java代码基于udp基础上实现可靠数据传输

在UDP协议中，由于其不可靠性，可能存在数据包丢失或乱序等问题。因此，为了实现可靠的数据传输，需要在UDP的基础上进行一些改进。下面是一份基于UDP协议的可靠数据传输的Java代码实现。

首先，定义一个Packet类，用于封装数据包：

public class Packet implements Serializable {
    private int seqNum; //序列号
    private byte[] data; //数据
    private long timeStamp; //时间戳
    private boolean ack; //是否是ACK包

    public Packet(int seqNum, byte[] data, long timeStamp, boolean ack) {
        this.seqNum = seqNum;
        this.data = data;
        this.timeStamp = timeStamp;
        this.ack = ack;
    }

    public int getSeqNum() {
        return seqNum;
    }

    public byte[] getData() {
        return data;
    }

    public long getTimeStamp() {
        return timeStamp;
    }

    public boolean isAck() {
        return ack;
    }
}

然后，定义一个UDPClient类，用于发送数据包和接收ACK包：

public class UDPClient {
    private DatagramSocket socket;
    private InetAddress address;
    private int port;

    public UDPClient(String ipAddress, int port) throws UnknownHostException, SocketException {
        this.socket = new DatagramSocket();
        this.address = InetAddress.getByName(ipAddress);
        this.port = port;
    }

    public void send(byte[] data) throws IOException {
        int seqNum = 0;
        int windowSize = 4;
        long timeout = 1000; //超时时间为1秒
        int base = 0;
        int nextSeqNum = 0;

        List<Packet> packets = new ArrayList<>();
        while (seqNum < data.length) {
            byte[] buffer = new byte[Math.min(data.length - seqNum, 1024)];
            System.arraycopy(data, seqNum, buffer, 0, buffer.length);
            Packet packet = new Packet(seqNum, buffer, System.currentTimeMillis(), false);
            packets.add(packet);
            seqNum += buffer.length;
        }

        while (base < packets.size()) {
            while (nextSeqNum < base + windowSize && nextSeqNum < packets.size()) {
                Packet packet = packets.get(nextSeqNum);
                sendPacket(packet, socket, address, port);
                nextSeqNum++;
            }

            boolean timeoutOccurred = false;
            byte[] receiveData = new byte[1024];
            DatagramPacket receivePacket = new DatagramPacket(receiveData, receiveData.length);

            try {
                socket.setSoTimeout((int) timeout);
                socket.receive(receivePacket);

                Packet ackPacket = (Packet) deserialize(receivePacket.getData());
                if (ackPacket.isAck()) {
                    base = Math.max(base, ackPacket.getSeqNum() + 1);
                }
            } catch (SocketTimeoutException e) {
                timeoutOccurred = true;
            }

            if (timeoutOccurred) {
                for (int i = base; i < nextSeqNum; i++) {
                    Packet packet = packets.get(i);
                    sendPacket(packet, socket, address, port);
                }
            }
        }
    }

    private void sendPacket(Packet packet, DatagramSocket socket, InetAddress address, int port) throws IOException {
        byte[] sendData = serialize(packet);
        DatagramPacket sendPacket = new DatagramPacket(sendData, sendData.length, address, port);
        socket.send(sendPacket);
    }

    private byte[] serialize(Object obj) throws IOException {
        ByteArrayOutputStream baos = new ByteArrayOutputStream();
        ObjectOutputStream oos = new ObjectOutputStream(baos);
        oos.writeObject(obj);
        oos.flush();
        return baos.toByteArray();
    }

    private Object deserialize(byte[] data) throws IOException, ClassNotFoundException {
        ByteArrayInputStream bais = new ByteArrayInputStream(data);
        ObjectInputStream ois = new ObjectInputStream(bais);
        return ois.readObject();
    }
}

在send方法中，首先将数据分成若干个Packet对象。然后，采用滑动窗口的方式发送Packet对象，每发送一个Packet对象，就将nextSeqNum加1。接着，等待ACK包的到来。如果ACK包到来，则将base设置为ACK包的序列号加1。如果超时，则重传从base到nextSeqNum-1的Packet对象。直到所有的Packet对象都被发送并且ACK包都被接收到为止。

最后，可以编写一个测试用例：

public class TestUDPClient {
    public static void main(String[] args) throws IOException {
        String ipAddress = "127.0.0.1";
        int port = 9999;
        UDPClient client = new UDPClient(ipAddress, port);
        byte[] data = "Hello, world!".getBytes();
        client.send(data);
    }
}

这里，我们将数据设置为“Hello, world!”，然后调用client.send方法将数据发送出去。在实际应用中，可以在UDPClient类中添加回调接口来处理接收到的数据。