SpringCloud - allowBeanDefinitionOverriding（spring.main.allow-bean-definition-overriding）分析

Thebean'characterEncodingFilter', definedinclasspathresource [zipkin/autoconfigure/ui/ZipkinUiAutoConfiguration.class], couldnotberegistered. Abeanwiththatnamehasalreadybeendefinedinclasspathresource [org/springframework/boot/autoconfigure/web/servlet/HttpEncodingAutoConfiguration.class] andoverridingisdisabled.
Action:
Considerrenamingoneofthebeansorenablingoverridingbysettingspring.main.allow-bean-definition-overriding=true

/** 是否允许使用相同名称重新注册不同的bean实现. 默认是允许*/privatebooleanallowBeanDefinitionOverriding=true;
/*** 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.* <p>Default is "true".【这里明确说明了默认是true】* @see #registerBeanDefinition*/publicbooleanisAllowBeanDefinitionOverriding() {
returnthis.allowBeanDefinitionOverriding;
}
@OverridepublicvoidregisterBeanDefinition(StringbeanName, BeanDefinitionbeanDefinition)
throwsBeanDefinitionStoreException {
Assert.hasText(beanName, "Bean name must not be empty");
Assert.notNull(beanDefinition, "BeanDefinition must not be null");
if (beanDefinitioninstanceofAbstractBeanDefinition) {
try {
            ((AbstractBeanDefinition) beanDefinition).validate();
        }
catch (BeanDefinitionValidationExceptionex) {
thrownewBeanDefinitionStoreException(beanDefinition.getResourceDescription(), beanName,
"Validation of bean definition failed", ex);
        }
    }
//bean加载到spring的工程中后，会存储在beanDefinitionMap中，key是bean的名称。BeanDefinitionexistingDefinition=this.beanDefinitionMap.get(beanName);
if (existingDefinition!=null) {//不为空，说明相同名称的bean已经存在了if (!isAllowBeanDefinitionOverriding()) {//如果不允许相同名称的bean存在，则直接抛出异常thrownewBeanDefinitionOverrideException(beanName, beanDefinition, existingDefinition);
        }
elseif (existingDefinition.getRole() <beanDefinition.getRole()) {
// e.g. was ROLE_APPLICATION, now overriding with ROLE_SUPPORT or ROLE_INFRASTRUCTUREif (logger.isInfoEnabled()) {
logger.info("Overriding user-defined bean definition for bean '"+beanName+"' with a framework-generated bean definition: replacing ["+existingDefinition+"] with ["+beanDefinition+"]");
            }
        }
elseif (!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+"]");
            }
        }
//可见，上面allowBeanDefinitionOverriding =true时，只是记录了一些日志，然后后来发现的这个bean，会覆盖之前老的bean。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=newArrayList<>(this.beanDefinitionNames.size() +1);
updatedDefinitions.addAll(this.beanDefinitionNames);
updatedDefinitions.add(beanName);
this.beanDefinitionNames=updatedDefinitions;
if (this.manualSingletonNames.contains(beanName)) {
Set<String>updatedSingletons=newLinkedHashSet<>(this.manualSingletonNames);
updatedSingletons.remove(beanName);
this.manualSingletonNames=updatedSingletons;
                }
            }
        }
else {
// Still in startup registration phasethis.beanDefinitionMap.put(beanName, beanDefinition);
this.beanDefinitionNames.add(beanName);
this.manualSingletonNames.remove(beanName);
        }
this.frozenBeanDefinitionNames=null;
    }
if (existingDefinition!=null||containsSingleton(beanName)) {
resetBeanDefinition(beanName);
    }
}

publicBeanDefinitionOverrideException(
StringbeanName, BeanDefinitionbeanDefinition, BeanDefinitionexistingDefinition) {
super(beanDefinition.getResourceDescription(), beanName,
"Cannot register bean definition ["+beanDefinition+"] for bean '"+beanName+"': There is already ["+existingDefinition+"] bound.");
this.beanDefinition=beanDefinition;
this.existingDefinition=existingDefinition;
}

//没有默认初始化就是falseprivatebooleanallowBeanDefinitionOverriding;
/*** Sets if bean definition overriding, by registering a definition with the same name* as an existing definition, should be allowed. Defaults to {@code false}.【这里写的很明白了，默认是false】* @param allowBeanDefinitionOverriding if overriding is allowed* @since 2.1* @see DefaultListableBeanFactory#setAllowBeanDefinitionOverriding(boolean)*/publicvoidsetAllowBeanDefinitionOverriding(booleanallowBeanDefinitionOverriding) {
this.allowBeanDefinitionOverriding=allowBeanDefinitionOverriding;
}
privatevoidprepareContext(ConfigurableApplicationContextcontext,
ConfigurableEnvironmentenvironment, SpringApplicationRunListenerslisteners,
ApplicationArgumentsapplicationArguments, BannerprintedBanner) {
context.setEnvironment(environment);
postProcessApplicationContext(context);
applyInitializers(context);
listeners.contextPrepared(context);
if (this.logStartupInfo) {
logStartupInfo(context.getParent() ==null);
logStartupProfileInfo(context);
    }
// Add boot specific singleton beansConfigurableListableBeanFactorybeanFactory=context.getBeanFactory();
beanFactory.registerSingleton("springApplicationArguments", applicationArguments);
if (printedBanner!=null) {
beanFactory.registerSingleton("springBootBanner", printedBanner);
    }
if (beanFactoryinstanceofDefaultListableBeanFactory) {
//**在此处给bean工程设置属性**        ((DefaultListableBeanFactory) beanFactory)
                .setAllowBeanDefinitionOverriding(this.allowBeanDefinitionOverriding);
    }
// Load the sourcesSet<Object>sources=getAllSources();
Assert.notEmpty(sources, "Sources must not be empty");
load(context, sources.toArray(newObject[0]));
listeners.contextLoaded(context);
}