networking with javan-1 outline client-server example steps required on the server side steps...

Networking with Java N-1

Outline• Client-Server Example

Steps required on the server sideSteps required on the client side


Server Using Sockets

Create a ServerSocket // 4000 is the only port available in our labs!// 4000 is the only port available in our labs! ServerSocket socket = new ServerSocket(4000);ServerSocket socket = new ServerSocket(4000);

• Establishes the port where the server waits for connections from clients


Server Using Sockets

Wait for a connection from the client with a Socket Socket client = socket.accept();Socket client = socket.accept();

• This waits for a connection • No further statements are executed in the server until

a client connects (shown later)• client is the server's way to communicate with the

client using ObjectInputStream and ObjectOutputStream


Writing using SocketsGet two IO streams to allow communication with the client and the server

Server can write objects to the client and read them ObjectOutputStream outputToClient = ObjectOutputStream outputToClient = new ObjectOutputStream(client.getOutputStream());new ObjectOutputStream(client.getOutputStream()); ObjectInputStream inputFromClient =ObjectInputStream inputFromClient =

new ObjectInputStream(client.getInputStream()); new ObjectInputStream(client.getInputStream()); Later, a client will be able to read this server's output when it write an object to the client with outputToClient.writeObject(new MyClass());outputToClient.writeObject(new MyClass());

and the client can write to this server that reads withMyClass c = (MyClass)inputFromClient.readObject();MyClass c = (MyClass)inputFromClient.readObject();

Assuming the client also has IO streams that is …


The ClientIn a separate program, create a socket to connect to the

server

String IPAddress = "localhost";String IPAddress = "localhost"; int port = 4000;int port = 4000; Socket server = new Socket(IPAddress, port);Socket server = new Socket(IPAddress, port);


The Client

Get references to the Server's IO streams of the server with which this client can now read from and write to

ObjectOutputStream outputToServer = new ObjectOutputStream outputToServer = new ObjectOutputStream(server.getOutputStream());ObjectOutputStream(server.getOutputStream());

ObjectInputStream inputFromServer = new ObjectInputStream inputFromServer = new ObjectInputStream(server.getInputStream());ObjectInputStream(server.getInputStream());

Do some Input

MyClass c = (MyClass) inputFromServer.readObject();MyClass c = (MyClass) inputFromServer.readObject();System.out.println(c.toString());System.out.println(c.toString());

Object read and written objects must be Serializable

public class MyClass implements Serializable {public class MyClass implements Serializable { public String toString() {public String toString() { return "This could be any of your types.";return "This could be any of your types."; }}}}



Use Object IO streams

• This example has one simple readObject with a writeObject from the other program

• The next example code could replace that simple read and write with loops

• This allows communication until some event occurs to terminate the looping (like a BattleBoat win or loss)

• The next example shows how a server can present the illusion of taking money form a client


Server with a loop replace simple write with loopBankAccount theClientsAccount = null;BankAccount theClientsAccount = null;// theClientsAccount will be read from client after connection to this server// theClientsAccount will be read from client after connection to this serverBankAccount theClientsAccount = null; BankAccount theClientsAccount = null; // This server's account will deposit the money withdrawn from the clients// This server's account will deposit the money withdrawn from the clients// account and then writes back the modified clients account.// account and then writes back the modified clients account.BankAccount theServersAccount = new BankAccount("Greedy", 0.00);BankAccount theServersAccount = new BankAccount("Greedy", 0.00); // Continue as long as the client sends a positive amount (as a double)// Continue as long as the client sends a positive amount (as a double)while (true) {while (true) { double amount = ((Double) inputFromClient.readObject()).doubleValue();double amount = ((Double) inputFromClient.readObject()).doubleValue(); if (amount <= 0.0)if (amount <= 0.0) break;break; // read the client's account// read the client's account theClientsAccount = (BankAccount) inputFromClient.readObject();theClientsAccount = (BankAccount) inputFromClient.readObject(); // "Take" the money (at least present the illusion)// "Take" the money (at least present the illusion) theClientsAccount.withdraw(amount);theClientsAccount.withdraw(amount); theServersAccount.deposit(amount);theServersAccount.deposit(amount); // and run. Send back the modified object// and run. Send back the modified object outputToClient.writeObject(theClientsAccount);outputToClient.writeObject(theClientsAccount);}}


Client with a Loop replace simple read with loop // The clients account// The clients account BankAccount myAccount = new BankAccount("Sucker", 5000.00);BankAccount myAccount = new BankAccount("Sucker", 5000.00); // Loop as long as the client wishes to give away money.// Loop as long as the client wishes to give away money. // (Okay, there really is no money being moved, it's just an illusion)// (Okay, there really is no money being moved, it's just an illusion) while (true) {while (true) { // Try to get a positive amount from the client using this program// Try to get a positive amount from the client using this program String amountAsString = JOptionPane.showInputDialog(null,String amountAsString = JOptionPane.showInputDialog(null, "You've won! Enter desired amount" + " you have ""You've won! Enter desired amount" + " you have " + myAccount.getBalance());+ myAccount.getBalance()); double amount = Double.parseDouble(amountAsString);double amount = Double.parseDouble(amountAsString);

// Write the amount this client thinks is winning// Write the amount this client thinks is winning outputToServer.writeObject(new Double(amount));outputToServer.writeObject(new Double(amount)); // Since the server is reading, the write is necessary to prevent// Since the server is reading, the write is necessary to prevent // an end of file exception when this client shuts down.// an end of file exception when this client shuts down. // Terminate this program when the client determines a positive// Terminate this program when the client determines a positive // input to the dialog box results in a decreasing balance for the client// input to the dialog box results in a decreasing balance for the client if (amount <= 0)if (amount <= 0) break;break; // And the account from which the server will "withdraw" money// And the account from which the server will "withdraw" money outputToServer.writeObject(myAccount);outputToServer.writeObject(myAccount); // Get a new version of this client's account back from the server // Get a new version of this client's account back from the server myAccount = (BankAccount) inputFromServer.readObject();myAccount = (BankAccount) inputFromServer.readObject(); }} }}


Summary• Networking is made much easier with Java’s extensive

API that hides a lot of networking details• Networking is integrated with input and output: both

use streams• You can read and write strings with BufferedReader

and PrintWriter, but Use ObjectOutputStream and ObjectInputStream to read and write any object the implements Serializable

• You can use loops in the client and server to communicate for a while

• If you read from one program, write from the other

networking with javan-1 outline client-server example steps required on the server side steps...

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