Java network concurrency

Objectives

• Program your own TCP client/server applications in Java with the Socket API
• Understand how to handle multiple clients at the same time
• Understand how to process data from streams

Your applications will be able to communicate over the network!

TCP

TCP is a transport protocol that is similar to a phone call:

1. A connection is established between two parties
2. Data sent is guaranteed to arrive in the same order
3. Data can be sent again

The Socket API

• Originally developed by Berkeley University
• Ported to Java and many other languages
• Provides a simple API to use TCP and UDP
• A socket is a connection between two parties using a protocol and a port

Client/server common functions

Client structure and functions

1. Create a Socket
2. Connect the socket to an IP address and a port number
3. Read and write data from/to the socket
4. Flush and close the socket

Server structure and functions

1. Create a ServerSocket
2. Bind the socket to an IP address and a port number
3. Listen for incoming connections
4. Loop
   1. Accept an incoming connection - creates a new Socket on a port
   2. Read and write data from/to the socket
   3. Flush and close the socket
5. Close the ServerSocket

To make it simple, a socket is just like a file that you can open, read from, write to and close. To exchange data, sockets on both sides must be connected.

Processing data from streams

• Sockets use data streams to send and receive data, just like files
• Get an input stream to read data from a socket
• Get an output stream to write data to a socket

Variable length data

Data sent can have a variable length. Manage this using one of the two methods:

• Use a delimiter
• Communicate a fixed length

This must be defined by your application protocol!

Using a delimiter:

// End of transmission character
String EOT = "\u0004";

// Read data until the delimiter is found
String line;
while ((line = in.readLine()) != null && !line.equals(EOT)) {
  System.out.println(
    "[Server " + SERVER_ID + "] received data from client: " + line
  );
}

Communicating a fixed length:

// Send the length of the data
out.write("DATA_LENGTH " + data.length() + "\n");

// Send the data
out.write(data);

// Read the length of the data
String[] parts = in.readLine().split(" ");
int dataLength = Integer.parseInt(parts[1]);

// Read the data
for (int i = 0; i < dataLength; i++) {
  System.out.print((char) in.read());
}

Handling one client at a time

1. Create a socket to listen to incoming connections
2. Create a new socket for the client
3. Handle the connection

Analogy: a restaurant with one table

Simple but quite useless...

Handling multiple clients at the same time

Handle multiple clients at the same time is called concurrency.

Concurrency can be achieved with:

• Multi-processing
• Multi-threading
• Asynchronous programming

Multi-processing

• Create an entirely new process for each client
• Heavyweight and slow
• Not recommended
• Analogy: a new restaurant for each customer, including the kitchen, waiters, etc.

Multi-threading

• Create a new thread for each client
• More lightweight and faster than multi-processing
• Recommended with thread pool to limit resource usage
• Not enough or too many threads can slow down the system
• Analogy: a new or limited number of tables for each customer

Asynchronous programming

• Handle multiple clients with a single thread
• Very performant!
• Analogy: a food truck - you get a ticket, wait, get your food and leave
• Out of scope for this course but interesting to know! Node.js is a good example for this approach

What will you do?

• Send an email using a SMTP client written in Java with the Socket API
• Run full client/server examples and understand how concurrent clients are handled

Finished? Was it easy? Was it hard?

Can you let us know what was easy and what was difficult for you during this chapter?

This will help us to improve the course and adapt the content to your needs. If we notice some difficulties, we will come back to you to help you.

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Sources

• Main illustration by Carl Nenzen Loven on Unsplash
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