3. Sockets and Streams

3. Sockets and Streams
	Chapter 11. Distributed Programming

3.1. Stream Sockets

3.1.1. What is a Socket?

Definition: A socket is one end-point of a two-way communication link between two programs running on the network.

Types:

Stream socket: class Socket.
Stream socket (To listen): class ServerSocket.
Datagram socket: class DatagramSocket.

3.1.2. Reading from and Writing to a Stream Socket

Example:

public class EchoTest {
   public static void main(String[] args) {
        Socket echoSocket = null;
        DataOutputStream os = null;
        DataInputStream is = null;
        DataInputStream stdIn = new DataInputStream(System.in);

        try {
              echoSocket = new Socket("taranis", 7);
              os = new DataOutputStream(echoSocket.getOutputStream());
              is = new DataInputStream(echoSocket.getInputStream());
        } catch (UnknownHostException e) { ... }
        catch (IOException e) { ... }

        if (echoSocket != null && os != null && is != null) {
           try {
                String userInput;

                while ((userInput = stdIn.readLine()) != null) {
                     os.writeBytes(userInput);
                     os.writeByte('\n');
                     System.out.println("echo: " + is.readLine());
                }
                os.close();
                is.close();
                echoSocket.close();
           } catch (IOException e) { ... } 
        }
   }
}

Audio in Portuguese

3.1.3. Writing the Server Side of a Stream Socket

Listening and Communicating:

Request channel: Server do the listening.
Communication channel: Created for each connection.

Figure 11.5. Listening and Communicating

Knock Knock Server:

Server: Knock! Knock!
Client: Who's there?
Server: Turnip
Client: Turnip who?
Server: Turnip the heat, it's cold in here! Want another? (y/n)

Classes:

KnockKnockServer: Implements the server
KKState: Implements the application. It keeps track of the current joke, the current state and serves up the various text pieces of the joke depending on the current state.
KnockKnockClient: Implements the client.

Audio in Portuguese

The KnockKnockServer Class:

Creates a ServerSocket to listen:

try {
    serverSocket = new ServerSocket(4444);
} catch (IOException e) { ... }

Waits until a clients connects:

Socket clientSocket = null;
try {
    clientSocket = serverSocket.accept();
} catch (IOException e) { ... }

Open an input and output stream to the socket:

DataInputStream is = new DataInputStream(
                 new BufferedInputStream(clientSocket.getInputStream()));
PrintStream os = new PrintStream(
             new BufferedOutputStream(clientSocket.getOutputStream(), 1024), false);
String inputLine, outputLine;
KKState kks = new KKState();

Read from and write to the socket

outputLine = kks.processInput(null);
os.println(outputLine);
os.flush();

while ((inputLine = is.readLine()) != null) {
    outputLine = kks.processInput(inputLine);
    os.println(outputLine);
    os.flush();
    if (outputLine.equals("Bye."))
        break;
}

Close all the input and output streams:

os.close();
is.close();
clientSocket.close();
serverSocket.close();

Audio in Portuguese

The Knock Knock Client:

Differences from EchoTest:

kkSocket = new Socket("taranis", 4444);
            . . .
while ((fromServer = is.readLine()) != null) {
    System.out.println("Server: " + fromServer);
    if (fromServer.equals("Bye."))
        break;
    while ((c = System.in.read()) != '\n') {
        buf.append((char)c);
    }
    System.out.println("Client: " + buf);
    os.println(buf.toString());
    os.flush();
    buf.setLength(0);
}

3.2. Supporting Multiple Clients

Requests treated by one execution thread.
Individual communication treated by:
- Another process
- Another thread.

Figure 11.6. Threads in Servers

Audio in Portuguese


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