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

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我是 spring boot 的新手，我在 spring boot rest 端点遇到了这个挑战。如果我尝试Autowired在字段上使用该组件，则会收到空异常。

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));
    
        }
    }

回答

查看以下文档@Autowired（通常总是尝试阅读 Javadoc 以了解 Spring 功能的工作原理）：

自动装配字段

在构建 bean 之后，在调用任何配置方法之前，立即注入字段。这样的配置字段不必是公开的。

正如您所看到的Common，在构建 bean之后注入该字段，而您尝试在字段初始值设定项中使用它，在构造函数完成之前。您可以改为尝试构造函数注入：

public String SUBJECT_EXISTS;

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

旁注：在 Java 中用大写命名字段是一种不好的做法，因为该字段不是最终的。应该更像subjectExists。

  • Well explained, you beat me by about a minute, +1
  • @NemugaDev if something is supposed to change dynamically a field is the wrong tool to use, you need a method instead that calls `translate` whenever asked instead of saving it into a local field. Note that this request is unrelated to the actual question because your original code would have had the same issue so far.
以上是org.springframework.beans.factory.UnsatisfiedDependencyException上的Spring启动空指针异常的全部内容。
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