Java I/O - 如何转换文件/路径和URL之间

import java.io.File;
import java.io.InputStream;
import java.net.URI;

public class Main {
  public static void main(String[] argv) throws Exception {
    File file = new File("filename");

    URI uri = file.toURI();
    System.out.println(uri);
    file = new File(uri.toURL().getFile());
    InputStream is = uri.toURL().openStream();
    is.close();
  }
}