Java Training Sockets and Network Programming

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Java TrainingSockets and Network Programming

• Written by Jeff Smith

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What is a Socket?

• A communications channel between two computers
• A socket on one computer can talk talk to a
  socket on another computer via this a
  communication channel
• It is a lower level communications mechanism than
  technologies such as web services, CORBA, RMI, or
  IIOP
• Two main types of sockets supported in Java
• TCP sockets (implemented by the Socket class)
• UDP sockets (implemented by the DatagramSocket
  class)
• TCP sockets are generally more commonly used and
  simpler.
• These slides will describe TCP sockets.

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TCP/IP Sockets -1

• TCP/IP Transfer Control Protocol Internet
  Protocol
• TCP is a reliable point-to-point communication
  protocol that client-server applications on the
  Internet use to communicate with each other
• To communicate over TCP, a client program and a
  server program establish a connection to one
  another
• The client and server programs both bind a socket
  to each end of the connection and communicate by
  reading and writing to those sockets
• There are three commonly used socket classes in
  the java.net package

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TCP/IP Sockets -2
Client requests connection
Client
Server
Port
Port
Client
Server
Connection accepted
Port
Port
Upon acceptance, the server gets a new socket
bound to the same local port and also has its
remote endpoint set to the address and port of
the client. It needs a new socket so that it can
continue to listen to the original socket
for connection requests while tending to the
needs of the connected client.
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URLConnection -1
URLConnection - abstract superclass for
communications links between an application and a
URL. HttpURLConnection is a commonly used
subclass
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URLConnection -2

• Basic Steps in Using URLConnection to retrieve
  web page
• Create URL object with a valid URL string. If the
  URL is invalid, youll get a MalformedURLException
  
• Open a connection on that URL (openConnection()
  method)
• Wrap the InputStream for that connection in a
  BufferedReader so you can read lines
• Read the web page using your BufferedReader
• Call your BufferedReaders close() method

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URLConnection -3

• URL url new URL(http//www.wrfportal.org/index.
  html)
• URLConnection urlCon url.openConnection()
  //connect to this URL
• //use the urlCons InputStream to create a
  BufferedReader
• InputStream inputStream urlCon.getInputStream()
  
• InputStreamReader isr new InputStreamReader(inpu
  tStream)
• BufferedReader bufReader new BufferedReader(isr)
  
• String line null
• while ((line bufReader.readLine()) ! null)
  //read lines from URL until null
• System.out.println(line "\n")
• bufReader.close() //close the BufferedReader
  for this urlCon InputStream

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Socket (Client) -1

• Besides using a URLConnection, you can use a
  lower level class, the Socket class, to retrieve
  a web page
• With the Socket class, you can connect to any
  program that is listening on a port
• HTTP typically listens on port 80
• FTP on port 21
• MySQL on port 3306
• SMTP on port 25
• Tomcat on port 8080
• You can write your own Java programs to
  communicate with programs listening on these
  ports

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Socket (Client) -2
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Socket (Client) -3

• Basic Steps Involved In Socket Programming
• Create InetAddress object for IP address/computer
  (e.g. sun.com)
• Create SocketAddress from this InetAddress and
  Port (e.g. 80)
• Create Socket and connect to SocketAddress
• Create a PrintWriter for this socket (for output)
• Create a BufferedReader for this socket (for
  input)
• Write some text messages to the socket (these
  messages are sent to the remote program which is
  listening on the port)
• Read some text messages back from the socket
  (this assumes that the remote program responds to
  your messages)

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Socket (Client) -4
//create InetAddress object InetAddress addr
InetAddress.getByName(sun.com) int port
80 SocketAddress sockaddr new
InetSocketAddress(addr, port) // Create an
unbound socket socket new Socket() // This
method will block no more than timeoutMS // If
the timeout occurs, SocketTimeoutException is
thrown int timeoutMS 2000 socket.connect(sock
addr, timeoutMS)
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Socket (Client) -5
boolean autoflush true PrintWriter out new
PrintWriter(socket.getOutputStream(),
autoflush) InputStream inputStream
socket.getInputStream() InputStreamReader
isReader new InputStreamReader(inputStream) Buf
feredReader rd new BufferedReader(isReader) //
send our HTTP request out.println("GET
/index.html HTTP/1.1") out.println("Host
localhost80") out.println("Connection
Close") out.println() //read the response
back String s null while ((s rd.readLine())
! null) System.out.println(s) rd.close()
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Socket (Client) -6

• Socket Exceptions
• UnknownHostException
• IOException
• Imports
• java.net.
• Java.io.

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ServerSocket -1
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ServerSocket -2

• Basic Steps For Writing Server Sockets
• Create ServerSocket object for the specific port
  you want to listen to (throws an Exception if
  there's a problem)
• Invoke ServerSocket.accept() method to block
  (stop there) while waiting for connection
• Get the input and output streams on that
  underlying Socket
• Create Printwriter and BufferedReader objects
  from those streams
• Read from and write to the Socket
• Close your open streams (always close the output
  or writer stream BEFORE the input or reader
  stream)

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ServerSocket -3
//This program waits for incoming client requests
//and when they come in, it processes them int
port 3000 //pick a port not used for anything
else ServerSocket server new ServerSocket(port)
Socket clientSocket null while (true)
//wait until client request comes (the accept()
method blocks) clientSocket server.accept()
//got a requestso process it
processRequest(clientSocket)
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ServerSocket -4
public void processRequest(Socket clientSocket)
try InputStream inputFromSocket
clientSocket.getInputStream() BufferedReader
streamReader new BufferedReader(new

InputStreamReader(inputFromSocket)) //read
one line from the client request String
clientMessage streamReader.readLine()
OutputStream outputToSocket clientSocket.getOutp
utStream() PrintWriter streamWriter new
PrintWriter(outputToSocket) //basically
just echo the request back to the client
streamWriter.println("Your request message was '"
clientMessage "'")
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ServerSocket -5
streamWriter.close() //close the writer
stream first streamReader.close()
catch (Exception e) System.out.println("E
rror handling client request " e)
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Socket Applications -1

• File Server
• Client socket connects to server and requests a
  file (by supplying the file name)
• Server returns contents of the file as a data
  stream
• Web Page Scraper
• Client socket connects to web server, requests
  html document
• Server returns the html document
• Client parses the document for information it
  wants to write to a file or database. Information
  could be email addresses, temperature readings in
  an HTML table, etc.

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Socket Applications -2

• Web Page Spider/Crawler
• Client socket connects to web server, requests
  html document
• Server returns the html document
• Client parses the document for hyperlinks to
  other pages. It stores these document links in a
  file or database.
• Client then creates sockets for connecting to the
  web servers specified in these hyperlinks,
  looking for yet more hyperlinks

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Socket Applications -3

• Meterological Information
• Client socket connects to NOAA server in
  Antarctica, requests temperature at certain
  location
• Server reads it sensor at given location, returns
  the temperature
• Load Balancer
• Client socket requests information that is
  obtained by running a time consuming program
• Server decides which computer on a cluster has
  the smallest load, and then creates its own
  client socket to that computer requesting the
  information
• Server returns the information to the original
  client

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Exercise 1

• Do a Google Search With The Socket class
• Write a program that allows you to search Google
  and retrieve the results.
• Here is the message/code your socket will have to
  send to google on port 80
• out.println("GET /search?q" searchString "
  HTTP/1.1")
• out.println("Host localhost80")
• out.println("Connection Close")
• out.println() //extra println is required
• Set searchStringJavaZoneScheduleGD309 and
  then wait for Googles
• response and you should see the names of three
  students in our class in the
• html you get back from Google.

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Exercise 2
Create A Simple Web Server Your web server
should listen on port 3000 for requests for html
documents. Create a few simple web pages in your
project directory that can be retrieved by your
web server. Youll need to create a client
program that requests documents in order to test
your web server. Your request message could Be
something simple like test1.html or you can get
fancier and support directories like
/html/subdir/test1.html. Step 1 Just dump
the html you get back from web server to the
console Step 2 Load the html you get back into a
browser (Internet Explorer) Hint
Runtime.getRuntime().exec(/path/to/browser.exe
somefile.html)