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2 Threads
• A Thread is a section of code executed independently from
other threads within a single program.
• Threads are lightweight processes (LWP): All threads within
the same process share the same address space.
• Threads can execute in parallel on a multi-core or
multi-processor system.
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2.1 Java Thread: Interface or Inheritance
There are 2 ways to define and start a Thread in Java:
• by implementing the Runnable interface,
• by subclassing (extending) the Thread class.
The main thread is started in the main method. All other threads
begin their execution in the run method.
Threads are terminated when they leave their run method.
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p u b l i c c l a s s ThreadEx implements Runnable {
@Override
p u b l i c v o i d run ( ) {
...
}
p u b l i c s t a t i c v o i d main ( String args [ ] ) {
( new Thread ( new ThreadEx ( ) ) ) . start ( ) ;
}
}
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2.2 Python Thread: Instance of Thread Class
Thread creation by passing a callable object to the constructor:
def thread_function ( p ) :
..
x = threading . Thread ( target=thread_function , args = ( 1 , ) )
x . start ( )
Alternative: overriding the run method in a subclass.
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2.3 Thread Interference: Race Condition
Interference happens when two operations, running in different
threads, but acting on the same data, interleave.
This means that the two operations consist of multiple steps, and
the sequences of steps overlap.
p u b l i c c l a s s Account {
p r i v a t e double balance = 0 ;
p u b l i c v o i d deposit ( double amount ) {
balance += amount ;
}
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p u b l i c boolean withdraw ( double amount ) {
i f ( balance − amount >= 0 ) {
// do some c o m p l i c a t e d p r o c e s s i n g
...
balance −= amount ;
return true ;
}
return f a l s e ;
}
}
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2.4 Synchronization
Race conditions can be prevented by using a mutual exclusion to
protect critical sections against concurrent access. The first thread
that enters a critical section grabs the lock. Any subsequent thread
will have to wait until the lock is released.
Java: synchronized methods and statements,
Python: threading.Lock object
lock = threading . Lock ( )
with lock :
...
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2.5 Deadlock
A deadlock occurs when multiple threads are waiting for an action
that only one of the others can perform:
• Thread 1 holds a lock on Object A and is waiting for the lock
on Object B,
• Thread 2 holds the lock on Object B and is waiting for the
lock on Object A.
Neither thread will ever acquire the second lock or release the first
lock. They will simply wait forever.
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2.6 Conclusion
Adantages of using multiple threads:
Responsiveness: Making the user interface more responsive
when performing long tasks,
Performance: Exploiting multiprocessor systems to handle
multiple tasks in parallel,
Design: Simplifying modeling of simulations or agent-based
systems, performing asynchronous or background processing
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Disadvantage:
Unpredictable Behaviour: concurrent data access can produce
race conditions and deadlocks which are difficult to debug.
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2.7 Exercises
1. Make the Java/Python class Account thread-safe.
2. Implement a console based multithreaded counter that
increments its counter value by 1 starting at 0 in 1 second
intervals and prints the resulting value in the console window.
When the user hits the ENTER key the counter value should
by decremented by 1. Each subsequent ENTER-Key event
should reverse the direction.
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2.8 Further Information
• Java Tutorials: Concurrency
docs.oracle.com/javase/tutorial/essential/concurrency
• Introduction to Java Threads
developer.ibm.com/technologies/java/tutorials/j-threads/
• Python threading – Thread-based parallelism
https://docs.python.org/3.8/library/threading.html
• An Intro to Threading in Python
realpython.com/intro-to-python-threading/
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