Multithreaded OO. Questions:

Size: px

Start display at page:

Download "Multithreaded OO. Questions:"

Paul Gallagher
5 years ago
Views:

1 Multithreaded OO Questions: 1

2 1. What s the following Applet code snippet doing? Also, which statement is establishing the network connection? public void actionperformed( ActionEvent evt ) { try { String str = evt.getactioncommand(); AppletContext context = getappletcontext(); int i = 1; String s; while ( ( s = getparameter( "item_" + i ) )!= null ) { if ( str.equals( s ) ) { URL u = new URL( getparameter( "url_" + i ) ); context.showdocument( u, "right" ); i++; catch( Exception e ) { showstatus( "Error " + e ); 2

3 2. From within a Java program, it is possible to determine the name of the operating system running on the machine, the name of the Java installation directory, the name of the user, etc. What method must be invoked for this purpose? 3

4 3. Which of the following properties is a Java applet allowed to read by invoking the System.getProperty method? key meaning java.version Java version number java.vendor.url Java vendor URL java.class.version Java class version number os.name Operating system name os.arch Operating system architecture os.version Operating system version file.separator File separator (eg, "/") path.separator Path separator (eg, ":") line.separator Line separator java.home Java installation directory java.class.path Java classpath user.name User account name user.home User home directory user.dir User s current working directory 4

5 4. Why is security a matter of concern with regard to Java applets embedded in web pages? 5

6 Multithreaded Object-Oriented Programming Unlike C++, Java has support for multithreading built right into the language. However, even with C++, it is possible to write multithreaded programs by using external thread classes. 6

7 public class ThreadDisplayer { public static void main( String[] args ) { ThreadGroup topgroup = Thread.currentThread().getThreadGroup().getParent(); topgroup.list(); java.lang.threadgroup[name=system,maxpri=10] Thread[Reference Handler,10,system] Thread[Finalizer,8,system] Thread[Signal Dispatcher,10,system] Thread[CompileThread0,10,system] java.lang.threadgroup[name=main,maxpri=10] Thread[main,5,main] 7

8 Multithreading facilitates the design of more responsive graphical interfaces, especially when they are intended for multimedia applications. 8

9 What s the difference between a process and a thread? 9

10 The distinction between multiple processes running concurrently and multiple threads running concurrently is crucial. A thread of execution takes place within a process. While a process typically requires its own address space, multiple threads of execution share the same address space, which is the address space of the process in which they reside. Sharing the same address space permits multiple threads to more easily access the same data in the memory. For the same reason, the time it takes to switch between threads is shorter than the time it takes to switch between processes. Additionally, inter-thread communication is faster than inter-process communication. 10

11 Thread Scheduling 11

12 In a multithreaded program, different priority levels can be attached to different threads. But how these priority levels are actually taken into account varies considerably from system to system. Ordinarily, the highest priority thread is selected for execution. Such a thread continues running until a thread of higher priority seeks attention. However, how a system handles multiple threads of equal priority depends on whether or not the system uses preemptive scheduling. Preemptive scheduling means that the system timeslices the threads and schedules them for execution in a round-robin fashion. An alternative way of dealing with threads of equal priority consists of letting a thread run to completion, unless the execution is preempted by a higher priority thread becoming available (in what may be referred to as priority-preemptive scheduling) or by a sleep or a wait command. 12

13 What s the relationship between Java threads and the native threads? 13

14 The performance of a multithreadable programming system depends ultimately on how the threads are mapped to the native threads of the underlying operating system. This mapping, kept mostly hidden from the application programmer, varies from one implementation to another of a programming system. Some implementations of the Java Virtual Machine use what are known as green threads, which are all executed in a single native thread of the operating system. Other implementations of JVM may use directly the native threads, which allows Java threads to execute in parallel on multi-processor machines. Yet other implementations may map n Java threads to m native threads dynamically at the discretion of the scheduler. In Linux, each Java thread may be mapped to a separate process of the operating system. 14

15 Creating and Executing Simple Threads in Java 15

16 class HelloThread extends Thread { String message; HelloThread( String message ) { this.message = message; public void run() { // int sleeptime = (int) ( Math.random() * 3000 ); // try { // sleep( sleeptime ); // catch( InterruptedException e ){ System.out.print( message ); public static void main( String[] args ) { HelloThread ht1 = new HelloThread( "Good" ); HelloThread ht2 = new HelloThread( " morning" ); HelloThread ht3 = new HelloThread( " to" ); ht1.start(); ht2.start(); ht3.start(); try { sleep( 1000 ); catch( InterruptedException e ){ System.out.println( " you!" ); 16

17 class HelloThread extends Thread { String message; HelloThread( String message ) { this.message = message; public void run() { System.out.print( message ); public static void main( String[] args ) throws InterruptedException { HelloThread ht1 = new HelloThread( "Good" ); HelloThread ht2 = new HelloThread( " morning" ); HelloThread ht3 = new HelloThread( " to" ); ht1.start(); ht2.start(); ht3.start(); ht1.join(); ht2.join(); ht3.join(); System.out.println( " you!" ); 17

18 The Runnable Interface in Java 18

19 class HelloThread implements Runnable { String message; HelloThread( String message ){ this.message = message; public void run() { // int sleeptime = (int) ( Math.random() * 3000 ); // try { // sleep( sleeptime ); // catch( InterruptedException e ){ System.out.print( message ); public static void main( String[] args ) { HelloThread ht1 = new HelloThread( "Good" ); Thread t1 = new Thread( ht1 ); HelloThread ht2 = new HelloThread( " morning" ); Thread t2 = new Thread( ht2 ); HelloThread ht3 = new HelloThread( " to" ); Thread t3 = new Thread( ht3 ); t1.start(); t2.start(); t3.start(); try { Thread.sleep( 1000 ); 19

20 catch( InterruptedException e ){ System.out.println( " you!" ); 20

21 Using sleep() for Thread Cooperation in Java One of the best ways for the threads to cooperate meaning give the other threads a chance at the processor is to fall asleep every once in a while. When a thread goes to sleep, the scheduler assigns the processor in a round-robin fashion to all the waiting threads of the same highest priority. A certain amount of thread cooperation is, of course, guaranteed by the time-slicing action which ensures that all threads of equal priority will have a go at the processor in a round robin fashion. But a more equitable access to the processor is ensured by briefly putting a thread to sleep whenever possible. Such sleep intervals are also come in handy if you do not wish for the processor usage to be completely dominated by the highest priority threads. 21

22 //filename: Friendly_And_Unfriendly_Threads.java ///////// class Friendly_And_Unfriendly_Threads ////////// class Friendly_And_Unfriendly_Threads { System.out.println( " System.out.println( " public static void main( String[] args ) { System.out.println( " System.out.println( " Friendly Unfriendly" System.out.println( " long timeorigin = System.currentTimeMillis(); System.out.println( "Created at: " + 0 ); FriendlyThreads ft = new FriendlyThreads( timeorigin ); long currenttime = System.currentTimeMillis(); long timefromorigin = currenttime - timeorigin; System.out.println( " " + "Created at: " + timefromorigin ); UnfriendlyThreads ut = new UnfriendlyThreads( timeorigin ); currenttime = System.currentTimeMillis(); timefromorigin = currenttime - timeorigin; System.out.println( "Started at: " + timefromorigin ); ft.start(); currenttime = System.currentTimeMillis(); timefromorigin = currenttime - timeorigin; System.out.println( " " + "Started at: " + timefromorigin ); ut.start(); 22

23 ////////////// class FriendlyThreads //////////// class FriendlyThreads extends Thread { protected long timeorigin; public FriendlyThreads( long time ) { timeorigin = time; public void busywork() { long curr = System.currentTimeMillis(); while ( System.currentTimeMillis() < curr + 2 ) ; public void run() { for (int i=1; i <= 10; i++ ) { busywork(); long currenttime = System.currentTimeMillis(); long interval = currenttime - timeorigin; System.out.println( "For i=" + i + " busywork ends at: " + interval ); try { Thread.sleep(5); catch( InterruptedException e) { 23

24 //////////////////// class UnfriendlyThreads ////////////////// class UnfriendlyThreads extends FriendlyThreads { public UnfriendlyThreads( long time ) { super( time ); public void run() { for (int i=1; i <= 10; i++ ) { busywork(); long currenttime = System.currentTimeMillis(); long interval = currenttime - timeorigin; System.out.println( " " + "For i=" + i + " busywork ends at: " + interval ); long t = System.currentTimeMillis(); while ( System.currentTimeMillis() < t + 5 ) ; 24

25 Friendly Unfriendly Created at: 0 Created at: 7 Started at: 9 Started at: 35 For i=1 busywork ends at: 38 For i=1 busywork ends at: 57 For i=2 busywork ends at: 69 For i=2 busywork ends at: 71 For i=3 busywork ends at: 87 For i=3 busywork ends at: 87 For i=4 busywork ends at: 97 For i=4 busywork ends at: 107 For i=5 busywork ends at: 124 For i=6 busywork ends at: 132 For i=5 busywork ends at: 137 For i=7 busywork ends at: 145 For i=6 busywork ends at: 158 For i=8 busywork ends at: 158 For i=9 busywork ends at: 167 For i=7 busywork ends at: 177 For i=10 busywork ends at: 179 For i=8 busywork ends at: 197 For i=9 busywork ends at: 217 For i=10 busywork ends at:

26 Thread States The different states of a thread are born runnable running waiting sleeping blocked dead 26

27 //filename: HelloThread.java import java.io.*; public class HelloThread extends Thread { HelloThread( String nameofthread ) { setname( nameofthread ); start(); public void run() { for ( int i=0; i<5; i++ ) { if ( getname().equals( "thread1" ) ) { System.out.print( "Enter your name: " ); String user = readstring(); System.out.println( " Hello to " + user + " from " + getname() ); else System.out.println( " Hello from " + getname() ); yield(); String readstring() { String word = ""; try { 27

28 while ( true ) { int ch = System.in.read(); if ( ch == \n ch == ) break; word += (char) ch; catch( IOException e ) { return word; public static void main( String[] arg ) { HelloThread ht1 = new HelloThread( "thread1" ); HelloThread ht2 = new HelloThread( "thread2" ); HelloThread ht3 = new HelloThread( "thread3" ); 28

29 Enter your name: avi <thread1 blocks until user has entered name> Hello to avi from thread1 Hello from thread2 Hello from thread3 Enter your name: zaphod <thread1 blocks until user has entered name> Hello to zaphod from thread1 Hello from thread2 Hello from thread3 Enter your name: orpheus <thread1 blocks until user has entered name> Hello to orpheus from thread1 Hello from thread2 Hello from thread3 Enter your name: artemis <thread1 blocks until user has entered name> Hello to artemis from thread1 Hello from thread2 Hello from thread3 Enter your name: zeus <thread1 blocks until user has entered name> Hello to zeus from thread1 Hello from thread2 Hello from thread3 29

Multithreading Pearson Education, Inc. All rights reserved.

Multithreading Pearson Education, Inc. All rights reserved. 1 23 Multithreading 2 23.1 Introduction Multithreading Provides application with multiple threads of execution Allows programs to perform tasks concurrently Often requires programmer to synchronize threads