Athipathy Threads In Java

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Threads

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Why use threads?? It is a powerful programming tool  Computer users take it for granted that their

systems can do more than one thing at a time.  They assume that they can continue to work in a word processor, while other applications download files, manage the print queue, and stream audio.  Even a single application is often expected to do more than one thing at a time.  Example of a web server: Many clients accessing….impossible to handle without threads! 04/16/09

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Use of threads cont..  In Internet: 100 simultaneous connections means

100 processes to handle. Just imagine 100 million internet users!!  Similarly with web servers..performance crawls with more simultaneous connections

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Another example  Streaming audio application must simultaneously

read the digital audio off the network, decompress it, manage playback, and update its display.  Even the word processor should always be ready to respond to keyboard and mouse events, no matter how busy it is reformatting text or updating the display.  Software that can do such things is known as concurrent software. 04/16/09

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Concurrent programming  The Java platform is designed from the ground up

to support concurrent programming, with basic concurrency support in the Java programming language and the Java class libraries.

 Since version 5.0, the Java platform has also

included high-level concurrency APIs in the java.util.concurrent packages.

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Concurrency vs. Parallelism CPU

main

CPU1 main

CPU2 RAM run

main this.count

run

run

main main

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run main

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Process & threads  In concurrent programming, there are two basic

units of execution: processes and threads.  In the Java programming language, concurrent programming is mostly concerned with threads. However, processes are also important. Threads are light weight processes!  A computer system normally has many active processes and threads. This is true even in systems that only have a single execution core, and thus only have one thread actually executing at any given moment.  Processing time for a single core is shared among processes and threads through an OS feature called time slicing. 04/16/09

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What is a thread?  A thread is like a sequential

program.  A single thread has a beginning, a sequence, and an end and at any given time during the runtime of the thread, there is a single point of execution.  However, a thread itself is not a program; it cannot run on its own. Rather, it runs within a program.

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Multiple threads

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Creating Threads Two ways of creating threads:  Extending a class.  Implementing an interface

Extending a class is the way Java inherits methods and variables from a parent class. In this case, one can only extend or inherit from a single parent class.  This limitation within Java can be overcome by implementing interfaces, which is the most common way to create threads. 

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Implementing thread  start()  run()  To get information from a finished thread is a

challenge especially in a multi-threaded system  return - is used

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Extending a class import java.util.*; class TimePrinter extends Thread { int pauseTime; String name; public TimePrinter(int x, String n) { pauseTime = x; name = n; } public void run() { while(true) { try { System.out.println(name + ":" + new Date(System.currentTimeMillis())); 12. Thread.sleep(pauseTime); } catch(Exception e) { } 13. } } 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.

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     

public static void main(String args[]) { TimePrinter tp1 = new TimePrinter(1000, "Fast Guy"); tp1.start(); TimePrinter tp2 = new TimePrinter(3000, "Slow Guy"); tp2.start(); }} 04/16/09

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Implementing Runnable interface import java.util.*; class TimePrinter implements Runnable { int pauseTime; String name; public TimePrinter (int x, String n) { pauseTime = x; name = n; } public void run() { while(true) { try { System.out.println(name + ":" + new Date(System.currentTimeMillis())); Thread.sleep(pauseTime);  } catch(Exception e) { }  04/16/09 Vimal } }         

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cont 1. public static void main(String

args[]) { 2. Thread t1 = new Thread(new TimePrinter(1000, "Fast Guy")); t1.start(); 3. Thread t2 = new Thread(new TimePrinter(3000, "Slow Guy")); t2.start(); 4. } } 04/16/09

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Creating Multiple threads Class newThread implements Runnable { String name;Thread t; NewThread(String threadname) { name=threadname; t=newThread(this,name); System.out.println(“New Thread:”+t); t.start(); } public void run() { 04/16/09

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Cont.. try{ For(int i=00;i>0;i--) { System.out.println(name +”:”+i); Thread.sleep(1000); }catch(IntrrruptedException e){} System.out.println(name +’exiting); } } 04/16/09

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Cont.. Class MultiThreadDemo { public static void main(String args[]) { new NewThread(“one”); new NewThread(“two”); new NewThread(“three”); try{ Thread.sleep(10000); }catch(InterruptedException e){} }} 04/16/09

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Synchronization  Two or more threads accessing the same data

simultaneously may lead to loss of data integrity.  Java uses the concept of semaphore or monitor to enable this.  Monitor is an object used as a mutually exclusive lock.  Synchronized 04/16/09

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 Class Consumer implements Runnable {  Stock c;  Thread t;  Consumer (Stock c)  { this.c=c;  t=new Thread(this,”consumer thread”);  t.start(); }  public void run(){  while(true){  try{  t.sleep(750);  }catch(InterruptedEcception e){}  c.getStock((int)(Math.random()*100));  } } 04/16/09 void stop(){ t.stop() } } Vimal

C o n s u m e r. j a v a21

 Class Producer implements Runnable {  Stock c;  Thread t;  Producer (Stock c)  { this.c=c;  t=new Thread(this,”producer thread”);  t.start(); }  public void run() {  while(true) {  try{  t.sleep(750);  }catch(InterruptedExcception e){}  c.getStock((int)(Math.random()*100));  } } 04/16/09  void stop(){ t.stop() } } Vimal

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Stock.java 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.

Class Stock { int goods=0; Public synchronized void addStock(int i) { goods=goods+i; System.out.println(“Stock added:”+i); System.out.println(“Present Stock:”+goods); notify(); } public synchronised int getStock(int j) { while(true) {

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• • • • • • • • • • • • •

if(goods >=j) { goods=goods=goods-j; System.out.println(“Stock taken away:”+i); System.out.println(“Present Stock:”+goods); break; }else { System.out.println(“Stock not enough:”+i); System.out.println(“Waiting for stocks to come:”+goods); try { wait(); }catch(InterruptedException e){} }} return goods; }

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• • • • • • • • • • •

public static void main(String args[]) { Stock= new Stock(); Producer p= new Producer(j); Consumer c=new Consumer(j); try{ Thread.sleep(10000); p.stop(); c.stop(); }catch(InterrruptedException e){} } }

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