Class Notes for Object Oriented Programming II | CMSC 132, Study notes of Computer Science

Client Programming

Basic steps

Determine server location – IP address & port

Open network connection to server

Write data to server (request)

Read data from server (response)

Close network connection

Stop client

Simple Server Programming

Basic steps

Determine server location - port (& IP address)

Create ServerSocket to listen for connections

Loop while (true) { Accept network connection from client Read data from client (request) Write data to client (response) Close network connection to client }

ServerSocket Class

Create socket on server Constructor specifies local port

Server listens to port

Usage

Begin waiting after invoking accept() Listen for connection (from client socket) Returns Socket for connection

Server Example public class Server { public static void main(String args[ ]) throws Exception { ServerSocket ss = new ServerSocket(4242); while (true) { Socket s = ss.accept(); BufferedReader r = new BufferedReader( new InputStreamReader(s.getInputStream())); PrintWriter out = new PrintWriter( new OutputStreamWriter(s.getOutputStream())); String name = r.readLine(); out.println("Hello " + name); out.flush(); s.close(); } } }

URL Class

Provides high-level access to network data Abstracts the notion of a connection Constructor opens network connection

To resource named by URL

Creating Threads in Java

Runnable Approach

Define class implementing Runnable interface public interface Runnable { public void run( ); }

Put work to be performed in run( ) method

Create instance of the “worker” class

Create thread to run it Create Thread object Pass worker object to Thread constructor Or hand the worker instance to an executor Alternative methods for running threads

Lock

Definition

Entity can be held by only one thread at a time

Properties

A type of synchronization Used to enforce mutual exclusion Thread can acquire / release locks Thread will wait to acquire lock (stop execution) If lock held by another thread

Lock Example public class DataRace extends Thread { static int common = 0; static Object o; // all threads use o’s lock public void run() { synchronized(o) { // single thread at once int local = common; // data race eliminated local = local + 1; common = local; } } public static void main(String[] args) { o = new Object(); … } }