1. ITEC324 Principle of CS III
Chapter 9 (Horstmann’s Book)
Multithreading
Hwajung Lee
ITEC324 Principle of CS III

2. Chapter Topics
Thread Basics
Thread Synchronization

3. Threads Thread: program unit that is executed independently
Multiple Threads: Program units what can be executed in parallel.
Multiple threads run simultaneously.
When executes
Multiprocessor computers: threads actually run in parallel OR
Illusion of threads running in parallel.
That is, Virtual machine executes each thread for short time slice. Thread scheduler activates, deactivates threads.

4. Threads and the Runnable Interface Type (1)
How to implement Running Threads
Define class that implements Runnable interface. Runnable has one method. void run()
Place thread action into run() method
Construct an object of the class which implements the Runnable Interface
Construct an object of Thread class
start() thread

5. Threads and the Runnable Interface Type (2)
public class MyRunnable implements Runnable { public void run() { thread action } ... Runnable r = new MyRunnable(); Thread t = new Thread(r); t.start();

6. Thread Example Run two threads in parallel Each thread
prints 10 greetings
for (int i = 1; i <= 10; i++) { System.out.println(i + ": " + greeting); Thread.sleep(100); }
After each printout, sleep for 100 milliseconds
All threads should occasionally yield control
sleep() throws InterruptedException

7. Thread Example (1) Ch9/greeting/GreetingProducer.java
Ch9/greeting/ThreadTester.java

8. Thread Example
Sample Output (Note: output not exactly interleaved.)
1: Hello, World! 1: Goodbye, World! 2: Hello, World! 2: Goodbye, World! 3: Hello, World! 3: Goodbye, World! 4: Hello, World! 4: Goodbye, World! 5: Hello, World! 5: Goodbye, World! 6: Hello, World! 6: Goodbye, World! 7: Hello, World! 7: Goodbye, World! 8: Goodbye, World! 8: Hello, World! 9: Goodbye, World! 9: Hello, World! 10: Goodbye, World! 10: Hello, World!
The thread scheduler gives no guarantee about the order in which threads are executed. Moreover, there will always be slight variations in running times, especially when calling operating system services (such as input and output). Thus, you should expect that the order in which each thread gains control appears to be somewhat random.

9. Starting Two Threads

10. Thread States Each thread has Thread states: thread state priority new
before start() called
runnable
blocked
dead
after run() method exits

11. Blocked Thread State Reasons for entering the blocked state:
Sleeping
Waiting for I/O
Waiting to acquire lock (later)
Waiting for condition (later)
Unblocks only if the reason for the blocking goes away

12. Scheduling Threads Scheduler activates a new (next) thread if
a thread has completed its time slice
a thread has blocked itself
a thread with higher priority has become runnable
Scheduler determines a new thread to run by
looking only at all runnable threads, and
picking one with the highest priority value. (The priority values are system-dependant and not adjustable by an application programmer.) OR
Picking one at random or using a round-robin scheme to gives each thread a chance.

13. Terminating Threads (1)
Normal way to terminate a thread
Thread terminates when run() exits
However, you sometimes need to terminate running thread
(Ex) you may have several threads attempting to find a solution to a problem. As soon as the first one has succeeded, you can terminate the other ones.

14. Thread Synchronization
Example
Two producers, one consumer
Each producer thread inserts 100 greetings into the same queue.
Each consumer thread removes all of the greetings in the queue.

15. Producer Thread In the try{ } block
int i = 1; while (i <= greetingCount) { if (!queue.isFull()) { queue.add(i + ": " + greeting); i++; } Thread.sleep((int)(Math.random() * DELAY)); }

16. Consumer Thread In the try{ } block int i = 1;
while (i <= greetingCount)
{ if (!queue.isEmpty()) { Object greeting = queue.remove(); System.out.println(greeting); i++; } Thread.sleep((int)(Math.random() * DELAY)); }

17. Expected Program Output
1: Hello, World! 1: Goodbye, World! 2: Hello, World! 3: Hello, World! : Goodbye, World! 100: Goodbye, World!

18. Why is Output Corrupted?
Sometimes, the program will corrupt the queue and not work correctly.
The consumer thread gets stuck and won’t complete. Even though 200 greetings were inserted in the queue, it can’t retrieve them all.
It may complete, but print the same greetings repeatedly.
Can you spot the problem?
Can see problem better when turning debugging on queue.setDebug(true);
Ch9/queue1/ThreadTester.java
Ch9/queue1/Producer.java
Ch9/queue1/Consumer.java
Ch9/queue1/BoundedQueue.java

19. Race Condition
Race Condition occurs if the effect of multiple threads on shared data depends on the order in which the threads are scheduled.
Multiple threads, in their race to complete their respective task, rush to win the race.

20. Race Condition Scenario
First thread calls add and executes elements[tail] = anObject;
First thread at end of time slice
Second thread calls add and executes elements[tail] = anObject; tail++;
Second thread at end of time slice
First thread executes tail++;

21. Resolve a Race Condition
To fix the race conditions, you need to ensure that only one thread manipulates the queue at any given moment.

22. Locks Locking Mechanism Two kinds of locks
Thread can temporarily acquire ownership of a lock
When another thread tries to acquire same lock, it is blocked.
When first thread releases the lock, other threads are unblocked and try again
Two kinds of locks
Objects of ReentrantLock class or another class implementing java.util.concurrent.Lock interface type in java.util.concurrent.locks packageLocks
Locks that are built into every Java object

23. Reentrant Locks
aLock = new ReentrantLock(); aLock.lock(); try { protected code } finally { aLock.unlock();
The finally clause ensures that the lock is unlocked even when an exception is thrown in the protected code.

24. Deadlocks
A deadlock occurs if no thread can proceed because each thread is waiting for another to do some work first.

25. Visualizing Locks Object = phone booth Thread = person
Locked object = closed booth
Blocked thread = person waiting for booth to open