This presentation introduces the Java concept of thread. It is the first part of a series of slides dedicated to concurrent programming. This slides introduces some basics concepts, such as: - Differences between processes and threads - Thread's basics - Thread's states - Introduction to UML sequence diagrams The presentation is took from the Java course I run in the bachelor-level informatics curriculum at the University of Padova.

1. CONCURRENT PROGRAMMING
THREAD’S BASICS
PROGRAMMAZIONE CONCORRENTE E DISTR.
Università degli Studi di Padova
Dipartimento di Matematica
Corso di Laurea in Informatica, A.A. 2015 – 2016
rcardin@math.unipd.it

2. Programmazione concorrente e distribuita
SUMMARY
 Introduction
 Thread basics
 Thread properties
 Thread states
 Thread interruption
 Sequence diagrams
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3. Programmazione concorrente e distribuita
INTRODUCTION
 Multitasking
 The ability to have more than a program working at
what seems like the same time
 The unity of this type of programming is a process
 A process has its own variables
 Communication between process is accomplished using
messages sent over a common channel (i.e. a socket)
 Very hard to accomplish and no so performant
 Types
 Cooperative: CPU control is left to the processes
 Time-sliced: the OS (scheduler) assigns a time slice to each
process
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4. Programmazione concorrente e distribuita
INTRODUCTION
 Multithread
 A programming model that allows multiple threads to
exists within the same context of a single process
 Every process will appear to do multiple tasks
 Threads share the same data and variables
 Every process as a main thread
 This thread can create other threads, called secondary
 Every thread as a priority order (1 .. 10)
 Types
 Cooperative
 Time-sliced: thread scheduling is left to the main thread, that
uses OS utilities
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5. Programmazione concorrente e distribuita
INTRODUCTION
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6. Programmazione concorrente e distribuita
INTRODUCTION
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Process
Memory
Thread 1
Task Task
Thread 2
Task
Thread 3
Task
Threads
share the
same
memory
chunks
Every thread
could be reused
to execute
different tasksThreads are
lighter than
processes, even
so their creation
is still time-
consuming
Each process
can execute
many threads
Threads are a
mechanism provided
by the OS

7. Programmazione concorrente e distribuita
DEFINITION
 Multithread programming
 A programming model that allows multiple threads to
exists within the same context of a single process
 Responsiveness
 Faster execution
 Lower resource consumption
 Parallelization
 Java has built-in support for concurrent programming
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A thread is the smallest sequence of programmed instructions that can
be managed independently. Multiple thread can exist within the same
process, executing concurrently and share resources such as memory.
- Wikipedia

8. Programmazione concorrente e distribuita
THREAD BASICS
 Threads in Java are the primitives that actually
execute tasks
 Runnable interface describes a task you want to
run, usually concurrently with others
 The code of the run method will be executed in a thread
 Tasks are short lived, so you don’t waste the time to start a
thread
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public interface Runnable {
void run();
}
Runnable task = () -> {
int i = 0;
while (i < 1000) i++;
}
new Thread(task).start(); // A thread running a task

9. Programmazione concorrente e distribuita
THREAD BASICS
 You should decouple the task that is to be run in
parallel from the mechanism of running it
 You can also define a subclass of Thread, but this
approach is no longer recommended
 If you have many task is to expensive create a single thread
for each one
 Do not call the run method on Thread or Runnable
instances
 The task is merely executed in the same thread
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class MyThread extends Thread {
public void run() {
// task code, don’t do this!!!
}
}

10. Programmazione concorrente e distribuita
THREAD BASICS
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11. Programmazione concorrente e distribuita
THREAD BASICS
 The main method executes in the main thread
 From the main thread they can be executed other
thread, called secondary
 They execute in pararrel wrt the main thread
 Threads can be of two type
 User threads
 JVM stops when there are no more user thread executing
 Deamon threads
 A deamon stops when its user thread stops
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public class Example {
public static void main(String[] args) {
// This code runs inside the main thread
}
}

12. Programmazione concorrente e distribuita
THREAD PROPERTIES
 Thread priorities
 Use setPriority method to give a thread a priority
 MIN_PRIORITY = 1 and MAX_PRIORITY = 10
 Don’t use priorities, they are too highly system-dependent
 Deamon threads
 Use setDeamon method
 Its only role is to serve other threads
 When only deamon threads remain, the JVM exits
 Handlers for uncaught exceptions
 The run method cannot throw any checked ex.
 Install an UncaughtExceptionHandler to manage ex.
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13. Programmazione concorrente e distribuita
THREAD STATES
 6 thread states
 New
 Runnable
 Blocked
 Waiting
 Time waiting
 Terminated
 Use getState
method
 Thread.State
13
No resources
associated
Runnable ≠
Running

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THREAD STATES
 New threads
 Just created with the new operator. Not yet running
 Runnable threads
 Once the start method is invoked.
 Resource creation, scheduling
 run method is invoked
 It may or not actually be running
 DO NOT CALL RUN METHOD DIRECTLY!
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public static void main(String[] args) {
// Not concurrent, but sequential
new MyThread().run();
for (int i = 0; i < 200; i++)
System.out.println("Cycle - " + i);
}

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THREAD STATES
 Blocked and Waiting threads
 Temporarily inactive
 A thread becomes blocked when it tries to acquire an
intrinsic object lock
 When a thread waits for another thread to notify of a
condition, it enters the waiting state
 The calling of a timeout parameters causes the thread to
enter the timed waiting (Thread.sleep)
 A thread waits for another thread to finish, calling the join
method on it
 Terminated threads
 It is not possible to reborn a thread in this state
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16. Programmazione concorrente e distribuita
THREADS INTERRUPTION
 A thread terminates when it’s run method:
 Returns by executing a return statement
 After executing the last statement in method body
 If an unhandled exception occurs
 It is possible to send an interruption request
 Use the interrupt method
 Thread states becomes interrupted, but thread is not
interrupted by the JVM
 Thread should check whether it has been interrupted
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while (!Thread.currentThread().isInterrupted() && more work to do) {
// do more work
}

17. Programmazione concorrente e distribuita
THREADS INTERRUPTION
 Waiting for another thread to finish
 Use join() or join(long millis)
 An instance of the joining thread must be available
 Passing a time interval to the method has the effect to limit
the waiting period to millis milliseconds
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Thread thread = new Thread(() -> {
// Do some heavy work
});
thread.start();
try {
// Waiting for at max 1 second the termination of thread
thread.join(1000L);
} catch(InterruptedException e) {
// thread was interrupted during sleep
} finally {
// cleanup, if required
}

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THREADS INTERRUPTION
 Implementing collaborative preemption
 Thread.yield() notifies the system that the current
thread is willing to "give up the CPU" for a while.
 Thread scheduler will select a different thread to run
 If no other thread is present, the statement has no effect
 When to use yield()? Practically NEVER
 Use Thread.sleep() (requires some self-computation)
 Use synchronization mechanisms if waiting for a process or
a resource
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while ( /* more work to do */ ) {
// do more work
Thread.yield();
}

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THREADS INTERRUPTION
 Interrupting a thread simply grabs its attention
 Use Thread.sleep(long time) to suspend
temporarily the thread
 If the interrupted thread was sleeping or waiting for
something, an InterruptedException is thrown
 ...and the thread status is cleared!
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try {
while ( /* more work to do */ ) {
// do more work
Thread.sleep(delay);
}
} catch(InterruptedException e) {
// thread was interrupted during sleep
} finally {
// cleanup, if required
}

20. Programmazione concorrente e distribuita
THREADS INTERRUPTION
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21. Programmazione concorrente e distribuita
SEQUENCE DIAGRAMS
 How can we reason about thread visually?
 UML gives use sequence diagrams
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Partecipant
Timepassing
Life line

22. Programmazione concorrente e distribuita
SEQUENCE DIAGRAMS
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A sequence diagram describes the cooperation between a group of
objects that have to interact with each other to fulfill an objective
Definition
Message
Find message
Internal call
Return
Object
creation

23. Programmazione concorrente e distribuita
EXAMPLES
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https://github.com/rcardin/pcd-snippets

24. Programmazione concorrente e distribuita
REFERENCES
 Chap. 14 «Multithreading», Core Java Volume I - Fundamentals, Cay
Horstmann, Gary Cornell, 2012, Prentice Hall
 Thread.yield
http://www.javamex.com/tutorials/threads/yield.shtml
 What are the main uses of yield(), and how does it differ from join()
and interrupt()?
http://stackoverflow.com/questions/6979796/what-are-the-main-
uses-of-yield-and-how-does-it-differ-from-join-and-interr
 Chap. 10 «Concurrent Programming», Core Java for the Impatient,
Cay Horstmann, 2015, Addison-Wesley
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