android BluetoothDevice.getName() return null

Finally, i found out the solution:

1.For device connected:

Read device name from gatt characteristic org.bluetooth.characteristic.gap.device_name of service org.bluetooth.service.generic_access.

2.For device no connected:

    /**
     * Get device name from ble advertised data
     */
    private LeScanCallback mScanCb = new LeScanCallback() {
        @Override
        public void onLeScan(final BluetoothDevice device, final int rssi,
            byte[] scanRecord) {
            final BleAdvertisedData badata = BleUtil.parseAdertisedData(scanRecord);
            String deviceName = device.getName();
            if( deviceName == null ){
                deviceName = badata.getName();
            }
    }


////////////////////// Helper Classes: BleUtil and BleAdvertisedData ///////////////
    final public class BleUtil {        
        private final static String TAG=BleUtil.class.getSimpleName();
        public static BleAdvertisedData parseAdertisedData(byte[] advertisedData) {      
            List<UUID> uuids = new ArrayList<UUID>();
            String name = null;
            if( advertisedData == null ){
                return new BleAdvertisedData(uuids, name);
            }

            ByteBuffer buffer = ByteBuffer.wrap(advertisedData).order(ByteOrder.LITTLE_ENDIAN);
            while (buffer.remaining() > 2) {
                byte length = buffer.get();
                if (length == 0) break;

                byte type = buffer.get();
                switch (type) {
                    case 0x02: // Partial list of 16-bit UUIDs
                    case 0x03: // Complete list of 16-bit UUIDs
                        while (length >= 2) {
                            uuids.add(UUID.fromString(String.format(
                                    "%08x-0000-1000-8000-00805f9b34fb", buffer.getShort())));
                            length -= 2;
                        }
                        break;
                    case 0x06: // Partial list of 128-bit UUIDs
                    case 0x07: // Complete list of 128-bit UUIDs
                        while (length >= 16) {
                            long lsb = buffer.getLong();
                            long msb = buffer.getLong();
                            uuids.add(new UUID(msb, lsb));
                            length -= 16;
                         }
                        break;
                    case 0x09:
                        byte[] nameBytes = new byte[length-1];
                        buffer.get(nameBytes);
                        try {
                            name = new String(nameBytes, "utf-8");
                        } catch (UnsupportedEncodingException e) {
                            e.printStackTrace();
                        }
                        break;
                    default:
                        buffer.position(buffer.position() + length - 1);
                        break;
                    }
                }
            return new BleAdvertisedData(uuids, name);
        }
    }


    public class BleAdvertisedData {
        private List<UUID> mUuids;
        private String mName;
        public BleAdvertisedData(List<UUID> uuids, String name){
            mUuids = uuids;
            mName = name;
        }

        public List<UUID> getUuids(){
            return mUuids;
        }

        public String getName(){
            return mName;
        }
    }