java 截取字符串（判断汉字）

/**
* 截取字符串 len为字节长度
* @param str
* @param len
* @return
* @throws UnsupportedEncodingException
*/
public static String getLimitLengthString( String str,int len){
try{
int counterOfDoubleByte = 0;
byte[] b = str.getBytes("gb2312");
if(b.length <= len)
return str;
for(int i = 0; i < len; i++){
if(b[i] < 0)
counterOfDoubleByte++;
}
if(counterOfDoubleByte % 2 == 0)
return new String(b,len,"gb2312");
else
return new String(b,len - 1,"gb2312");
}catch(Exception ex){
DBTools.error(ex);
return "";
}
}