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public static String toHexString(byte[] b) {
    StringBuffer sb = new StringBuffer();
    for (int i = 0; i < b.length; i++) {
        //& 0x0F  取出后四位
        sb.append(HexUtil.HEX_CHARS.charAt(b[i] >>> 4 & 0x0F));
        sb.append(HexUtil.HEX_CHARS.charAt(b[i] & 0x0F));
    }
    return sb.toString();
}

private static final String HEX_CHARS = "0123456789abcdef";