Java HTML/XML - 如何将XML字符串转换为XML文件

import java.io.BufferedWriter;
import java.io.File;
import java.io.FileWriter;
import java.io.Writer;

public class Main {
  public static void main(String args[]) throws Exception {
    String s = "<xmltag atr=value>tag data</xmltag>";
    FileWriter fr = new FileWriter(new File("a.txt"));
    Writer br = new BufferedWriter(fr);
    br.write(s);
    br.close();
  }
}