Java 网络编程之TCP(四)：基于NIO中的selector实现服务端，解决客户端异常断开导致服务端不断读取OP_READ问题

上一篇文章中，没有使用Selector，实习服务端的读取多个客户端的数据；本文先使用Selector实现读取多个客户单数据的功能，然后做些扩展。

一、基于NIO Selector读取多个客户的数据

1.服务端：基于Selector处理客户端的连接事件：OP_READ，处理客户端的数据具备事件：OP_READ

2.客户端：和上一篇一样，基于BIO实现连接和发送数据

服务端代码：

java 复制代码

import java.io.IOException;
import java.net.InetSocketAddress;
import java.nio.ByteBuffer;
import java.nio.channels.SelectionKey;
import java.nio.channels.Selector;
import java.nio.channels.ServerSocketChannel;
import java.nio.channels.SocketChannel;
import java.util.Iterator;
import java.util.Set;

/**
 * 基于NIO实现服务端，通过Selector基于事件驱动客户端的读取
 *
 */
class NIOSelectorServer {
    Selector selector;

    public static void main(String[] args) throws IOException {
        NIOSelectorServer server = new NIOSelectorServer();
        server.start(); // 开启监听和事件处理
    }

    public void start() {
        initServer();
        // selector非阻塞轮询有哪些感兴趣的事件到了
        doService();
    }

    private void doService() {
        if (selector == null) {
            System.out.println("server init failed, without doing read/write");
            return;
        }
        try {
            while (true) {
                while (selector.select() > 0) {
                    Set<SelectionKey> keys = selector.selectedKeys(); // 感兴趣且准备好的事件
                    Iterator<SelectionKey> iterator = keys.iterator(); // 迭代器遍历处理，后面要删除集合元素
                    while (iterator.hasNext()) {
                        SelectionKey key = iterator.next();
                        iterator.remove(); // 删除当前元素，防止重复处理
                        // 下面根据事件进行分别处理
                        if (key.isAcceptable()) {
                            // 客户端连接事件
                            acceptHandler(key);
                        } else if (key.isReadable()) {
                            // 读取客户端数据
                            readHandler(key);
                        }
                    }
                }
            }
        } catch (IOException exception) {
            exception.printStackTrace();
        }
    }

    private void initServer() {
        try {
            ServerSocketChannel serverSocketChannel = ServerSocketChannel.open();
            serverSocketChannel.configureBlocking(false);
            serverSocketChannel.bind(new InetSocketAddress(9090));

            // 此时在selector上注册感兴趣的事件
            // 这里先注册OP_ACCEPT: 客户端连接事件
            selector = Selector.open();
            serverSocketChannel.register(selector, SelectionKey.OP_ACCEPT);
            System.out.println("server init success");
        } catch (IOException exception) {
            exception.printStackTrace();
            System.out.println("server init failied");
        }
    }

    public void acceptHandler(SelectionKey key) {
        ServerSocketChannel server = (ServerSocketChannel) key.channel(); // 获取客户端的channel
        try {
            SocketChannel client = server.accept();
            client.configureBlocking(false); // 设置client非阻塞
            System.out.println("server receive a client :" + client);
            // 注册OP_READ事件，用于从客户端读取数据
            // 给Client分配一个buffer，用于读取数据，注意buffer的线程安全
            ByteBuffer buffer = ByteBuffer.allocate(1024); // buffer这个参数注册的时候也可以不用
            client.register(key.selector(), SelectionKey.OP_READ, buffer);
        } catch (IOException exception) {
            exception.printStackTrace();
        }
    }

    public void readHandler(SelectionKey key) {
        System.out.println("read handler");
        SocketChannel client = (SocketChannel) key.channel(); // 获取客户端的channel
        ByteBuffer buffer = (ByteBuffer) key.attachment(); // 获取Client channel关联的buffer
        buffer.clear(); // 使用前clear

        // 防止数据分包，需要while循环读取
        try {
            while (true) {
                int readLen = client.read(buffer);
                if (readLen > 0) {
                    // 读取到数据了
                    buffer.flip();
                    byte[] data = new byte[buffer.limit()];
                    buffer.get(data);
                    System.out.println("server read data from " + client + ", data is :" + new String(data));
                } else if (readLen == 0) {
                    // 没读到数据
                    System.out.println(client + " : no data");
                    break;
                } else if (readLen == -1) {
                    // client 关闭连接
                    System.out.println(client + " close");
                    break;
                }
            }
        } catch (IOException exception) {
            // exception.printStackTrace();
            // client 关闭连接
            System.out.println(client + " disconnect");
            // todo:disconnect 导致一直有read事件，怎么办？
        }

    }
}

客户端代码：

java 复制代码

import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.net.Socket;

/**
 * 基于BIO的TCP网络通信的客户端，接收控制台输入的数据，然后通过字节流发送给服务端
 *
 * @author freddy
 */
class ChatClient {
    public static void main(String[] args) throws IOException {
        // 连接server
        Socket serverSocket = new Socket("localhost", 9090);
        System.out.println("client connected to server");

        // 读取用户在控制台上的输入，并发送给服务器
        new Thread(new ClientThread(serverSocket)).start();

        // 接收服务端发送过来的数据
        try (InputStream serverSocketInputStream = serverSocket.getInputStream();) {
            byte[] buffer = new byte[1024];
            int len;
            while ((len = serverSocketInputStream.read(buffer)) != -1) {
                String data = new String(buffer, 0, len);
                System.out.println(
                    "client receive data from server" + serverSocketInputStream + " data size：" + len + ": " + data);
            }
        }

    }
}

class ClientThread implements Runnable {
    private Socket serverSocket;

    public ClientThread(Socket serverSocket) {
        this.serverSocket = serverSocket;
    }

    @Override
    public void run() {
        // 读取用户在控制台上的输入，并发送给服务器
        InputStream in = System.in;
        byte[] buffer = new byte[1024];
        int len;
        try (OutputStream outputStream = serverSocket.getOutputStream();) {
            // read操作阻塞，直到有数据可读，由于后面还要接收服务端转发过来的数据，这两个操作都是阻塞的，所以需要两个线程
            while ((len = in.read(buffer)) != -1) {
                String data = new String(buffer, 0, len);
                System.out.println("client receive data from console" + in + " : " + new String(buffer, 0, len));
                if ("exit\n".equals(data)) {
                    // 模拟客户端关闭连接
                    System.out.println("client close :" + serverSocket);
                    // 这里跳出循环后，try-with-resources 会自动关闭outputStream
                    break;
                }
                // 发送数据给服务器端
                outputStream.write(new String(buffer, 0, len).getBytes()); // 此时buffer中是有换行符
            }
        } catch (IOException e) {
            throw new RuntimeException(e);
        }
    }
}

测试：

先启动服务端，再启动2个客户端，客户端发送数据

bash 复制代码

server init success
server receive a client :java.nio.channels.SocketChannel[connected local=/127.0.0.1:9090 remote=/127.0.0.1:13982]
server receive a client :java.nio.channels.SocketChannel[connected local=/127.0.0.1:9090 remote=/127.0.0.1:13989]
read handler
server read data from java.nio.channels.SocketChannel[connected local=/127.0.0.1:9090 remote=/127.0.0.1:13982], data is :client1

java.nio.channels.SocketChannel[connected local=/127.0.0.1:9090 remote=/127.0.0.1:13982] : no data
read handler
server read data from java.nio.channels.SocketChannel[connected local=/127.0.0.1:9090 remote=/127.0.0.1:13989], data is :client2

java.nio.channels.SocketChannel[connected local=/127.0.0.1:9090 remote=/127.0.0.1:13989] : no data

客户端1，exit 关闭连接

客户端日志：

bash 复制代码

exit
client receive data from consolejava.io.BufferedInputStream@72cdfe9a : exit

client close :Socket[addr=localhost/127.0.0.1,port=9090,localport=13982]
Exception in thread "main" java.net.SocketException: Socket closed
	at java.base/sun.nio.ch.NioSocketImpl.endRead(NioSocketImpl.java:248)
	at java.base/sun.nio.ch.NioSocketImpl.implRead(NioSocketImpl.java:327)
	at java.base/sun.nio.ch.NioSocketImpl.read(NioSocketImpl.java:350)
	at java.base/sun.nio.ch.NioSocketImpl$1.read(NioSocketImpl.java:803)
	at java.base/java.net.Socket$SocketInputStream.read(Socket.java:966)
	at java.base/java.io.InputStream.read(InputStream.java:218)
	at com.huawei.io.chatroom.bio.ChatClient.main(ChatClient.java:30)

Process finished with exit code 1

服务端日志：

bash 复制代码

read handler
java.nio.channels.SocketChannel[connected local=/127.0.0.1:9090 remote=/127.0.0.1:13982] close

客户端2，异常关闭

直接关闭客户端2的服务；如果是用nc命令模拟的，直接Ctrl+C

服务端日志：

bash 复制代码

java.net.SocketException: Connection reset
	at java.base/sun.nio.ch.SocketChannelImpl.throwConnectionReset(SocketChannelImpl.java:394)
	at java.base/sun.nio.ch.SocketChannelImpl.read(SocketChannelImpl.java:411)
	at com.huawei.io.chatroom.nio.sel.NIOSelectorServer.readHandler(NIOSelectorServer.java:105)
	at com.huawei.io.chatroom.nio.sel.NIOSelectorServer.doService(NIOSelectorServer.java:54)
	at com.huawei.io.chatroom.nio.sel.NIOSelectorServer.start(NIOSelectorServer.java:32)
	at com.huawei.io.chatroom.nio.sel.NIOSelectorServer.main(NIOSelectorServer.java:26)
java.net.SocketException: Connection reset
	at java.base/sun.nio.ch.SocketChannelImpl.throwConnectionReset(SocketChannelImpl.java:394)
	at java.base/sun.nio.ch.SocketChannelImpl.read(SocketChannelImpl.java:411)
	at com.huawei.io.chatroom.nio.sel.NIOSelectorServer.readHandler(NIOSelectorServer.java:105)
	at com.huawei.io.chatroom.nio.sel.NIOSelectorServer.doService(NIOSelectorServer.java:54)
	at com.huawei.io.chatroom.nio.sel.NIOSelectorServer.start(NIOSelectorServer.java:32)
	at com.huawei.io.chatroom.nio.sel.NIOSelectorServer.main(NIOSelectorServer.java:26)
java.nio.channels.SocketChannel[connected local=/127.0.0.1:9090 remote=/127.0.0.1:16672] disconnect
read handler
java.nio.channels.SocketChannel[connected local=/127.0.0.1:9090 remote=/127.0.0.1:16672] disconnect
read handler
java.nio.channels.SocketChannel[connected local=/127.0.0.1:9090 remote=/127.0.0.1:16672] disconnect
read handler
java.nio.channels.SocketChannel[connected local=/127.0.0.1:9090 remote=/127.0.0.1:16672] disconnect

可以看到，客户端2的异常关闭，会导致服务器端一直不断收到客户端的OP_READ事件，然后去调用java.nio.channels.SocketChannel#read(java.nio.ByteBuffer)导致抛出异常java.net.SocketException: Connection reset

问题分析：

出现该问题的原因是，客户端2的异常关闭后，服务器端第一次java.nio.channels.SocketChannel#read(java.nio.ByteBuffer)时收到java.net.SocketException: Connection reset，就应该识别出这是客户但异常关闭，需要调用对应的SocketChannel.close()方法关闭客户端；此方法会在对应的Selector上取消之前注册的事件；

修复后的代码如下：

// try {

// client.close();

// } catch (IOException ex) {

// System.out.println("close ex");

// }

此时客户端异常关闭后，不会再持续收到该客户端的OP_READ事件，而且新的客户端可以正常连接发送数据

服务端日志：

bash 复制代码

read handler
java.nio.channels.SocketChannel[connected local=/127.0.0.1:9090 remote=/127.0.0.1:26783] disconnect
java.net.SocketException: Connection reset
	at java.base/sun.nio.ch.SocketChannelImpl.throwConnectionReset(SocketChannelImpl.java:394)
	at java.base/sun.nio.ch.SocketChannelImpl.read(SocketChannelImpl.java:426)
	at com.huawei.io.chatroom.nio.sel.NIOSelectorServer.readHandler(NIOSelectorServer.java:105)
	at com.huawei.io.chatroom.nio.sel.NIOSelectorServer.doService(NIOSelectorServer.java:54)
	at com.huawei.io.chatroom.nio.sel.NIOSelectorServer.start(NIOSelectorServer.java:32)
	at com.huawei.io.chatroom.nio.sel.NIOSelectorServer.main(NIOSelectorServer.java:26)
server receive a client :java.nio.channels.SocketChannel[connected local=/127.0.0.1:9090 remote=/127.0.0.1:26892]
read handler
server read data from java.nio.channels.SocketChannel[connected local=/127.0.0.1:9090 remote=/127.0.0.1:26892], data is :client3

java.nio.channels.SocketChannel[connected local=/127.0.0.1:9090 remote=/127.0.0.1:26892] : no data

待优化：

可以看到上面，在上面客户端异常端口连接的异常捕获中，再次捕获了client.close();的异常，这很不优雅呀。。。

需要看下别人的优秀代码怎么搞的

todo