SpringBoot创建系统应用上下文是在run方法，第301行。

ConfigurableEnvironment environment = prepareEnvironment(listeners, bootstrapContext, applicationArguments);

private ConfigurableEnvironment prepareEnvironment(SpringApplicationRunListeners listeners,
			DefaultBootstrapContext bootstrapContext, ApplicationArguments applicationArguments) {
		// Create and configure the environment
	ConfigurableEnvironment environment = getOrCreateEnvironment();
	configureEnvironment(environment, applicationArguments.getSourceArgs());
	ConfigurationPropertySources.attach(environment);
	listeners.environmentPrepared(bootstrapContext, environment);
	DefaultPropertiesPropertySource.moveToEnd(environment);
	Assert.state(!environment.containsProperty("spring.main.environment-prefix"),
			"Environment prefix cannot be set via properties.");
	bindToSpringApplication(environment);
	if (!this.isCustomEnvironment) {
		EnvironmentConverter environmentConverter = new EnvironmentConverter(getClassLoader());
		environment = environmentConverter.convertEnvironmentIfNecessary(environment, deduceEnvironmentClass());
	}
	ConfigurationPropertySources.attach(environment);
	return environment;
}

private ApplicationContextFactory applicationContextFactory = ApplicationContextFactory.DEFAULT;
private ConfigurableEnvironment getOrCreateEnvironment() {
	if (this.environment != null) {
		return this.environment;
	}
	ConfigurableEnvironment environment = this.applicationContextFactory.createEnvironment(this.webApplicationType);
	if (environment == null && this.applicationContextFactory != ApplicationContextFactory.DEFAULT) {
		environment = ApplicationContextFactory.DEFAULT.createEnvironment(this.webApplicationType);
	}
	return (environment != null) ? environment : new ApplicationEnvironment();
}

ApplicationContextFactory DEFAULT = new DefaultApplicationContextFactory();

最后具体的加载逻辑在

@Override
	public ConfigurableEnvironment createEnvironment(WebApplicationType webApplicationType) {
		return getFromSpringFactories(webApplicationType, ApplicationContextFactory::createEnvironment, null);
	}

仍然是一个工厂类：ApplicationContextFactory
这个工厂类有两个实现。

# Application Context Factories
org.springframework.boot.ApplicationContextFactory=\
org.springframework.boot.web.reactive.context.AnnotationConfigReactiveWebServerApplicationContext.Factory,\
org.springframework.boot.web.servlet.context.AnnotationConfigServletWebServerApplicationContext.Factory

算上默认的ApplicationEnvironment，有三个上下文环境类型，刚好对应了WebApplicationType的三个枚举值。

AnnotationConfigReactiveWebServerApplicationContext.Factory

注释：

接受带注释类作为输入的ReactiveWebServerApplicationContext，特别是@Configuration带注释类，还接受使用javax.inject注释的普通@Component类和JSR-330兼容类。允许一个接一个地注册类（将类名指定为配置位置）以及类路径扫描（将基本包指定为配置地点）。

注意：如果有多个@Configuration类，稍后的@Bean定义将覆盖先前加载的文件中定义的定义。这可以用来通过一个额外的Configuration类故意覆盖某些bean定义。

如果是REACTIVE类型，就创建一个ApplicationReactiveWebEnvironment类。

AnnotationConfigServletWebServerApplicationContext.Factory

注释：

ServletWebServerApplicationContext，它接受带注释的类作为输入，特别是@Configuration注释的类，也接受普通的@Component类和使用javax.inject注释的JSR-330兼容类。允许一个接一个地注册类（将类名指定为配置位置）以及类路径扫描（将基本包指定为配置地点）

注意：如果有多个@Configuration类，稍后的@Bean定义将覆盖先前加载的文件中定义的定义。这可以用来通过一个额外的Configuration类故意覆盖某些bean定义。

如果应用类型是SERVLET，就创建一个ApplicationServletEnvironment类。

java中实例化一个类，会同时调用父类的构造方法。

ApplicationServletEnvironment

继承关系：StandardServletEnvironment、StandardEnvironment、AbstractEnvironment
在AbstractEnvironment抽象类中，创建了一个类。

public AbstractEnvironment() {
	this(new MutablePropertySources());
}

protected AbstractEnvironment(MutablePropertySources propertySources) {
	this.propertySources = propertySources;
	this.propertyResolver = createPropertyResolver(propertySources);
	customizePropertySources(propertySources);
}

最终，propertySources是一个MutablePropertySources 类型。
propertyResolver则是被使用子类createPropertyResolver方法返回的值。

@Override
protected ConfigurablePropertyResolver createPropertyResolver(MutablePropertySources propertySources) {
	return ConfigurationPropertySources.createPropertyResolver(propertySources);
}

创建了一个ConfigurationPropertySourcesPropertyResolver类，再创建一个DefaultResolver，持有propertySources。

然后调用子类customizePropertySources方法。

public static final String SERVLET_CONTEXT_PROPERTY_SOURCE_NAME = "servletContextInitParams";
public static final String SERVLET_CONFIG_PROPERTY_SOURCE_NAME = "servletConfigInitParams";
public static final String JNDI_PROPERTY_SOURCE_NAME = "jndiProperties";
private static final boolean jndiPresent = ClassUtils.isPresent(
			"javax.naming.InitialContext", StandardServletEnvironment.class.getClassLoader());
@Override
protected void customizePropertySources(MutablePropertySources propertySources) {
	propertySources.addLast(new StubPropertySource(SERVLET_CONFIG_PROPERTY_SOURCE_NAME));
	propertySources.addLast(new StubPropertySource(SERVLET_CONTEXT_PROPERTY_SOURCE_NAME));
	if (jndiPresent && JndiLocatorDelegate.isDefaultJndiEnvironmentAvailable()) {
		propertySources.addLast(new JndiPropertySource(JNDI_PROPERTY_SOURCE_NAME));
	}
	super.customizePropertySources(propertySources);
}

把servlet上下文参数和servlet配置参数放入propertySources，占位。
然后调用父类方法

public static final String SYSTEM_ENVIRONMENT_PROPERTY_SOURCE_NAME = "systemEnvironment";
public static final String SYSTEM_PROPERTIES_PROPERTY_SOURCE_NAME = "systemProperties";
@Override
protected void customizePropertySources(MutablePropertySources propertySources) {
	propertySources.addLast(
			new PropertiesPropertySource(SYSTEM_PROPERTIES_PROPERTY_SOURCE_NAME, getSystemProperties()));
	propertySources.addLast(
			new SystemEnvironmentPropertySource(SYSTEM_ENVIRONMENT_PROPERTY_SOURCE_NAME, getSystemEnvironment()));
}

把系统环境变量和JVM环境变量添加到propertySources中。