一.传输层:

1.UDP:无连接,不可靠,面向数据报,全双工.

2.TCP:有连接,可靠,面向字节流,全双工.

注意:这里的可不可靠是相对的,并且和安不安全无关.

二.UDP数据报套接字编程:

1.socket文件:表示网卡的这类文件.

 2.DatagramPacket:表示一个UDP数据报.

 

三.代码实现:

1.回显服务器代码:

import java.io.IOException;
import java.net.DatagramPacket;
import java.net.DatagramSocket;
import java.net.SocketException;
import java.nio.charset.StandardCharsets;

public class EchoUDPServer {
    // 服务器
    private DatagramSocket socket = null;

    public EchoUDPServer(int port) throws SocketException {
        // 服务器端指定端口号
        socket = new DatagramSocket(port);
    }

    public void start() throws IOException {
        System.out.println("启动服务器");
        // 由于服务器不会停止工作,所以使用死循环
        while (true) {
            // 1.读取请求,并解析
            // 类似于数组的输出型参数,不过要手动输入数组类型和大小
            DatagramPacket requestPacket = new DatagramPacket(new byte[4096], 4096);
            socket.receive(requestPacket);
            // 客户端的数据是一个文本字符串
            String request = new String(requestPacket.getData(), 0, requestPacket.getLength());
            // 2.根据请求,计算机相应
            String response = this.process(request);
            // 3.把相应写会给客户端
            // 注意第二个参数的长度
            DatagramPacket responsePacket = new DatagramPacket(response.getBytes(), response.getBytes().length,
                    requestPacket.getSocketAddress());
            socket.send(responsePacket);
            System.out.println(responsePacket.getSocketAddress() + "  接受内容:" + request + "  回复内容:" + response);
        }
    }

    public String process(String request) {
        return request;
    }

    public static void main(String[] args) throws IOException {
        // 服务器端口号由自己指定(1025~65535),0~1024最好不要使用
        EchoUDPServer server = new EchoUDPServer(1999);
        //EchoUDPServer server = new DictionaryUDPServer(1999);
        server.start();
    }
}

2.回显客户端代码:

import java.io.IOException;
import java.net.*;
import java.nio.charset.StandardCharsets;
import java.util.Scanner;

public class EchoUDPClient {
    // 客户端
    private DatagramSocket socket = null;
    private String serverIp = null;
    private int serverPort;

    public EchoUDPClient(String serverIp, int serverPort) throws SocketException {
        socket = new DatagramSocket();
        this.serverIp = serverIp;
        this.serverPort = serverPort;
    }

    public void start() throws IOException {
        Scanner scanner = new Scanner(System.in);
        System.out.println("客户端启动");
        while (true) {
            System.out.println("->");
            // 1.从控制台获取用户输入
            String request = scanner.nextLine();
            // 2.构造请求对象,发送给服务器
            DatagramPacket requestPacket = new DatagramPacket(request.getBytes(), 0, request.getBytes().length,
                    InetAddress.getByName(serverIp), serverPort);
            socket.send(requestPacket);
            // 3.读取服务器响应,解析响应内容
            DatagramPacket responsePacket = new DatagramPacket(new byte[4096], 4096);
            socket.receive(responsePacket);
            String response = new String(responsePacket.getData(), 0, responsePacket.getLength());
            // 4.显示在屏幕上
            System.out.println(response);
        }
    }

    public static void main(String[] args) throws IOException {
        EchoUDPClient client = new EchoUDPClient("127.0.0.1", 1999);
        client.start();
    }
}

3.字典服务器代码: 

import java.io.IOException;
import java.net.SocketException;
import java.util.HashMap;
import java.util.Map;

public class DictionaryUDPServer extends EchoUDPServer{

    private Map<String, String> dictionary = new HashMap<>();

    public DictionaryUDPServer(int port) throws SocketException {
        super(port);
        dictionary.put("I", "我");
        dictionary.put("love", "爱");
        dictionary.put("China", "中国");
    }

    @Override
    public String process(String request) {
        return dictionary.getOrDefault(request, "没找到");
    }

    public static void main(String[] args) throws IOException {
        EchoUDPServer server = new DictionaryUDPServer(1999);
        server.start();
    }
}

四.调试问题:

同时启动多个相同的代码: