code: public Resource getResource(final RelativeResource relativeResource)
  		throws IOException {
  		Resource root = getRoot(relativeResource.getRoot());
  		if (root == null) {
  			throw new IllegalStateException("根路径 " + relativeResource.getRoot() + " 未注册");
  		}
  		return root.createRelative(relativeResource.getPath());
  	}
docs: 定位相对资源。
  
  @param relativeResource
  需要定位的资源
  @return 已定位的绝对路径资源
  @throws IOException
  定位失败
  @throws IllegalStateException
  根路径没有注册