Spring扩展接口-ApplicationContextInitializer

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简介

上下文（ConfigurableApplicationContext）刷新（当spring框架内部执行 ConfigurableApplicationContext#refresh() 方法的时候或者在SpringBoot的run()执行时）之前，Spring提供了扩展接口ApplicationContextInitializer，允许对ConfigurableApplicationContext做进一步的处理和设置。

通常用于需要对应用程序上下文进行编程初始化的web应用程序中。例如，根据上下文环境注册属性源或激活配置文件。参考ContextLoader和FrameworkServlet中支持定义contextInitializerClasses作为context-param或定义init-param。
ApplicationContextInitializer支持Order注解，表示执行顺序，越小越早执行；

public interface ApplicationContextInitializer<C extends ConfigurableApplicationContext> {

    /**

     * Initialize the given application context.

     * @param applicationContext the application to configure

     */

    void initialize(C applicationContext);

}

用途示例

SpringBoot内置的一些ApplicationContextInitializer

下面列出了一个使用缺省配置的Springboot web应用默认所使用到的ApplicationContextInitializer实现们：
DelegatingApplicationContextInitializer
使用环境属性context.initializer.classes指定的初始化器(initializers)进行初始化工作，如果没有指定则什么都不做。通过它使得我们可以把自定义实现类配置在application.properties里成为了可能。
ContextIdApplicationContextInitializer
设置Spring应用上下文的ID,会参照环境属性。至于Id设置为啥值会参考环境属性：
spring.application.name
vcap.application.name
spring.config.name
spring.application.index
vcap.application.instance_index
如果这些属性都没有，ID使用application。
ConfigurationWarningsApplicationContextInitializer
对于一般配置错误在日志中作出警告
ServerPortInfoApplicationContextInitializer
将内置servlet容器实际使用的监听端口写入到Environment环境属性中。这样属性local.server.port就可以直接通过@Value注入到测试中，或者通过环境属性Environment获取。
SharedMetadataReaderFactoryContextInitializer
创建一个SpringBoot和ConfigurationClassPostProcessor共用的CachingMetadataReaderFactory对象。实现类为：ConcurrentReferenceCachingMetadataReaderFactory
ConditionEvaluationReportLoggingListener
将ConditionEvaluationReport写入日志。

自定义实现示例

public class FirstInitializer implements ApplicationContextInitializer {

    @Override

    public void initialize(ConfigurableApplicationContext applicationContext) {

        ConfigurableEnvironment environment = applicationContext.getEnvironment();

        Map<String, Object> map = new HashMap<>();

        map.put("key1", "First");

        MapPropertySource mapPropertySource = new MapPropertySource("firstInitializer", map);

        environment.getPropertySources().addLast(mapPropertySource);

        System.out.println("run firstInitializer");

    }

}

三种添加方式：
1.在 main函数中编程方式添加

@SpringBootApplication

public class SpringbootApplication {

    public static void main(String[] args) {

        SpringApplication springApplication = new SpringApplication(SpringbootApplication.class);

        springApplication.addInitializers(new FirstInitializer());

        springApplication.run();

    }

}

2.在application.properties配置

context.initializer.classes=com.learn.springboot.initializer.FirstInitializer

3.SPI扩展方式，在resources/META-INF/spring.factories 中配置

org.springframework.context.ApplicationContextInitializer=com.learn.springboot.initializer.FirstInitializer

更多信息请移步Spring专栏：www.yuque.com/mrhuang-ire…

参考：
[1].www.cnblogs.com/duanxz/p/11…
[2].zhuanlan.zhihu.com/p/650183190