网络变成中的两个要素：
    1、IP和端口号；
    2、提供网络通信协议：TP\IP参考模型。

    通信要素一：IP和端口号
    1、IP地址唯一的去标识Internet上的计算机；
    2、再JAVA中使用InetAddress类代表IP；
    3、IP分类：IPv4和IPv6；万维网和局域网；
    4、域名；
    5、本地回路地址：127.0.0.1 对应着：localhost；
    6、如何实例化InetAddress：两个方法：getByName(String host)、getLocalHost()；两个常用方法：getHostName()\getHostAddress()。

    端口号标识正在计算机上运行的进程（程序），分为公认端口、注册端口、动态/私有端口。

    Socket：端口号与IP地址的组合得出的一个网络套接字。

    TCP协议与UDP协议的区别。
    TCP：要三次握手四次挥手、可靠、效率低；
    UDP：不握手、不可靠、效率高。

    实现TCP网络编程的例子：

    1. package com.JavaWebTest;
    3. import org.junit.Test;
    5. import java.io.ByteArrayOutputStream;
    6. import java.io.IOException;
    7. import java.io.InputStream;
    8. import java.io.OutputStream;
    9. import java.net.InetAddress;
    10. import java.net.ServerSocket;
    11. import java.net.Socket;
    12. import java.net.UnknownHostException;
    14. /**
    15.  * 实现TCP的网络编程
    16.  *
    17.  * 客户短发送信息给服务端，服务端将数据显示在控制台上
    18.  */
    20. public class TCPTest1 {
    21.     // 客户端
    22.     @Test
    23.     public void client() {
    24.         Socket socket = null;
    25.         OutputStream os = null;
    26.         try {
    27.             //1、创建Socket对象，指明服务器端的IP和端口号
    28.             InetAddress inet = InetAddress.getByName("127.0.0.1");
    29.             socket = new Socket(inet,8899);
    30.             //2、获取输出流，用于输出数据
    31.             os = socket.getOutputStream();
    32.             //3、写出数据
    33.             os.write("你好，我是客户端mm".getBytes());
    34.         } catch (IOException e) {
    35.             e.printStackTrace();
    36.         } finally {
    37.             //4、关闭资源
    38.             if(os !=null) {
    39.                 try {
    40.                     os.close();
    41.                 } catch (IOException e) {
    42.                     e.printStackTrace();
    43.                 }
    44.             }
    45.             if(socket != null){
    46.                 try {
    47.                     socket.close();
    48.                 } catch (IOException e) {
    49.                     e.printStackTrace();
    50.                 }
    51.             }
    52.         }
    54.     }
    56.     // 服务端
    57.     @Test
    58.     public void server() {
    60.         ServerSocket ss = null;
    61.         Socket socket = null;
    62.         InputStream is = null;
    63.         ByteArrayOutputStream baos = null;
    64.         try {
    65.             //1、创建服务器端的ServerSocket，指明自己的端口号
    66.             ss = new ServerSocket(8899);
    67.             //2、调用accept()表示接受来自于客户端的socket
    68.             socket = ss.accept();
    69.             //3、获取输入流
    70.             is = socket.getInputStream();
    71.             //4、获取输入流的数据
    72.             baos = new ByteArrayOutputStream();
    73.             byte[] buffer = new byte[5];
    74.             int len;
    75.             while((len = is.read(buffer)) != -1){
    76.                 baos.write(buffer,0,len);
    77.             }
    79.             System.out.println(baos.toString());
    81.         } catch (IOException e) {
    82.             e.printStackTrace();
    83.         } finally {
    84.             //5、关闭资源
    85.             if(baos != null){
    86.                 try {
    87.                     baos.close();
    88.                 } catch (IOException e) {
    89.                     e.printStackTrace();
    90.                 }
    91.             }
    92.             if(is != null){
    93.                 try {
    94.                     is.close();
    95.                 } catch (IOException e) {
    96.                     e.printStackTrace();
    97.                 }
    98.             }
    99.             if(socket != null){
    100.                 try {
    101.                     socket.close();
    102.                 } catch (IOException e) {
    103.                     e.printStackTrace();
    104.                 }
    105.             }
    106.             if(ss != null){
    107.                 try {
    108.                     ss.close();
    109.                 } catch (IOException e) {
    110.                     e.printStackTrace();
    111.                 }
    112.             }
    113.         }
    114.     }
    115. }

    用JAVA SOCKET编程，读服务器几个字符，再写入本地显示，代码：

    1. //服务端
    2. public class Server {
    3.     private ServerSocket ss;
    4.     private Socket socket;
    5.     private BufferedReader in;
    6.     private PrintWriter out;
    8.     public Server() {
    9.         try {
    10.             ss = new ServerSocket(10000);
    11.             while (true) {
    12.                 socket = ss.accept();
    13.                 String RemoteIP = socket.getInetAddress().getHostAddress();
    14.                 String RemotePort = ":" + socket.getLocalPort();
    15.                 System.out.println("A client come in!IP:" + RemoteIP + RemotePort);
    16.                 in = new BufferedReader(new InputStreamReader(socket.getInputStream()));
    17.                 String line = in.readLine();
    18.                 System.out.println("Cleint send is :" + line);
    19.                 out = new PrintWriter(socket.getOutputStream(), true);
    20.                 out.println("Your Message Received!");
    21.                 out.close();
    22.                 in.close();
    23.                 socket.close();
    24.             }
    25.         } catch (IOException e) {
    26.             out.println("wrong");
    27.         }
    28.     }
    30.     publicstaticvoid main(String[] args) {
    31.         new Server();
    32.     }
    33. };
    35. //客户端
    36. public class Client {
    37.     Socket socket;
    38.     BufferedReader in;
    39.     PrintWriter out;
    41.     public Client() {
    42.         try {
    43.             System.out.println("Try to Connect to 127.0.0.1:10000");
    44.             socket = new Socket("127.0.0.1", 10000);
    45.             System.out.println("The Server Connected!");
    46.             System.out.println("Please enter some Character:");
    47.             BufferedReader line = new BufferedReader(new InputStreamReader(System.in));
    48.             out = new PrintWriter(socket.getOutputStream(), true);
    49.             out.println(line.readLine());
    50.             in = new BufferedReader(new InputStreamReader(socket.getInputStream()));
    51.             System.out.println(in.readLine());
    52.             out.close();
    53.             in.close();
    54.             socket.close();
    55.         } catch (IOException e) {
    56.             out.println("Wrong");
    57.         }
    58.     }
    60.     public static void main(String[] args) {
    61.         new Client();
    62.     }
    63. }

    UDP网络通信：

    1. package com.JavaWebTest;
    3. import org.junit.Test;
    5. import java.io.IOException;
    6. import java.net.*;
    8. public class UDPTest {
    10.     // 发送端
    11.     @Test
    12.     public void sender() throws IOException {
    13.         DatagramSocket socket = new DatagramSocket();
    15.         String str = "我是你爹";
    16.         byte[] data = str.getBytes();
    17.         InetAddress inet = InetAddress.getLocalHost();
    18.         DatagramPacket packet = new DatagramPacket(data,0,data.length,inet,9090);
    20.         socket.send(packet);
    22.         socket.close();
    23.     }
    25.     //接收端
    26.     @Test
    27.     public void receiver() throws IOException {
    29.         DatagramSocket socket = new DatagramSocket(9090);
    31.         byte[] buffer = new byte[100];
    32.         DatagramPacket packet = new DatagramPacket(buffer,0,buffer.length);
    34.         socket.receive(packet);
    36.         System.out.println(new String(packet.getData(),0,packet.getLength()));
    38.         socket.close();
    40.     }
    41. }

    URL（Uniform Resourece Locator）编程：
    URL的基本结构由5部分组成：
    <传输协议>://<主机名>:<端口号>/<文件名>#片段名?参数列表
    例如: http://192.168.1.100:8080/helloworld/index.jsp#a?username=shkstart&password=123

    URL编程例子：

    1. package com.JavaWebTest;
    3. import java.io.FileOutputStream;
    4. import java.io.IOException;
    5. import java.io.InputStream;
    6. import java.net.HttpURLConnection;
    7. import java.net.MalformedURLException;
    8. import java.net.URL;
    10. public class URLTest1 {
    12.     public static void main(String[] args) throws IOException {
    13.         HttpURLConnection urlConnection = null;
    14.         InputStream is = null;
    15.         FileOutputStream fos = null;
    16.         try {
    17.             URL url = new URL("http://192.168.1.100:8080/helloworld/index.jsp#a?username=shkstart&password=123");
    19.             urlConnection = (HttpURLConnection) url.openConnection();
    21.             urlConnection.connect();
    23.             is = urlConnection.getInputStream();
    24.             fos = new FileOutputStream("beauty3.jpg");
    26.             byte[] buffer = new byte[1024];
    27.             int len;
    28.             while((len = is.read(buffer)) != -1){
    29.                 fos.write(buffer,0,len);
    30.             }
    31.         } catch (IOException e) {
    32.             e.printStackTrace();
    33.         } finally {
    34.             // 关闭资源
    35.             if(is != null){
    36.                 try {
    37.                     is.close();
    38.                 } catch (IOException e) {
    39.                     e.printStackTrace();
    40.                 }
    41.             }
    42.             if(is != null){
    43.                 try {
    44.                     fos.close();
    45.                 } catch (IOException e) {
    46.                     e.printStackTrace();
    47.                 }
    48.             }
    49.         }
    50.         urlConnection.disconnect();
    51.     }
    52. }