不要设置allow-bean-definition-overriding=true

不要设置allow-bean-definition-overriding=true

Description:

The bean 'FeignClientSpecification' could not be registered. A bean with that name has already been defined and overriding is disabled.

Action:

Consider renaming one of the beans or enabling overriding by setting spring.main.allow-bean-definition-overriding=true

你是不是也会经常遇到上面的问题，这时候你就会去百度，百度会很友好的回复你spring已经给你的提示，设置 spring.main.allow-bean-definition-overriding=true，设置了真的就没问题了吗？是的，没问题了，好了，不管了上线。但是这里的坑，你知道吗？

allow-bean-definition-overriding 到底是个啥？

DefaultListableBeanFactory.class

/**
 * Set whether it should be allowed to override bean definitions by registering
 * a different definition with the same name, automatically replacing the former.
 * If not, an exception will be thrown. This also applies to overriding aliases.
 * 
Default is "true".
 * @see #registerBeanDefinition
 */
public void setAllowBeanDefinitionOverriding(boolean allowBeanDefinitionOverriding) {
   this.allowBeanDefinitionOverriding = allowBeanDefinitionOverriding;
}

我们先看下spring源码对于该字段的描述，发现默认就是true，哎，那为啥我们还要设置呢？那是因为springboot封装的时候默认成了false，源码如下：

SpringApplication.class

private boolean allowBeanDefinitionOverriding;
... ...
private void prepareContext(ConfigurableApplicationContext context,
      ConfigurableEnvironment environment, SpringApplicationRunListeners listeners,
      ApplicationArguments applicationArguments, Banner printedBanner) {
   context.setEnvironment(environment);
   postProcessApplicationContext(context);
   applyInitializers(context);
   listeners.contextPrepared(context);
   if (this.logStartupInfo) {
      logStartupInfo(context.getParent() == null);
      logStartupProfileInfo(context);
   }
   // Add boot specific singleton beans
   ConfigurableListableBeanFactory beanFactory = context.getBeanFactory();
   beanFactory.registerSingleton("springApplicationArguments", applicationArguments);
   if (printedBanner != null) {
      beanFactory.registerSingleton("springBootBanner", printedBanner);
   }
   if (beanFactory instanceof DefaultListableBeanFactory) {
      ((DefaultListableBeanFactory) beanFactory)
            .setAllowBeanDefinitionOverriding(this.allowBeanDefinitionOverriding);
   }
   // Load the sources
   Set<Object> sources = getAllSources();
   Assert.notEmpty(sources, "Sources must not be empty");
   load(context, sources.toArray(new Object[0]));
   listeners.contextLoaded(context);
}

那设置为true，spring会进行什么样的操作呢，我们继续看对应源码

DefaultListableBeanFactory.class

@Override
public void registerBeanDefinition(String beanName, BeanDefinition beanDefinition)
      throws BeanDefinitionStoreException {

   Assert.hasText(beanName, "Bean name must not be empty");
   Assert.notNull(beanDefinition, "BeanDefinition must not be null");

   if (beanDefinition instanceof AbstractBeanDefinition) {
      try {
         ((AbstractBeanDefinition) beanDefinition).validate();
      }
      catch (BeanDefinitionValidationException ex) {
         throw new BeanDefinitionStoreException(beanDefinition.getResourceDescription(), beanName,
               "Validation of bean definition failed", ex);
      }
   }

   BeanDefinition existingDefinition = this.beanDefinitionMap.get(beanName);
   if (existingDefinition != null) {
      if (!isAllowBeanDefinitionOverriding()) {
         throw new BeanDefinitionOverrideException(beanName, beanDefinition, existingDefinition);
      }
      else if (existingDefinition.getRole() < beanDefinition.getRole()) {
         // e.g. was ROLE_APPLICATION, now overriding with ROLE_SUPPORT or ROLE_INFRASTRUCTURE
         if (logger.isInfoEnabled()) {
            logger.info("Overriding user-defined bean definition for bean '" + beanName +
                  "' with a framework-generated bean definition: replacing [" +
                  existingDefinition + "] with [" + beanDefinition + "]");
         }
      }
      else if (!beanDefinition.equals(existingDefinition)) {
         if (logger.isDebugEnabled()) {
            logger.debug("Overriding bean definition for bean '" + beanName +
                  "' with a different definition: replacing [" + existingDefinition +
                  "] with [" + beanDefinition + "]");
         }
      }
      else {
         if (logger.isTraceEnabled()) {
            logger.trace("Overriding bean definition for bean '" + beanName +
                  "' with an equivalent definition: replacing [" + existingDefinition +
                  "] with [" + beanDefinition + "]");
         }
      }
      this.beanDefinitionMap.put(beanName, beanDefinition);
   }
   else {
      if (hasBeanCreationStarted()) {
         // Cannot modify startup-time collection elements anymore (for stable iteration)
         synchronized (this.beanDefinitionMap) {
            this.beanDefinitionMap.put(beanName, beanDefinition);
            List<String> updatedDefinitions = new ArrayList<>(this.beanDefinitionNames.size() + 1);
            updatedDefinitions.addAll(this.beanDefinitionNames);
            updatedDefinitions.add(beanName);
            this.beanDefinitionNames = updatedDefinitions;
            removeManualSingletonName(beanName);
         }
      }
      else {
         // Still in startup registration phase
         this.beanDefinitionMap.put(beanName, beanDefinition);
         this.beanDefinitionNames.add(beanName);
         removeManualSingletonName(beanName);
      }
      this.frozenBeanDefinitionNames = null;
   }

   if (existingDefinition != null || containsSingleton(beanName)) {
      resetBeanDefinition(beanName);
   }
   else if (isConfigurationFrozen()) {
      clearByTypeCache();
   }
}

看源码可以看到，如果设置为true，再方法最后会 this.beanDefinitionMap.put(beanName, beanDefinition) 进行覆盖，这样就会导致bean被覆盖掉了，这是隐藏的大坑啊，如果同一个项目两个人默契的写了同一个bean，这时候有一个可怜虫就要被覆盖掉了，可能还不知道什么原因，这可太惨了。如果最后发现是你干的，你猜他会不会拿起40米的大刀。。。所以，遇到文章开始的问题，找对应重复bean根据实际使用删除或者修改，不要设置allow-bean-definition-overriding=true，切记切记~