网络编程的基本模型是Client/Server模型，也就是两个进程之间进行相互通信，其中服务端提供位置信息（绑定的IP地址和监听端口），客户端通过连接操作向服务端监听的地址发起连接请求，通过三次握手建立连接，如果连接建立成功，双方就可以通过网络套接字（Socket）进行通信。
在基于传统同步阻塞模型开发中，ServerSocket负责绑定IP地址，启动监听端口；Socket负责发起连接操作。连接成功之后，双方通过输入和输出流进行同步阻塞式通信。
传统的BIO通信
采用BIO通信模型的服务端，通常由一个独立的Acceptor线程负责监听客户端的连接，它接收到客户端连接请求之后为每个客户端创建一个新的线程进行链路处理，处理完成之后，通过输出流返回应答给客户端，线程销毁。这就是典型的一请求一应答通信模型。
服务端代码示例：
import java.io.IOException; import java.net.ServerSocket; import java.net.Socket; public class TimeServer { public static void main(String[] args) throws IOException { int port = 8080; if (args != null && args.length > 0) { try { port = Integer.valueOf(args[0]); } catch (NumberFormatException e) { // 采用默认值 } } ServerSocket server = null; try { server = new ServerSocket(port); System.out.println("The time server is start in port : " + port); Socket socket = null; while (true) { socket = server.accept(); new Thread(new TimeServerHandler(socket)).start(); } } finally { if (server != null) { System.out.println("The time server close"); server.close(); server = null; } } } } import java.io.BufferedReader; import java.io.IOException; import java.io.InputStreamReader; import java.io.PrintWriter; import java.net.Socket; public class TimeServerHandler implements Runnable { private Socket socket; public TimeServerHandler(Socket socket) { this.socket = socket; } @Override public void run() { BufferedReader in = null; PrintWriter out = null; try { in = new BufferedReader(new InputStreamReader( this.socket.getInputStream())); out = new PrintWriter(this.socket.getOutputStream(), true); String currentTime = null; String body = null; while (true) { body = in.readLine(); if (body == null) break; System.out.println("The time server receive order : " + body); currentTime = "QUERY TIME ORDER".equalsIgnoreCase(body) ? new java.util.Date( System.currentTimeMillis()).toString() : "BAD ORDER"; out.println(currentTime); } } catch (Exception e) { if (in != null) { try { in.close(); } catch (IOException e1) { e1.printStackTrace(); } } if (out != null) { out.close(); out = null; } if (this.socket != null) { try { this.socket.close(); } catch (IOException e1) { e1.printStackTrace(); } this.socket = null; } } } }
客户端示例：
import java.io.BufferedReader; import java.io.IOException; import java.io.InputStreamReader; import java.io.PrintWriter; import java.net.Socket; public class TimeClient { public static void main(String[] args) { int port = 8080; if (args != null && args.length > 0) { try { port = Integer.valueOf(args[0]); } catch (NumberFormatException e) { // 采用默认值 } } Socket socket = null; BufferedReader in = null; PrintWriter out = null; try { socket = new Socket("127.0.0.1", port); in = new BufferedReader(new InputStreamReader( socket.getInputStream())); out = new PrintWriter(socket.getOutputStream(), true); out.println("QUERY TIME ORDER"); System.out.println("Send order 2 server succeed."); String resp = in.readLine(); System.out.println("Now is : " + resp); } catch (Exception e) { //不需要处理 } finally { if (out != null) { out.close(); out = null; } if (in != null) { try { in.close(); } catch (IOException e) { e.printStackTrace(); } in = null; } if (socket != null) { try { socket.close(); } catch (IOException e) { e.printStackTrace(); } socket = null; } } } }
运行结果：
服务端
The time server is start in port : 8080 //阻塞等待客户的连接
The time server receive order : QUERY TIME ORDER //runable线程处理请求
客户端
Send order 2 server succeed. //连接后给服务端发数据
Now is : Mon Dec 12 13:17:12 CST 2016 //接受服务端响应的数据
