:119) ~[spring-web-5.2.2.RELEASE.jar:5.2.2.RELEASE]
    at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:334) ~[spring-security-web-5.2.1.RELEASE.jar:5.2.1.RELEASE]
    at org.springframework.security.web.FilterChainProxy.doFilterInternal(FilterChainProxy.java:215) ~[spring-security-web-5.2.1.RELEASE.jar:5.2.1.RELEASE]
    at org.springframework.security.web.FilterChainProxy.doFilter(FilterChainProxy.java:178) ~[spring-security-web-5.2.1.RELEASE.jar:5.2.1.RELEASE]
    at org.springframework.web.filter.DelegatingFilterProxy.invokeDelegate(DelegatingFilterProxy.java:358) ~[spring-web-5.2.2.RELEASE.jar:5.2.2.RELEASE]
    at org.springframework.web.filter.DelegatingFilterProxy.doFilter(DelegatingFilterProxy.java:271) ~[spring-web-5.2.2.RELEASE.jar:5.2.2.RELEASE]
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) ~[tomcat-embed-core-9.0.29.jar:9.0.29]
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) ~[tomcat-embed-core-9.0.29.jar:9.0.29]
    at org.springframework.web.filter.RequestContextFilter.doFilterInternal(RequestContextFilter.java:100) ~[spring-web-5.2.2.RELEASE.jar:5.2.2.RELEASE]
    at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:119) ~[spring-web-5.2.2.RELEASE.jar:5.2.2.RELEASE]
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) ~[tomcat-embed-core-9.0.29.jar:9.0.29]
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) ~[tomcat-embed-core-9.0.29.jar:9.0.29]
    at org.springframework.web.filter.FormContentFilter.doFilterInternal(FormContentFilter.java:93) ~[spring-web-5.2.2.RELEASE.jar:5.2.2.RELEASE]
    at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:119) ~[spring-web-5.2.2.RELEASE.jar:5.2.2.RELEASE]
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) ~[tomcat-embed-core-9.0.29.jar:9.0.29]
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) ~[tomcat-embed-core-9.0.29.jar:9.0.29]
    at org.springframework.web.filter.CharacterEncodingFilter.doFilterInternal(CharacterEncodingFilter.java:201) ~[spring-web-5.2.2.RELEASE.jar:5.2.2.RELEASE]
    at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:119) ~[spring-web-5.2.2.RELEASE.jar:5.2.2.RELEASE]
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) ~[tomcat-embed-core-9.0.29.jar:9.0.29]
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) ~[tomcat-embed-core-9.0.29.jar:9.0.29]
    at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:202) ~[tomcat-embed-core-9.0.29.jar:9.0.29]
    at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:96) ~[tomcat-embed-core-9.0.29.jar:9.0.29]
    at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:526) ~[tomcat-embed-core-9.0.29.jar:9.0.29]
    at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:139) ~[tomcat-embed-core-9.0.29.jar:9.0.29]
    at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:92) ~[tomcat-embed-core-9.0.29.jar:9.0.29]
    at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:74) ~[tomcat-embed-core-9.0.29.jar:9.0.29]
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:343) ~[tomcat-embed-core-9.0.29.jar:9.0.29]
    at org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:367) ~[tomcat-embed-core-9.0.29.jar:9.0.29]
    at org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:65) ~[tomcat-embed-core-9.0.29.jar:9.0.29]
    at org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:860) ~[tomcat-embed-core-9.0.29.jar:9.0.29]
    at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1591) ~[tomcat-embed-core-9.0.29.jar:9.0.29]
    at org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:49) ~[tomcat-embed-core-9.0.29.jar:9.0.29]
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149) ~[na:1.8.0_232]
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624) ~[na:1.8.0_232]
    at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61) ~[tomcat-embed-core-9.0.29.jar:9.0.29]
    at java.lang.Thread.run(Thread.java:748) ~[na:1.8.0_232]

更新:

这就是我注入(inject) StockCrud 类的方式

@Route("")
@Push
@Viewport("width=device-width, minimum-scale=1, initial-scale=1, user-scalable=yes, viewport-fit=cover")
@PWA(name = "My Application", shortName = "My App")
public class MainView extends AppLayout {

    /**
     * 
     */
    private static final long serialVersionUID = 1L;
    private WeeksGrid weeksGrid;
    private StocksCrud stocksCrud;

    @PostConstruct
    public void init() {

        weeksGrid = new WeeksGrid();
        stocksCrud = new StocksCrud();
    }

}

Answer 1

您没有通过 DI 容器创建 StocksCrud 实例，因此 spring 无法注入(inject)任何依赖项。将 StocksCrud 注入(inject)到 MainView 中，它应该得到正确初始化。

一些解释:为了让 Spring(或者任何 DI 容器，实际上)发挥其魔力，容器需要创建相关的 bean。在所提供的代码中，StocksCrud 的实例是通过 new 创建的，即不是通过 DI 容器创建的。 DI 容器的使用不会改变 Java 的语义，因此 new StocksCrud() 具有与任何 Java 程序中相同的语义。 DI 容器分析带注释的 bean 以检查某些注释(例如 @Inject 或 @Autowired)，并尝试注入(inject)其他合适的 bean 作为依赖项。

<小时/>

对您的代码的评论:我强烈建议使用 constructor injection 而不是字段注入(inject)。 .