使用NIO编写服务端代码，客户端关闭后，服务端无限循环获取读事件

问题描述

服务端代码如下：

package com.ethan.nio;

import lombok.extern.slf4j.Slf4j;

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

/**
 * 关于Buffer的Scattering与Gathering
 */
@Slf4j
public class NioTest12 {

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

        int[] ports = new int[] {10000, 10001, 10002, 10003, 10004};

        Selector selector = Selector.open();

        for (int i = 0; i < ports.length; i++) {
            ServerSocketChannel serverSocketChannel = ServerSocketChannel.open();
            serverSocketChannel.configureBlocking(false);
            ServerSocket serverSocket = serverSocketChannel.socket();
            serverSocket.bind(new InetSocketAddress(ports[i]));

            serverSocketChannel.register(selector, SelectionKey.OP_ACCEPT);

            log.info("监听端口：{}", ports[i]);
        }

        while (true) {
            int numbers = selector.select();
            log.info("numbers:{}", numbers);

            Set selectionKeys = selector.selectedKeys();
            Iterator iterator = selectionKeys.iterator();
            while (iterator.hasNext()) {
                SelectionKey selectionKey = iterator.next();
                iterator.remove();
                log.info("selectionKey interestOps:{}", selectionKey.interestOps());
                if (selectionKey.isAcceptable()) {
                    ServerSocketChannel serverSocketChannel = (ServerSocketChannel) selectionKey.channel();
                    SocketChannel socketChannel = serverSocketChannel.accept();
                    socketChannel.configureBlocking(false);
                    socketChannel.register(selector, SelectionKey.OP_READ);
                    log.info("获得客户端连接:{}", socketChannel);
                } else if (selectionKey.isReadable()) {
                    SocketChannel socketChannel = (SocketChannel) selectionKey.channel();
                    int bytesRead = 0;
                    while (true) {
                        ByteBuffer byteBuffer = ByteBuffer.allocate(512);
                        byteBuffer.clear();
                        int read = socketChannel.read(byteBuffer);
                        if (read <= 0) {
                            break;
                        }
                        byteBuffer.flip();
                        socketChannel.write(byteBuffer);
                        bytesRead += read;
                    }
                    log.info("读取{}个字符串，来自于:{}", bytesRead, socketChannel);
                }
            }
            selectionKeys.clear();
        }
    }

}

通过nc或telnet命令，连接任意服务端口，如10000，当端口连接后，服务端不断接受客户端的读事件。

问题分析

通过调试，发现当客户端关闭后，从channel中读取的内容长度为-1，经过查阅资料，等值内容长度为-1时，表示客户端关闭，需要将服务端保留的channel关闭或取消关注SelectKey.OP_READ事件。

解决办法

判断读取长度是否为-1，若为-1，则手动关闭channel，完整代码如下：

package com.ethan.nio;

import lombok.extern.slf4j.Slf4j;

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

/**
 * 关于Buffer的Scattering与Gathering
 */
@Slf4j
public class NioTest12 {

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

        int[] ports = new int[] {10000, 10001, 10002, 10003, 10004};

        Selector selector = Selector.open();

        for (int i = 0; i < ports.length; i++) {
            ServerSocketChannel serverSocketChannel = ServerSocketChannel.open();
            serverSocketChannel.configureBlocking(false);
            ServerSocket serverSocket = serverSocketChannel.socket();
            serverSocket.bind(new InetSocketAddress(ports[i]));

            serverSocketChannel.register(selector, SelectionKey.OP_ACCEPT);

            log.info("监听端口：{}", ports[i]);
        }

        //TODO 如果解决selector.select 一直有返回值，并且是最后关闭的socket
        while (true) {
            int numbers = selector.select();
            log.info("numbers:{}", numbers);

            Set selectionKeys = selector.selectedKeys();
            Iterator iterator = selectionKeys.iterator();
            while (iterator.hasNext()) {
                SelectionKey selectionKey = iterator.next();
                iterator.remove();
                log.info("selectionKey interestOps:{}", selectionKey.interestOps());
                if (selectionKey.isAcceptable()) {
                    ServerSocketChannel serverSocketChannel = (ServerSocketChannel) selectionKey.channel();
                    SocketChannel socketChannel = serverSocketChannel.accept();
                    socketChannel.configureBlocking(false);
                    socketChannel.register(selector, SelectionKey.OP_READ);
                    log.info("获得客户端连接:{}", socketChannel);
                } else if (selectionKey.isReadable()) {
                    SocketChannel socketChannel = (SocketChannel) selectionKey.channel();
                    int bytesRead = 0;
                    while (true) {
                        ByteBuffer byteBuffer = ByteBuffer.allocate(512);
                        byteBuffer.clear();
                        int read = socketChannel.read(byteBuffer);
                        if (read == -1) {
                            // 当读取长度为-1时，表示客户端断开连接，服务端应当关闭channel，或解除对 {@link SelectionKey.OP_READ} 的监听
                            socketChannel.close();
                            break;
                        }
                        if (read == 0) {
                            break;
                        }
                        byteBuffer.flip();
                        socketChannel.write(byteBuffer);
                        bytesRead += read;
                    }
                    log.info("读取{}个字符串，来自于:{}", bytesRead, socketChannel);
                }
            }
            selectionKeys.clear();
        }
    }

}