Writing a WebSocket server in Java
==================================

This example shows you how to create a WebSocket API server using Oracle Java.

Although other server-side languages can be used to create a WebSocket server, this example uses Oracle Java to simplify the example code.

This server conforms to RFC 6455, so it only handles connections from Chrome version 16, Firefox 11, IE 10 and higher.

First steps
-----------

WebSockets communicate over a TCP (Transmission Control Protocol) connection. Java's ServerSocket class is located in the `java.net` package.

### ServerSocket

The `ServerSocket` constructor accepts a single parameter `port` of type `int`.

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

Here's an implementation split into parts:


```java
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.");
```

### Socket Methods

`java.net.Socket.getInputStream()`

Returns an input stream for this socket.

`java.net.Socket.getOutputStream()`

Returns an output stream for this socket.

### OutputStream Methods


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


Writes `len` bytes from the specified byte array starting at offset `off` to this output stream.

### InputStream Methods


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


Reads up to *len* bytes of data from the input stream into an array of bytes.

Let us extend our example.


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

Handshaking
-----------

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.


```java
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 of *Sec-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 of *Sec-WebSocket-Accept* response header as part of an HTTP response.


```java
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);
```

Decoding messages
-----------------

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.
 Opcode *0x1* 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.
### Decoding algorithm

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

Example in Java:


```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();
    }
  }
}
```

Related
-------

* Writing WebSocket servers