MD5 加密

引用：http://blog.sina.com.cn/s/blog_48a6d7ba0100unkd.html

public final static String MD5(String s) {
        try {
            byte[] btInput = s.getBytes();
            MessageDigest mdInst = MessageDigest.getInstance("MD5");
            mdInst.update(btInput);
            byte[] md = mdInst.digest();
            StringBuffer sb = new StringBuffer();
            for (int i = 0; i < md.length; i++) {
                int val = ((int) md[i]) & 0xff;
                if (val < 16)
                    sb.append("0");
                sb.append(Integer.toHexString(val));
            }
            return sb.toString();
        } catch (Exception e) {
            return null;
        }
    }