EE2E1. JAVA Programming Lecture 8 Multi-threading

Contents Introduction Introduction Creating a thread – the Thread class Creating a thread – the Thread class Starting and running threads Starting and running threads Thread states Thread states Thread priorities and scheduling Thread priorities and scheduling Thread synchronisation Thread synchronisation The Runnable interface The Runnable interface

Introduction You are used to computers (operating systems) being multi-tasking You are used to computers (operating systems) being multi-tasking  You can browse the web whilst editing your Java programs! Multi-tasking refers to an operating system running several processes concurrently Multi-tasking refers to an operating system running several processes concurrently  Each process has its own completely independent data  Multi-tasking is difficult to incorporate in application programs requiring system programming primitives

Java is the only commonly used programming language that enables concurrency to be implemented within application programs Java is the only commonly used programming language that enables concurrency to be implemented within application programs  Multi-threading An application program can be written as a set of threads which run concurrently (in parallel) An application program can be written as a set of threads which run concurrently (in parallel) A thread is different from a process in that threads share the same data A thread is different from a process in that threads share the same data  Switching between threads involves much less overhead than switching between programs  Sharing data can lead to programming complications (for example in reading/writing databases)

Creating threads – the Thread class To run a piece of code in a separate thread it is placed in the run() method of a class which extends Thread To run a piece of code in a separate thread it is placed in the run() method of a class which extends Thread class ThreadApp extends Thread { public void run() { // This code runs in a new thread } public static void main(String args[]) { ThreadApp m=new ThreadApp(); m.start(); }

The Thread class has a start() method which automatically calls run() The Thread class has a start() method which automatically calls run() main() thread new thread new thread created

Example The following simple application creates separate threads to update a count within a window The following simple application creates separate threads to update a count within a window 0Stuff/ThreadTestApplet/ThreadTestApplet.html 0Stuff/ThreadTestApplet/ThreadTestApplet.html 0Stuff/ThreadTestApplet/ThreadTestApplet.html 0Stuff/ThreadTestApplet/ThreadTestApplet.html

Main thread Create counter thread …..

Class Counter stores the current count and a frame to display the text Class Counter stores the current count and a frame to display the text It has a a run() method which simply slowly counts upwards within a separate thread It has a a run() method which simply slowly counts upwards within a separate thread class Counter extends Thread { public Counter(LabelFrame frame) {…} public void run() {..} private int count=0; private LabelFrame labelFrame; }

public void run() { try { do { count++; labelFrame.getPanel().setCount(count); sleep(10); labelFrame.repaint(); } while (true); } catch(InterruptedException e){} }

public class ThreadTestFrame extends JFrame { private int nthreads=0; public ThreadTestFrame() { Container contentPane=getContentPane(); JPanel p=new JPanel(); addButton(p,"Start Count", new ActionListener() { public void actionPerformed(ActionEvent evt) { // Select frame colour Counter c=new Counter(new LabelFrame(color)); c.start(); nthreads++; } }); contentPane.add(p,"South"); } public void addButton(Container c, String title, ActionListener a) { // Add button to a panel } }

The sleep(int t) method is a static method of Thread which puts the currently running thread to sleep for t milliseconds The sleep(int t) method is a static method of Thread which puts the currently running thread to sleep for t milliseconds  This allows other counters running in separate threads to resume counting  sleep() throws an exception when the thread is interrupted by another thread wanting to run – hence the try-catch clause

Each Counter object is created in the actionPerformed() method of the button attached to the outer frame (applet) Each Counter object is created in the actionPerformed() method of the button attached to the outer frame (applet)  The thread is started by calling the start() method  The number of threads started is counted in the nthreads variable and the frame colour selected accordingly

Thread states A thread can exist in one of 4 states A thread can exist in one of 4 states  new  The thread has just been created  runnable  The start() method has been called for the thread and it is now ready to run

 blocked  Either the sleep() method has been called  The thread has blocked on an I/O operation  The thread has called the wait() method  The thread tries to lock an object that is locked by another thread  dead  Either the run() method for the thread has completed or an uncaught exception has terminated the run() method

new runnable blocked dead start() sleep() run() method exits block on I/O Done sleeping wait() notify() I/O complete

Thread priorities and scheduling Every thread has a priority which can be increased or decreased by calling the setPriority() method Every thread has a priority which can be increased or decreased by calling the setPriority() method  Thread.MIN_PRIORITY is the minimum priority (defined 1)  Thread.MAX_PRIORITY is the maximum priority (defined as 10)  When a thread is created, it is given a priority of 5 – defined as Thread.NORM_PRIORITY

Thread scheduling Differs depending on the operating system Differs depending on the operating system  Windows  Each thread is given a timeslice after which it is pre-empted by another thread of higher or equal priority  UNIX, LINUX  A thread can only be pre-empted by a thread of higher priority. If one is not available, the thread runs to completion In both cases, lower priority threads can only run if all higher priority threads are blocked In both cases, lower priority threads can only run if all higher priority threads are blocked

Runnable threadsBlocked threads P=5 Blocks Un-blocks P=5P=3

Thread synchronisation Threads need to share access to objects and may update shared objects Threads need to share access to objects and may update shared objects  For example multiple threads may access a database for an online flight booking system  One thread may be updating a database entry whilst another is reading it may lead to problems We can synchronise threads so that they must complete their action before another thread is scheduled We can synchronise threads so that they must complete their action before another thread is scheduled  We do this by tagging methods as synchronized  When a synchronised method is being executed, the object is locked and no other method can access the object until the method completes

Unsynchronised threads Thread 1Thread 2 Update database Read database Pre-empt

Synchronised threads Thread 1Thread 2 Update database Read database

Example – An online seat reservation system Seats for a concert can be reserved through booking agents Seats for a concert can be reserved through booking agents The processing for each booking agent runs in a separate thread – possibly on a different processor The processing for each booking agent runs in a separate thread – possibly on a different processor Each booking transaction must check seat availability before reserving the seat Each booking transaction must check seat availability before reserving the seat

Seat reservation database Booking agent 1 Booking agent 2 ….. Booking agent n Check availability Reserve seat Check availability Reserve seat Check availability Reserve seat

Pseudo-code for booking a seat Pseudo-code for booking a seat if seat n is available book seat n; Code not atomic Code not atomic  For an unsynchronised thread it can be pre- empted by another thread after the if statement  The thread might think the seat is available but it then might be booked by the pre-empting thread  The seat will be double booked /SeatBookingApplet/SeatBookingApplet.html /SeatBookingApplet/SeatBookingApplet.html /SeatBookingApplet/SeatBookingApplet.html /SeatBookingApplet/SeatBookingApplet.html

Booking agent 1 Booking agent 2 check availability book seat pre-empted check availability book seat re-schedule Unsynchronised threads

2 main classes 2 main classes  SeatBookings  Contains the seat booking information (just a simple array)  Contains isAvailable() and bookSeat() methods which access the seat booking information  BookingAgent  Extends Thread and thus contains a run() method  Contains a reference to a SeatBookings object which is thus a shared object between all of the BookingAgent objects The code is as follows (code to output to frames has been omitted) The code is as follows (code to output to frames has been omitted)

class SeatBookings { private int[] seats; public SeatBookings(int[] s) { seats=s; } public boolean isAvailable(int seatno) { return (seats[seatno]==0); } public void bookSeat(int seatno) { if (isAvailable(seatno)) seats[seatno]++; } public boolean doubleBooked(int seatno) { return (seats[seatno]>1); }

class BookingAgent extends Thread { private SeatBookings sb;// shared object public BookingAgent(SeatBookings s) { sb=s; } public void run() { do { int seatno=(int)(Math.random()*20000); sb.bookSeat(seatno); if (sb.doubleBooked(seatno)) // generate a warning dialog } while (true); }

We can synchronise the threads by tagging the SeatBookings.bookSeat() method as synchronized We can synchronise the threads by tagging the SeatBookings.bookSeat() method as synchronized  The SeatBookings object is then locked by the thread which has current access SeatBookings object Booking agent 1 Booking agent 2 locked locked out

Synchronised threads Booking agent 1Booking agent 2 check availability book seat check availability book seat check availability book seat

Simple change to code Simple change to code class SeatBookings {. public synchronized void bookSeat(int seatno) { … }. } No double booking then takes place No double booking then takes place  Stuff/SeatBooking%20SynchApplet/SeatBook ingSynchApplet.html Stuff/SeatBooking%20SynchApplet/SeatBook ingSynchApplet.html Stuff/SeatBooking%20SynchApplet/SeatBook ingSynchApplet.html

The Runnable interface So far, to develop classes which support multi- threading we have extended the Thread class and overwritten the run() method So far, to develop classes which support multi- threading we have extended the Thread class and overwritten the run() method This causes problems if the class is already extended from another class This causes problems if the class is already extended from another class  Java doesn’t support multiple inheritance MyClass SuperClassThread

This is generally a problem if we want outer containers (JFrame or JApplet) objects to support multi-threading This is generally a problem if we want outer containers (JFrame or JApplet) objects to support multi-threading The simple solution is to make our class implement the Runnable interface The simple solution is to make our class implement the Runnable interface  It would then need to implement a run() method in the usual way class myClass extends superClass implements Runnable { … public void run() {…} }

A thread is created and MyClass.run() started as follows : A thread is created and MyClass.run() started as follows : MyClass myObject=new Myclass(); Thread t=new Thread(myObject); t.start(); The call to Thread(Runnable r) creates a thread The call to Thread(Runnable r) creates a thread  Calling Thread.start() then automatically calls the run() method of the Runnable object

Example – a counter applet We will create an applet which has 2 buttons We will create an applet which has 2 buttons  Start Count  Reset Count The thread to do the counting must run in a separated thread to the one which handles button presses The thread to do the counting must run in a separated thread to the one which handles button presses  Swing programs create an event dispatch thread which handles all the events generated by the program (including window events for repainting graphics)

Event dispatch thread Main threadCounting thread Construct applet container show container exits Start button pressed Start counting … Reset button pressed Reset count

The following applet doesn’t use a separate thread to do the counting The following applet doesn’t use a separate thread to do the counting  Button press events or repainting events can’t be handled once counting begins  0Stuff/CounterApplet/CounterApplet.html 0Stuff/CounterApplet/CounterApplet.html 0Stuff/CounterApplet/CounterApplet.html  The following applet generates a separate thread for the counter  0Stuff/RunnableTestApplet/RunnableTestAp plet.html 0Stuff/RunnableTestApplet/RunnableTestAp plet.html 0Stuff/RunnableTestApplet/RunnableTestAp plet.html

public class RunnableTestApplet extends JApplet implements Runnable { public void init() { // Generate outer container … addButton(p,"Start Count", new ActionListener() { public void actionPerformed(ActionEvent evt) { startCount(); } }); addButton(p,"Reset Count", new ActionListener() { public void actionPerformed(ActionEvent evt) { resetCount(); } }); // Add buttons to container … }

public void startCount() { runner=new Thread(this); runner.start(); } public void resetCount() { runner.interrupt(); count=0; // repaint frame } public void run() { while (!Thread.interrupted()) { count++; // repaint frame } private Thread runner; }// end of class RunnableTestApplet

And finally….. Multi-threading is complex Multi-threading is complex Make sure you understand the code for the first example we looked at Make sure you understand the code for the first example we looked at Run the applet from my web site and scrutinize the code Run the applet from my web site and scrutinize the code Multi-threading is the basis of many applications and, as we have seen, particularly crucial in graphical/event driven programming Multi-threading is the basis of many applications and, as we have seen, particularly crucial in graphical/event driven programming We have only scratched the surface. For an excellent and thorough description of threading see We have only scratched the surface. For an excellent and thorough description of threading see In the last lab exercise, you will have the opportunity to write a multi-threading server for a network-based game! In the last lab exercise, you will have the opportunity to write a multi-threading server for a network-based game!