1. Network Programming in Java
Networking
Network Programming in Java
©SoftMoore Consulting

2. A Network Is... node host address host name any device on the network
Networking
A Network Is...
node
any device on the network
host
a computer on the network
address
computer-readable name for host
host name
human-readable name for host
©SoftMoore Consulting

3. A Network Does... Packet (or datagram) Protocol IP
Networking
A Network Does...
Packet (or datagram)
little bundle of information
sent from one node to another
each packet is delivered separately (possibly by different routes)
Protocol
roles, vocabulary, rules for communication
network protocols (e.g., IP) versus application protocol (e.g., HTTP) IP
Internet Protocol
enables different local area networks to communicate
basis for connecting computers around the world
©SoftMoore Consulting

4. Two Types of Information
Networking
Two Types of Information
Application data
the information one computer wants to send to another
Network protocol data
describes how to reach the intended computer
describes how to check for errors in the transmission
©SoftMoore Consulting

5. TCP/UDP/IP IP (Internet Protocol) UDP (User Datagram Protocol)
Networking
TCP/UDP/IP
IP (Internet Protocol)
raw packets
the “Internet Layer”
UDP (User Datagram Protocol)
packets
unreliable, unordered
the “Transport Layer”
TCP (Transmission Control Protocol)
data stream
reliable, ordered (retry on failure; notify sender of success or failure)
©SoftMoore Consulting

6. TCP/IP: The Internet Protocol
Networking
TCP/IP: The Internet Protocol
Application Layer (HTTP, FTP, SMTP)
Transport Layer (TCP, UDP)
Internet Layer (IP)
Physical Network
©SoftMoore Consulting

7. TCP vs UDP UDP is unreliable TCP addresses these problems
Networking
TCP vs UDP
UDP is unreliable
Packets can arrive out of order
Packets can be lost
Not acknowledged by recipient
No provisions for flow control
Packet size limited by IP
TCP addresses these problems
Ensures that all packets show up, in correct order
Connection oriented
More like a telephone connection than a letter
Con: uses more resources: memory, speed, bandwidth
©SoftMoore Consulting

8. The Three “I”s and an “E”
Networking
The Three “I”s and an “E”
internet
any IP-based network
Internet
the big, famous, world-wide IP network
intranet
a corporate LAN-based IP network
extranet
accessing corporate data across the Internet
©SoftMoore Consulting

9. IP Addresses IP Address Domain name Domain Naming Service (DNS)
Networking
IP Addresses
IP Address
identifies a host
serves as a destination address for a packet
IPv4 (most widely deployed)
4 bytes/32 bits for an IP address
Expressed using “dot” notation:
IPv6 (the next generation)
16 bytes/128 bits for an IP address
Domain name
easy-to-remember name for a host
Example:
Domain Naming Service (DNS)
translates from domain names to IP addresses
©SoftMoore Consulting

10. Socket Two-way connection
Networking
Socket
Two-way connection
Provides inter-process communication using IP
Derived from BSD (UNIX) sockets
Read/write streams between hosts
use either UDP and TCP protocols (layered on top of IP)
uses IP addresses and port numbers to locate servers
©SoftMoore Consulting

11. Port Meeting place on a host
Networking
Port
Meeting place on a host
one service per port
Port numbers identify services on a server
Range from 1 to (use type int)
Port numbers 1 through 255 are reserved for well-known services
HTTP is 80
TELNET is 23
etc.
Port numbers less than 1024 are privileged
©SoftMoore Consulting

12. Sockets and Ports (Diagram)
Networking
Sockets and Ports (Diagram)
Client
Time Service
port 13
Web Service
port 80
Socket
Socket
Server
©SoftMoore Consulting

13. Well-Known Ports 13: time 20,21: FTP 23: telnet 25: SMTP 43: whois
Networking
Well-Known Ports
13: time
20,21: FTP
23: telnet
25: SMTP
43: whois
80: HTTP
119: NNTP
443 HTTPS
989/990: SFTP
1099: RMI (Java)
For additional information on port numbers see
“List of TCP and UDP port numbers,” Wikipedia,
©SoftMoore Consulting

14. Networking
Time of Day Service
From the command prompt, enter the following command:
telnet time-A.timefreq.bldrdoc.gov 13
Connects to the time-of-day service on a computer operated by the National Institute of Standards and Technology in Boulder, Colorado
Gives time based on a Cesium atomic clock
result not completely accurate due to network delays
By default, the telnet client is not installed on Windows.
To install telnet
Start → Control Panel → Programs and Features
→ Turn Windows features on or off
→ Telnet Client option
©SoftMoore Consulting

15. HTTP Example From command prompt, enter the following commands: telnet
Networking
HTTP Example
From command prompt, enter the following commands:
telnet
set localecho
open 80
GET /index.html HTTP/1.1
Host:
<enter>
©SoftMoore Consulting

16. HTTP Protocol for the World Wide Web port 80 default Client: Server:
Networking
HTTP
Protocol for the World Wide Web
port 80 default
Client:
GET path HTTP/1.1
Host: URL:port
Header: value
blank line
Server:
HTTP/1.1 OK 200
data
©SoftMoore Consulting

17. Java and Networking Built into language One of the Java buzzwords
Network ClassLoader
Package java.net
Based on TCP/IP, the Internet Protocol
©SoftMoore Consulting

18. Networking
The Socket Class
Hides the complexities of establishing a network connection and sending information across it
Provides the same programming interface used for working with files
Primary constructor/methods
Socket(String host, int port)
InputStream getInputStream()
OutputStream getOutputStream()
void close()
©SoftMoore Consulting

19. Byte Streams A byte stream is a sequence of bytes.
Networking
Byte Streams
A byte stream is a sequence of bytes.
All other stream types are built on byte streams.
I/O from disk, network, memory, etc. is handled in exactly the same way.
Abstract class InputStream
abstract int read() // reads next byte (-1 for EOF)
abstract void close()
Abstract class OutputStream
abstract void write(int b) // (ignores 24 high-order bits)
©SoftMoore Consulting

20. Character Streams A character stream is a sequence of characters
Networking
Character Streams
A character stream is a sequence of characters
Abstract class Reader
abstract int read() // reads next character (-1 for EOF)
abstract void close()
Abstract class Writer
abstract void write(int c) // ignores 16 high-order bits
abstract write(String str) // writes a string
Java naming conventions for I/O classes:
…InputStream/…OutputStream – for reading/writing bytes
…Reader/…Writer – for reading/writing characters
©SoftMoore Consulting

21. Networking
Class PrintStream
Method print() outputs an object, primitive, or string.
Method println() does the same thing, but it adds a newline at the end.
Object System.out is a PrintStream.
Example
OutputStream os = ...;
PrintStream ps = new PrintStream(os);
©SoftMoore Consulting

22. Class BufferedReader Provides method Examples String readLine()
Networking
Class BufferedReader
Provides method
String readLine()
Examples
InputStreamReader isReader
= new InputStreamReader(System.in);
BufferedReader reader = new BufferedReader(isReader)
©SoftMoore Consulting

23. Example: Using Sockets
Networking
Example: Using Sockets
import java.io.*;
import java.net.*;
...
Socket s = new Socket("time-A.timefreq.bldrdoc.gov", 13);
InputStream is = s.getInputStream();
InputStreamReader isReader = new InputStreamReader(is);
BufferedReader in = new BufferedReader(isReader);
String line;
while ((line = in.readLine()) != null)
System.out.println(line);
in.close();
©SoftMoore Consulting

24. Example: Simple Browser Using Sockets
Networking
Example: Simple Browser Using Sockets
import java.net.*;
import java.io.*;
public class SimpleHttpBrowser {
public static void main(String[] args) throws Exception
String host = args[0];
int port = Integer.parseInt(args[1]);
Socket s = new Socket(host, port);
InputStream inStream = s.getInputStream();
OutputStream outStream = s.getOutputStream();
PrintWriter out = new PrintWriter(
new OutputStreamWriter(outStream), true);
host and port
are command
line arguments
©SoftMoore Consulting

25. Example: Simple Browser Using Sockets (continued)
Networking
Example: Simple Browser Using Sockets (continued)
BufferedReader in = new BufferedReader(
new InputStreamReader(inStream));
out.println("GET / HTTP/1.1");
out.println("Host: " + host + ":" + port);
out.println();
String line;
while ((line = in.readLine()) != null)
System.out.println(line);
in.close();
out.close();
s.close(); }
©SoftMoore Consulting

26. Client/Server Overview
Networking
Client/Server Overview
Can communicate with remote file systems using a client/server model
Server listens for connection requests from clients
Clients know how to connect to the server via a port number
Upon connection, server processes the request coming across the network from the client
Connection can remain open or shut down after each transaction
©SoftMoore Consulting

27. Client-Server Difference between client and server is
Networking
Client-Server
Client – initiates connection
retrieves data
displays data
responds to user input
requests more data
Examples
web browser
chat program
PC accessing files
Server – responds to connection
receives data request
formulates response
sends response to client
Examples
web server
database server
server
domain name server
Difference between client and server is
relative. It’s just peers talking to each other.
©SoftMoore Consulting

28. Creating Servers using TCP
Networking
Creating Servers using TCP
A server listens on a port and accepts connections.
only one service per port for the entire host
Java throws an exception if you try to open more than one.
Create a ServerSocket attached to port.
port number of 0 creates a socket on any free port
optional second parameter sets maximum queue length for incoming connection requests (default is 50)
ServerSocket server = new ServerSocket(portNum);
Wait for connections from clients requesting connections to that port.
Socket s = server.accept();
©SoftMoore Consulting

29. Creating Servers using TCP (continued)
Networking
Creating Servers using TCP (continued)
Get input and output streams associated with socket.
outStream = s.getOutputStream();
inStream = s.getInputStream();
Use filters if performing text I/O
PrintWriter out = new PrintWriter(
new OutputStreamWriter(outStream));
BufferedReader in = new BufferedReader(
new InputStreamReader(inStream));
©SoftMoore Consulting

30. Creating Clients using TCP
Networking
Creating Clients using TCP
Create a Socket object attached to host/port
Socket client = new Socket(host, portNum);
When the constructor returns, you have a connection
Get input and output streams associated with socket.
outStream = client.getOutputStream();
inStream = client.getInputStream();
Use filters if performing text I/O
PrintWriter out = new PrintWriter(
new OutputStreamWriter(outStream));
BufferedReader in = new BufferedReader(
new InputStreamReader(inStream));
©SoftMoore Consulting

31. What to Do Upon Connection?
Networking
What to Do Upon Connection?
Can read/write to socket, thus, communicating with the server or client
Server talks to client:
out.println("Watson! Come here...");
String data = in.readLine();
Client talks to server:
out.println("I heard you over socket!");
©SoftMoore Consulting

32. EchoServer.java ServerSocket ss = new ServerSocket(1234);
Networking
EchoServer.java
ServerSocket ss = new ServerSocket(1234);
Socket s = ss.accept();
InputStream in = s.getInputStream();
OutputStream out = s.getOutputStream();
int ch;
while ((ch = in.read()) != -1)
out.write(ch);
out.close();
in.close();
©SoftMoore Consulting

33. EchoServer Demo Connect Type some stuff Disconnect Problem:
Networking
EchoServer Demo
Connect
Type some stuff
Disconnect
Problem:
multiple simultaneous connections
connections are made, but server doesn’t respond
even if we loop, can still handle only one at a time
Solution: Java Threads
©SoftMoore Consulting

34. Multiple Clients Must spawn thread for each connected client
Networking
Multiple Clients
Must spawn thread for each connected client
Pass input/output streams to client handler
Client handler implements protocol between client and server
©SoftMoore Consulting

35. ThreadedEchoServer.java // in ThreadedEchoServer method main()
Networking
ThreadedEchoServer.java
// in ThreadedEchoServer method main()
int port = 8189;
int count = 0;
try (ServerSocket server = new ServerSocket(port);) {
System.out.println("EchoServer listening on port " + port);
while(true)
Socket socket = server.accept();
++count;
System.out.println("Spawning " + count);
Thread handler =
new ThreadedEchoHandler(socket, count);
handler.start(); }
catch (Exception e)
...
©SoftMoore Consulting

36. Networking
ThreadedEchoHandler.java
// in ThreadedEchoHandler method run() (within a try block)
InputStream is = socket.getInputStream();
InputStreamReader isReader = new InputStreamReader(is);
BufferedReader in = new BufferedReader(isReader);
OutputStream os = socket.getOutputStream();
PrintWriter out = new PrintWriter(os, true); // autoFlush
out.println("Hello! Enter BYE to exit.");
String line;
while ((line = in.readLine()) != null) {
out.println("Echo (" + count + "): " + line);
if (line.trim().equals("BYE"))
break; }
socket.close();
...
©SoftMoore Consulting

37. Networking
Server Scaling Issues
A threaded server works well as long as there aren’t too many threads.
Thread objects use a significant amount of memory.
Allocating and deallocating many thread objects creates a significant memory management overhead.
Problem: How to scale a server even more without introducing too many threads?
Possible solutions:
thread pools
package nio (multiple data channels per thread)
©SoftMoore Consulting

38. Networking
Fixed Thread Pools
A fixed thread pool always has a specified number of threads running.
Tasks are submitted to the pool via an internal queue, which holds extra tasks whenever there are more active tasks than threads.
A simple way to create a fixed thread pool is to use the capabilities provided in java.util.concurrent. Specifically, class Executors has a factory method named newFixedThreadPool().
©SoftMoore Consulting

39. Networking
Executors
The java.util.concurrent package defines three executor interfaces
Executor – a simple interface that supports launching new tasks
ExecutorService – a subinterface of Executor, which adds features that help manage the lifecycle, both of the individual tasks and of the executor itself.
ScheduledExecutorService – a subinterface of ExecutorService, supports future and/or periodic execution of tasks.
The Executor interface provides a single method, execute(), designed to be a drop-in replacement for a common thread-creation idiom.
©SoftMoore Consulting

40. Example: Using an Executor
Networking
Example: Using an Executor
Runnable r … ; Executor e … ; e.execute(r); // replaces (new Thread(r)).start()
©SoftMoore Consulting

41. ExecutorEchoServer.java // in EchoServer method main()
Networking
ExecutorEchoServer.java
// in EchoServer method main()
ExecutorService pool = Executors.newFixedThreadPool(POOL_SIZE);
int port = 8189;
int count = 0;
try (ServerSocket server = new ServerSocket(port);) {
System.out.println("EchoServer listening on port " + port);
while(true)
Socket socket = server.accept();
++count;
System.out.println("Spawning " + count);
Runnable handler = new RunnableEchoHandler(socket, count);
pool.execute(handler); }
catch (Exception e)
...
©SoftMoore Consulting

42. Networking
RunnableEchoHandler.java
… same as ThreadedEchoHandler except that RunnableEchoHandler must “implement” interface Runnable rather than “extend” class Thread.
©SoftMoore Consulting

43. Sending Objects Use serialization ObjectOutputStream to write objects
Networking
Sending Objects
Use serialization
ObjectOutputStream to write objects
ObjectInputStream to read objects
©SoftMoore Consulting

44. Example: Sending Objects
Networking
Example: Sending Objects
// First, define an object to send:
public class Message implements Serializable {
private int senderID;
private String messageText;
public Message(int id, String text)
senderID = id;
messageText = text; }
public String getText()
return messageText; }
©SoftMoore Consulting

45. Example: Sending Objects (continued)
Networking
Example: Sending Objects (continued)
// Send the message across the socket:
Message msg = new Message( , "Hello");
out = new ObjectOutputStream(socket.getOutputStream());
out.writeObject(msg);
// On the other side of the socket, retrieve object:
in = new ObjectInputStream(socket.getInputStream());
Message messageObject = (Message) in.readObject();
String messageText = messageObject.getText();
©SoftMoore Consulting

46. URL Format Examples protocol://host[:port][/path/][file][#anchor]
Networking
URL
Format
protocol://host[:port][/path/][file][#anchor]
Examples
ftp://ftp.cs.rpi.edu/pub/stl/
(same as
©SoftMoore Consulting

47. java.net.URL URL(String) URL(String, String, int, String)
Networking
java.net.URL
URL(String)
creates a URL object from the String representation
URL(String, String, int, String)
creates a URL object from the specified protocol, host, port number, and file
URL(URL context, String spec)
creates a URL by parsing the specification spec within a specified context.
InputStream openStream()
opens a connection to this URL and returns an InputStream for reading from that connection
©SoftMoore Consulting

48. java.net.URL (continued)
Networking
java.net.URL (continued)
URLConnection openConnection()
Returns a URLConnection object that represents a connection to the remote object referred to by the URL.
MalformedURLException
thrown if you try to create a bogus URL
usually means bad user input, so fail gracefully and informatively
accessor methods
getProtocol() – getHost()
getPort() – getFile()
getRef()
©SoftMoore Consulting

49. java.net.URLConnection
Networking
java.net.URLConnection
InputStream getInputStream()
returns an input stream that reads from this open connection
makes the connection if it has not already been made
URLConnection creates the socket and handles the protocol (e.g. HTTP) for you
There are also handlers for other protocols
ftp, gopher, file, mailto, or you can write your own
getOutputStream()
Handles relative URLs via CodeBase/DocumentBase
©SoftMoore Consulting

50. java.net.URLConnection (continued)
Networking
java.net.URLConnection (continued)
Object getContent()
Makes the connection if it hasn’t already been made
Returns an object that’s appropriate for this file’s MIME type
For text, returns an InputStream
For images, returns an ImageObserver
Only supports text/plain, text/Generic, image/gif, and image/jpeg
You can add your own ContentHandler if you like
©SoftMoore Consulting

51. Example: Simple Browser Using URLs
Networking
Example: Simple Browser Using URLs
import java.net.*;
import java.io.*;
public class SimpleBrowser {
public static void main(String[] args) throws Exception
URL url = new URL(args[0]);
InputStream is = url.openStream();
InputStreamReader isReader = new InputStreamReader(is);
BufferedReader in = new BufferedReader(isReader);
String line;
while ((line = in.readLine()) != null)
System.out.println(line);
in.close(); }
©SoftMoore Consulting

52. java.net.HttpURLConnection
Extends URLConnection with additional support for HTTP connections.
Usage: Cast a URLConnection
URL url = new URL("
HttpURLConnection urlConnection =
(HttpURLConnection) url.openConnection();
Selected methods in HttpURLConnection
int getResponseCode()
String getResponseMessage()
String getHeaderField(int n)
String getHeaderFieldKey(int n)
©SoftMoore Consulting

53. Networking
java.net.URLEncoder
static String encode(String)
The ASCII characters ‘a’ through ‘z’, ‘A’ through ‘Z’, and ‘0’ through ‘9’ remain the same.
The space character is converted into a plus sign ‘+’.
All other characters are converted into the 3-character string “%xy”, where xy is the two-digit hexadecimal representation of the lower 8-bits of the character.
©SoftMoore Consulting

54. Networking
Applets
An applet is a small application that is delivered to the browser in the form of Java bytecode.
Applets are usually written in Java, but they can be written in any programming language that compiles to Java bytecode.
The applet is launched from a web page and is executed within a Java Virtual Machine (JVM) in a process separate from the web browser itself.
Applets have been available since the first version of Java and were widely used in the beginning, but their usage is now declining.
©SoftMoore Consulting

55. Networking Application: Checking URLs
URLConnection conn = url.openConnection();
if (conn instanceof HttpURLConnection) {
HttpURLConnection httpConn = (HttpURLConnection) conn;
// response code 400 or higher is bad
if (httpConn.getResponseCode() >= 400)
printBadUrl(url, httpConn.getResponseCode(),
httpConn.getResponseMessage()); }
else
printFeedback("Skipping " + url + ": not HTTP protocol");
©SoftMoore Consulting

56. Getting Stock Quotes from Yahoo
You can use HTTP to obtain stock quotes from yahoo.com in a comma-separated values (csv) file format. The URL has the form
<stock symbols separated by "+“>&f=<special tags>
Selected special tags:
s : symbol – o : open
l : last trade with time – c : change and percent change
w: 52-week range – n : name
r : P/E ratio – x : stock exchange
©SoftMoore Consulting

57. Example: Getting Stock Quotes from Yahoo
Format for returned line:
last trade time & price change/percent name
"3:21pm - <b>610.97</b>"," %", "Google Inc."
©SoftMoore Consulting

58. Networking Application: Checking Stocks
public class Stock {
private static final String QUOTE_FORMAT = "&f=lcwn";
private String symbol;
private String lastTradePrice;
private String lastTradeTime;
private String change;
private String range;
private String name;
public Stock(String symbol)
this.symbol = symbol.toUpperCase(); }
... // continued on next several slides
©SoftMoore Consulting

59. Networking Application: Checking Stocks (continued)
public void load() throws MalformedURLException, IOException {
String line = null;
URL url = new
URL("
+ symbol + QUOTE_FORMAT);
URLConnection connection = url.openConnection();
BufferedReader in = new BufferedReader(new
InputStreamReader(connection.getInputStream()));
line = in.readLine();
in.close();
... // continued }
©SoftMoore Consulting

60. Networking Application: Checking Stocks (continued)
if (line != null) {
// parse the line and remove quotes where necessary
String[] values = line.split(",");
change = values[1].substring(1, values[1].length() - 1);
range = values[2].substring(1, values[2].length() - 1);
name = values[3].substring(1, values[3].length() - 1);
// parse last trade time and price
String lastTrade = values[0];
int start = 1; // skip opening quote
int end = lastTrade.indexOf(" - ");
lastTradeTime = lastTrade.substring(start, end);
start = lastTrade.indexOf(">") + 1;
end = lastTrade.indexOf("<", start);
lastTradePrice = lastTrade.substring(start, end); }
©SoftMoore Consulting