關於@PropertySources註解的理解：http://www.imooc.com/article/252889?block_id=tuijian_wz

public interface PropertySources extends Iterable<PropertySource<?>> {

	/**
	 * Return a sequential {@link Stream} containing the property sources.
	 * @since 5.1
	 */
	default Stream<PropertySource<?>> stream() {
		return StreamSupport.stream(spliterator(), false);
	}
	/**
	 * Return whether a property source with the given name is contained.
	 * @param name the {@linkplain PropertySource#getName() name of the property source} to find
	 */
	boolean contains(String name);

	/**
	 * Return the property source with the given name, {@code null} if not found.
	 * @param name the {@linkplain PropertySource#getName() name of the property source} to find
	 */
	@Nullable
	PropertySource<?> get(String name);
這就是PropertySources的介面，那麼它的實現類在哪裡？是MutablePropertySources,
}

　　PropertySources是由工廠類生成的，看看DefaultPropertySourceFactory

public class DefaultPropertySourceFactory implements PropertySourceFactory {

    @Override
    public PropertySource<?> createPropertySource(@Nullable String name, EncodedResource resource) throws IOException {
        return (name != null
 
 ? new ResourcePropertySource(name, resource) : new ResourcePropertySource(resource));
    }

}

然後又呼叫ResourcePropertySource類，在該類下，實現了對配置檔案的載入loadProperties(resource).

public class ResourcePropertySource extends PropertiesPropertySource {

	/** The original resource name, if different from the given name. */
	@Nullable
	private final String resourceName;


	/**
	 * Create a PropertySource having the given name based on Properties
	 * loaded from the given encoded resource.
	 */
	public ResourcePropertySource(String name, EncodedResource resource) throws IOException {
		super(name, PropertiesLoaderUtils.loadProperties(resource));
		this.resourceName = getNameForResource(resource.getResource());
	}

　　剩下的就是java的檔案的載入過程，細節就不再討論了