Client Server Example in Java

1
Client Server Example in Java

• The next 11 slides highlight the steps needed to
  have one server wait for one client application
• Five steps required on the server slide
• Four steps required on the client slide
• They also show example code from
• AccountServer.java (adds the try catch)
• AccountClient.java (adds the try catch)

2
Server Using Sockets

• Five steps
• 1. Create a ServerSocket
• int port 4000 // Only port available in
  our labs!
• ServerSocket socket null
• socket new ServerSocket(port)
• Establishes the port where the server waits for
  connections from clients

3
Server Using Sockets

• 2. Wait for a connection from the client with a
  Socket
• Socket connection socket.accept()
• This waits for a connection
• A Connection is the server's way to communicate
  with the client, which can be done using
  ObjectInputStream and ObjectOutputStream

4
Server Using Sockets

• 3. Get two IO streams to allow communication
  with the client and the server
• These two stream objects allow the server to
  write objects to and read objects from the client
  
• ObjectOutputStream output
• new ObjectOutputStream(connection.getOutput
  Stream())
• ObjectInputStream input
• new ObjectInputStream(connection.getInputS
  tream())
• Now a client can read the servers output (with
  input) and write to the server's input (with
  output)
• Assuming the client also has IO streams

5
Server Using Sockets

• 4. Let the client and server communicate (silly
  example)
• BankAccount theClientsAccount null
• BankAccount theServersAccount new
  BankAccount("Greedy", 0.00)
• while (true)
• double amount ((Double) input.readObject()).d
  oubleValue()
• if (amount lt 0.0)
• break
• theClientsAccount (BankAccount)
  input.readObject()
• if (theClientsAccount.withdraw(amount))
• theServersAccount.deposit(amount)
• System.out.println(theClientsAccount.toString()
  " "
• theServersAccount.toString()
  )
• output.writeObject(theClientsAccount)
• Take money from the client, but leave at least 100

6
Server Using Sockets

• 5. Shut down the connection
• connection.close()
• Any object that is read from or written to a
  socket must implement serializable
• String already does this
• To BankAccount, add this
• public class BankAccount implements
  ComparableltBankAccountgt, Serializable

7
The Client

• Four Steps
• 1. Create a socket to connect to the server
• String IPAddress "localhost" // The
  machine you're on
• int port 4000
• Socket server new Socket(IPAddress, port)

8
The Client

• 2. Get references to the Server's IO streams
• ObjectOutputStream output new
• ObjectOutputStream(server.getOutputStrea
  m())
• ObjectInputStream input new
  ObjectInputStream(server.getInputStream())

9
The Client

• 3. Do the processing
• BankAccount myAccount new BankAccount("Sucker",
  5000.00)
• while (true)
• String amountAsString JOptionPane.showInputDi
  alog(null,
• "You've won! Enter desired amount" "
  you have "
• 
  myAccount.getBalance())
• double amount Double.parseDouble(amountAsStri
  ng)
• // First write a double to the server
• output.writeObject(new Double(amount))
• if (amount gt 0)
• // First write the BankAccount to the server
• output.writeObject(myAccount)
• // Read the BankAccount back from the server
  (with less money)
• myAccount (BankAccount) input.readObject()
  
• else
• break
• Writes an amount an an account to the server

10
The Client

• 4. Close the connection
• server.close()
• This example wrote objects with ObjectOutStream
• If you only need to pass strings around (as in
  BattleBoat perhaps), you can also readLine with a
  BufferedReader and write strings with PrintWriter
• This is like reading text from a file and writing
  text to a file

11
Summary

• Networking is made much easier with Javas
  extensive API that hides a lot of networking
  details
• Networking is integrated with input and output
  both use streams.
• Decorators make your life much easier, you can
  write and read serialized objects (also encrypt
  and/or zip)
• You can read and write strings with
  BufferedReader and PrintWriter