Socket 编程 - 单个客户端

Socket编程就是根据服务器/客户端模型，在Socket API的基础上开发的网络计算机和进程间相互通信的应用。

1. 服务端绑定一个端口号，并在此端口监听客户端的连接，线程会block直到有客户连接请求，读取请求信息，然后处理并返回。
2. 客户端和指定的服务器的指定端口建立连接，发送请求信息，并等待返回信息。

服务端：

public class MyServer {
	ServerSocket server;
	Socket client;
	BufferedReader in;
	PrintWriter out;
	
	public MyServer() {
		try {
			// Instantiate a ServerSocket with specified port
			server = new ServerSocket(4451);
			
			// Listen on the port to receive the connection request
			client = server.accept();
			
			// Construct a reader to get the information from the client
			in = new BufferedReader(new InputStreamReader(client.getInputStream()));
			
			// Construct a writer to respond to the client
			out = new PrintWriter(client.getOutputStream());
			
			// Continually to retrieve the message from the client
			while (true) {
				String input = in.readLine();
	
				System.out.println(input);
				out.println("has received: " + input);
				out.flush();
				
				if (input.equals("end"))
					break;
			}
		} catch (IOException ex) {
			ex.printStackTrace();
		} finally {
			// Release the resources - omitted
		}
	}

	public static void main(String[] args) {
		new MyServer();
	}
}

客户端：

public class MyClient {
	Socket connection;
	BufferedReader in;
	PrintWriter out;
	BufferedReader reader;
	
	public MyClient() {
		try {
			// Establish a connection to the Socket server
			connection = new Socket(InetAddress.getLocalHost(),4451);
			
			// Construct a reader to get the response from the server
			in = new BufferedReader(new InputStreamReader(connection.getInputStream()));
			
			// Construct a writer to send messages to the server
			out = new PrintWriter(connection.getOutputStream());
			
			// Construct a reader to accept the user input
			reader = new BufferedReader(new InputStreamReader(System.in));
			
			// Continuously send out the messages to the server and receive the response 
			while (true) {
				String str = reader.readLine();
				out.println(str);
				out.flush();
				System.out.println("sent out: " + str);
				
				if (str.equals("end"))
					break;
				
				System.out.println(in.readLine());
			}
		} catch (UnknownHostException e) {
			e.printStackTrace();
		} catch (IOException e) {
			e.printStackTrace();
		} finally {
			// Release the resources - omitted
		}
	}

	public static void main(String[] args) {
		new MyClient();
	}

}

其中：
1. out = new PrintWriter(connection.getOutputStream()); 使用系统默认的编码格式。
如果默认编码不符合要求，使用OutputStreamWriter指定：
out = new PrintWriter(new OutputStreamWriter(connection.getOutputStream(), "utf-8"));

2. 可以把手动flush改成自动的：
out = new PrintWriter(connection.getOutputStream(), true);
这样，当println, printf, format方法调用时，将缓存输出到底层流并清空缓存。

3. 必须首先开启服务端，否则客户端建立连接的时候会报java.net.ConnectException: Connection refused: connect