Java Socket几个简单的入门示例-Toy模板网

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概述

Socket难的地方是服务端的编写，首先要合理地管理客户端连接，能让客户端持续不断地连接进来。其次每个连接的读写不能互相干扰，不能因为一个连接在传输数据，别的连接就得挂着。搞定了这两点，基本上就解决了Socket编程80%的问题。

以下是根据个人经验，写了几个示例，希望对看官有所帮助。

开发环境搭建，请点这里！！

纯Socket的实现

java的Socket有两个版本，一个服务端的（ServerSocket），一个客户端的（Socket ）。服务端版本可以启用监听，接受客户端连接；客户端版本只能发起连接，概念上应该不难理解。

下边是官方服务端及客户端，分别启动后，客户端里打上几个字，两边在控制台里都会产生相应的输出。为了让程序代码能看得明白一点，我加了一些输出代码，有兴趣的可以运行起来看一下。

注：为节省篇幅，本文从头至尾只写了一个客户端代码。所以后边示例代码只例出服务端的。

初始代码

以下代码可能是大多数学习Socket编程的入门代码，也就是创建一个服务端和客户端，然后启动服务端，启动客户端，如果两端不报错，就意味着这个代码运行成功了。由于入门代码只演示了如何监听，建立连接以及简单的读写，所以这种代码在实现生产中没有一点用处。

有点编程基础的人，我想下边的代码还是比较容易看懂的，如果看不懂，就调试模式运行起，单步走一走，就知道每行代码是怎么触发的了。

服务端

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.net.ServerSocket;
import java.net.Socket;

public class AppServer {
	public static void main(String[] args) throws IOException {
		System.out.println("服务启动");
		try (ServerSocket serverSocket = new ServerSocket(8888);

				Socket clientSocket = serverSocket.accept();
				PrintWriter out = new PrintWriter(clientSocket.getOutputStream(), true);
				BufferedReader in = new BufferedReader(new InputStreamReader(clientSocket.getInputStream()));) {

			System.out.println("客户端连接");
			
			String inputLine;
			while ((inputLine = in.readLine()) != null) {
				System.out.println("消息:" + inputLine);
				out.println(inputLine);
			}
			
		} catch (IOException e) {
			System.out.println(
					"Exception caught when trying to listen on port " + 8888 + " or listening for a connection");
			System.out.println(e.getMessage());
		}
		System.out.println("服务退出");

	}
}

客户端

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.net.Socket;
import java.net.UnknownHostException;

public class AppClient {
	public static void main(String[] args) throws UnknownHostException, IOException {
		String hostName = "localhost";
		int portNumber = 8888;

		try (Socket echoSocket = new Socket(hostName, portNumber);
				PrintWriter out = new PrintWriter(echoSocket.getOutputStream(), true);
				BufferedReader in = new BufferedReader(new InputStreamReader(echoSocket.getInputStream()));
				BufferedReader stdIn = new BufferedReader(new InputStreamReader(System.in))) {
			
			System.out.println("已连接，请输入内容");
			String userInput;
			while (!(userInput = stdIn.readLine()).equals("exit")) {
				out.println(userInput);
				System.out.println("回显: " + in.readLine());
			}
		} catch (UnknownHostException e) {
			System.err.println("Don't know about host " + hostName);
			System.exit(1);
		} catch (IOException e) {
			System.err.println("Couldn't get I/O for the connection to " + hostName);
			System.exit(1);
		}

		System.out.println("客户端退出");
	}
}

遗憾的是，这个服务端是一次性的，随着客户端退出，服务器端就挂了。其原因是只调用accept()了一次，为了解决这个问题，我们要让服务端不停地accept()。可以在accept()之上，套一层无限循环，这样就可以不停地接受客户端了。

第一次改进

这里改进其实是为了更深入理解服务端是如何接受客户端连接的。服务端起动监听后，还要显式地调用accept()方法，表明这个服务开始接受客户端的连接。不过，每次accept()只能接收一个客户端socket，而且在没有新的客户端连接之前，这个方法就会一直挂起在那里。为了能够不停地接受客户端，那么就要尽快地将连接过来的socket给接收下来，然后进入下一个等待。

因为调用会挂起（专业术语叫阻塞），所以不用考虑递归调用，使用一个无限循环就可以了。

注：
一个完整的应用程序，应该考虑程序的初始化，关闭，异常等逻辑。这里仅演示方便，并未考虑这些细节。所以，尽量避免将这样的代码直接应用到你的生产项目中去。

服务端改进1

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.net.ServerSocket;
import java.net.Socket;

public class AppServer {
	public static void main(String[] args) throws IOException {
		System.out.println("服务启动");
		try (ServerSocket serverSocket = new ServerSocket(8888);
		) {
			//增加一个无限循环
			while (true) {
				Socket clientSocket = serverSocket.accept();
				PrintWriter out = new PrintWriter(clientSocket.getOutputStream(), true);
				BufferedReader in = new BufferedReader(new InputStreamReader(clientSocket.getInputStream()));
				System.out.println("客户端连接");

				String inputLine;
				while ((inputLine = in.readLine()) != null) {
					System.out.println("消息:" + inputLine);
					out.println(inputLine);
				}
				in.close();
				out.close();
				clientSocket.close();
			}
		} catch (IOException e) {
			System.out.println(
					"Exception caught when trying to listen on port " + 8888 + " or listening for a connection");
			System.out.println(e.getMessage());
		}
		System.out.println("服务退出");

	}
}

但这个服务端还是有点问题，每次只能连接一个客户端，只有等到前一个客户端退出了，后边的才能正常连接。为了能够迸发连接，就会用到另一个技术——线程。我们可以将读写部分放到一个单独线程中，这样主线程就可以立即接受下一个连接了。

二次改进

在读取客户端连接的时候也是阻塞的，因而前边的代码就会出现一个问题。在你没放弃读取当前连接的数据时，其它客户端依然是连不进来的。

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.net.ServerSocket;
import java.net.Socket;

public class AppServer {
	public static void main(String[] args) throws IOException {
		System.out.println("服务启动");
		try (ServerSocket serverSocket = new ServerSocket(8888);) {
			// 增加一个无限循环
			while (true) {
				Socket clientSocket = serverSocket.accept();
				Thread clientThread = new Thread(new Runnable() {

					@Override
					public void run() {
						PrintWriter out;
						try {
							out = new PrintWriter(clientSocket.getOutputStream(), true);

							BufferedReader in = new BufferedReader(
									new InputStreamReader(clientSocket.getInputStream()));
							System.out.println("客户端连接");

							String inputLine;
							while ((inputLine = in.readLine()) != null) {
								System.out.println("消息:" + inputLine);
								out.println(inputLine);
							}
							in.close();
							out.close();
							clientSocket.close();
						} catch (IOException e) {
							// TODO Auto-generated catch block
							e.printStackTrace();
						}

					}
				});
				clientThread.start();
			}
		} catch (IOException e) {
			System.out.println(
					"Exception caught when trying to listen on port " + 8888 + " or listening for a connection");
			System.out.println(e.getMessage());
		}
		System.out.println("服务退出");

	}
}

不过，据说不停地new Thread()比较奢侈，具体原因跟操作系统线程原理有关，具体这里不展开讲，总之线程多了即降低性能，又浪费内存。这时考虑使用线程池的方式来实现，线程池可以用JDK的现成类库。

三次改进

这次改进的重点是性能上的优化。因为接受客户端，以及读取客户端数据都是阻塞的，为了使每个客户端能并行执行，不可避免地要用到线程。但线程的使用要有个度，适当地使用线程能提升性能，将单线程的阻塞，变成并行的阻塞，确实是提升了CPU的复用率。但这里有个风险，就是线程数量超过某个阀值的时候，反而会降低性能。具体原理，网上有很多大神已经讲解得非常细致了，这里不赘述。有个原则就是在不阻塞的情况下，尽量少地去使用线程。

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.net.ServerSocket;
import java.net.Socket;
import java.util.concurrent.Executors;

public class AppServer {
	public static void main(String[] args) throws IOException {
		System.out.println("Service starting...");
		try (ServerSocket serverSocket = new ServerSocket(8888);) {
			// 创建线程池，数量跟CPU的线程数相当
			var pool = Executors.newFixedThreadPool(8);
			// 增加一个无限循环
			while (true) {
				Socket clientSocket = serverSocket.accept();
				pool.submit(new Runnable() {

					@Override
					public void run() {
						PrintWriter out;
						try {
							out = new PrintWriter(clientSocket.getOutputStream(), true);

							BufferedReader in = new BufferedReader(
									new InputStreamReader(clientSocket.getInputStream()));
							System.out.println("Client connected...");

							String inputLine;
							while ((inputLine = in.readLine()) != null) {
								System.out.println("Message:" + inputLine);
								out.println(inputLine);
							}
							in.close();
							out.close();
							clientSocket.close();
							System.out.println("Client disconnected...");
						} catch (IOException e) {
							// TODO Auto-generated catch block
							e.printStackTrace();
						}
					}
				});
			}
		} catch (IOException e) {
			System.out.println(
					"Exception caught when trying to listen on port " + 8888 + " or listening for a connection");
			System.out.println(e.getMessage());
		}
		System.out.println("服务退出");

	}
}

至此服务端已经改进不少，但有新的问题出现，如果长连接的客户端足够多，那么线程必然又要增加。我们的目标是提升服务器的性能，而不是无谓的增加线程数量。可以考虑使用非阻塞的方式来接收客户端的数据。

再次改进

代码改进如下：

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.net.ServerSocket;
import java.net.Socket;
import java.util.ArrayList;
import java.util.Collections;
import java.util.LinkedList;
import java.util.List;
import java.util.concurrent.Executors;

public class AppServer {
	public static void main(String[] args) throws IOException {
		System.out.println("Service starting...");
		try (ServerSocket serverSocket = new ServerSocket(8888);) {
			// 创建线程池
			var pool = Executors.newFixedThreadPool(8);

			List<Socket> clients = Collections.synchronizedList(new ArrayList<Socket>(0));
			pool.submit(new Runnable() {

				@Override
				public void run() {
					while (true) {
						var iterator = clients.iterator();

						while (iterator.hasNext()) {
							var socket = iterator.next();
							if (socket.isClosed()) {
								System.out.println("Client disconnected...");
								iterator.remove();
								continue;
							}
							try {
								//如果此处有更为复杂的逻辑要处理，则可以创建一个Runnalbe对象，塞进线程池中
								PrintWriter out = new PrintWriter(socket.getOutputStream(), true);
								if (socket.getInputStream().available() > 0) {
									System.out.println("Message received...");

									BufferedReader in = new BufferedReader(
											new InputStreamReader(socket.getInputStream()));

									String inputLine = in.readLine();
									System.out.println("Message:" + inputLine);
									out.println(inputLine);
									if (inputLine.equals("exit"))
										socket.close();
								} else {
									socket.getOutputStream().write(0);
								}

							} catch (IOException e) {
								e.printStackTrace();
								iterator.remove();
							}
						}
						try {
							Thread.sleep(50);
						} catch (InterruptedException e) {
							e.printStackTrace();
						}
					}
				}
			});
			// 增加一个无限循环
			while (true) {
				Socket clientSocket = serverSocket.accept();
				System.out.println("Client connected...");
				clients.add(clientSocket);
			}
		} catch (IOException e) {
			System.out.println(
					"Exception caught when trying to listen on port " + 8888 + " or listening for a connection");
			System.out.println(e.getMessage());
		}
		System.out.println("服务退出");

	}
}

至此，纯Socket的写法已经有相当强的处理能力了。当然，这还是示例，并不能解决实际的业务，不过，方向是对的，实际项目中往这个思路上靠就是了。

非阻塞NIO的实现

NIO的实现大同小异，只是Accecpt()换成了select()，至于读消息是不是也可以使用select()就不得而知了，笔者才疏学浅没找到相关的API。

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.net.InetAddress;
import java.net.InetSocketAddress;
import java.net.ServerSocket;
import java.net.Socket;
import java.nio.channels.SelectionKey;
import java.nio.channels.Selector;
import java.nio.channels.ServerSocketChannel;
import java.nio.channels.spi.SelectorProvider;
import java.util.ArrayList;
import java.util.Collections;
import java.util.Date;
import java.util.Iterator;
import java.util.List;
import java.util.Set;
import java.util.concurrent.Executors;

public class AppServer {
	public static void main(String[] args) throws IOException {
		System.out.println("Service starting...");
		try {
			// 创建线程池
			var pool = Executors.newFixedThreadPool(8);

			List<Socket> clients = Collections.synchronizedList(new ArrayList<Socket>(0));
			pool.submit(new Runnable() {

				@Override
				public void run() {
					while (true) {
						var iterator = clients.iterator();

						while (iterator.hasNext()) {
							var socket = iterator.next();
							if (socket.isClosed()) {
								System.out.println("Client disconnected...");
								iterator.remove();
								continue;
							}
							try {
								// 如果此处有更为复杂的逻辑要处理，则可以创建一个Runnalbe对象，塞进线程池中
								PrintWriter out = new PrintWriter(socket.getOutputStream(), true);
								if (socket.getInputStream().available() > 0) {
									System.out.println("Message received...");

									BufferedReader in = new BufferedReader(
											new InputStreamReader(socket.getInputStream()));

									String inputLine = in.readLine();
									System.out.println("Message:" + inputLine);
									out.println(inputLine);
									if (inputLine.equals("exit"))
										socket.close();
								} else {
									socket.getOutputStream().write(0);
								}

							} catch (IOException e) {
								e.printStackTrace();
								iterator.remove();
							}
						}
						try {
							Thread.sleep(50);
						} catch (InterruptedException e) {
							e.printStackTrace();
						}
					}
				}
			});

			// Selector for incoming time requests
			Selector acceptSelector = SelectorProvider.provider().openSelector();

			// Create a new server socket and set to non blocking mode
			ServerSocketChannel ssc = ServerSocketChannel.open();
			ssc.configureBlocking(false);

			// Bind the server socket to the local host and port

			InetSocketAddress isa = new InetSocketAddress(8888);
			ssc.socket().bind(isa);

			// Register accepts on the server socket with the selector. This
			// step tells the selector that the socket wants to be put on the
			// ready list when accept operations occur, so allowing multiplexed
			// non-blocking I/O to take place.
			SelectionKey acceptKey = ssc.register(acceptSelector, SelectionKey.OP_ACCEPT);

			int keysAdded = 0;

			// Here's where everything happens. The select method will
			// return when any operations registered above have occurred, the
			// thread has been interrupted, etc.
			while ((keysAdded = acceptSelector.select()) > 0) {
				// Someone is ready for I/O, get the ready keys
				Set<SelectionKey> readyKeys = acceptSelector.selectedKeys();
				Iterator<SelectionKey> i = readyKeys.iterator();

				// Walk through the ready keys collection and process date requests.
				while (i.hasNext()) {
					SelectionKey sk = (SelectionKey) i.next();
					i.remove();
					// The key indexes into the selector so you
					// can retrieve the socket that's ready for I/O
					ServerSocketChannel nextReady = (ServerSocketChannel) sk.channel();
					// Accept the date request and send back the date string
					Socket s = nextReady.accept().socket();
					System.out.println("Client connected...");
					clients.add(s);
					// Write the current time to the socket

				}
			}

		} catch (IOException e) {
			System.out.println(
					"Exception caught when trying to listen on port " + 8888 + " or listening for a connection");
			System.out.println(e.getMessage());
		}
		System.out.println("Service exited");

	}
}

Netty实现

使用Netty后，代码结构清爽了不少。不用自己写线程，数据读写方面只要会用ByteBuf就可以了。

import java.io.IOException;

import io.netty.bootstrap.ServerBootstrap;
import io.netty.buffer.ByteBuf;
import io.netty.channel.ChannelFuture;
import io.netty.channel.ChannelHandlerContext;
import io.netty.channel.ChannelInboundHandlerAdapter;
import io.netty.channel.ChannelInitializer;
import io.netty.channel.ChannelOption;
import io.netty.channel.EventLoopGroup;
import io.netty.channel.nio.NioEventLoopGroup;
import io.netty.channel.socket.SocketChannel;
import io.netty.channel.socket.nio.NioServerSocketChannel;

public class AppServer extends ChannelInboundHandlerAdapter {

	@Override
	public void channelUnregistered(ChannelHandlerContext ctx) throws Exception {
		System.out.println("Client disconnected...");
	}

	@Override
	public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {
		ByteBuf in = ((ByteBuf) msg).copy();
		ctx.write(msg); // (1)
		ctx.flush(); // (2)

		String str = "";
		while (in.isReadable()) { // (1)
			str += (char) in.readByte();
		}
		System.out.print("message:"+str);

	}

	public static void main(String[] args) throws IOException, InterruptedException {
		System.out.println("Service starting...");
		EventLoopGroup bossGroup = new NioEventLoopGroup(); // (1)
		EventLoopGroup workerGroup = new NioEventLoopGroup();
		try {
			ServerBootstrap b = new ServerBootstrap(); // (2)
			b.group(bossGroup, workerGroup).channel(NioServerSocketChannel.class) // (3)
					.childHandler(new ChannelInitializer<SocketChannel>() { // (4)
						@Override
						public void initChannel(SocketChannel ch) throws Exception {
							System.out.println("Client connected...");
							ch.pipeline().addLast(new AppServer());
						}

					}).option(ChannelOption.SO_BACKLOG, 128) // (5)
					.childOption(ChannelOption.SO_KEEPALIVE, true); // (6)

			// Bind and start to accept incoming connections.
			ChannelFuture f = b.bind(8888).sync(); // (7)

			// Wait until the server socket is closed.
			// In this example, this does not happen, but you can do that to gracefully
			// shut down your server.
			f.channel().closeFuture().sync();
		} finally {
			workerGroup.shutdownGracefully();
			bossGroup.shutdownGracefully();
		}
		System.out.println("Service exited");

	}
}

Vert.x 实现

Vert.x是建立在Netty之上的，但对程序员更加友好。相同的功能，明显代码少了不少。文章来源地址https://www.toymoban.com/news/detail-464105.html

import java.io.IOException;

import io.vertx.core.Vertx;
import io.vertx.core.net.NetServer;

public class AppServer {

	public static void main(String[] args) throws IOException, InterruptedException {
		System.out.println("Service starting...");
		Vertx vertx = Vertx.vertx();
		NetServer server = vertx.createNetServer();
		server.connectHandler(socket -> {
			System.out.println("Client connected!");
			socket.handler(buffer -> {
				System.out.println("Message received!");
				var c = buffer.getString(0, buffer.length());
				socket.write(c);
				System.out.print("Message:"+c);
			});

		});

		server.listen(8888, res -> {
			if (res.succeeded()) {
				System.out.println("Server is now listening!");
			} else {
				System.out.println("Failed to bind!");
			}
		});
		System.out.println("Service exited");

	}
}