Spring Autowired如果未找到，则返回null

问题描述：

默认@Autowired未定义应该自动装配的Bean时，Spring实现抛出错误。有没有可能配置Spring，这将分配空对象而不是抛出异常？Spring Autowired如果未找到，则返回null

编辑：

我已经添加到required=falseAutowired但它仍然无法正常工作。那是我的代码：

@Autowired 
private ApplicationContext applicationContext; 

@Autowired(required = false) 
private HelloService helloService; 

public HelloController() { 
    message = "Hello World"; 
    System.out.println("Controller constructor"); 
} 

@RequestMapping(method = RequestMethod.GET) 
public ModelAndView helloWorld() { 
    ModelAndView modelAndView = new ModelAndView("hello"); 
    if (helloService == null) { 
     System.out.println(message); 
    } else { 
     helloService.hello(); 
     BeanDefinitionRegistry factory = (BeanDefinitionRegistry) applicationContext.getAutowireCapableBeanFactory(); 
     factory.removeBeanDefinition("helloService"); 
    } 
    return modelAndView; 
}

在第一次请求它的自动连接，但与factory.removeBeanDefinition("helloService")去除豆后下一个请求，控制器豆再次施工，我得到NoSuchBeanDefinitionException

EDIT2：

我已经创建了以下机构的另一个控制器：

@Autowired(required = false) 
private TestService testService; 

@RequestMapping(method = RequestMethod.GET) 
public ModelAndView hello() { 
    ModelAndView modelAndView = new ModelAndView("hello"); 
    return modelAndView; 
}

它正常工作 - Obj ect为空并且不会出错。也许我应该使用不同的方法从Spring上下文中删除bean？

堆栈跟踪：

org.springframework.beans.factory.NoSuchBeanDefinitionException: No bean named 'helloService' is defined 
at org.springframework.beans.factory.support.DefaultListableBeanFactory.getBeanDefinition(DefaultListableBeanFactory.java:698) ~[spring-beans-4.2.7.RELEASE.jar:4.2.7.RELEASE] 
at org.springframework.beans.factory.support.AbstractBeanFactory.getMergedLocalBeanDefinition(AbstractBeanFactory.java:1175) ~[spring-beans-4.2.7.RELEASE.jar:4.2.7.RELEASE] 
at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:284) ~[spring-beans-4.2.7.RELEASE.jar:4.2.7.RELEASE] 
at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:197) ~[spring-beans-4.2.7.RELEASE.jar:4.2.7.RELEASE] 
at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor.resolvedCachedArgument(AutowiredAnnotationBeanPostProcessor.java:508) ~[spring-beans-4.2.7.RELEASE.jar:4.2.7.RELEASE] 
at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor.access$200(AutowiredAnnotationBeanPostProcessor.java:115) ~[spring-beans-4.2.7.RELEASE.jar:4.2.7.RELEASE] 
at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor$AutowiredFieldElement.inject(AutowiredAnnotationBeanPostProcessor.java:538) ~[spring-beans-4.2.7.RELEASE.jar:4.2.7.RELEASE] 
...

重现步骤：

https://github.com/nowszy94/Autowired-null

请告诉我们你的异常 – xenteros

的[@Autowired可能重复的堆栈跟踪的构造函数（必需= FALSE）给NoSuchBeanDefinitionException]（http://stackoverflow.com/questions/23267440/autowiredrequired-false-on-constructor-giving-nosuchbeandefinitionexception） – xenteros

我不能重现你的问题。请发布[mcve]。 –

答

问题是AutowiredAnnotationBeanPostProcessor缓存注入结果。所以，当你从上下文中删除bean时，这个类认为这个对象实际上是存在的（参见私有类AutowiredFieldElement在AutowiredAnnotationBeanPostProcessor.class和方法注入中扩展了InjectionMetadata.InjectedElement）。所以，你应该清除缓存。

最笨的办法，我发现的是，但看起来像你想要做

@Controller 
@RequestMapping("/hello") 
public class HelloController { 

    @Autowired(required = false) 
    private HelloService helloService; 

    @Autowired 
    private ApplicationContext applicationContext; 

    @RequestMapping(method = RequestMethod.GET) 
    public ModelAndView modelAndView() { 
     ModelAndView modelAndView = new ModelAndView("hello"); 
     if (helloService != null) { 
      helloService.hello(); 
      removeBean("helloService"); 
     } 

     return modelAndView; 
    } 

    private void removeBean(String beanName) { 
     BeanDefinitionRegistry factory = (BeanDefinitionRegistry) applicationContext 
       .getAutowireCapableBeanFactory(); 
     factory.removeBeanDefinition(beanName); 
     clearCache(factory); 
    } 

    private void clearCache(BeanDefinitionRegistry beanFactory){ 
     AutowiredAnnotationBeanPostProcessor processor = null; 

     for (BeanPostProcessor beanPostProcessor : ((DefaultListableBeanFactory) beanFactory).getBeanPostProcessors()){ 
      if (beanPostProcessor.getClass().equals(AutowiredAnnotationBeanPostProcessor.class)){ 
       processor = (AutowiredAnnotationBeanPostProcessor) beanPostProcessor; 
      } 
     } 

     try { 
      Field injectionMetadataCache = processor.getClass().getDeclaredField("injectionMetadataCache"); 
      injectionMetadataCache.setAccessible(true); 
      Method clear = Map.class.getMethod("clear"); 
      clear.invoke(injectionMetadataCache.get(processor)); 
     } catch (NoSuchFieldException e) { 
      e.printStackTrace(); 
     } catch (NoSuchMethodException e) { 
      e.printStackTrace(); 
     } catch (IllegalAccessException e) { 
      e.printStackTrace(); 
     } catch (InvocationTargetException e) { 
      e.printStackTrace(); 
     } 


    } 

}

这个巨大的反射只是调用'processor.injectionMetadataCache.clear（）;'但这是私人的方法，所以用反射做到这一点 –

谢谢，这是有效的。你能否在这里发布你的答案 - http://stackoverflow.com/questions/39043038/autowiredrequired-false-fails-after-removing-bean？ – nowszy94

答

您可以通过required属性设置为false禁用此。

@Autowired(required=false)

如果春天找不到豆，它会离开现场未设置为空。

请参阅我的版本的问题 – nowszy94

Spring Autowired如果未找到，则返回null

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