Java - Concurrent programming - Thread's basics

CONCURRENT PROGRAMMINGTHREAD’S BASICSPROGRAMMAZIONE CONCORRENTE E DISTR.Università degli Studi di Padova

Dipartimento di Matematica

Corso di Laurea in Informatica, A.A. 2015 – [email protected]

2Programmazione concorrente e distribuita

SUMMARY Introduction Thread basics

Thread properties Thread states

Thread interruption Sequence diagrams

Riccardo Cardin


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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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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INTRODUCTION

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INTRODUCTION

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Process

Mem

ory

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


DEFINITION

Multithread programmingA 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


THREAD BASICS Threads in Java are the primitives that actually

execute tasksRunnable 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


THREAD BASICS You should decouple the task that is to be run in

parallel from the mechanism of running itYou 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 Riccardo Cardin

class MyThread extends Thread { public void run() { // task code, don’t do this!!! }}


THREAD BASICS

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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 typeUser threads

JVM stops when there are no more user thread executingDeamon 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 }}


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 threadsUse setDeamon method

Its only role is to serve other threads When only deamon threads remain, the JVM exits

Handlers for uncaught exceptionsThe run method cannot throw any checked ex.

Install an UncaughtExceptionHandler to manage ex.

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THREAD STATES 6 thread states

NewRunnableBlockedWaitingTime waitingTerminated

Use getState method

Thread.State

No resources associated

Runnable ≠ Running


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);}


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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THREADS INTERRUPTION A thread terminates when it’s run method:

Returns by executing a return statementAfter executing the last statement in method body If an unhandled exception occurs

It is possible to send an interruption requestUse 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}


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}


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 resourceRiccardo Cardin

while ( /* more work to do */ ) { // do more work Thread.yield();}


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}


THREADS INTERRUPTION

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SEQUENCE DIAGRAMS How can we reason about thread visually?

UML gives use sequence diagrams

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Partecipant

Time passing

Life line


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 messageInternal

call

Return

Object creation


EXAMPLES

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https://github.com/rcardin/pcd-snippets


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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http://www.javamex.com/tutorials/threads/yield.shtml

http://www.javamex.com/tutorials/threads/yield.shtml

http://stackoverflow.com/questions/6979796/what-are-the-main-uses-of-yield-and-how-does-it-differ-from-join-and-interr



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