Springboot默认加载application.yml原理以及扩展

SpringApplication.run(…)默认会加载classpath下的application.yml或application.properties配置文件。公司要求搭建的框架默认加载一套默认的配置文件demo.properties，让开发人员实现“零”配置开发，但是前提如果开发人员在application.yml或application.properties文件中自定义配置，则会“覆盖”默认的demo.properties文件，按照Springboot外部化配置的特性（优先使用先加载的），只要demo.properties配置在application.yml或application.properties 配置之后加载到environment中即可。

一、SpirngApplication.run(…)源码分析

通过源码分析，得知Springboot加载配置文件，是利用Spring的事件机制，通过EventPublishingRunListener取发布准备资源事件ApplicationEnvironmentPreparedEvent，被ConfigFileApplicationListener监听到，从而来实现资源的加载

具体源码如下：

    public ConfigurableApplicationContext run(String... args) {StopWatch stopWatch = new StopWatch();stopWatch.start();ConfigurableApplicationContext context = null;Collection<SpringBootExceptionReporter> exceptionReporters = new ArrayList<>();configureHeadlessProperty();//这里是扩展的关键点SpringApplicationRunListeners listeners = getRunListeners(args);listeners.starting();try {ApplicationArguments applicationArguments = new DefaultApplicationArguments(args);//这里是加载资源的关键ConfigurableEnvironment environment = prepareEnvironment(listeners,applicationArguments);....}

//从方法名称来看就是准备environment的即配置信息

   private ConfigurableEnvironment prepareEnvironment(SpringApplicationRunListeners listeners,ApplicationArguments applicationArguments) {// Create and configure the environmentConfigurableEnvironment environment = getOrCreateEnvironment();configureEnvironment(environment, applicationArguments.getSourceArgs());//这里默认EventPublishingRunListener发布ApplicationEnvironmentPreparedEvent事件//让监听器ConfigFileApplicationListener加载配置文件//这个listeners就是我们扩展的地方listeners.environmentPrepared(environment);bindToSpringApplication(environment);if (this.webApplicationType == WebApplicationType.NONE) {environment = new EnvironmentConverter(getClassLoader()).convertToStandardEnvironmentIfNecessary(environment);}ConfigurationPropertySources.attach(environment);return environment;}
SpirngApplication.run(...)方法中有个重要的扩展点方法getRunListeners(args);private SpringApplicationRunListeners getRunListeners(String[] args) {Class<?>[] types = new Class<?>[] { SpringApplication.class, String[].class };return new SpringApplicationRunListeners(logger, getSpringFactoriesInstances(SpringApplicationRunListener.class, types, this, args));}//可扩展的关键点SpringFactoriesLoader//SpringFactoriesLoader会去加载META-INF/spring.factories文件，并根据//type过滤出符合要求的类//比如这里的type对应的是：SpringApplicationRunListenerprivate <T> Collection<T> getSpringFactoriesInstances(Class<T> type,Class<?>[] parameterTypes, Object... args) {ClassLoader classLoader = Thread.currentThread().getContextClassLoader();// Use names and ensure unique to protect against duplicatesSet<String> names = new LinkedHashSet<>(SpringFactoriesLoader.loadFactoryNames(type, classLoader));List<T> instances = createSpringFactoriesInstances(type, parameterTypes,classLoader, args, names);AnnotationAwareOrderComparator.sort(instances);return instances;}

Springboot默认提供的META-INF/spring.factories，这里就是我们可以扩展的地方

Run Listeners

org.springframework.boot.SpringApplicationRunListener=\
org.springframework.boot.context.event.EventPublishingRunListener

至此资源加载的大概流程就分析完了，下面是我们的扩展

二、扩展——自定义加载配置文件（demo.properties）

通过上述源码分析得知：只需要在项目中添加META-INF/spring.factories，并配置SpringApplicationRunListener为我们自定义的来即可

1、在项目中的resources下创建META-INF/spring.factories

org.springframework.boot.SpringApplicationRunListener=\
com.demo.module.ApplicatonEnvironDemoListener

2、ApplicatonEnvironDemoListener的代码

  package com.chyjr.hyboot.demo.module;import org.springframework.boot.SpringApplication;import org.springframework.boot.SpringApplicationRunListener;import org.springframework.context.ConfigurableApplicationContext;import org.springframework.core.PriorityOrdered;import org.springframework.core.env.ConfigurableEnvironment;import org.springframework.core.env.MutablePropertySources;import org.springframework.core.env.PropertiesPropertySource;import org.springframework.core.env.PropertySource;import java.io.IOException;import java.util.Properties;public class ApplicatonEnvironDemoListener implements SpringApplicationRunListener,PriorityOrdered {private SpringApplication application;private String[] args;/*** 通过反射创建该实例对象的，构造方法中的参数要加上如下参数*/public ApplicatonEnvironDemoListener(SpringApplication application,String[] args){this.application = application;this.args = args;}/*** 在准备环境之间调用* SpringApplication#run -> listeners.starting();*/@Overridepublic void starting() {System.out.println("starting-----");}@Overridepublic void environmentPrepared(ConfigurableEnvironment environment) {Properties properties = new Properties();try {//demo.properties就是我们自定义的配置文件，extension是自定义目录properties.load(this.getClass().getClassLoader().getResourceAsStream("extension/demo.properties"));PropertySource propertySource =new PropertiesPropertySource("demo",properties);//PropertySource是资源加载的核心MutablePropertySources propertySources = environment.getPropertySources();//这里添加最后propertySources.addLast(propertySource);} catch (IOException e) {e.printStackTrace();}}//省略其他方法.../*** 这里可以设置该配置文件加载的顺序，在application.yml之前还是之后* EventPublishingRunListener#getOrder方法返回 “0”，按照需求这里我们这是比0大，* 即在application.yml之后加载，这样在application.yml配置时，可以“覆盖”my.yml* 这里用“覆盖”可能不合适，意思到了就好*/@Overridepublic int getOrder() {return 1;}}