Springboot扩展点之ApplicationContextInitializer

Bertha 。 2024-03-30 13:48 83阅读 0赞

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Springboot扩展点系列实现方式、工作原理集合：

[Springboot扩展点之ApplicationContextInitializer][Springboot_ApplicationContextInitializer]

[Springboot扩展点之BeanFactoryPostProcessor][Springboot_BeanFactoryPostProcessor]

[Springboot扩展点之BeanDefinitionRegistryPostProcessor][Springboot_BeanDefinitionRegistryPostProcessor]

[Springboot扩展点之BeanPostProcessor][Springboot_BeanPostProcessor]

[Springboot扩展点之InstantiationAwareBeanPostProcessor][Springboot_InstantiationAwareBeanPostProcessor]

[Springboot扩展点之SmartInstantiationAwareBeanPostProcessor][Springboot_SmartInstantiationAwareBeanPostProcessor]

[Springboot扩展点之ApplicationContextAwareProcessor][Springboot_ApplicationContextAwareProcessor]

[Springboot扩展点之@PostConstruct][Springboot_PostConstruct]

[Springboot扩展点之InitializingBean][Springboot_InitializingBean]

[Springboot扩展点之SmartInitializingSingleton][Springboot_SmartInitializingSingleton]

[Springboot扩展点之CommandLineRunner和ApplicationRunner][Springboot_CommandLineRunner_ApplicationRunner]

[Springboot扩展点之FactoryBean][Springboot_FactoryBean]

[Springboot扩展点之DisposableBean][Springboot_DisposableBean]

[Springboot扩展点系列之终结篇：Bean的生命周期][Springboot_Bean]

--------------------

## 前言 ##

一提到Spring、Springoboot，很多人马上就会想到依赖注入、控制反转、自动装配、约定大于配置、使开发变得简单等等。但是如果仅仅会使用Springboot、SpringMVC完成一些增删改查，解决一些bug，那么实际上你并没有真的懂Spring、Springboot。Spring的核心是容器，Springboot更是封装了Spring，把复杂隐藏在内部，让其在使用上更简单，同时又预留了很多的扩展。所以我认为学会Springboot的简单使用只是一个开始，对业务开发更有参考和学习意义的是Springboot如何把复杂变得简单、预留的扩展接口又是如何使用的。

## 1. ApplicationContextInitializer的功能作用 ##

1.  在Spring容器初始化开始的时候，ApplicationContextInitializer接口的所有实现在类会被实例化；
2.  在Spring容器刷新前，所有实现类的org.springframework.context.ApplicationContextInitializer\#initialize方法会被调用，initialize方法的形参类型是ConfigurableApplicationContext，因此可以认为ApplicationContextInitializer实际上是Spring容器初始化前ConfigurableApplicationContext的回调接口，可以对上下文环境作一些操作，如运行环境属性注册、激活配置文件等

public interface ApplicationContextInitializer<C extends ConfigurableApplicationContext> {
    
       /**
        * Initialize the given application context.
        * @param applicationContext the application to configure
        */
       void initialize(C applicationContext);
    
    }

## 2. 三种实现方式 ##

下面通过一个具体实现类实现ApplicationContextInitializer接口，来分享一下其具体的实现方式。Springboot的扩展点ApplicationContextInitializer接口的实现主要分为两步：

1、实现ApplicationContextInitializer接口；

MyApplicationContextInitializer实现ApplicationContextInitializer接口，并打印一下系统相关属性的key、value

public class MyApplicationContextInitializer implements ApplicationContextInitializer<ConfigurableApplicationContext> {
        @Override
        public void initialize(ConfigurableApplicationContext applicationContext) {
            Map<String, Object> systemProperties = applicationContext.getEnvironment().getSystemProperties();
            System.out.println("---------------start------------------");
            systemProperties.forEach((key,value)->{
                System.out.println("key:"+key+",value:"+value);
            }); System.out.println("---------------end------------------");
        }
    }

2、把实现类注册到Spring容器中

把实现类MyApplicationContextInitializer注册到Spring容器中，主要有三种方式：

### 2.1 spring.factories ###

在resources目录新建/META-INFI/spring.factories文件，并预置以下内容，即可完成自定义MyApplicationContextInitializer的注册；

org.springframework.context.ApplicationContextInitializer=com.fanfu.config.MyApplicationContextInitializer

### 2.2 application.properties ###

在application.properties文件中预置以下配置内容，即可完成自定义MyApplicationContextInitializer的注册；

context.initializer.classes=com.fanfu.config.MyApplicationContextInitializer

### 2.3 springApplication.addInitializers() ###

在springboot的启动类中，使用springApplication.addInitializers(new MyApplicationContextInitializer())，完成自定义MyApplicationContextInitializer的注册；

@SpringBootApplication
    public class FanfuApplication {
        public static void main(String[] args) {
            SpringApplication springApplication = new SpringApplication(FanfuApplication.class);
            springApplication.addInitializers(new MyApplicationContextInitializer());
            springApplication.run(args);
        }
    }

执行结果如下：（user.dir=项目的根目录）

![3f273cc2104748ddb84b66c24c5d9450.png][]

## 3. 初始化时机 ##

ApplicationContextInitializer接口的实现类的初始化，是在SpringApplication类的构造函数中，即spring容器初始化开始前，先通过 getSpringFactoriesInstances(...)得到所有实现类的集合，然后通过 setInitializers(...)注入到SpringApplication类的initializersn属性中；

public SpringApplication(ResourceLoader resourceLoader, Class<?>... primarySources) {
       this.resourceLoader = resourceLoader;
       Assert.notNull(primarySources, "PrimarySources must not be null");
       this.primarySources = new LinkedHashSet<>(Arrays.asList(primarySources));
       this.webApplicationType = WebApplicationType.deduceFromClasspath();
       setInitializers((Collection) getSpringFactoriesInstances(ApplicationContextInitializer.class));
       setListeners((Collection) getSpringFactoriesInstances(ApplicationListener.class));
       this.mainApplicationClass = deduceMainApplicationClass();
    }

## 4. 执行时机 ##

ApplicationContextInitializer接口的实现类的执行时机是在org.springframework.boot.SpringApplication\#prepareContext-->org.springframework.boot.SpringApplication\#applyInitializers中，即spring容器正式刷新前，准备上下文环境时；

1.  getInitializers()得到，SpringApplication类的构造函数中通过 setInitializers(...)注入到SpringApplication类的initializersn属性中的所有ApplicationContextInitializer接口的实现类；
2.  遍历这些实现类，并调用initialize()方法；

protected void applyInitializers(ConfigurableApplicationContext context) {
       for (ApplicationContextInitializer initializer : getInitializers()) {
          Class<?> requiredType = GenericTypeResolver.resolveTypeArgument(initializer.getClass(),
                ApplicationContextInitializer.class);
          Assert.isInstanceOf(requiredType, context, "Unable to call initializer.");
          initializer.initialize(context);
       }
    }

![098f598dc7b04e8eb7466c2678e3dd96.png][]

## 5. 内置的实现类 ##

Springboot内部也有一些内置的实现类，用于辅助Spring相关功能的实现，常见的实现类如下：

### 5.1 DelegatingApplicationContextInitializer ###

ApplicationContextInitializer的第二种实现方式（application.properties），在application.properties文件中配置context.initializer.classes=com.fanfu.config.MyApplicationContextInitializer，DelegatingApplicationContextInitializer的作用就是找到application.properties文件中配置的实现类实例化，并执行initialize()方法

![f2f51fcf2de14df1a6871ea674bab782.png][]

### 5.2 ContextIdApplicationContextInitializer ###

ContextIdApplicationContextInitializer用于设置 Spring 应用上下文 ID，如果在application.properties中未设置spring.application.name，则默认为“application”

![cf0a4485a1b44818a459928cd120710f.png][]

### 5.3 ConfigurationWarningsApplicationContextInitializer ###

ConfigurationWarningsApplicationContextInitializer用于报告 Spring 容器的一些常见的错误配置，可以看出，该初始化器为 context 增加了一个 Bean 的后置处理器。这个处理器是在注册 BeanDefinition 实例之后生效的，用于处理注册实例过程中产生的告警信息，其实就是通过日志打印出告警信息。

![30af0b2776a4492fa44c7b5b24358711.png][]

### 5.4 ServerPortInfoApplicationContextInitializer ###

ServerPortInfoApplicationContextInitializer除了实现了ApplicationContextInitializer接口外，还实现了ApplicationListener接口，ServerPortInfoApplicationContextInitializer作用就是把自己作为一个监听器注册到Spring的上下文环境中；

![ad0dfa3291014ad3b43083d0309518d1.png][]

### 5.5 SharedMetadataReaderFactoryContextInitializer ###

实例化了一个org.springframework.boot.autoconfigure.SharedMetadataReaderFactoryContextInitializer.CachingMetadataReaderFactoryPostProcessor对象并注册到了Spring的上下文环境中，CachingMetadataReaderFactoryPostProcessor是SharedMetadataReaderFactoryContextInitializer的一个内部类，这里就不具体展开了，有兴趣的小伙伴可以继续跟一下org.springframework.boot.autoconfigure.SharedMetadataReaderFactoryContextInitializer.CachingMetadataReaderFactoryPostProcessor\#postProcessBeanDefinitionRegistry方法了解一下具体的内容；

![ff809f3fe7ae41a3b7dfcea101c7ac24.png][]

### 5.6 ConditionEvaluationReportLoggingListener ###

ConditionEvaluationReportLoggingListener从名字看是一个监听器，实际上不是，其内部类org.springframework.boot.autoconfigure.logging.ConditionEvaluationReportLoggingListener.ConditionEvaluationReportListener才是实际的监听器，而ConditionEvaluationReportLoggingListener的作用就是把内部类ConditionEvaluationReportListener注册到Spring上下文环境中；

![2cce73378ed244cba15f431ddc45e748.png][]

## 6. 总结 ##

通过这篇文章，可以了解到：

第一，在Spring容器被刷新前，可以通过实现ApplicationContextInitializer接口对Spring上下文环境作一些配置或操作；

第二，ApplicationContextInitializer接口的实现方式有三种，可以根据项目需要选择合适的；

第三，了解了ApplicationContextInitializer接口实现类的初始化时机和执行时机，以及Springboot内置的具体实现类，可以学习到spring本身是如何利用扩展接口实现一些功能的，对实际的项目开发具有一定的参考意义。

示例代码地址：[凡夫贩夫 / fanfu-web · GitCode][_ fanfu-web _ GitCode]（ApplicationContextInitializer分支）

[f3c0952eb2434701b9742cb61164448a.png]: https://image.dandelioncloud.cn/pgy_files/images/2024/03/30/fd5ea99dc0f149f0bc53d776e0f548a1.png
[Springboot_ApplicationContextInitializer]: https://blog.csdn.net/fox9916/article/details/128522598
[Springboot_BeanFactoryPostProcessor]: https://blog.csdn.net/fox9916/article/details/128823328
[Springboot_BeanDefinitionRegistryPostProcessor]: https://blog.csdn.net/fox9916/article/details/128538625
[Springboot_BeanPostProcessor]: https://blog.csdn.net/fox9916/article/details/128917992
[Springboot_InstantiationAwareBeanPostProcessor]: https://blog.csdn.net/fox9916/article/details/128941464
[Springboot_SmartInstantiationAwareBeanPostProcessor]: https://blog.csdn.net/fox9916/article/details/128976944
[Springboot_ApplicationContextAwareProcessor]: https://blog.csdn.net/fox9916/article/details/128990305
[Springboot_PostConstruct]: https://blog.csdn.net/fox9916/article/details/128998846
[Springboot_InitializingBean]: https://blog.csdn.net/fox9916/article/details/129016083
[Springboot_SmartInitializingSingleton]: https://blog.csdn.net/fox9916/article/details/129042652
[Springboot_CommandLineRunner_ApplicationRunner]: https://blog.csdn.net/fox9916/article/details/129086542?spm=1001.2014.3001.5502
[Springboot_FactoryBean]: https://blog.csdn.net/fox9916/article/details/129070764?spm=1001.2014.3001.5502
[Springboot_DisposableBean]: https://blog.csdn.net/fox9916/article/details/129026156?spm=1001.2014.3001.5501
[Springboot_Bean]: https://blog.csdn.net/fox9916/article/details/129101918
[3f273cc2104748ddb84b66c24c5d9450.png]: https://image.dandelioncloud.cn/pgy_files/images/2024/03/30/f1d3f54096ca4132a7f1d0d11300d13a.png
[098f598dc7b04e8eb7466c2678e3dd96.png]: https://image.dandelioncloud.cn/pgy_files/images/2024/03/30/73083963b7e94c6da7c8be6e774a7c77.png
[f2f51fcf2de14df1a6871ea674bab782.png]: https://image.dandelioncloud.cn/pgy_files/images/2024/03/30/c25fc20c7ac44671a1f313efdb31a2b6.png
[cf0a4485a1b44818a459928cd120710f.png]: https://image.dandelioncloud.cn/pgy_files/images/2024/03/30/993cddffef2e4831b3592c979dc8a3d0.png
[30af0b2776a4492fa44c7b5b24358711.png]: https://image.dandelioncloud.cn/pgy_files/images/2024/03/30/eba9a01f299e4910bf1805a09a612195.png
[ad0dfa3291014ad3b43083d0309518d1.png]: https://image.dandelioncloud.cn/pgy_files/images/2024/03/30/4028d8185b474975bfbfb35382475f75.png
[ff809f3fe7ae41a3b7dfcea101c7ac24.png]: https://image.dandelioncloud.cn/pgy_files/images/2024/03/30/edcde66341114b02ae610ced595c951f.png
[2cce73378ed244cba15f431ddc45e748.png]: https://image.dandelioncloud.cn/pgy_files/images/2024/03/30/a481c6d2bdd649b69066e1ea4eead3b3.png
[_ fanfu-web _ GitCode]: https://gitcode.net/fox9916/fanfu-web.git