面试题：Java AsynchronousSocketChannel在复杂网络场景下的应用

Question

假设存在一个复杂的网络场景，网络延迟不稳定且存在丢包情况，需要使用AsynchronousSocketChannel实现可靠的数据传输。请详细描述你的设计思路，包括如何处理连接中断重连、数据丢失重传等问题，同时给出关键部分的代码示例或伪代码。

Answer 1

设计思路

1. 连接管理：
   • 使用AsynchronousSocketChannel创建异步连接。当连接中断时，启动重连机制。可以设置一个重试次数和重试间隔，通过定时任务（如ScheduledExecutorService）来尝试重新连接。
   • 维护一个连接状态标志，用于判断当前连接是否正常。
2. 数据传输：
   • 为保证数据可靠传输，为每个发送的数据块添加序列号。接收端根据序列号来判断数据是否有序，若发现数据丢失，发送重传请求。
   • 发送端维护一个已发送但未确认的数据块列表，当收到重传请求或超时未收到确认时，重传相应的数据块。
3. 丢包处理：
   • 引入超时机制，对于发送的数据块，设置一个超时时间。若超时未收到确认，判定为丢包，进行重传。
   • 接收端定期检查接收数据的序列号连续性，发现不连续则向发送端请求重传丢失的数据块。

关键部分代码示例（Java）

1. 连接建立与重连

import java.net.InetSocketAddress;
import java.nio.ByteBuffer;
import java.nio.channels.AsynchronousSocketChannel;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Future;
import java.util.concurrent.ScheduledExecutorService;
import java.util.concurrent.TimeUnit;

public class ReliableSocket {
    private static final String HOST = "127.0.0.1";
    private static final int PORT = 8080;
    private static final int MAX_RETRY = 5;
    private static final int RETRY_INTERVAL = 5; // seconds
    private AsynchronousSocketChannel socketChannel;
    private boolean isConnected = false;
    private final ExecutorService executorService = Executors.newFixedThreadPool(10);
    private final ScheduledExecutorService scheduledExecutorService = Executors.newScheduledThreadPool(1);

    public void connect() {
        int retryCount = 0;
        while (!isConnected && retryCount < MAX_RETRY) {
            try {
                socketChannel = AsynchronousSocketChannel.open();
                Future<Void> future = socketChannel.connect(new InetSocketAddress(HOST, PORT));
                future.get();
                isConnected = true;
                System.out.println("Connected to server.");
            } catch (InterruptedException | ExecutionException e) {
                retryCount++;
                System.out.println("Connection failed, retry attempt " + retryCount);
                try {
                    Thread.sleep(RETRY_INTERVAL * 1000);
                } catch (InterruptedException ex) {
                    Thread.currentThread().interrupt();
                }
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
        if (!isConnected) {
            System.out.println("Failed to connect after multiple attempts.");
        }
    }

    public void handleDisconnect() {
        isConnected = false;
        System.out.println("Connection lost.");
        scheduledExecutorService.schedule(() -> connect(), RETRY_INTERVAL, TimeUnit.SECONDS);
    }
}

2. 数据发送与重传

import java.nio.ByteBuffer;
import java.nio.channels.AsynchronousSocketChannel;
import java.util.HashMap;
import java.util.Map;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Future;
import java.util.concurrent.ScheduledExecutorService;
import java.util.concurrent.TimeUnit;

public class DataSender {
    private static final int TIMEOUT = 3; // seconds
    private final AsynchronousSocketChannel socketChannel;
    private final ExecutorService executorService = Executors.newFixedThreadPool(10);
    private final ScheduledExecutorService scheduledExecutorService = Executors.newScheduledThreadPool(1);
    private final Map<Integer, ByteBuffer> unacknowledgedData = new HashMap<>();

    public DataSender(AsynchronousSocketChannel socketChannel) {
        this.socketChannel = socketChannel;
    }

    public void sendData(ByteBuffer data, int sequenceNumber) {
        unacknowledgedData.put(sequenceNumber, data.duplicate());
        Future<Integer> future = socketChannel.write(data);
        try {
            future.get();
            scheduleAckCheck(sequenceNumber);
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    private void scheduleAckCheck(int sequenceNumber) {
        scheduledExecutorService.schedule(() -> {
            if (unacknowledgedData.containsKey(sequenceNumber)) {
                System.out.println("Timeout for sequence number " + sequenceNumber + ", retransmitting.");
                ByteBuffer data = unacknowledgedData.get(sequenceNumber);
                data.rewind();
                sendData(data, sequenceNumber);
            }
        }, TIMEOUT, TimeUnit.SECONDS);
    }

    public void receiveAck(int sequenceNumber) {
        unacknowledgedData.remove(sequenceNumber);
        System.out.println("Received ACK for sequence number " + sequenceNumber);
    }
}

3. 数据接收与重传请求

import java.nio.ByteBuffer;
import java.nio.channels.AsynchronousSocketChannel;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Future;

public class DataReceiver {
    private static final int BUFFER_SIZE = 1024;
    private final AsynchronousSocketChannel socketChannel;
    private final ExecutorService executorService = Executors.newFixedThreadPool(10);
    private int expectedSequenceNumber = 0;

    public DataReceiver(AsynchronousSocketChannel socketChannel) {
        this.socketChannel = socketChannel;
    }

    public void receiveData() {
        ByteBuffer buffer = ByteBuffer.allocate(BUFFER_SIZE);
        Future<Integer> future = socketChannel.read(buffer);
        try {
            int bytesRead = future.get();
            if (bytesRead > 0) {
                buffer.flip();
                int sequenceNumber = buffer.getInt();
                if (sequenceNumber == expectedSequenceNumber) {
                    // Process data
                    System.out.println("Received data with sequence number " + sequenceNumber);
                    expectedSequenceNumber++;
                    // Send ACK
                    sendAck(sequenceNumber);
                } else {
                    // Request retransmission
                    System.out.println("Missing data, expected sequence number " + expectedSequenceNumber + ", received " + sequenceNumber);
                    requestRetransmission(expectedSequenceNumber);
                }
            }
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    private void sendAck(int sequenceNumber) {
        ByteBuffer ackBuffer = ByteBuffer.wrap(("ACK " + sequenceNumber).getBytes());
        Future<Integer> future = socketChannel.write(ackBuffer);
        try {
            future.get();
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    private void requestRetransmission(int sequenceNumber) {
        ByteBuffer requestBuffer = ByteBuffer.wrap(("RETRANSMIT " + sequenceNumber).getBytes());
        Future<Integer> future = socketChannel.write(requestBuffer);
        try {
            future.get();
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

上述代码实现了在不稳定网络场景下使用AsynchronousSocketChannel进行可靠数据传输的核心功能，包括连接建立与重连、数据发送与重传以及数据接收与重传请求等部分。实际应用中还需根据具体需求进行完善和优化。