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

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：

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channeltry {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channelByteBuffer buffer = (ByteBuffer) key.attachment(); // 获取Client channel关联的bufferbuffer.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：

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serverSocket 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;}@Overridepublic 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outputStreambreak;}// 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

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 :client1java.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 :client2java.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：

exit
client receive data from consolejava.io.BufferedInputStream@72cdfe9a : exitclient close :Socket[addr=localhost/127.0.0.1,port=9090,localport=13982]
Exception in thread "main" java.net.SocketException: Socket closedat 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：

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：

java.net.SocketException: Connection resetat 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 resetat 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：

read handler
java.nio.channels.SocketChannel[connected local=/127.0.0.1:9090 remote=/127.0.0.1:26783] disconnect
java.net.SocketException: Connection resetat 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 :client3java.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