Java Socket如何處理并發連接

在Java中，使用Socket處理并發連接通常有兩種方法：多線程和NIO（非阻塞I/O）。下面分別介紹這兩種方法的實現。

1. 多線程方法：

在這種方法中，為每個客戶端連接創建一個新的線程。當客戶端連接到服務器時，服務器將創建一個新的線程來處理該連接。這種方法簡單易實現，但在大量并發連接的情況下可能會導致性能問題。

以下是一個簡單的多線程服務器示例：

import java.io.*;
import java.net.*;

public class MultiThreadedServer {
    public static void main(String[] args) throws IOException {
        ServerSocket serverSocket = new ServerSocket(8888);
        System.out.println("Server started on port 8888");

        while (true) {
            Socket socket = serverSocket.accept();
            System.out.println("New client connected");

            ClientHandler clientHandler = new ClientHandler(socket);
            new Thread(clientHandler).start();
        }
    }
}

class ClientHandler implements Runnable {
    private Socket socket;

    public ClientHandler(Socket socket) {
        this.socket = socket;
    }

    @Override
    public void run() {
        try {
            InputStream input = socket.getInputStream();
            BufferedReader reader = new BufferedReader(new InputStreamReader(input));

            OutputStream output = socket.getOutputStream();
            PrintWriter writer = new PrintWriter(output, true);

            String message;
            while ((message = reader.readLine()) != null) {
                System.out.println("Received message from client: " + message);
                writer.println("Server received: " + message);
            }

            socket.close();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

2. NIO（非阻塞I/O）方法：

NIO提供了更高效的I/O操作，可以在單個線程中處理多個并發連接。這種方法使用Selector來監聽多個通道（Channel），當某個通道上有事件發生時（例如數據可讀或可寫），Selector會通知相應的處理器進行處理。這種方法在大量并發連接的情況下性能更好。

以下是一個簡單的NIO服務器示例：

import java.io.*;
import java.net.*;
import java.nio.*;
import java.nio.channels.*;
import java.util.*;

public class NIOServer {
    public static void main(String[] args) throws IOException {
        ServerSocketChannel serverSocketChannel = ServerSocketChannel.open();
        serverSocketChannel.socket().bind(new InetSocketAddress(8888));
        serverSocketChannel.configureBlocking(false);

        Selector selector = Selector.open();
        serverSocketChannel.register(selector, SelectionKey.OP_ACCEPT);
        System.out.println("Server started on port 8888");

        while (true) {
            selector.select();
            Set<SelectionKey> selectedKeys = selector.selectedKeys();
            Iterator<SelectionKey> iterator = selectedKeys.iterator();

            while (iterator.hasNext()) {
                SelectionKey key = iterator.next();

                if (key.isAcceptable()) {
                    handleAccept(key, selector);
                } else if (key.isReadable()) {
                    handleRead(key);
                }

                iterator.remove();
            }
        }
    }

    private static void handleAccept(SelectionKey key, Selector selector) throws IOException {
        ServerSocketChannel serverSocketChannel = (ServerSocketChannel) key.channel();
        SocketChannel socketChannel = serverSocketChannel.accept();
        socketChannel.configureBlocking(false);
        socketChannel.register(selector, SelectionKey.OP_READ);
        System.out.println("New client connected");
    }

    private static void handleRead(SelectionKey key) throws IOException {
        SocketChannel socketChannel = (SocketChannel) key.channel();
        ByteBuffer buffer = ByteBuffer.allocate(1024);
        int bytesRead = socketChannel.read(buffer);

        if (bytesRead > 0) {
            buffer.flip();
            byte[] data = new byte[buffer.remaining()];
            buffer.get(data);
            String message = new String(data).trim();
            System.out.println("Received message from client: " + message);

            ByteBuffer responseBuffer = ByteBuffer.wrap(("Server received: " + message).getBytes());
            socketChannel.write(responseBuffer);
        } else if (bytesRead < 0) {
            socketChannel.close();
        }
    }
}

這兩種方法都可以處理并發連接，具體選擇哪種方法取決于你的需求和性能要求。對于大量并發連接的情況，NIO方法通常更合適。