以下是使用Java语言和ACR122U-A9读取NFC卡片7字节UID的示例代码：

import java.util.Arrays;
import javax.smartcardio.CardException;
import javax.smartcardio.TerminalFactory;
import javax.smartcardio.Card;
import javax.smartcardio.CardChannel;
import javax.smartcardio.CommandAPDU;
import javax.smartcardio.ResponseAPDU;

public class NFCReader {
    public static void main(String[] args) {
        // 获取智能卡终端列表
        try {
            TerminalFactory factory = TerminalFactory.getDefault();
            List terminals = factory.terminals().list();
            
            // 获取第一个智能卡终端
            CardTerminal terminal = terminals.get(0);

            // 等待卡片插入
            while (true) {
                if (terminal.isCardPresent()) {
                    break;
                }
            }

            // 获取智能卡
            Card card = terminal.connect("*");
            CardChannel channel = card.getBasicChannel();
            
            // 发送APDU指令读取卡片UID
            byte[] command = new byte[] { (byte)0xFF, (byte)0xCA, (byte)0x00, (byte)0x00, (byte)0x07 };
            CommandAPDU apdu = new CommandAPDU(command);
            ResponseAPDU response = channel.transmit(apdu);
            
            // 获取卡片UID并以十六进制字符串格式输出
            if (response.getSW() == 0x9000) {
                byte[] uid = Arrays.copyOfRange(response.getBytes(), 0, 7);
                String uidStr = javax.xml.bind.DatatypeConverter.printHexBinary(uid);
                System.out.println("卡片UID: " + uidStr);
            } else {
                System.out.println("APDU指令发送失败");
            }
            
            // 断开智能卡连接
            card.disconnect(true);
        } catch (CardException e) {
            e.printStackTrace();
        }
    }
}