org.springframework.beans.factory.UnsatisfiedDependencyException上的Spring Boot空指针异常

Nemuga Dev 发表于 Dev

Nemuga Dev

我是Spring Boot的新手，我在Spring Boot Rest端点中遇到了这一挑战。如果尝试Autowired在字段上使用组件，则会收到null异常。

这是我得到的例外

  org.springframework.beans.factory.UnsatisfiedDependencyException: Error creating bean with name 'homeContoller': Unsatisfied dependency expressed through field 'common'; nested exception is org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'common' defined in file Common.class]: Bean instantiation via constructor failed; nested exception is org.springframework.beans.BeanInstantiationException: Failed to instantiate [Common]: Constructor threw exception; nested exception is java.lang.NullPointerException
        at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor$AutowiredFieldElement.inject(AutowiredAnnotationBeanPostProcessor.java:643) ~[spring-beans-5.2.10.RELEASE.jar:5.2.10.RELEASE]
        at org.springframework.beans.factory.annotation.InjectionMetadata.inject(InjectionMetadata.java:130) ~[spring-beans-5.2.10.RELEASE.jar:5.2.10.RELEASE]
        at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor.postProcessProperties(AutowiredAnnotationBeanPostProcessor.java:399) ~[spring-beans-5.2.10.RELEASE.jar:5.2.10.RELEASE]
        at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.populateBean(AbstractAutowireCapableBeanFactory.java:1420) ~[spring-beans-5.2.10.RELEASE.jar:5.2.10.RELEASE]
        at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:593) ~[spring-beans-5.2.10.RELEASE.jar:5.2.10.RELEASE]
        at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:516) ~[spring-beans-5.2.10.RELEASE.jar:5.2.10.RELEASE]
        at org.springframework.beans.factory.support.AbstractBeanFactory.lambda$doGetBean$0(AbstractBeanFactory.java:324) ~[spring-beans-5.2.10.RELEASE.jar:5.2.10.RELEASE]
        at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:234) ~[spring-beans-5.2.10.RELEASE.jar:5.2.10.RELEASE]
        at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:322) ~[spring-beans-5.2.10.RELEASE.jar:5.2.10.RELEASE]
        at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:202) ~[spring-beans-5.2.10.RELEASE.jar:5.2.10.RELEASE]
        at org.springframework.beans.factory.support.DefaultListableBeanFactory.preInstantiateSingletons(DefaultListableBeanFactory.java:897) ~[spring-beans-5.2.10.RELEASE.jar:5.2.10.RELEASE]
        at org.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:879) ~[spring-context-5.2.10.RELEASE.jar:5.2.10.RELEASE]
        at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:551) ~[spring-context-5.2.10.RELEASE.jar:5.2.10.RELEASE]
        at org.springframework.boot.web.servlet.context.ServletWebServerApplicationContext.refresh(ServletWebServerApplicationContext.java:141) ~[spring-boot-2.2.11.RELEASE.jar:2.2.11.RELEASE]
        at org.springframework.boot.SpringApplication.refresh(SpringApplication.java:747) [spring-boot-2.2.11.RELEASE.jar:2.2.11.RELEASE]
        at org.springframework.boot.SpringApplication.refreshContext(SpringApplication.java:405) [spring-boot-2.2.11.RELEASE.jar:2.2.11.RELEASE]
        at org.springframework.boot.SpringApplication.run(SpringApplication.java:315) [spring-boot-2.2.11.RELEASE.jar:2.2.11.RELEASE]
        at org.springframework.boot.SpringApplication.run(SpringApplication.java:1226) [spring-boot-2.2.11.RELEASE.jar:2.2.11.RELEASE]
        at org.springframework.boot.SpringApplication.run(SpringApplication.java:1215) [spring-boot-2.2.11.RELEASE.jar:2.2.11.RELEASE]
        at Application.main(Application.java:23) [classes/:na]
        at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:1.8.0_172]
        at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[na:1.8.0_172]
        at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[na:1.8.0_172]
        at java.lang.reflect.Method.invoke(Method.java:498) ~[na:1.8.0_172]
        at org.springframework.boot.devtools.restart.RestartLauncher.run(RestartLauncher.java:49) [spring-boot-devtools-2.2.11.RELEASE.jar:2.2.11.RELEASE]
    Caused by: org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'common' defined in file Common.class]: Bean instantiation via constructor failed; nested exception is org.springframework.beans.BeanInstantiationException: Failed to instantiate [Common]: Constructor threw exception; nested exception is java.lang.NullPointerException
        at org.springframework.beans.factory.support.ConstructorResolver.instantiate(ConstructorResolver.java:313) ~[spring-beans-5.2.10.RELEASE.jar:5.2.10.RELEASE]
        at org.springframework.beans.factory.support.ConstructorResolver.autowireConstructor(ConstructorResolver.java:294) ~[spring-beans-5.2.10.RELEASE.jar:5.2.10.RELEASE]
        at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.autowireConstructor(AbstractAutowireCapableBeanFactory.java:1356) ~[spring-beans-5.2.10.RELEASE.jar:5.2.10.RELEASE]
        at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBeanInstance(AbstractAutowireCapableBeanFactory.java:1203) ~[spring-beans-5.2.10.RELEASE.jar:5.2.10.RELEASE]
        at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:556) ~[spring-beans-5.2.10.RELEASE.jar:5.2.10.RELEASE]
        at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:516) ~[spring-beans-5.2.10.RELEASE.jar:5.2.10.RELEASE]
        at org.springframework.beans.factory.support.AbstractBeanFactory.lambda$doGetBean$0(AbstractBeanFactory.java:324) ~[spring-beans-5.2.10.RELEASE.jar:5.2.10.RELEASE]
        at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:234) ~[spring-beans-5.2.10.RELEASE.jar:5.2.10.RELEASE]
        at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:322) ~[spring-beans-5.2.10.RELEASE.jar:5.2.10.RELEASE]
        at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:202) ~[spring-beans-5.2.10.RELEASE.jar:5.2.10.RELEASE]
        at org.springframework.beans.factory.config.DependencyDescriptor.resolveCandidate(DependencyDescriptor.java:276) ~[spring-beans-5.2.10.RELEASE.jar:5.2.10.RELEASE]
        at org.springframework.beans.factory.support.DefaultListableBeanFactory.doResolveDependency(DefaultListableBeanFactory.java:1307) ~[spring-beans-5.2.10.RELEASE.jar:5.2.10.RELEASE]
        at org.springframework.beans.factory.support.DefaultListableBeanFactory.resolveDependency(DefaultListableBeanFactory.java:1227) ~[spring-beans-5.2.10.RELEASE.jar:5.2.10.RELEASE]
        at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor$AutowiredFieldElement.inject(AutowiredAnnotationBeanPostProcessor.java:640) ~[spring-beans-5.2.10.RELEASE.jar:5.2.10.RELEASE]
        ... 24 common frames omitted
    Caused by: org.springframework.beans.BeanInstantiationException: Failed to instantiate [Common]: Constructor threw exception; nested exception is java.lang.NullPointerException
        at org.springframework.beans.BeanUtils.instantiateClass(BeanUtils.java:217) ~[spring-beans-5.2.10.RELEASE.jar:5.2.10.RELEASE]
        at org.springframework.beans.factory.support.SimpleInstantiationStrategy.instantiate(SimpleInstantiationStrategy.java:117) ~[spring-beans-5.2.10.RELEASE.jar:5.2.10.RELEASE]
        at org.springframework.beans.factory.support.ConstructorResolver.instantiate(ConstructorResolver.java:309) ~[spring-beans-5.2.10.RELEASE.jar:5.2.10.RELEASE]
        ... 37 common frames omitted
    Caused by: java.lang.NullPointerException: null
        at Common.<init>(Common.java:39) ~[classes/:na]
        at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method) ~[na:1.8.0_172]
        at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62) ~[na:1.8.0_172]
        at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45) ~[na:1.8.0_172]
        at java.lang.reflect.Constructor.newInstance(Constructor.java:423) ~[na:1.8.0_172]
        at org.springframework.beans.BeanUtils.instantiateClass(BeanUtils.java:204) ~[spring-beans-5.2.10.RELEASE.jar:5.2.10.RELEASE]
        ... 39 common frames omitted

这是我的java代码

  @Component
    public class Common {
    
/* tried this and still not working(does not change locale per request it pick default language)
     @Autowired
public Common(LanguageTranslator translator)
{
   this.translator=translator;
    SUBJECT_EXISTS =translator.translate("SUBJECT_EXISTS");
}
*/
        @Autowired
        LanguageTranslator translator;
    
        public  String SUBJECT_EXISTS =translator.translate("SUBJECT_EXISTS") ;//failing error
    
    }   
    
    @Component
    public class LanguageTranslator {
    
    
        private  ResourceBundleMessageSource messageSource;
    
        @Autowired
        public LanguageTranslator(ResourceBundleMessageSource resourceBundleMessageSource)
        {
            this.messageSource=resourceBundleMessageSource;
        }
    
    
    
        public  String translate(String msg) {
            Locale locale = LocaleContextHolder.getLocale();
            return messageSource.getMessage(msg, null, locale);
        }
    
    
    }
    
    @RestController
    @RequestMapping("/home")
    public class HomeContoller {
    
    
    
        @Autowired
        Common  common;
    
    
        @GetMapping("/")
        public String home() {
    
            return "Home page";
        }
    
        @RequestMapping(value = "/demo", method = RequestMethod.POST)
        public ResponseEntity all_menu_assignment(HttpServletRequest req)  {
    
    
            return  ResponseEntity
                    .ok().body(new ServerResponse(00,common.SUBJECT_EXISTS));
    
        }
    }

嘛

看一看文档@Autowired（通常总是尝试阅读Javadocs来了解Spring功能的工作原理）：

自动接线领域

在构造任何bean之后，调用任何config方法之前，都将立即注入字段。这样的配置字段不必是公共的。

如您所见，在Common构造bean之前，您试图在字段初始化程序中使用它时，将在构造bean之后注入该字段。您可以尝试使用构造函数注入：

public String SUBJECT_EXISTS;

@Autowired
public Common(LanguageTranslator translator) {
    this.translator = translator;
    SUBJECT_EXISTS = translator.translate("SUBJECT_EXISTS");
}

旁注：在Java中，使用大写字母命名字段是一种不好的做法，因为该字段不是最终的。它应该更像subjectExists。

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