Spring 启动时如何加载Environment和属性配置呢?

 spring启动时，在构造函数中即可实现env和属性的加载
 
 AbstractApplicationContext -> refresh ()
    -> prepareRefresh () ->
	getEnvironment ().validateRequiredProperties ();

getEnvironment()函数内容

 
public ConfigurableEnvironment getEnvironment() {
		if (this.environment == null) {
			this.environment = createEnvironment();
		}
		return this.environment;
}

protected ConfigurableEnvironment createEnvironment() {
		return new StandardEnvironment();
}

StandardEnvironment 继承自 AbstractEnvironment，看下 AbstractEnvironment 的构造方法

/********************* AbstractEnvironment ***************************/
private final MutablePropertySources propertySources = new MutablePropertySources();

public AbstractEnvironment() {
		customizePropertySources(this.propertySources);
}

// AbstractEnvironment自己没实现，由子类实现
protected void customizePropertySources(MutablePropertySources propertySources) {

}

public class MutablePropertySources implements PropertySources {
	private final list<PropertySource<?>> propertySourceList = new CopyOnWriteArrayList<>();
    public MutablePropertySources() {}
    public MutablePropertySources(PropertySources propertySources) {
		this();
		for (PropertySource<?> propertySource : propertySources) {
			addLast(propertySource);
		}
	}

    /**
	 * Add the given property source object with highest precedence.
	 */
    public void addFirst(PropertySource<?> propertySource) {
		removeIfPresent(propertySource);
		this.propertySourceList.add(0, propertySource);
	}

	/**
	 * Add the given property source object with lowest precedence.
	 */
	public void addLast(PropertySource<?> propertySource) {
		removeIfPresent(propertySource);
		this.propertySourceList.add(propertySource);
	}

	/**
	 * Add the given property source object with precedence immediately higher
	 * than the named relative property source.
	 */
	public void addBefore(String relativePropertySourceName, PropertySource<?> propertySource) {
		assertLegalRelativeAddition(relativePropertySourceName, propertySource);
		removeIfPresent(propertySource);
		int index = assertPresentAndGetIndex(relativePropertySourceName);
		addAtIndex(index, propertySource);
	}
}

StandardEnvirontment中customizePropertySources函数实现

public class StandardEnvironment extends AbstractEnvironment {

	/** System environment property source name: {@value}. */
	public static final String SYSTEM_ENVIRONMENT_PROPERTY_SOURCE_NAME = "systemEnvironment";

	/** JVM system properties property source name: {@value}. */
	public static final String SYSTEM_PROPERTIES_PROPERTY_SOURCE_NAME = "systemProperties";


	/**
	 * 实现了AbstractEnvironment的customizePropertySources方法，添加了系统环境和属性
	 * @see AbstractEnvironment#customizePropertySources(MutablePropertySources)
	 * @see #getSystemProperties()
	 * @see #getSystemEnvironment()
	 */
	@Override
	protected void customizePropertySources(MutablePropertySources propertySources) {
        // 注意这里添加是有顺序的，
		propertySources.addLast(new MapPropertySource(SYSTEM_PROPERTIES_PROPERTY_SOURCE_NAME, getSystemProperties()));
		propertySources.addLast(new SystemEnvironmentPropertySource(SYSTEM_ENVIRONMENT_PROPERTY_SOURCE_NAME, getSystemEnvironment()));
	}

}

public class MapPropertySource extends EnumerablePropertySource<Map<String, Object>> {

	public MapPropertySource(String name, Map<String, Object> source) {
		super(name, source);
	}


	@Override
	@Nullable
	public Object getProperty(String name) {
		return this.source.get(name);
	}

	@Override
	public boolean containsProperty(String name) {
		return this.source.containsKey(name);
	}

	@Override
	public String[] getPropertyNames() {
		return StringUtils.toStringArray(this.source.keySet());
	}

}

public class SystemEnvironmentPropertySource extends MapPropertySource {
    public SystemEnvironmentPropertySource(String name, Map<String, Object> source) {
		super(name, source);
	}

    @Override
	public boolean containsProperty(String name) {
		return (getProperty(name) != null);
	}

    @Override
	@Nullable
	public Object getProperty(String name) {
		String actualName = resolvePropertyName(name);
		return super.getProperty(actualName);
	}

    protected final String resolvePropertyName(String name) {
		Assert.notNull(name, "Property name must not be null");
		String resolvedName = checkPropertyName(name);
		if (resolvedName != null) {
			return resolvedName;
		}
		String uppercasedName = name.toUpperCase();
		if (!name.equals(uppercasedName)) {
			resolvedName = checkPropertyName(uppercasedName);
			if (resolvedName != null) {
				return resolvedName;
			}
		}
		return name;
	}

    @Nullable
	private String checkPropertyName(String name) {
		// Check name as-is
		if (containsKey(name)) {
			return name;
		}
		// Check name with just dots replaced
		String noDotName = name.replace('.', '_');
		if (!name.equals(noDotName) && containsKey(noDotName)) {
			return noDotName;
		}
		// Check name with just hyphens replaced
		String noHyphenName = name.replace('-', '_');
		if (!name.equals(noHyphenName) && containsKey(noHyphenName)) {
			return noHyphenName;
		}
		// Check name with dots and hyphens replaced
		String noDotNoHyphenName = noDotName.replace('-', '_');
		if (!noDotName.equals(noDotNoHyphenName) && containsKey(noDotNoHyphenName)) {
			return noDotNoHyphenName;
		}
		// Give up
		return null;
	}

	private boolean containsKey(String name) {
		return (isSecurityManagerPresent() ? this.source.keySet().contains(name) : this.source.containsKey(name));
	}

	protected boolean isSecurityManagerPresent() {
		return (System.getSecurityManager() != null);
	}
}