工具方法
//生成随即字符串
private static Random random = new Random();
private static final String RAND_CHARS
public static String getRandStr(int length, boolean isOnlyNum) {
int size=isOnlyNum?10:62;
StringBuffer hash = new StringBuffer(length);
for (int i = 0; i < length; i++) {
hash.append(RAND_CHARS.charAt(random.nextInt(size)));
}
return hash.toString();
}