springboot1.X 和2.X 关于Bean名字相同时覆盖问题

在2版本之前的版本，项目中有两个相同名字的bean是可以启动成功的，但是会有覆盖问题

但是在2.X版本的时候会报错：

could not be registered. A bean with that name has already been defined in class path resource

这时候解决办法可以在配置文件中添加：

spring.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*/
private boolean allowBeanDefinitionOverriding = 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
 */
public boolean isAllowBeanDefinitionOverriding() {
	return this.allowBeanDefinitionOverriding;
}

@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);
		}
	}

	//bean加载到spring的工程中后，会存储在beanDefinitionMap中，key是bean的名称。
	BeanDefinition existingDefinition = 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
			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 + "]");
			}
		}
		//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 = new ArrayList<>(this.beanDefinitionNames.size() + 1);
				updatedDefinitions.addAll(this.beanDefinitionNames);
				updatedDefinitions.add(beanName);
				this.beanDefinitionNames = updatedDefinitions;
				if (this.manualSingletonNames.contains(beanName)) {
					Set<String> updatedSingletons = new LinkedHashSet<>(this.manualSingletonNames);
					updatedSingletons.remove(beanName);
					this.manualSingletonNames = updatedSingletons;
				}
			}
		}
		else {
			// Still in startup registration phase
			this.beanDefinitionMap.put(beanName, beanDefinition);
			this.beanDefinitionNames.add(beanName);
			this.manualSingletonNames.remove(beanName);
		}
		this.frozenBeanDefinitionNames = null;
	}

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

 

上面贴出来的是spring的代码，而springboot2.X对这个参数又进行了二次封装，springboot中的allowBeanDefinitionOverriding是没有初始化默认值的，我们知道，java中的boolean类型不初始化时是false。
springboot中源代码：

//没有默认初始化就是false
private boolean allowBeanDefinitionOverriding;
/**
 * 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)
 */
public void setAllowBeanDefinitionOverriding(boolean allowBeanDefinitionOverriding) {
	this.allowBeanDefinitionOverriding = 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) {
		//**在此处给bean工程设置属性**
		((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);
}