NIO复习(3)：selector

今天继续来复习NIO三剑客的最后1个：selector

selector的工作原理，简单来看，就是上面这张图，Channel必须先向Selector注册(注：register的时候，可以选择关注哪些事件，比如：有新连接 或 有数据可读 等)，注册成功后，Selector通过select方法来检查这些Channel上是否有事件发生，比如：有数据发过来，channel就可以把数据读到Buffer中。

这三者在类设计的层面是怎么串起来的？下面是Selector类的示意图：

当Channel注册上来时，会被包装成一个SelectionKey放到Set中，通过keys()方法可以得到所有注册的SelectionKey。当Channel上有事件发生时，通过selectedKeys()方法，可以得到所有当前有事件发生的Set集合。很显然，selectedKeys是keys的子集。

SelectionKey类中，又通过channel()方法，持有Channel的引用，这样就能通过该引用来向Buffer读/写数据（注：记住NIO中，向Buffer中写入数据，在网络编程中其实就是向对方发数据）。

看起来并不复杂，但真正用NIO写一个基本的Server端Demo，还是要很多步骤的，正所谓知易行难，梳理了一张图，大家可以感受一下：

值得一提的是，这里用到二类Channel，它们都继承自SelectableChannel

下面来热热身，写一个最基本的ServerDemo:

package test.nio.study;

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.Arrays;
import java.util.Iterator;

/**
 * @author 菩提树下的杨过(http://yjmyzz.cnblogs.com)
 */
public class ServerDemo {

    public static void main(String[] args) throws IOException {

        InetSocketAddress addr = new InetSocketAddress(8086);

        ServerSocketChannel serverSocketChannel = ServerSocketChannel.open();
        //绑定ip:port
        serverSocketChannel.bind(addr);
        //配置为非阻塞
        serverSocketChannel.configureBlocking(false);

        //获取Selector
        Selector selector = Selector.open();

        //将channel注册到Selector(仅关注:新连接事件)
        serverSocketChannel.register(selector, SelectionKey.OP_ACCEPT);

        //轮询事件
        while (true) {
            //每100ms轮一次
            if (selector.select(100) == 0) {
                continue;
            }

            //如果有事件发生，则拿到一个SelectionKey集合的迭代器
            Iterator<SelectionKey> iterator = selector.selectedKeys().iterator();

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

                try {
                    //判断事件类型(当有新连接请求时)
                    if (key.isAcceptable()) {
                        //(接受)创建新连接，同时返回新连接的Channel（注：accept方法是阻塞的）
                        SocketChannel socketChannel = serverSocketChannel.accept();
                        socketChannel.configureBlocking(false);
                        System.out.println(socketChannel.getRemoteAddress() + " is connected");
                        //向client回显一句话
                        socketChannel.write(ByteBuffer.wrap(("hello:" + socketChannel.getRemoteAddress() + "\n").getBytes()));
                        //新连接的Channel，也要注册到Selector，并关注读取事件(以响应客户端发过来的消息)
                        socketChannel.register(selector, SelectionKey.OP_READ, ByteBuffer.allocate(1024));
                    }

                    //有数据可读取时
                    if (key.isReadable()) {
                        SocketChannel channel = (SocketChannel) key.channel();
                        ByteBuffer buffer = (ByteBuffer) key.attachment();
                        try {
                            int count = channel.read(buffer);
                            if (count != -1) {
                                System.out.println(channel.getRemoteAddress() + " say:" + new String(Arrays.copyOf(buffer.array(), count)));
                            }
                        } catch (IOException e) {
                            System.err.println(channel.getRemoteAddress() + " disconnected");
                            continue;
                        }
                        buffer.clear();
                    }
                } finally {
                    //处理完后，一定要将自身移除，否则下一次select有事件触发时，无法正常处理
                    iterator.remove();
                }
            }
        }
    }
}

关键地方都加了详细注释，应该不难理解，把这个程序跑起来。

然后开2个终端(我是mac环境)，都输入 telnet localhost 8160，相当于2个client端连接上来，然后每个终端里打几个字，向Server端发点数据。

接下来改造一下，写一个多人聊天室的原型，要实现的基本功能如下：
server端：

1、client连接成功时，server自动发问候语

2、新client上线时，通知其它client

3、有client说话时，转发给其它client(即：所有人，都能看到其它人发的最新消息)

4、有client下线(或断网时)，通知其它人

client端：

1、连接到server端

2、能正常收发消息

先写client端代码，这个相对容易点：

package test.nio.study;

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.SocketChannel;
import java.util.Arrays;
import java.util.Iterator;
import java.util.Set;

/**
 * 菩提树下的杨过
 */
public class ChatRoomClient {

    public static void main(String[] args) {
        new ChatRoomClient("localhost", 8086).start();
    }

    SocketChannel socketChannel;
    Selector selector;
    InetSocketAddress serverAddress;
    boolean isConnected = false;

    public ChatRoomClient(String host, int port) {
        serverAddress = new InetSocketAddress(host, port);
    }

    /**
     * 连接到server
     *
     * @param address
     */
    private void connect(InetSocketAddress address) {
        try {
            socketChannel = SocketChannel.open();
            socketChannel.configureBlocking(false);
            boolean connect = socketChannel.connect(address);
            if (!connect) {
                //注：建立连接是需要时间的，调用完connect方法后，这里返回的connect大概率是false
                int timeout = 5000;
                long start = System.currentTimeMillis();
                //所以要通过finishConnect来轮询，才能知道最终是否连接成功，
                while (!socketChannel.finishConnect()) {
                    Thread.sleep(50);
                    if (System.currentTimeMillis() - start > timeout) {
                        System.err.println("connect to " + address + " timeout");
                        return;
                    }
                }
            }
            selector = Selector.open();
            //注册，只关注读事件，同时关联一个buffer
            socketChannel.register(selector, SelectionKey.OP_READ, ByteBuffer.allocate(1024));
        } catch (Exception e) {
            System.err.println(e);
        }
        isConnected = true;
        System.err.println(address + " connect successfully");

    }

    /**
     * 接收消息
     *
     * @param selector
     */
    private void readMessage(Selector selector) {
        if (!isConnected) {
            return;
        }
        while (true) {
            try {
                //注：这里使用了阻塞版本的select方法（不想阻塞的话，可以用selectNow）
                if (selector.select() > 0) {
                    Set<SelectionKey> selectionKeys = selector.selectedKeys();
                    Iterator<SelectionKey> iterator = selectionKeys.iterator();
                    while (iterator.hasNext()) {
                        SelectionKey key = iterator.next();
                        SocketChannel channel = (SocketChannel) key.channel();
                        ByteBuffer buffer = (ByteBuffer) key.attachment();
                        int count = channel.read(buffer);
                        if (count != -1) {
                            System.out.println(new String(Arrays.copyOf(buffer.array(), count)));
                        }
                        buffer.clear();
                        //千万要记得这个，不然下次事件触发，无法正常处理
                        iterator.remove();
                    }
                }
            } catch (IOException e) {
                System.err.println("read message error:" + e);
            }
        }
    }

    /**
     * 发送消息
     *
     * @param channel
     */
    private void sendMessage(SocketChannel channel) {
        if (!isConnected) {
            return;
        }
        byte[] buffer = new byte[1024];
        while (true) {
            try {
                /**
                 * 等待键盘输入内容
                 */
                int count = System.in.read(buffer);
                if (count > 0) {
                    channel.write(ByteBuffer.wrap(Arrays.copyOf(buffer, count - 1)));
                }
            } catch (IOException e) {
                System.err.println("send message error:" + e);
            }
        }
    }

    /**
     * 启动
     */
    private void start() {
        connect(serverAddress);
        new Thread(() -> {
            sendMessage(socketChannel);
        }, "send-thread").start();

        new Thread(() -> {
            readMessage(selector);
        }, "read-thread").start();
    }
}

　　

服务端：

package test.nio.study;

import java.io.IOException;
import java.net.InetSocketAddress;
import java.nio.ByteBuffer;
import java.nio.channels.*;
import java.util.Arrays;
import java.util.Iterator;
import java.util.Set;

/**
 * @author 菩提树下的杨过
 */
public class ChatRoomServer {

    ServerSocketChannel serverSocketChannel;
    Selector selector;

    public ChatRoomServer(int port) {
        InetSocketAddress addr = new InetSocketAddress(port);
        try {
            serverSocketChannel = ServerSocketChannel.open();
            //绑定ip:port
            serverSocketChannel.bind(addr);
            //配置为非阻塞
            serverSocketChannel.configureBlocking(false);
            //获取Selector
            selector = Selector.open();
            //将channel注册到Selector(仅关注:新连接事件)
            serverSocketChannel.register(selector, SelectionKey.OP_ACCEPT);
        } catch (IOException e) {
            System.err.println("server init fail:" + e);
        }
        System.out.println("server started");
    }

    void connHandle(SelectionKey key) throws IOException {
        //(接受)创建新连接，同时返回新连接的Channel（注：accept方法是阻塞的）
        SocketChannel socketChannel = serverSocketChannel.accept();
        socketChannel.configureBlocking(false);
        System.err.println(socketChannel.getRemoteAddress() + " connected");
        //向client回显一句话
        String message = "hello," + socketChannel.getRemoteAddress() + "\n";
        socketChannel.write(ByteBuffer.wrap(message.getBytes()));
        sendMessageToOther(key, socketChannel, "hi, all");
        //新连接的Channel，也要注册到Selector，并关注读取事件(以响应客户端发过来的消息)
        socketChannel.register(selector, SelectionKey.OP_READ, ByteBuffer.allocate(1024));

    }

    void sendMessageToOther(SelectionKey key, SocketChannel channel, String message) throws IOException {
        Set<SelectionKey> keys = selector.keys();
        //转发到其它client
        for (SelectionKey otherKey : keys) {
            if (!otherKey.equals(key)) {
                SelectableChannel otherChannel = otherKey.channel();
                if (otherChannel instanceof SocketChannel) {
                    ((SocketChannel) otherChannel).write(ByteBuffer.wrap((channel.getRemoteAddress() + " say:" + message).getBytes()));
                }
            }
        }
    }

    boolean readHandle(SelectionKey key) throws IOException {
        SocketChannel channel = (SocketChannel) key.channel();
        ByteBuffer buffer = (ByteBuffer) key.attachment();
        try {
            int count = channel.read(buffer);
            if (count != -1) {
                String message = new String(Arrays.copyOf(buffer.array(), count));
                System.out.println(channel.getRemoteAddress() + " say:" + message);
                //转发到其它人
                sendMessageToOther(key, channel, message);
            }
        } catch (IOException e) {
            String message = channel.getRemoteAddress() + " disconnected";
            System.err.println(message);
            sendMessageToOther(key, channel, "disconnected");
            return false;
        }
        buffer.clear();
        return true;
    }

    void start() {
        //轮询事件
        while (true) {
            try {
                //每100ms轮一次
                if (selector.select(100) == 0) {
                    continue;
                }

                //如果有事件发生，则拿到一个SelectionKey集合的迭代器
                Iterator<SelectionKey> iterator = selector.selectedKeys().iterator();

                while (iterator.hasNext()) {
                    SelectionKey key = iterator.next();
                    try {
                        //判断事件类型(当有新连接请求时)
                        if (key.isAcceptable()) {
                            connHandle(key);
                        }

                        //有数据可读取时
                        if (key.isReadable()) {
                            if (!readHandle(key)) {
                                continue;
                            }
                        }

                    } finally {
                        //处理完后，一定要将自身移除，否则下一次select有事件触发时，无法正常处理
                        iterator.remove();
                    }
                }
            } catch (IOException e) {
                System.err.println(e);
            }
        }
    }

    public static void main(String[] args) {
        new ChatRoomServer(8086).start();
    }
}

跑起来看看，不过这里有1个小技巧，使用idea的话，默认情况下，每个程序都是单实例运行，如果要同时启用多个client是不行，可参考下图设置：

server端运行截图：

client-1运行截图：

client-2截图：

client-3运行截图：

参考文章：

https://docs.oracle.com/en/java/javase/13/docs/api/java.base/java/nio/channels/Selector.html