Spring覆盖已初始化bean

现在在配置类里面创建一个bean date

@Configuration
public class DateConfig {

    @Bean("date")
    public Date  date(){
        return new Date();
    }
}

时间并不是一成不变的，我想要获取当前时间呢，应该怎么覆盖已经在容器内bean。我一开始想到使用org..cloud.context.scope.refresh.RefreshScope,但是Spring boot项目并没有使用到Spring Cloud包，这个走不通，就试着registerBean动态注册相同名字bean，想着能不能覆盖容器内bean，毕竟所谓容器只不过是Map而已，只要通过机制覆盖掉Map 上value 就可以实现动态刷新了。

    private ApplicationContext applicationContext;

    @GetMapping("setting/now")
    public void dkd(){
        GenericApplicationContext gac = (GenericApplicationContext)applicationContext;
        gac.registerBean("date",Date.class);
    }

执行这个请求，直接报错了，抛出了一个BeanDefinitionOverrideException异常，bean不能被覆盖。在DefaultListableBeanFactory.registerBeanDefinition可以看到其中原因

	public void registerBeanDefinition(String beanName, BeanDefinition beanDefinition)
			throws BeanDefinitionStoreException {
                // 省略多余代码
		BeanDefinition existingDefinition = this.beanDefinitionMap.get(beanName);
		if (existingDefinition != null) { //对于已经存在bean
			if (!isAllowBeanDefinitionOverriding()) { //如果allowBeanDefinitionOverriding 这个值为false 这里就会抛出异常
				throw new BeanDefinitionOverrideException(beanName, beanDefinition, existingDefinition);
			}
			else if (existingDefinition.getRole() < beanDefinition.getRole()) { //这里是BeanDefinition  
				// 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);
		}
	 //省略。。

然后发现这个allowBeanDefinitionOverriding 在SpringBoot 刚初始化时，在SpringApplication 会初始化这个值，在SpringApplication.prepareContext

	private void prepareContext(DefaultBootstrapContext bootstrapContext, ConfigurableApplicationContext context,
			ConfigurableEnvironment environment, SpringApplicationRunListeners listeners,
			ApplicationArguments applicationArguments, Banner printedBanner) {
		context.setEnvironment(environment);
		postProcessApplicationContext(context);
		applyInitializers(context);
		listeners.contextPrepared(context);
		bootstrapContext.close(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 AbstractAutowireCapableBeanFactory) {
			((AbstractAutowireCapableBeanFactory) beanFactory).setAllowCircularReferences(this.allowCircularReferences);
			if (beanFactory instanceof DefaultListableBeanFactory) {
				((DefaultListableBeanFactory) beanFactory)
						.setAllowBeanDefinitionOverriding(this.allowBeanDefinitionOverriding); //设置到DefaultListableBeanFactory中
			}
		}
		if (this.lazyInitialization) { //开启懒加载配置
			context.addBeanFactoryPostProcessor(new LazyInitializationBeanFactoryPostProcessor());
		}
		context.addBeanFactoryPostProcessor(new PropertySourceOrderingBeanFactoryPostProcessor(context));
		// 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);
	}

接着看下配置文件值如何设置到SpringApplication.allowBeanDefinitionOverriding

	private ConfigurableEnvironment prepareEnvironment(SpringApplicationRunListeners listeners,
			DefaultBootstrapContext bootstrapContext, ApplicationArguments applicationArguments) {
		// Create and configure the environment
		ConfigurableEnvironment environment = getOrCreateEnvironment();
		configureEnvironment(environment, applicationArguments.getSourceArgs());
		ConfigurationPropertySources.attach(environment);
		listeners.environmentPrepared(bootstrapContext, environment);
		DefaultPropertiesPropertySource.moveToEnd(environment);
		Assert.state(!environment.containsProperty("spring.main.environment-prefix"),
				"Environment prefix cannot be set via properties.");
		bindToSpringApplication(environment); //将配置环境bind到属性中
		if (!this.isCustomEnvironment) {
			environment = convertEnvironment(environment);
		}
		ConfigurationPropertySources.attach(environment);
		return environment;
	}

	protected void bindToSpringApplication(ConfigurableEnvironment environment) {
		try {
                        //将配置文件绑定到当前属性上
                        //看起来就有ConfigurationProperties 那味了
			Binder.get(environment).bind("spring.main", Bindable.ofInstance(this)); 
		}
		catch (Exception ex) {//略}
	}

在application.properties 添加下面配置

spring.main.allow-bean-definition-overriding=true

重启后重新执行HTTP 请求，没有报错了，重新获取date 这个bean，时间也变成最新值了。

心得

添加多这一个配置估计为了兼容不同组件间可能存在一些bean 冲突情况，后面初始化bean组件可以覆盖Spring 内部已经创建组件。假如现在Spring 内部已经初始化bean A，并且成功加入到容器中了，这时加载再加载Spring 组件也有一个Class 继承bean A，这是需要添加到容器中。如果没有beanName 相同覆盖的机制，组件在初始化就会失败。
还有一点值得注意的，registerBean 这个方法只有在容器中删除这个bean 缓存，如何已经将bean注入到对象属性中，这时这个值不会变化的，需要手动调用beanFactory.getBean("beanName")，因为只有这个bean不存在时候才会执行初始化。如果有这种bean刷新场景可以使用@Lookup来生成一个代理方法。

    @Lookup
    public Date initDate() { //这里会将容器内Date类型注入，每次调用方法，重新从容器获取一次
        return null;
    }