java - 关于springMvc-MultipartFile，我有点迷茫

Question

@Service
public class upService {

public void save(MultipartFile multipartFile) throws IOException {

        multipartFile.getInputStream();

    }

这是我的方法，调用MultipartFile接口的getInputStream()

@SuppressWarnings("serial")

private static class StandardMultipartFile implements MultipartFile, Serializable {

    private final Part part;

    private final String filename;

    public StandardMultipartFile(Part part, String filename) {
        this.part = part;
        this.filename = filename;

      ....................

    @Override
    public InputStream getInputStream() throws IOException {
        return this.part.getInputStream();
    }

    }

debug后发现实现类就是这个StandardMultipartFile，我没有指定实现类，springmvc怎么调用StandardMultipartFile中的实现方法？这是多态吗？它是如何做到的？