WebSockets communicate over aTCP (Transmission Control Protocol) connection. Java'sServerSocket class is located in thejava.net package.

TheServerSocket constructor accepts a single parameterport of typeint.

When you instantiate the ServerSocket class, it is bound to the port number you specified by theport argument.

Here's an implementation split into parts:

import java.io.IOException;import java.io.InputStream;import java.io.OutputStream;import java.net.ServerSocket;import java.net.Socket;import java.security.MessageDigest;import java.security.NoSuchAlgorithmException;import java.util.Base64;import java.util.Scanner;import java.util.regex.Matcher;import java.util.regex.Pattern;public class WebSocket {  public static void main(String[] args) throws IOException, NoSuchAlgorithmException {    ServerSocket server = new ServerSocket(80);    try {      System.out.println("Server has started on 127.0.0.1:80.\r\nWaiting for a connection…");      Socket client = server.accept();      System.out.println("A client connected.");

java.net.Socket.getInputStream()

Returns an input stream for this socket.

java.net.Socket.getOutputStream()

Returns an output stream for this socket.

write(byte[] b, int off, int len)

Writeslen bytes from the specified byte array starting at offsetoff to this output stream.

read(byte[] b, int off, int len)

Reads up tolen bytes of data from the input stream into an array of bytes.

Let us extend our example.

InputStream in = client.getInputStream();OutputStream out = client.getOutputStream();Scanner s = new Scanner(in, "UTF-8");

When a client connects to a server, it sends a GET request to upgrade the connection to a WebSocket from a simple HTTP request. This is known as handshaking.

try {  String data = s.useDelimiter("\\r\\n\\r\\n").next();  Matcher get = Pattern.compile("^GET").matcher(data);

Creating the response is easier than understanding why you must do it in this way.

You must,

1. Obtain the value ofSec-WebSocket-Key request header without any leading and trailing whitespace
2. Link it with "258EAFA5-E914-47DA-95CA-C5AB0DC85B11"
3. Compute SHA-1 and Base64 code of it
4. Write it back as value ofSec-WebSocket-Accept response header as part of an HTTP response.

if (get.find()) {  Matcher match = Pattern.compile("Sec-WebSocket-Key: (.*)").matcher(data);  match.find();  byte[] response = ("HTTP/1.1 101 Switching Protocols\r\n"    + "Connection: Upgrade\r\n"    + "Upgrade: websocket\r\n"    + "Sec-WebSocket-Accept: "    + Base64.getEncoder().encodeToString(MessageDigest.getInstance("SHA-1").digest((match.group(1) + "258EAFA5-E914-47DA-95CA-C5AB0DC85B11").getBytes("UTF-8")))    + "\r\n\r\n").getBytes("UTF-8");  out.write(response, 0, response.length);

After a successful handshake, client can send messages to the server, but now these are encoded.

If we send "abcdef", we get these bytes:

129 134 167 225 225 210 198 131 130 182 194 135

• 129:

  FIN (Is this the whole message?)	RSV1	RSV2	RSV3	Opcode
  1	0	0	0	0x1=0001

  FIN: You can send your message in frames, but now keep things simple.Opcode0x1 means this is a text.Full list of Opcodes

• 134:

  If the second byte minus 128 is between 0 and 125, this is the length of the message. If it is 126, the following 2 bytes (16-bit unsigned integer), if 127, the following 8 bytes (64-bit unsigned integer, the most significant bit MUST be 0) are the length.

  Note:It can take 128 because the first bit is always 1.

• 167, 225, 225 and 210 are the bytes of the key to decode. It changes every time.

• The remaining encoded bytes are the message.

decoded byte = encoded byte XOR (position of encoded byte BITWISE AND 0x3)th byte of key

Example in Java:

          byte[] decoded = new byte[6];          byte[] encoded = new byte[] { (byte) 198, (byte) 131, (byte) 130, (byte) 182, (byte) 194, (byte) 135 };          byte[] key = new byte[] { (byte) 167, (byte) 225, (byte) 225, (byte) 210 };          for (int i = 0; i < encoded.length; i++) {            decoded[i] = (byte) (encoded[i] ^ key[i & 0x3]);          }        }      } finally {        s.close();      }    } finally {      server.close();    }  }}

• Writing WebSocket servers