SpringBoot应用中涉及配置相关的东西，例如配置文件、环境变量、系统变量等都是基于类ApplicationServletEnvironment的构造器开启的，如下其抽象父类AbstractEnvironment构造器：

public abstract class AbstractEnvironment{
    private final MutablePropertySources propertySources;

	private final ConfigurablePropertyResolver propertyResolver;

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

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

抽象类PropertySource 内部存在两个属性 name & source：

public abstract class PropertySource<T> {

	protected final String name;

	protected final T source;
}

其中，泛型source是指压缩后的不同类型的配置value。

抽象类PropertySource的常用子类如下所示：