Understanding Multithreading and Synchronization in Java, Lecture notes of Java Programming

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Multithreading

Chapter Goals

• To understand how multiple threads can execute in parallel
• To learn how to implement threads
• To understand race conditions and deadlocks
• To be able to avoid corruption of shared objects by using locks

and conditions

• To be able to use threads for programming animations

Running a Thread

• Implement a class that implements the Runnable interface:

public interface Runnable

void run();

• Place the code for your task into the run method of your class:

public class MyRunnable implements Runnable

public void run()

Task statements

Running a Thread

• Create an object of your subclass:

Runnable r = new MyRunnable();

• Construct a Thread object from the runnable object:

Thread t = new Thread(r);

• Call the start method to start the thread:

t.start();

GreetingRunnable Outline

public class GreetingRunnable implements Runnable

private String greeting;

public GreetingRunnable(String aGreeting)

greeting = aGreeting;

public void run()

Task statements

Thread Action for GreetingRunnable

• Print a time stamp
• Print the greeting
• Wait a second

Running Threads

• sleep puts current thread to sleep for given number of

milliseconds:

Thread.sleep(milliseconds)

• When a thread is interrupted, most common response is to

terminate run

Generic run method

public void run()

try

Task statements

catch (InterruptedException exception)

Clean up, if necessary

ch20/greeting/GreetingRunnable.java (cont.)

22 public void run() 23 { 24 try 25 { 26 for (int i = 1; i <= REPETITIONS; i++) 27 { 28 Date now = new Date(); 29 System.out.println(now + " " + greeting); 30 Thread.sleep(DELAY); 31 } 32 } 33 catch (InterruptedException exception) 34 { 35 } 36 } 37 }

To Start the Thread

• Construct an object of your runnable class:

Runnable t = new GreetingRunnable("Hello World");

• Then construct a thread and call the start method:

Thread t = new Thread(r);

t.start();

ch20/greeting/GreetingThreadRunner.java (cont.)

Program Run:

Mon Dec 28 12:04:46 PST 2009 Hello, World! Mon Dec 28 12:04:46 PST 2009 Goodbye, World! Mon Dec 28 12:04:47 PST 2009 Hello, World! Mon Dec 28 12:04:47 PST 2009 Goodbye, World! Mon Dec 28 12:04:48 PST 2009 Hello, World! Mon Dec 28 12:04:48 PST 2009 Goodbye, World! Mon Dec 28 12:04:49 PST 2009 Hello, World! Mon Dec 28 12:04:49 PST 2009 Goodbye, World! Mon Dec 28 12:04:50 PST 2009 Hello, World! Mon Dec 28 12:04:50 PST 2009 Goodbye, World! Mon Dec 28 12:04:51 PST 2009 Hello, World! Mon Dec 28 12:04:51 PST 2009 Goodbye, World! Mon Dec 28 12:04:52 PST 2009 Goodbye, World! Mon Dec 28 12:04:52 PST 2009 Hello, World! Mon Dec 28 12:04:53 PST 2009 Hello, World! Mon Dec 28 12:04:53 PST 2009 Goodbye, World! Mon Dec 28 12:04:54 PST 2009 Hello, World! Mon Dec 28 12:04:54 PST 2009 Goodbye, World! Mon Dec 28 12:04:55 PST 2009 Hello, World! Mon Dec 28 12:04:55 PST 2009 Goodbye, World!

Thread Scheduler

• Thread scheduler: runs each thread for a short amount of time

(a time slice )

• Then the scheduler activates another thread
• There will always be slight variations in running times -

especially when calling operating system services (e.g. input

and output)

• There is no guarantee about the order in which threads are

executed

Self Check 20.

What would be the result of the program if the main method

called

r1.run();

r2.run();

instead of starting threads?

Answer: The first call to run would print ten “Hello”

messages, and then the second call to run would print ten

“Goodbye” messages

Terminating Threads

• A thread terminates when its run method terminates
• Do not terminate a thread using the deprecated stop method
• Instead, notify a thread that it should terminate:

t.interrupt();

• interrupt does not cause the thread to terminate – it sets a

boolean variable in the thread data structure