Java NIO 套接字：服务器没有通过同一个套接字接收第二条消息答案

【问题标题】：Java NIO sockets: the server doesn't receive the second message over the same socketJava NIO 套接字：服务器没有通过同一个套接字接收第二条消息
【发布时间】：2019-11-25 15:06:50
【问题描述】：

我有使用 Java NIO 套接字构建的简单服务器和客户端。这个想法是客户端向服务器发送5，服务器回复1。然后，客户端再次使用同一个套接字发送1，服务器应该再次回复5。

但是，服务器没有收到第二条消息。这是我的实现：

Server.java

import java.io.IOException;
import java.net.InetSocketAddress;
import java.nio.ByteBuffer;
import java.nio.channels.*;

public class Server {

    public static void main(String[] args) throws IOException {
        Selector selector = Selector.open();
        ServerSocketChannel serverChannel = ServerSocketChannel.open();
        serverChannel.configureBlocking(false);
        serverChannel.bind(new InetSocketAddress(4444));
        serverChannel.register(selector, SelectionKey.OP_ACCEPT);

        while (true) {
            if (selector.select() > 0) {
                for (SelectionKey key : selector.selectedKeys()) {
                    if (key.isReadable()) {
                        SocketChannel clientChannel = (SocketChannel) key.channel();
                        ByteBuffer buffer = ByteBuffer.allocate(1);
                        clientChannel.read(buffer);
                        System.out.println("Received: " + buffer.array()[0]);
                        clientChannel.write(ByteBuffer.wrap(new byte[]{1}));
                        System.out.println("Replied: 1");
                    }
                    if (key.isAcceptable()) {
                        SocketChannel channel = serverChannel.accept();
                        if (channel != null) {
                            channel.configureBlocking(false);
                            channel.register(selector, SelectionKey.OP_READ);
                            System.out.println("Accepted");
                        }
                    }
                }
            }
        }
    }
}

Client.java

import java.io.IOException;
import java.net.InetAddress;
import java.net.InetSocketAddress;
import java.nio.ByteBuffer;
import java.nio.channels.SelectionKey;
import java.nio.channels.Selector;
import java.nio.channels.SocketChannel;

public class Client {

    public static void main(String[] args) throws IOException {
        Selector selector = Selector.open();
        SocketChannel sc = SocketChannel.open();
        sc.configureBlocking(false);
        sc.connect(new InetSocketAddress(InetAddress.getByName("127.0.0.1"), 4444));
        sc.register(selector, SelectionKey.OP_CONNECT);

        while (true) {
            if (selector.select() > 0) {
                for (SelectionKey key : selector.selectedKeys()) {
                    SocketChannel channel = (SocketChannel) key.channel();
                    if (key.isConnectable()) {
                        while (channel.isConnectionPending()) {
                            channel.finishConnect();
                        }
                        key.interestOps(SelectionKey.OP_WRITE);
                        System.out.println("Connected");
                    }
                    if (key.isWritable()) {
                        channel.write(ByteBuffer.wrap(new byte[] { 5 }));
                        key.interestOps(SelectionKey.OP_READ);
                        System.out.println("Sending: 5");
                    }
                    if (key.isReadable()) {
                        ByteBuffer buffer = ByteBuffer.allocate(1);
                        channel.read(buffer);
                        System.out.println("Received: " + buffer.array()[0]);
                        key.interestOps(SelectionKey.OP_WRITE);
                    }
                }
            }
        }
    }
}

服务器输出

Accepted
Received: 5
Replied: 1

客户端输出

Connected
Sending: 5
Received: 1
Sending: 5

【问题讨论】：

标签： java sockets

【解决方案1】：

我在这里找到了解决问题的方法：https://stackoverflow.com/a/9940133/4167198

select() 返回已更改的键数。因此，如果一个密钥在 select() 调用之前已经准备好，那么它可能会返回 0，但 selectedKeys 可能是非空的。

所以检查select()是否返回大于0是没有必要的。

Server.java

import java.io.IOException;
import java.net.InetSocketAddress;
import java.nio.ByteBuffer;
import java.nio.channels.*;

public class Server {

    public static void main(String[] args) throws IOException {
        Selector selector = Selector.open();
        ServerSocketChannel serverChannel = ServerSocketChannel.open();
        serverChannel.configureBlocking(false);
        serverChannel.bind(new InetSocketAddress(4444));
        serverChannel.register(selector, SelectionKey.OP_ACCEPT);

        while (true) {
            selector.select();
            for (SelectionKey key : selector.selectedKeys()) {
                if (key.isReadable()) {
                    SocketChannel clientChannel = (SocketChannel) key.channel();
                    ByteBuffer buffer = ByteBuffer.allocate(1);
                    clientChannel.read(buffer);
                    System.out.println("Received: " + buffer.array()[0]);
                    clientChannel.write(ByteBuffer.wrap(new byte[]{1}));
                    System.out.println("Replied: 1");
                }
                if (key.isAcceptable()) {
                    SocketChannel channel = serverChannel.accept();
                    if (channel != null) {
                        channel.configureBlocking(false);
                        channel.register(selector, SelectionKey.OP_READ);
                        System.out.println("Accepted");
                    }
                }
            }
        }
    }
}

Client.java

import java.io.IOException;
import java.net.InetAddress;
import java.net.InetSocketAddress;
import java.nio.ByteBuffer;
import java.nio.channels.SelectionKey;
import java.nio.channels.Selector;
import java.nio.channels.SocketChannel;

public class Client {

    public static void main(String[] args) throws IOException {
        Selector selector = Selector.open();
        SocketChannel sc = SocketChannel.open();
        sc.configureBlocking(false);
        sc.connect(new InetSocketAddress(InetAddress.getByName("127.0.0.1"), 4444));
        sc.register(selector, SelectionKey.OP_CONNECT);

        while (true) {
            selector.select();
            for (SelectionKey key : selector.selectedKeys()) {
                SocketChannel channel = (SocketChannel) key.channel();
                if (key.isConnectable()) {
                    while (channel.isConnectionPending()) {
                        channel.finishConnect();
                    }
                    key.interestOps(SelectionKey.OP_WRITE);
                    System.out.println("Connected");
                }
                if (key.isWritable()) {
                    channel.write(ByteBuffer.wrap(new byte[] { 5 }));
                    key.interestOps(SelectionKey.OP_READ);
                    System.out.println("Sending: 5");
                }
                if (key.isReadable()) {
                    ByteBuffer buffer = ByteBuffer.allocate(1);
                    channel.read(buffer);
                    System.out.println("Received: " + buffer.array()[0]);
                    key.interestOps(SelectionKey.OP_WRITE);
                }
            }
        }
    }
}

【讨论】：