Spring扩展点之EnvironmentPostProcessor

前言

EnvironmentPostProcessor是一个Environment后置处理器，是spring中比较重要的一个扩展点。为什么重要呢？因为这个扩展点，是在日志模块之前启动的。例如根据这个特性，我们可以在日志启动之前，利用EnvironmentPostProcessor接口来实现对日志模块的准备工作。

源码链路

以springboot启动为例，（2.7.3版本，我回去看了2.1.4版本，逻辑没啥变化）

1. run()方法
// 准备环境
2. ConfigurableEnvironment environment = this.prepareEnvironment(listeners, bootstrapContext, applicationArguments);
// 所有的实现了SpringApplicationRunListener的listeners
3. listeners.environmentPrepared(bootstrapContext, (ConfigurableEnvironment)environment);
// 轮询所有listener发送事件
4. listener.environmentPrepared(bootstrapContext, environment);
// 默认的EventPublishingRunListener发布ApplicationEnvironmentPreparedEvent事件
5. this.initialMulticaster.multicastEvent(new ApplicationEnvironmentPreparedEvent(bootstrapContext, this.application, this.args, environment));

事件发布完毕，找一下订阅方，2.4版本以后是在EnvironmentPostProcessorApplicationListener， 2.4版本之前是在ConfigFileApplicationListener
流程是一样的：

1. if (event instanceof ApplicationEnvironmentPreparedEvent) {
            onApplicationEnvironmentPreparedEvent((ApplicationEnvironmentPreparedEvent) event);
        }
2. private void onApplicationEnvironmentPreparedEvent(ApplicationEnvironmentPreparedEvent event) {
        ConfigurableEnvironment environment = event.getEnvironment();
        SpringApplication application = event.getSpringApplication();
        for (EnvironmentPostProcessor postProcessor : getEnvironmentPostProcessors(application.getResourceLoader(),
                event.getBootstrapContext())) {
            postProcessor.postProcessEnvironment(environment, application);
        }
    }
3. List getEnvironmentPostProcessors(ResourceLoader resourceLoader,
            ConfigurableBootstrapContext bootstrapContext) {
        ClassLoader classLoader = (resourceLoader != null) ? resourceLoader.getClassLoader() : null;
        EnvironmentPostProcessorsFactory postProcessorsFactory = this.postProcessorsFactory.apply(classLoader);
        return postProcessorsFactory.getEnvironmentPostProcessors(this.deferredLogs, bootstrapContext);
    }
4. public static List loadFactoryNames(Class factoryType, @Nullable ClassLoader classLoader) {
        ClassLoader classLoaderToUse = classLoader;
        if (classLoader == null) {
            classLoaderToUse = SpringFactoriesLoader.class.getClassLoader();
        }

        String factoryTypeName = factoryType.getName();
        return (List)loadSpringFactories(classLoaderToUse).getOrDefault(factoryTypeName, Collections.emptyList());
    }
5. Enumeration urls = classLoader.getResources("META-INF/spring.factories");

总结从spring.factories文件里找key为org.springframework.boot.env.EnvironmentPostProcessor的类，以spi的方式注入的类。然后遍历执行postProcessEnvironment()方法。

举例

apollo

1.  将Apollo配置加载提到初始化日志系统之前(1.2.0+)

从1.2.0版本开始，如果希望把日志相关的配置（如`logging.level.root=info`或`logback-spring.xml`中的参数）也放在Apollo管理，那么可以额外配置`apollo.bootstrap.eagerLoad.enabled=true`来使Apollo的加载顺序放到日志系统加载之前，更多信息可以参考[PR 1614](https://github.com/apolloconfig/apollo/pull/1614)。参考配置示例如下：
     # will inject 'application' namespace in bootstrap phase
     apollo.bootstrap.enabled = true
     # put apollo initialization before logging system initialization
     apollo.bootstrap.eagerLoad.enabled=true

上面这一段这是从apollo官方文档copy过来的，我们看看如何实现的。
ApolloApplicationContextInitializer实现了EnvironmentPostProcessor接口， 然后在springboot启动发布ApplicationEnvironmentPreparedEvent事件的时候，触发回调到ApolloApplicationContextInitializer的postProcessEnvironment()方法

@Override
  public void postProcessEnvironment(ConfigurableEnvironment configurableEnvironment, SpringApplication springApplication) {

    // should always initialize system properties like app.id in the first place
    initializeSystemProperty(configurableEnvironment);

    Boolean eagerLoadEnabled = configurableEnvironment.getProperty(PropertySourcesConstants.APOLLO_BOOTSTRAP_EAGER_LOAD_ENABLED, Boolean.class, false);

    //EnvironmentPostProcessor should not be triggered if you don't want Apollo Loading before Logging System Initialization
    if (!eagerLoadEnabled) {
      return;
    }

    Boolean bootstrapEnabled = configurableEnvironment.getProperty(PropertySourcesConstants.APOLLO_BOOTSTRAP_ENABLED, Boolean.class, false);

    if (bootstrapEnabled) {
      initialize(configurableEnvironment);
    }

  }

代码通俗易懂，直接拿配置的apollo.bootstrap.eagerLoad.enabled，如果是true，触发apollo初始化工作。也就是说当你需要使用到apollo作为自己的配置中心，并且需要在日志模块之前完成初始化，利用的就是Spring的EnvironmentPostProcessor扩展点。小伙伴们可以举一反三，合理的运用到自己项目中。