4003-707-02 Advanced Programming
Module 7. Networking & Concurrency -- Lecture Notes

Networking

• Socket: Reliable, two-way communication of byte streams
  • One process, the server, listens for connections
  • Another process, the client, sets up a connection to the server
  • Each process has a socket
  • Each socket has an output stream and an input stream
  • The bytes that the client writes to its socket output stream are the bytes that the server reads from its socket input stream
  • The bytes that the server writes to its socket output stream are the bytes that the client reads from its socket input stream

• Internet address: host name, port number
  • Class java.net.InetSocketAddress

• Client side connection setup
  • Create an instance of class java.net.Socket with the no-argument constructor
  • Call the socket's connect() method, passing in the Internet address to which to connect

• Server side connection setup
  • Create an instance of class java.net.ServerSocket with the no-argument constructor
  • Call the server socket's bind() method, passing in the Internet address to which to listen for connections
  • Call the server socket's accept() method
  • When a client sets up a connection, the accept() method returns a socket

• Client side and server side socket communication
  • Call the socket's getOutputStream() method, write data to the output stream
  • Call the socket's getInputStream() method, read data from the input stream

• Often the server has a thread that calls accept() in a loop to accept multiple client socket connections

• Often the server (and the client) creates a separate thread to read from the socket and process incoming messages

Concurrency

• Class java.lang.Thread
  • Used to create and control a separate thread of execution
  • The thread itself is inside the JVM and operating system

• To define a thread:
  • Write a class that extends class Thread
  • Use the constructor to pass parameters into the thread subclass if necessary
  • Override the run() method
    • Put the code the thread will execute in the body of the run() method
    • The run() method returns void and throws no checked exceptions

• To create a thread:
  • Create an instance of the thread subclass
  • Call the start() method on that instance
  • The start() method returns immediately to its caller
  • The (separate) thread executes the run() method

• To terminate a thread:
  • Return from the run() method

Multiple Thread Safety

• If two or more threads call methods (any methods) on the same object, the object must be multiple thread safe

• Alternative 1: Declare each method to be synchronized

  public class Xyz
      {
      public synchronized void abc()
          {
          . . .
          }
      public synchronized int def
          (int i)
          {
          . . .
          }
      }
  

• Alternative 2: Declare the critical section of code in each method to be synchronized

  public class Xyz
      {
      public void abc()
          {
  	synchronized (this)
  		{
  		. . .
  		}
          . . .
          }
      public int def
          (int i)
          {
  	synchronized (this)
  		{
  		. . .
  		}
          . . .
          }
      }
  

• If one thread is executing a synchronized method or synchronized block on an object, any other thread that tries to execute a synchronized method or synchronized block on that object will wait until the first thread finishes executing the synchronized method or synchronized block

Advanced Programming • 4003-707-02 • Fall Quarter 2008

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Alan Kaminsky • Department of Computer Science • Rochester Institute of Technology • 4486 + 2220 = 6706

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Copyright © 2008 Alan Kaminsky. All rights reserved. Last updated 31-Oct-2008. Please send comments to ark­@­cs.rit.edu.