code: public static String readString(File file, Charset charset) throws IOException {
          if (file == null || !file.isFile()) {
              return null;
          }
          StringBuilder fileContent = new StringBuilder();
  
          BufferedReader reader = null;
          try {
              InputStreamReader is = new InputStreamReader(new FileInputStream(file), charset);
              reader = new BufferedReader(is);
              String line = null;
              while ((line = reader.readLine()) != null) {
                  if (!fileContent.toString().equals("")) {
                      fileContent.append("\r\n");
                  }
                  fileContent.append(line);
              }
              reader.close();
              return fileContent.toString();
          } finally {
              closeQuietly(reader);
          }
      }
docs: read file
  
  @param file    file
  @param charset The name of a supported {@link Charset </code>charset<code>}
  @return if file not exist, return null, else return content of file
  @throws IOException if an error occurs while operator BufferedReader