请教大家一个IO问题

我想把“明朝的那些事儿.txt”这个文件分成若干个小文件，代码如下，不过这个代码生成的小文件打开之后是乱码，我用的是Winddows操作系统，明朝的那些事儿.txt 的编码方式是GB2312，我知道是BufferedWriter的默认编码方式UTF-8造成的错误，但不知道如何作改，请教高手，谢谢

import java.io.BufferedReader;
import java.io.BufferedWriter;
import java.io.File;
import java.io.FileNotFoundException;
import java.io.FileReader;
import java.io.FileWriter;
import java.io.IOException;

/**

• @author ht
  
• 预处理
  
• /
  public class FilePreprocess {
  public static void main(String[] arg){
  String outputpath = "D:\test\small\";//小文件存放路径
  String filename = "D:\test\明朝的那些事儿.txt";//原文件存放路径
  if(!new File(outputpath).exists()){
  new File(outputpath).mkdirs();
  }
  splitToSmallFiles(new File(filename), outputpath);
  }
  /*大文件切割为小的
  
• @param file
  

• @param outputpath
  
  */
  
  public static void splitToSmallFiles(File file ,String outputpath){
  
  int filePointer = 0;
  
  int MAX_SIZE = 10240;
  
  String filename = "output";

  BufferedWriter writer = null;
  
  try {
  
  BufferedReader reader = new BufferedReader(new FileReader(file));
  
  StringBuffer buffer = new StringBuffer();
  
  String line = reader.readLine();
  
  while(line != null){
  
  buffer.append(line).append("\r\n");
  
  if(buffer.toString().getBytes().length>=MAX_SIZE){
  
  writer = new BufferedWriter(new FileWriter(outputpath+filename+filePointer+".txt"));
  
  writer.write(buffer.toString());
  
  writer.close();
  
  filePointer++;
  
  buffer=new StringBuffer();
  
  }
  
  line = reader.readLine();
  
  }
  
  writer = new BufferedWriter(new FileWriter(outputpath+filename+filePointer+".txt"));
  
  writer.write(buffer.toString());
  
  writer.close();
  
  System.out.println("The file hava splited to small files !");
  
  } catch (FileNotFoundException e) {
  
  System.out.println("file not found !");
  
  e.printStackTrace();
  
  } catch (IOException e) {
  
  e.printStackTrace();
  
  }
  
  }
  
  }

javaAPI上对FileReader的描述：用来读取字符文件的便捷类。此类的构造方法假定默认字符编码和默认字节缓冲区大小都是适当的。要自己指定这些值，可以先在 FileInputStream 上构造一个 InputStreamReader。因为FileReader假定默认字符编码和默认字节缓冲区都是适当的，所以你要做的是不要用FileReader，而是自己创建InputStreamReader.