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;
}
}