code: public synchronized static void append(File file,String content) throws IOException {
          Writer writer = writerCache.getOrDefault(file.getPath(), new FileWriter(file,true));
          writer.write(content);
          writer.flush();
          writerCache.put(file.getPath(), writer);
      }
docs: 文件中追加内容
  @param file
  @param content
  @throws IOException