code: public void setSources(Set<String> sources) {
  		Assert.notNull(sources, "Sources must not be null");
  		this.sources = new LinkedHashSet<>(sources);
  	}
docs: Set additional sources that will be used to create an ApplicationContext. A source
  can be: a class name, package name, or an XML resource location.
  <p>
  Sources set here will be used in addition to any primary sources set in the
  constructor.
  @param sources the application sources to set
  @see #SpringApplication(Class...)
  @see #getAllSources()