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;
}