一开始是android6.0的问题，可以用下面的'MacUtils'类解决，但是当用华为mate 9时（android7.0），结果返回值又是空，只好另想办法。(android8.0下面方法已无效，暂时没想到解决办法)

public class MacUtils {
/**
 * 获取手机的MAC地址
 *
 * @return
 */
public static String getMac() {
    String str = "";
    String macSerial = "";
    try {
        Process pp = Runtime.getRuntime().exec(
                "cat /sys/class/net/wlan0/address ");
        InputStreamReader ir = new InputStreamReader(pp.getInputStream());
        LineNumberReader input = new LineNumberReader(ir);
        for (; null != str; ) {
            str = input.readLine();
            if (str != null) {
                macSerial = str.trim();// 去空格
                break;
            }
        }
    } catch (Exception ex) {
        ex.printStackTrace();
    }
    if ("".equals(macSerial)) {
        try {
            return loadFileAsString("/sys/class/net/eth0/address")
                    .toUpperCase().substring(0, 17);
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
    return macSerial;
}

private static String loadFileAsString(String fileName) throws Exception {
    FileReader reader = new FileReader(fileName);
    String text = loadReaderAsString(reader);
    reader.close();
    return text;
}

private static String loadReaderAsString(Reader reader) throws Exception {
    StringBuilder builder = new StringBuilder();
    char[] buffer = new char[4096];
    int readLength = reader.read(buffer);
    while (readLength >= 0) {
        builder.append(buffer, 0, readLength);
        readLength = reader.read(buffer);
    }
    return builder.toString();
}
}

解决华为mate 9(不知道其他手机是否一样)

 /**
 * 获取蓝牙地址
 */
public static String getMac(Context context) {
    return android.provider.Settings.Secure.getString(context.getContentResolver(), "bluetooth_address");
}