Spring覆蓋已初始化bean

現在在配置類裏面創建一個bean date

@Configuration
public class DateConfig {

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

時間並不是一成不變的，我想要獲取當前時間呢，應該怎麼覆蓋已經在容器內bean。我一開始想到使用org..cloud.context.scope.refresh.RefreshScope,但是Spring boot項目並沒有使用到Spring Cloud包，這個走不通，就試着registerBean動態註冊相同名字bean，想着能不能覆蓋容器內bean，畢竟所謂容器只不過是Map而已，只要通過機制覆蓋掉Map 上value 就可以實現動態刷新了。

    private ApplicationContext applicationContext;

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

執行這個請求，直接報錯了，拋出了一個BeanDefinitionOverrideException異常，bean不能被覆蓋。在DefaultListableBeanFactory.registerBeanDefinition可以看到其中原因

    public void registerBeanDefinition(String beanName, BeanDefinition beanDefinition)
            throws BeanDefinitionStoreException {
                // 省略多餘代碼
        BeanDefinition existingDefinition = this.beanDefinitionMap.get(beanName);
        if (existingDefinition != null) { //對於已經存在bean
            if (!isAllowBeanDefinitionOverriding()) { //如果allowBeanDefinitionOverriding 這個值爲false 這裏就會拋出異常
                throw new BeanDefinitionOverrideException(beanName, beanDefinition, existingDefinition);
            }
            else if (existingDefinition.getRole() < beanDefinition.getRole()) { //這裏是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。。

然後發現這個allowBeanDefinitionOverriding 在SpringBoot 剛初始化時，在SpringApplication 會初始化這個值，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); //設置到DefaultListableBeanFactory中
            }
        }
        if (this.lazyInitialization) { //開啓懶加載配置
            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設置到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); //將配置環境bind到屬性中
        if (!this.isCustomEnvironment) {
            environment = convertEnvironment(environment);
        }
        ConfigurationPropertySources.attach(environment);
        return environment;
    }

    protected void bindToSpringApplication(ConfigurableEnvironment environment) {
        try {
                        //將配置文件綁定到當前屬性上
                        //看起來就有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

重啓後重新執行HTTP 請求，沒有報錯了，重新獲取date 這個bean，時間也變成最新值了。

心得

添加多這一個配置估計爲了兼容不同組件間可能存在一些bean 衝突情況，後面初始化bean組件可以覆蓋Spring 內部已經創建組件。假如現在Spring 內部已經初始化bean A，並且成功加入到容器中了，這時加載再加載Spring 組件也有一個Class 繼承bean A，這是需要添加到容器中。如果沒有beanName 相同覆蓋的機制，組件在初始化就會失敗。
還有一點值得注意的，registerBean 這個方法只有在容器中刪除這個bean 緩存，如何已經將bean注入到對象屬性中，這時這個值不會變化的，需要手動調用beanFactory.getBean("beanName")，因爲只有這個bean不存在時候纔會執行初始化。如果有這種bean刷新場景可以使用@Lookup來生成一個代理方法。

    @Lookup
    public Date initDate() { //這裏會將容器內Date類型注入，每次調用方法，重新從容器獲取一次
        return null;
    }