SUMMARY INTRODUCTION CONCURRENT PROGRAMMING THREAD S BASICS. Introduction Thread basics. Thread states. Sequence diagrams

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SUMMARY CONCURRENT PROGRAMMING THREAD S BASICS PROGRAMMAZIONE CONCORRENTE E DISTR.

1 SUMMARY CONCURRENT PROGRAMMING THREAD S BASICS PROGRAMMAZIONE CONCORRENTE E DISTR. Introduction Thread basics Thread properties Thread states Thread interruption Sequence diagrams Università degli Studi di Padova Dipartimento di Matematica Corso di Laurea in Informatica, A.A rcardin@math.unipd.it 2 INTRODUCTION 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 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 3 4

INTRODUCTION INTRODUCTION Threads are lighter than processes, even so their creation is still timeconsuming Every thread could be reused to execute different tasks Thread 2 Thread 3 Threads share the

programmed instructions that can be managed independently. Multiple thread can exist within the same process, executing concurrently and share resources such as memory.

2 INTRODUCTION INTRODUCTION Threads are lighter than processes, even so their creation is still timeconsuming Every thread could be reused to execute different tasks Thread 2 Thread 3 Threads share the same memory chunks Threads are a mechanism provided by the OS Memory Thread 1 Process Each process can execute many threads 5 6 DEFINITION THREAD BASICS 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 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 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 public interface Runnable { void run(); s are short lived, so you don t waste the time to start a thread Runnable task = () -> { int i = 0; while (i < 1000) i++; new Thread(task).start(); // A thread running a task 7 8

THREAD BASICS 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

!! Do not call the run method on Thread or Runnable instances The task is merely executed in the same thread 9 10 THREAD BASICS THREAD PROPERTIES The main method executes in the main thread public

3 THREAD BASICS 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 class MyThread extends Thread { public void run() { // task code, don t do this!!! Do not call the run method on Thread or Runnable instances The task is merely executed in the same thread 9 10 THREAD BASICS THREAD PROPERTIES The main method executes in the main thread public class Example { public static void main(string[] args) { // This code runs inside 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 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

Resource creation, scheduling run method is invoked It may or not actually be running DO NOT CALL RUN METHOD DIRECTLY!

4 THREAD STATES THREAD STATES 6 thread states New Runnable Blocked Waiting Time waiting Terminated Use getstate method Thread.State Runnable Running No resources associated 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! 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 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 while (!Thread.currentThread().isInterrupted() && more work to do) { // do more work 15 16

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

join(1000l); catch(interruptedexception e) { // thread was interrupted during sleep finally { // cleanup, if required Implementing collaborative preemption Thread.

5 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 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 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 while ( /* more work to do */ ) { // do more work Thread.yield(); When to use yield()? Practically NEVER Use Thread.sleep() (requires some self computation) Use synchronization mechanisms if waiting for a process or a resource 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! try { while ( /* more work to do */ ) { // do more work Thread.sleep(delay); catch(interruptedexception e) { // thread was interrupted during sleep finally { // cleanup, if required 19 20

6 SEQUENCE DIAGRAMS How can we reason about thread visually? UML gives use sequence diagrams SEQUENCE DIAGRAMS Definition A sequence diagram describes the cooperation between a group of objects that have to interact with each other to fulfill an objective Partecipant Life line Time passing Find message Message Object creation Internal call Return EXAMPLES REFERENCES snippets Chap. 14 «Multithreading», Core Java Volume I Fundamentals, Cay Horstmann, Gary Cornell, 2012, Prentice Hall Thread.yield What are the main uses of yield(), and how does it differ from join() and interrupt()? are the mainuses 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 23 24

SUMMARY FUTURES CALLABLES CONCURRENT PROGRAMMING THREAD S ADVANCED CONCEPTS

SUMMARY FUTURES CALLABLES CONCURRENT PROGRAMMING THREAD S ADVANCED CONCEPTS SUMMARY CONCURRENT PROGRAMMING THREAD S ADVANCED CONCEPTS Callable tasks Futures Executors Executor services Deadlocks PROGRAMMAZIONE CONCORRENTE E DISTR. Università degli Studi di Padova Dipartimento