1. CSCD 330 Network Programming
Spring 2012
Lecture 7
Socket Programming in Java
UDP and Threads
Reading: Chapter 2 and see Relevant Links
Some Material in these slides from J.F Kurose and K.W. Ross All material copyright 1 1

2. Review Client/Server Programming
Firefox
HTTP Port: 80
Review Client/Server Programming
So far,
Host has IP Address
Network Layer identifier
Every network “device” has this identifier
Phone, Toaster, etc.
Processes running on Hosts
Assigned a port number
Port numbers identify processes
Some port numbers reserved
System reserves these
Other port numbers reserved for widely recognized processes

3. Review Client/Server Programming
Communication Between Client/Server
Uses object, “Socket”
Socket is the API between a program and the TCP/IP stack of the OS
It has an input stream and an output stream built in to it
Both the client and server define different ends of this socket
Actual link to Java .net Package
Link to Java .net Package API explained

4. TCP/IP Client/Server How this works in Java Server Socket
1. Binds socket to specific port number
2. Listens for incoming connections on that port
3. When connection attempted, it accepts
connection, creates a regular socket for
communication to client
4. Port number is different and selected by TCP/IP
stack software

5. TCP/IP Client/Server Java Code for Server
Server socket listens on a port
Java Code for Server
ss = new ServerSocket (port);
// Loop forever
While (true) {
// Get a connection
Socket newSocket = ss.accept ();
// Deal with the connection
// .... }
Inside loop waits for connection
Creates a new socket object representing new connection
What is not obvious is that the new connection is through a different port number

6. TCP/IP Client/Server Java code for Client, Send/Receive Data
// Create a socket for communicating with server
Socket clientSocket = new Socket ("hostname", 6789);
// Create data streams for communicating through the socket
BufferedReader in = new BufferedReader
(new InputStreamReader (clientSocket.getInputStream ());
PrintWriter out = new PrintWriter (clientSocket.getOutputStream ());
System.out.println (in.readLine ()); // Print output to screen
Create a client TCP socket, with host and port
Then, create streams to send input and get it back from server

7. Java Package .NET Can look at the Java Package API Another Tutorial
Another Tutorial

8. Example of Server Creating Socket Object
Demo a Server that listens on a port for a connection, and creates a “socket” object for talking to client
Has Ephemeral port numbers
Can use telnet to play the role of a client
Steps:
1. Start my Server: HelloServer 5555
2. telnet localhost 5555
I am running my server, HelloServer on 5555

9. UDP Socket Programming

10. UDP Socket Programming
UDP no real “connection” between client and server
No handshaking
Sender attaches IP address and destination port to each packet
Server must extract IP address and port of sender from received packet, so answer can be sent back!
Tansmitted data may be received
Out of order
Lost 10

11. UDP Summary No connection setup No open “pipe” as in TCP
Main Differences, TCP
Each batch of bytes sent with attached
address information
No special ServerSocket class in java 11

12. UDP Summary Create a packet Mail is a lot like UDP
Push it out into the network through a socket
Server accepts the packet addressed to him
Mail is a lot like UDP
Each letter needs the address of the destination
Independent letters sent to the same address

13. Java Classes for UDP Datagrams for connectionless protocol
Two classes implement datagrams in Java:
java.net.DatagramPacket
java.net.DatagramSocket
DatagramPacket is actual packet of information, an array of bytes, that is transmitted over the network.
DatagramSocket is socket that sends and receives DatagramPackets across the network.
Think of DatagramPacket as the letter and DatagramSocket as mailbox where letters
are delivered
Need both classes for UDP sockets 13

14. Java Classes for UDP
DatagramPacket class provides programmer with two constructors.
First for DatagramPackets
that receives data
Constructor needs
Array to store the data
Amount of data to receive
Second is used for DatagramPackets
that sends data
Same information as before
Plus destination address and port number 14

15. Java Classes for UDP DatgramSocket represents connectionless socket
It provides two constructors,
1. Programmer can specify a port OR
2. Allow system to randomly use a port
Methods
Two most important methods, send() and receive()‏
Each takes an argument of a constructed DatagramPacket
send() method
Data in packet is sent to specified host and port
receive() method
Will block execution until a packet is received by underlying socket, then data copied into packet provided 15

16. UDP Clients and Servers are Similar
TCP
Server
ServerSocket establishes the server at a specific port number
Socket then connects to the client on a separate port number, somewhat behind the scenes
Client
Socket is used to connect to server at a specific port number
UDP
Uses DatagramSocket to listen for client on specific port number
Gets DatagramPacket with IP address and port number of client, uses this information for reply to client
Uses DatagramSocket to send DatagramPacket to server on specific port number
Gets answer from Server and saves in DatagramPacket

17. Sentence Capitalizer (Again)‏ Example: Java client (UDP)‏
import java.io.*;
import java.net.*;
class UDPClient {
public static void main(String args[]) throws Exception {
BufferedReader inFromUser =
new BufferedReader(new InputStreamReader(System.in));
DatagramSocket clientSocket = new DatagramSocket();
InetAddress IPAddress = InetAddress.getByName("hostname");
byte[] sendData = new byte[1024];
byte[] receiveData = new byte[1024];
String sentence = inFromUser.readLine();
sendData = sentence.getBytes();
Create
input stream
Create
client socket
Translate
hostname to IP
address using DNS 17

18. Example: Java client (UDP), cont.
Create datagram with data-to-send,
length, IP addr, port
DatagramPacket sendPacket =
new DatagramPacket(sendData, sendData.length,
IPAddress, 9876);
clientSocket.send(sendPacket);
DatagramPacket receivePacket =
new DatagramPacket(receiveData, receiveData.length);
clientSocket.receive(receivePacket);
String modifiedSentence =
new String(receivePacket.getData());
System.out.println("FROM SERVER:" + modifiedSentence);
clientSocket.close(); }
Send datagram
to server
Read datagram
from server 18

19. Sentence Capitalizer (Again)‏ Example: Java server (UDP)‏
import java.io.*;
import java.net.*;
class UDPServer {
public static void main(String args[]) throws Exception {
DatagramSocket serverSocket = new DatagramSocket(9876);
byte[] receiveData = new byte[1024];
byte[] sendData = new byte[1024];
while(true)
DatagramPacket receivePacket =
new DatagramPacket(receiveData, receiveData.length);
serverSocket.receive(receivePacket);
Create
datagram socket
at port 9876
Create space for
received datagram
Receive
datagram 19

20. Example: Java server (UDP), cont
String sentence = new String(receivePacket.getData());
InetAddress IPAddress = receivePacket.getAddress();
int port = receivePacket.getPort();
String capitalizedSentence = sentence.toUpperCase();
sendData = capitalizedSentence.getBytes();
DatagramPacket sendPacket =
new DatagramPacket(sendData, sendData.length, IPAddress,
port);
serverSocket.send(sendPacket); }
Get IP addr
port #, of
sender
Create datagram
to send to client
Write out
datagram
to socket
End of while loop,
loop back and wait for
another datagram 20

21. I/O and Sockets

22. More on I/O and Sockets
Move on and study Input/Output and then threads
Study client-server programs
Also, go over the Input/Output of Sockets
Sockets – with threads
Allow multiple clients in parallel 22

23. Details of Socket I/O New Example DailyAdviceServer
Server provides valuable advice to clients
You connect as a client and it sends back
the “advice of the day”
Random selection of advice messages
Still one client at a time
For connection-oriented, what type of socket are we using? 23

24. Daily Advice Client/Server
I need Advice
“You need to rethink that hairdo”
Client
Server

25. Algorithm for Daily Advice Client
Creates a socket to Advice Server
Waits for Server to provide excellent advice
Reads advice from server
Prints advice to screen
Quits

26. Socket I/O - Client What IP is this? Number is a what?
import java.io.*;
import java.net.*;
//The client
public class DailyAdviceClient {
public void go() {
try {
Socket s = new Socket(" ", 4200);
InputStreamReader streamReader =
new InputStreamReader(s.getInputStream());
BufferedReader reader =
new BufferedReader(streamReader);
String advice = reader.readLine();
System.out.println("Today you should: " + advice);
reader.close ();
} catch(IOException ex) {
ex.printStackTrace(); }
} // close go
What IP is this?
Number is a what? 26

27. Socket I/O - Client //Main method
public static void main(String[] args) {
DailyAdviceClient client = new DailyAdviceClient();
client.go(); } 27

28. Socket I/O - Client Input InputStreamReader acts like a bridge between
low-level byte stream, getInputStream() and
high-level character stream like BufferedReader
InputStreamReader streamReader =
new InputStreamReader(s.getInputStream());
Converts bytes to characters
Then ..... 28

29. Details of Socket I/O - Client
Input Continued ...
We chain high-level character stream like BufferedReader to the InputStreamReader to read buffered characters
BufferedReader reader =
new BufferedReader(streamReader);
String advice = reader.readLine();
Chaining of input streams from server to client looks like … 29

30. Details of Socket I/O - Client
Chain of input from server to client
Client
InputStreamReader
getInputStream
Server
BufferedReader
Buffered
Characters
Characters
Sockets input stream in bytes
Characters are buffered
Converted to characters
Why do you want to use a Buffered Reader class? 30

31. Buffered Readers are Efficient
Reason to use buffered reader
More efficient I/O
Each time you read a character, must access the disk
Buffered reader gets several characters at once.
Explains I/O performance
1st_edition/html/JPIOPerformance.fm.html

32. Socket I/O - Server import java.io.*; import java.net.*;
public class DailyAdviceServer {
String[] adviceList = {"Take smaller bites", "Go for the tight jeans. No they do NOT make you look fat.", "One word: inappropriate", "Just for today, be honest. Tell your boss what you *really* think", "You might want to rethink that haircut."};
public static void main(String[] args) {
DailyAdviceServer server = new DailyAdviceServer();
server.go(); // Main Server code in this routine } 32

33. Socket I/O - Server What is this number? What does this do?
public void go() {
try {
ServerSocket serverSock = new ServerSocket (4200);
while(true) {
Socket sock = serverSock.accept();
PrintWriter writer =
new PrintWriter(sock.getOutputStream());
String advice = getAdvice(); // select advice string
writer.println(advice);
writer.close(); // need THIS or flush() or never writes...
System.out.println(advice); // Writes to screen too }
} catch(IOException ex) {
ex.printStackTrace();
} // close go
What does this do? 33

34. Socket I/O - Server private String getAdvice() {
int random = (int) (Math.random() * adviceList.length);
return adviceList[random]; } 34

35. Socket I/O - Server Output
Create a PrintWriter object gets chained to low-level socket output getOutputStream
PrintWriter acts like its own bridge between character data and the bytes it gets from the Socket’s low-level output stream
Can then write strings to the socket connection
PrintWriter writer =
new PrintWriter(sock.getOutputStream());
String advice = getAdvice();
writer.println(advice); //adds a newline to string 35

36. Socket I/O - Server Chain of output from server to client Server
getOutputStream
Clien t
PrintWriter
“message …”
Message is in characters
Sockets output stream in bytes 36

37. Threads in Java 37

38. Threads in Java What is a thread? Separate thread of execution
Called from an existing program
A java class, Thread represents a thread in Java
Has a separate stack !!!
Allows you to run separate processes from main thread
Can do concurrency 38

39. Threads What does it mean to have more than one stack?
Get the appearance of having multiple things happen at once
Execution is actually moving back and forth between stacks
It might look like this … 39

40. Threads Main gets started, invokes Dog method
User starts a thread, cat
Execution switches between main and user thread
Active thread
Active thread
cat.go()‏
Dog()‏
run()‏
main()‏
User thread
Main thread 40

41. Threads in Java Two Classes in Java Threads
Runnable – Interface
Thread – Class from which a thread is declared
You link these two together
The actual thread and
The run method that runs the thread
Other Way to Create a Thread
Extending the Thread class (java.lang.Thread)‏ 41

42. Threads in Java
Advice is to implement Runnable not Extend Thread class, why?
Answer,
if thread class is to be subclass of some other class, it can’t extend from the Thread class
Java does not allow a class to inherit from more than one class ...
Use Runnable interface to implement threads

43. Threads in Java What are the Advantages of Thread?
Multithreading several advantages over Multiprocessing
Threads are lightweight compared to processes
Threads share same address space and
can share both data and code
Context switching between threads is less
expensive than between processes
Cost of thread intercommunication relatively
low than that of process intercommunication
Threads allow different tasks to be performed
concurrently

44. Single Threaded and Multi-threaded Process Models

45. Threads in Java How to Launch
Runnable interface makes a job runnable as a thread
First create a runnable job
Create a thread object and hand it a runnable job to work on
Then, the thread is started
Example Runnable linked to Thread
Runnable threadJob = new MyRunnable ();
Thread myThread = new Thread (threadJob); 45

46. Example Threads via Runnable
Thread uses the run() method of the runnable interface.
Example below
public class MyRunnable implements Runnable {
public void run () {
go (); }
public void go () {
doMore ();
public void doMore ();
System.out.println (“Top o’ the stack”);
Runnable has one method, run()‏
This is where you put the job the thread is supposed to run 46

47. Example Threads via Runnable
Example Continued
class ThreadTester {
public static void main (String[] args) {
Runnable threadJob = new MyRunnable ();
Thread myThread = new Thread(threadJob);
myThread.start();
System.out.println (“back in Main”); }
Pass the Runnable instance to thread constructor
Start the new thread 47

48. Thread States Running! Thread t = new Thread (r); t.start();
Thread ready to run,
runnable state
Waiting to be selected for
execution
Thread selected to run and
is the currently running
thread
Thread created
but not started
Once thread is runnable, can go back and forth between running, runnable and blocked 48

49. Threads
The JVM thread scheduler is responsible for deciding who gets to run next
You, have little control over the decision
Blocked Threads
Thread might be blocked for many reasons
Examples
Executing code to read from socket input stream, but no data to read
Executing code told thread to sleep
Tried to call a method but object was “locked” 49

50. Scheduling Threads
Don’t base your program’s correctness on the scheduler working in a particular way
Can’t control it or even predict the scheduler behavior
Implementations are different with different JVM’s
Compile and run previous code several times to see this
Results will differ even on the same machine 50

51. Multiple Clients Original Problem
Need to handle multiple requests for service from a given server
Without threads
One client at a time, process each until done
With threads
Many clients, process one, move on and process another etc. 51

52. Threads and multiple clients
Same problem, DailyAdvice Server
Now,
Add threads so we can service multiple clients at once
Everyone will have the benefit of having great advice to start their day 52

53. ServerSocket serverSock = new ServerSocket(4200); while(true) {
public void go() {
try {
ServerSocket serverSock = new ServerSocket(4200);
while(true) {
AdviceRequest request =
new AdviceRequest (serverSock.accept());
Thread thread = new Thread (request);
thread.start (); }
} catch(IOException ex) {
ex.printStackTrace();
} // close go
AdviceRequest is our Runnable object
Pass an extension of Runnable to thread constructor 53

54. Inside run, processRequest does all the work is the “job” to be run
final class AdviceRequest implements Runnable {
Socket socket;
public AdviceRequest (Socket socket) throws Exception {
this.socket = socket; }
// Implement the run method of the Runnable interface
public void run() {
try {
processRequest ();
} catch (Exception e) {
System.out.println (e);
private void processRequest () throws Exception {
// Attach a PrintWriter to socket's output stream
PrintWriter writer =
new PrintWriter(this.socket.getOutputStream());
String advice = getAdvice();
writer.println(advice);
writer.close(); // must have THIS or flush() or it never writes...
System.out.println(advice);
AdviceRequest
implements the
Runnable interface
One method - run ()‏
Inside run, processRequest does all the work is the “job” to be run
Gets output stream and sends advice to one client 54

55. Summary Brief coverage of sockets with threads
Should be enough for you to get started
This example will be available to download
Put links to code on the main class page
We will be implementing .....
A multi-threaded Web Server!!!!
Also, practice client-server in the lab
Can read references in RelevantLinks for more information 55

56. Webserver assignment will allow practice of these ideas
Lots of fun!!!! 56