java 按字节长度截取字符串

public static String subStr(String str,int start,int end) throws Exception{
  byte[] bytes=str.getBytes("GBK");
  String  string="";
  end=end-start;
  if(start>=0&&end>0&&bytes.length>=(end+start)){
  if(start!=0){
  String tempString=new String(bytes,0,start,"GBK");
  String endTemp=tempString.substring(tempString.length()-1);
  String endTemp1=new String(endTemp.getBytes("GBK"),"GBK");
  if(endTemp.equals(endTemp1)){
  string= new String(bytes,start,end,"GBK");
  }else{
  string= new String(bytes,start+1,end-1,"GBK");
  }
  }else{
  string= new String(bytes,start,end,"GBK");
  }
  String laString=string.substring(string.length()-1);
  String lastTemp=new String(laString.getBytes("GBK"),"GBK");
  if(!laString.equals(lastTemp)){
  string=string.substring(0,string.length()-1);
  }
  }
  return string;
  }