Springboot默认加载application.yml原理以及扩展

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 environment
		ConfigurableEnvironment 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：SpringApplicationRunListener
	private <T> Collection<T> getSpringFactoriesInstances(Class<T> type,
			Class<?>[] parameterTypes, Object... args) {
		ClassLoader classLoader = Thread.currentThread().getContextClassLoader();
		// Use names and ensure unique to protect against duplicates
		Set<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();
	     */
		@Override
		public void starting() {
			System.out.println("starting-----");
		}

		@Override
		public 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
	     */
		@Override
		public int getOrder() {
			return 1;
		}
	}