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 ""; 
} 
}