java api for distributed computing

Spring/2002 Distributed Software EngineeringC:\unocourses\4350\slides\DefiningThreads

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Java API for distributed computing


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Two pure Java mechanisms

• Socket-based communication– Endpoints of a two-way communication

between two distributed components communicating with each other.

– Communication must be explicitly established by the two parties


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Two pure Java mechanisms

• RMI– Makes the network transparent– Allows distributed component manipulation

almost as if they were on same host– Programmers do not deal with interhost

communication


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Socket-based communication

• Two kinds of sockets:– Server sockets: wait for requests from clients– Client sockets: used to send data to a server socket and receive

data from it.

• Server socket listens at a specific port. • This port completely distinguishes different servers

running on the same host. • Server socket must be running on server host before any

communicating client. • After contact by client

– communication is established– Client socket is created for the app running on server

host to communicate with client.


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Server socket API

• ConstructorsServerSocket(int port)

port: specifies port number at which server socket will be listening for clients

When multiple client access server socket they will be placed in a queue.

ServerSocket(int port, in backlog)

backlog specifies max length of queue.


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Common server socket methods

Socket accept();Waits for connection request

Thread executing this call will be blocked until request is received, at which time returns a client socket.

void close();Stop waiting for requests from clients.


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Typical code structure for server socket

try{ServerSocket server = new ServerSocket(port);for ( ; ; ){

Socket client = server.accept();// handle client …

} } catch(IOException exc) {

// handle failure to create server socket}


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Client socket

• An instance of the Socket class.

• Can be created one of two ways:– Using constructor:

Socket( String host, int port);– On the server side, clients sockets returned by

the accept method.


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Socket communication

• Communication between server and client handled by client sockets at both ends.

• Each client socket has – an InputStream for receiving data– An OutputStream for sending data


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Common socket API

InputStream getInputStream();Returns an input stream for this socket.

OutputStream getOutputStream();Returns an output stream for this socket

void close(); Closes this socket.


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Typical client socket code

try{Socket client = new Socket(host, port);PrintWriter out = new PrintWriter(

new OutputStreamWriter(client.getOutputStream()));BufferedReader in = new BufferedReader(

new InputStreamReader(client.getInputStream()));

// send and receive data ….

in.close();out.close();client.close();

}catch( IOException exc) {

// handle failure of connection}


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import java.io.*;import java.net.*;

class EchoServer { public static void main(String[] args) { System.out.println("EchoServer started."); try { ServerSocket server = new ServerSocket(8008); Socket incoming = server.accept();

System.out.println("Connected to: " + incoming.getInetAddress() +

" at port: " + incoming.getLocalPort()); BufferedReader in = new BufferedReader(new InputStreamReader(incoming.getInputStream())); PrintWriter out = new PrintWriter(new OutputStreamWriter(incoming.getOutputStream()));

out.println("Hello! This is Java EchoServer. "); out.println("Enter BYE to exit."); out.flush();


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// while (true) {

String str = in.readLine(); if (str == null) { break; // client close } else { out.println("Echo: " + str); out.flush(); System.out.println("Received: " + str); if (str.trim().equals("BYE")) break; }

}//end while incoming.close(); } catch (Exception e) { System.out.println("Error: " + e); } System.out.println("EchoServer stopped."); }//end main

}


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• To run a server – Enter the java command to execute it

• To run a client– Enter the command to execute client followed by name

of server host and port number where server is executing.

• You can run for test purposes server and client on same host; client invocation will look:telnet localhost 8008

Or telnet 127.0.0.1 8008


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class EchoClient {

public static void main(String[] args) { try { String host; if (args.length > 0 && args[0] != null) {

host = args[0]; } else {

host = "localhost"; } Socket client = new Socket(host, 8008); BufferedReader in = new BufferedReader(

new InputStreamReader(client.getInputStream())); PrintWriter out = new PrintWriter(

new OutputStreamWriter(client.getOutputStream()));


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//sending data to server:for (int i = 1; i <= 10; i++) {

System.out.println("Sending: line " + i); out.println("line " + i); out.flush();

} out.println("BYE"); out.flush();

//receiving data from server: for (;;) { String str = in.readLine(); if (str == null) { break; }

else { System.out.println(str); } }//end for } catch (Exception e) { System.out.println("Error: " + e); } }//end main}


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A multi-echo server

• Using threads we can create sessions to handle clients.

• The session needs a reference to the client socket


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public class ClientHandler extends Thread {

protected Socket incoming;

public ClientHandler(Socket incoming) { this.incoming = incoming; }

public void run() { try { BufferedReader in

= new BufferedReader(new InputStreamReader(incoming.getInputStream()));

PrintWriter out = new PrintWriter(new OutputStreamWriter(incoming.getOutputStream()));

out.println("Hello! This is Java MultiEchoServer."); out.println("Enter BYE to exit."); out.flush();


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for (;;) {String str = in.readLine(); if (str == null) { break; }else { out.println("Echo: " + str); out.flush(); System.out.println("Received: " + str); if (str.trim().equals("BYE")) break; }

} //end for incoming.close(); } catch (Exception e) { System.out.println("Error: " + e); } }//end run

}


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import java.net.*; public class MultiEchoServer {

public static void main(String[] args) { System.out.println("MultiEchoServer started."); try { ServerSocket s = new ServerSocket(8009); for (;;) {

Socket incoming = s.accept(); new ClientHandler(incoming).start();

} } catch (Exception e) { System.out.println("Error: " + e); }

System.out.println("MultiEchoServer stopped."); }}


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Broadcast echo server


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import java.util.*;import java.io.*;import java.net.*;

class BroadcastClientHandler extends Thread {

protected Socket incoming; protected int id; protected BufferedReader in; protected PrintWriter out; public BroadcastClientHandler(Socket incoming, int id) { this.incoming = incoming; this.id = id; try { if (incoming != null) {

in = new BufferedReader(new InputStreamReader(incoming.getInputStream())); out = new PrintWriter(new OutputStreamWriter(incoming.getOutputStream()));

} } catch (Exception e) { System.out.println("Error: " + e); } }


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public synchronized void putMessage(String msg) { if (out != null) { out.println(msg); out.flush(); } }

public synchronized void broadcastClients(String str){

Enumeration enum = BroadcastEchoServer.activeThreads.elements(); while (enum.hasMoreElements()) { BroadcastClientHandler client =

(BroadcastClientHandler) enum.nextElement(); if (client != this) {

client.putMessage("Broadcast(" + id + "): " + str); } }

}


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public void run() { System.out.println("Client handler " + id + " started."); if (in != null && out != null) { putMessage("Hello! This is Java BroadcastEchoServer. Enter BYE to exit."); try {

for (;;) { String str = in.readLine(); if (str == null) { break; } else { // str is not null putMessage("Echo: " + str); System.out.println("Received (" + id + "): " + str); if (str.trim().equals("BYE")) { break; } else {

broadcastClients(str); } } //end else of not null str}//end forincoming.close(); BroadcastEchoServer.activeThreads.removeElement(this);

} catch (IOException e) {} } System.out.println("Client thread " + id + " stopped."); } // end run}


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public class BroadcastEchoServer {

static protected Vector activeThreads;

public static void main(String[] args) { System.out.println("BroadcastEchoServer started."); activeThreads = new Vector(); int i = 1; try { ServerSocket s = new ServerSocket(8010); for (;;) {

Socket incoming = s.accept(); System.out.println("Spawning client thread " + i);BroadcastClientHandler newThread = new BroadcastClientHandler(incoming, i); activeThreads.addElement(newThread); newThread.start(); i++;

} } catch (Exception e) { System.out.println("Error: " + e); }

System.out.println("BroadcastEchoServer stopped."); }}

java api for distributed computing

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