[Java Advanced Programming] Network Programming

1. Use of InetAddress class

1.1. Two problems need to be solved to realize network communication

• How to accurately locate one or more hosts on the network; locate specific applications on a host
• How to transfer data reliably and efficiently after finding the host

1.2. Two elements of network communication

• Corresponding question 1: IP and port number
• Corresponding question 2: Provide network communication protocol: TCP/IP reference model (application layer, transport layer, network layer, physical + data link layer)

1.3. Communication element 1: IP and port number

• Understanding of IP
  • IP: uniquely identifies a computer (communication entity) on the Internet
  • Use InetAddress class to represent IP in Java
  • IP Classification: IPv4 and IPv6; World Wide Web and Local Area Network
  • Area name: www.baidu.com, www.mi.com
    • Domain name resolution: The domain name is easy to remember. After entering the domain name of a host when connecting to the network, the domain name server (DNS) is responsible for converting the domain name into an IP address, so that a connection can be established with the host.
  • Local loop address: 127.0.0.1 corresponds to: localhost
• InetAddress class: an object of this class represents a specific IP address
  • Instantiate:
    • getByName(String host)
    • getLocalHost()
  • Common methods:
    • getHostName()
    • getHostAddress()
• Port number: the process running on the computer
  • Requirement: different port numbers for different processes
  • Range: specified as a 16-bit integer 0~65535
• The combination of port number and IP address results in a network socket: Socket

1.4. Communication Element 2: Network Communication Protocol

OSI reference model	TCP/IP Reference Model	Protocols corresponding to each layer of the TCP/IP reference model
application layer, presentation layer, session layer	application layer	HTTP、FTP、Telnet、DNS…
transport layer	transport layer	TCP、UDP...
Network layer	Network layer	IP、ICMP、ARP…
data link layer, physical layer	Physical + Data Link Layer	Link

1.5, the difference between TCP and UDP

• TCP protocol:
  • Before using the TCP protocol, a TCP connection must be established to form a transmission data channel
  • Before transmission, the "three-way handshake" method, point-to-point communication, is reliable
  • Two application processes for TCP protocol communication: client and server
  • A large amount of data can be transferred in the connection
  • After the transmission is completed, the established connection needs to be released, which is inefficient
• UDP protocol:
  • Encapsulate data, source, and destination into packets without establishing a connection
  • The size of each datagram is limited to 64K
  • Regardless of whether the sender is ready or not, the receiver does not confirm receipt, so it is unreliable
  • Can broadcast
  • There is no need to release resources at the end of sending data, with low overhead and fast speed

1.6, TCP three-way handshake and four-way wave

• three handshake

• waved four times

2. TCP network programming

/**
 * 客户端发送信息给服务端，服务端将数据显示在控制台上
 */
public class TCPTest1 {
    // 客户端
    @Test
    public void client() {
        Socket socket = null;
        OutputStream os = null;
        try {
            // 1、创建Socket对象，指明服务器端的IP和端口号
            InetAddress inet = InetAddress.getByName("127.0.0.1");
            socket = new Socket(inet, 8899);
            // 2、获取一个输出流，用于输出数据
            os = socket.getOutputStream();
            // 3、写出数据的操作
            os.write("你好，我是客户端mm".getBytes());
        } catch (IOException e) {
            e.printStackTrace();
        } finally {
            // 4、资源的关闭
            if (os != null) {
                try {
                    os.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }

            if (socket != null) {
                try {
                    socket.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
        }
    }
    //服务端
    @Test
    public void server () {
        ServerSocket ss = null;
        Socket socket = null;
        InputStream is = null;
        ByteArrayOutputStream baos = null;
        try {
            // 1、创建服务器端的ServerSocket，指明自己的端口号
            ss = new ServerSocket(8899);
            // 2、调用accept()表示接收来自客户端的socket
            socket = ss.accept();
            // 3、获取输入流
            is = socket.getInputStream();
            // 4、读取输入流中的数据
            baos = new ByteArrayOutputStream();
            byte[] buffer = new byte[5];
            int len;
            while ((len = is.read(buffer)) != -1) {
                baos.write(buffer, 0, len);
            }

            System.out.println(socket.getInetAddress().getHostAddress() + "：" + baos.toString());
        } catch (IOException e) {
            e.printStackTrace();
        } finally {
            // 5、关闭资源
            if (baos != null) {
                try {
                    baos.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
            if (is != null) {
                try {
                    is.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
            if (socket != null) {
                try {
                    socket.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
            if (ss != null) {
                try {
                    ss.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
        }
    }
}

/**
 * 客户端发送文件给服务端，服务端将文件保存在本地
 */
public class TCPTest2 {
    // 客户端
    @Test
    public void client () {
        Socket socket = null;
        OutputStream os = null;
        FileInputStream fis = null;
        try {
            socket = new Socket(InetAddress.getByName("127.0.0.1"), 9090);
            os = socket.getOutputStream();
            fis = new FileInputStream(new File("beauty.jpg"));
            byte[] buffer = new byte[1024];
            int len;
            while ((len = fis.read(buffer)) != -1) {
                os.write(buffer, 0, len);
            }
        } catch (IOException e) {
            e.printStackTrace();
        } finally {
            if (fis != null) {
                try {
                    fis.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
            if (os != null) {
                try {
                    os.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
            if (socket != null) {
                try {
                    socket.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
        }
    }

    //服务端
    @Test
    public void server () {
        ServerSocket ss = null;
        Socket socket = null;
        InputStream is = null;
        FileOutputStream fos = null;
        try {
            ss = new ServerSocket(9090);
            socket = ss.accept();
            is = socket.getInputStream();
            fos = new FileOutputStream(new File("beauty1.jpg"));

            byte[] buffer = new byte[1024];
            int len;
            while ((len = is.read(buffer)) != -1) {
                fos.write(buffer, 0, len);
            }
        } catch (IOException e) {
            e.printStackTrace();
        } finally {
            if (fos != null) {
                try {
                    fos.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
            if (is != null) {
                try {
                    is.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
            if (socket != null) {
                try {
                    socket.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
            if (ss != null) {
                try {
                    ss.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
        }
    }
}

/**
 * 从客户端发送文件给服务器，服务端保存到本地。并返回“发送成功”给客户端
 * 并关闭相关的连接
 */
public class TCPTest3 {
    // 客户端
    @Test
    public void client ()
    {
        Socket socket = null;
        OutputStream os = null;
        FileInputStream fis = null;
        InputStream is = null;
        ByteArrayOutputStream baos = null;
        try {
            socket = new Socket(InetAddress.getByName("127.0.0.1"), 9090);
            os = socket.getOutputStream();
            fis = new FileInputStream(new File("beauty.jpg"));

            byte[] buffer = new byte[1024];
            int len;
            while ((len = fis.read(buffer)) != -1) {
                os.write(buffer, 0, len);
            }
            // 关闭数据的输出
            socket.shutdownOutput();

            // 接收来自于服务器端的数据，并显示到控制台上
            is = socket.getInputStream();
            baos = new ByteArrayOutputStream();
            byte[] buffer1 = new byte[20];
            int len1;
            while ((len1 = is.read(buffer1)) != -1) {
                baos.write(buffer1, 0, len1);
            }

            System.out.println(baos.toString());
        } catch (IOException e) {
            e.printStackTrace();
        } finally {
            if (baos != null) {
                try {
                    baos.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
            if (is != null) {
                try {
                    is.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
            if (fis != null) {
                try {
                    fis.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
            if (os != null) {
                try {
                    os.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
            if (socket != null) {
                try {
                    socket.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }

        }
    }

    // 服务端
    @Test
    public void server () {
        ServerSocket ss = null;
        Socket socket = null;
        InputStream is = null;
        FileOutputStream fos = null;
        OutputStream os = null;
        try {
            ss = new ServerSocket(9090);
            socket = ss.accept();
            is = socket.getInputStream();
            fos = new FileOutputStream(new File("beauty2.jpg"));

            byte[] buffer = new byte[1024];
            int len;
            while ((len = is.read(buffer)) != -1) {
                fos.write(buffer, 0, len);
            }

            System.out.println("图片传输完成");

            // 服务器端给予客户单反馈
            os = socket.getOutputStream();
            os.write("你好，美女，照片我已收到，非常漂亮！".getBytes());
        } catch (IOException e) {
            e.printStackTrace();
        } finally {
            if (os != null) {
                try {
                    os.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
            if (fos != null) {
                try {
                    fos.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
            if (is != null) {
                try {
                    is.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
            if (socket != null) {
                try {
                    socket.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
            if (ss != null) {
                try {
                    ss.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
        }
    }
}

3. UDP network programming

public class UDPTest {
    // 发送端
    @Test
    public void sender () {
        DatagramSocket socket = null;
        try {
            socket = new DatagramSocket();

            String str = "我是UDP方式发送的导弹";
            byte[] data = str.getBytes();
            InetAddress inet = InetAddress.getLocalHost();
            DatagramPacket packet = new DatagramPacket(data, 0, data.length, inet, 9090);

            socket.send(packet);
        } catch (IOException e) {
            e.printStackTrace();
        } finally {
            if (socket != null) {
                socket.close();
            }
        }
    }
    // 接收端
    @Test
    public void receiver () {
        DatagramSocket socket = null;
        try {
            socket = new DatagramSocket(9090);

            byte[] buffer = new byte[100];
            DatagramPacket packet = new DatagramPacket(buffer, 0, buffer.length);

            socket.receive(packet);

            System.out.println(new String(packet.getData(), 0, packet.getLength()));
        } catch (IOException e) {
            e.printStackTrace();
        } finally {
            socket.close();
        }
    }
}

4. URL programming

4.1. Understanding of URL (Uniform Resource Locator)

• Uniform resource locator, corresponding to a resource address on the Internet

4.2. Five basic structures of URL

• protocol
• CPU name
• The port number
• resource address
• parameter list

4.3, how to instantiate

• URL url = new URL("http://localhost:8080/examples/beauty.jpg?username=Tom")

4.4 Common methods

• public String getProtocol(): Get the protocol name of the URL
• public String getHost(): Get the host name of the URL
• public String getPort(): Get the port number of the URL
• public String getPath(): Get the file path of the URL
• public String getFile(): Get the file name of the URL
• public String getQuery(): Get the query name of the URL

4.5, can read and download the corresponding url resources

public static void main(String[] args) {
    HttpURLConnection urlConnection = null;
    InputStream is = null;
    FileOutputStream fos = null;
    try {
        URL url = new URL("http://localhost:8080/examples/beauty.jpg");
        urlConnection = (HttpURLConnection) url.openConnection();
        urlConnection.connect();

        is = urlConnection.getInputStream();
        fos = new FileOutputStream("day10\\beauty3.jpg");

        byte[] buffer = new byte[1024];
        int len;
        while ((len = is.read(buffer)) != -1) {
            fos.write(buffer, 0, len);
        }

        System.out.println("下载完成");
    } catch (IOException e) {
        e.printStackTrace();
    } finally {
        if (fos != null) {
            try {
                fos.close();
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
        if (is != null) {
            try {
                is.close();
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
        if (urlConnection != null) {
            urlConnection.disconnect();
        }
    }
}