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, 0, len, "gb2312");
else
return new string(b, 0, len - 1, "gb2312");
}catch(exception ex){
dbtools.error(ex);
return "";
}
}