面试题：Java Socket服务端高并发处理机制

Question

当Java Socket服务端面临大量客户端连接请求的高并发场景时，描述你所了解的优化策略和机制。例如，如何利用多线程、线程池或者NIO（New I/O）技术来提升服务端的性能和并发处理能力，详细说明每种技术的原理和具体实现方式。

Answer 1

多线程

• 原理：为每个客户端连接分配一个独立的线程来处理。这样多个客户端的请求可以并行处理，避免一个连接的处理阻塞其他连接。
• 具体实现方式：

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.net.ServerSocket;
import java.net.Socket;

public class MultiThreadedServer {
    public static void main(String[] args) {
        try (ServerSocket serverSocket = new ServerSocket(12345)) {
            while (true) {
                Socket clientSocket = serverSocket.accept();
                new Thread(new ClientHandler(clientSocket)).start();
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    static class ClientHandler implements Runnable {
        private final Socket clientSocket;

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

        @Override
        public void run() {
            try (
                BufferedReader in = new BufferedReader(new InputStreamReader(clientSocket.getInputStream()));
                PrintWriter out = new PrintWriter(clientSocket.getOutputStream(), true)
            ) {
                String inputLine;
                while ((inputLine = in.readLine()) != null) {
                    System.out.println("Received: " + inputLine);
                    out.println("Echo: " + inputLine);
                    if ("quit".equals(inputLine)) {
                        break;
                    }
                }
            } catch (IOException e) {
                e.printStackTrace();
            } finally {
                try {
                    clientSocket.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
        }
    }
}

线程池

• 原理：预先创建一组线程，这些线程可以被重复使用来处理客户端请求。避免了频繁创建和销毁线程带来的开销，提高了系统资源的利用率。
• 具体实现方式：

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.net.ServerSocket;
import java.net.Socket;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

public class ThreadPoolServer {
    private static final ExecutorService executor = Executors.newFixedThreadPool(10);

    public static void main(String[] args) {
        try (ServerSocket serverSocket = new ServerSocket(12345)) {
            while (true) {
                Socket clientSocket = serverSocket.accept();
                executor.submit(new ClientHandler(clientSocket));
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    static class ClientHandler implements Runnable {
        private final Socket clientSocket;

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

        @Override
        public void run() {
            try (
                BufferedReader in = new BufferedReader(new InputStreamReader(clientSocket.getInputStream()));
                PrintWriter out = new PrintWriter(clientSocket.getOutputStream(), true)
            ) {
                String inputLine;
                while ((inputLine = in.readLine()) != null) {
                    System.out.println("Received: " + inputLine);
                    out.println("Echo: " + inputLine);
                    if ("quit".equals(inputLine)) {
                        break;
                    }
                }
            } catch (IOException e) {
                e.printStackTrace();
            } finally {
                try {
                    clientSocket.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
        }
    }
}

NIO（New I/O）

• 原理：NIO采用基于通道（Channel）和缓冲区（Buffer）的I/O操作方式，使用多路复用器（Selector）来管理多个通道。它是非阻塞I/O，允许一个线程管理多个连接，大大提高了并发处理能力。
• 具体实现方式：

import java.io.IOException;
import java.net.InetSocketAddress;
import java.nio.ByteBuffer;
import java.nio.CharBuffer;
import java.nio.channels.*;
import java.nio.charset.StandardCharsets;
import java.util.Iterator;
import java.util.Set;

public class NIOServer {
    public static void main(String[] args) {
        try (ServerSocketChannel serverSocketChannel = ServerSocketChannel.open();
             Selector selector = Selector.open()) {
            serverSocketChannel.bind(new InetSocketAddress(12345));
            serverSocketChannel.configureBlocking(false);
            serverSocketChannel.register(selector, SelectionKey.OP_ACCEPT);

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

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

                    if (key.isAcceptable()) {
                        ServerSocketChannel ssc = (ServerSocketChannel) key.channel();
                        SocketChannel sc = ssc.accept();
                        sc.configureBlocking(false);
                        sc.register(selector, SelectionKey.OP_READ);
                    } else if (key.isReadable()) {
                        SocketChannel sc = (SocketChannel) key.channel();
                        ByteBuffer byteBuffer = ByteBuffer.allocate(1024);
                        int bytesRead = sc.read(byteBuffer);
                        if (bytesRead > 0) {
                            byteBuffer.flip();
                            CharBuffer charBuffer = StandardCharsets.UTF_8.decode(byteBuffer);
                            System.out.println("Received: " + charBuffer.toString());
                            ByteBuffer responseBuffer = StandardCharsets.UTF_8.encode("Echo: " + charBuffer.toString());
                            sc.write(responseBuffer);
                        } else if (bytesRead == -1) {
                            sc.close();
                        }
                    }
                    keyIterator.remove();
                }
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}