How does SpringBoot choose to load its own beans first?

Implementation method

DependsOn annotation

There are many ways to implement @DependsOn online. If the number of dependent beans is small, it is easier to do. , but when the number increases, it becomes more troublesome. Each class needs to be rewritten, so the second method is recommended.

ApplicationContextInitializer

After registering ApplicationContextInitializer, you can register BeanDefinitionRegistryPostProcessor in Spring. Finally, implement BeanDefinitionRegistryPostProcessor to register the target bean.

 class DemoApplicationContextInitializer implements ApplicationContextInitializer<ConfigurableApplicationContext> {
  @Override
  public void initialize(ConfigurableApplicationContext applicationContext) {
      applicationContext.addBeanFactoryPostProcessor(new DemoBeanDefinitionRegistryPostProcessor());
  }
}

Implementing BeanDefinitionRegistryPostProcessor:

public class DemoBeanDefinitionRegistryPostProcessor implements BeanDefinitionRegistryPostProcessor, PriorityOrdered {
  // from BeanDefinitionRegistryPostProcessor interface
  @Override
  public void postProcessBeanDefinitionRegistry(BeanDefinitionRegistry beanDefinitionRegistry) throws BeansException {
// 重点在这里，这里可以把自己的 想要提起 实现的 或者初始化的 bean  加到这里
beanDefinitionRegistry.registerBeanDefinition("DemoService",new RootBeanDefinition(DemoService.class));
  }
  // from BeanDefinitionRegistryPostProcessor interface
  @Override
  public void postProcessBeanFactory(ConfigurableListableBeanFactory configurableListableBeanFactory) throws BeansException {
  }
// from PriorityOrdered  interface
  @Override
  public int getOrder() {
  // 排在 ConfigurationClassPostProcessor 之前即可
      return Ordered.LOWEST_PRECEDENCE - 1;
  }
}

It should be noted that @Componet or other annotations cannot be used to register BeanDefinitionRegistryPostProcessor.

Because the @Componet annotation method can be registered only if it is scanned by ConfigurationClassPostProcessor. Now, we need to consider our bean registration before these beans, so it must not be managed by "it-ConfigurationClassPostProcessor". Thinking about it from another angle, if the beans registered by "it" management class must not be ranked in front of ConfigurationClassPostProcessor.

Note: @Order can only control the order of spring's own beans. It cannot control the annotations @Service @Component and @Repository.

Simple Demo

Requirements: The author wants TestService to be registered in advance, and only after execution can other beans be registered.

public class TestService {
    // 存放系统配置
    private static Map<String, String> GLOBAL_CONF = new HashMap<>();
    @PostConstruct
    public void init() {
        // 先做初始化 GLOBAL_CONF 或者其他 IO操作
        // GLOBAL_CONF.put(key, value);
    }
    //其他 bean 通过这个方法获得系统配置
    public static String getGlobalConfig(String key) {
        return GLOBAL_CONF.get(key);
    }
}

Implementation: (For simplicity, directly implement the above three interfaces)

public class DemoBeanDefinitionRegistryPostProcessor implements BeanDefinitionRegistryPostProcessor, 
PriorityOrdered, 
ApplicationContextInitializer<ConfigurableApplicationContext> {
    /**
     *  第二步： 注册 自己的 bean
     *
     * @param beanDefinitionRegistry
     */
    // from BeanDefinitionRegistryPostProcessor interface
    @Override
    public void postProcessBeanDefinitionRegistry(BeanDefinitionRegistry beanDefinitionRegistry) throws BeansException {
        beanDefinitionRegistry.registerBeanDefinition("TestService",new RootBeanDefinition(TestService.class));
    }
    // from BeanDefinitionRegistryPostProcessor interface
    @Override
    public void postProcessBeanFactory(ConfigurableListableBeanFactory configurableListableBeanFactory) throws BeansException {
    }
// from PriorityOrdered  interface
    @Override
    public int getOrder() {
        return Ordered.LOWEST_PRECEDENCE - 1;
    }
    /**
     *  第一步 先注册 到 configurableApplicationContext 中
     *
     * @param configurableApplicationContext
     */
    // from ApplicationContextInitializer  interface
    @Override
    public void initialize(ConfigurableApplicationContext configurableApplicationContext) {
        configurableApplicationContext.addBeanFactoryPostProcessor(new DemoBeanDefinitionRegistryPostProcessor());
    }
}

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