java - concurrent programming - thread's advanced concepts

CONCURRENT PROGRAMMINGTHREAD’S ADVANCED CONCEPTSPROGRAMMAZIONE 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 Callable tasks Futures Executors Executor services Deadlocks

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CALLABLES A Callable is a Runnable, that returns a value

The Callable type is parametrized on the type of its return value

Callable<Integer> represents an asynchronous computation that will produce an Integer value

The value computed is not directly available We need a type to represents a value that will be available in the

future...Represents a deferred computation

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public interface Callable<V> { // The method can throw an exception, // unlike Runnable run method public V call() throws Exception;}


FUTURES A Future represents a computation whose result

will be available at some future timeStart the computation, give someone the Future object,

and forget about it To obtain the result a synchronization is needed

The get method blocks until the result is available Or until a timeout has been reached

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public interface Future<V> { V get() throws . . .; V get(long timeout, TimeUnit unit) throws . . .; void cancel(boolean mayInterrupt); boolean isCancelled(); boolean isDone();}

Programmazione concorrente e distribuita

FUTURES Using FutureTask is possible to run a Callable, obtaining a Future as resultAdapter of the Runnable and Future interfacesUsing FutureTask is possible to run a Callable using

a Thread

Exception semantics ExecutionException: error during execution CancellationException: task was cancelled

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Callable<Integer> myComputation = . . .;FutureTask<Integer> task = new FutureTask<Integer>(myComputation);Thread t = new Thread(task); // it's a Runnablet.start();// . . .Integer result = task.get(); // it's a Future


FUTURES

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FUTURES A Future have some interesting characteristics

Immutable Once a future is completed, it will be completed forever

Lets treat asynchronous programming in a synchronous way Simplify the division of complex task into smaller ones, that

can be executed concurrently

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Future<Integer> future = /* Initialization */ ;System.out.println(future.get()); // Blocks and print 42System.out.println(future.get()); // Prints 42 again

produceSomething

startDoingSomething

doSomethingWithResult

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FUTURES

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EXECUTORS Usually it doesn’t make sense to have a one-to-

one relationship between a task and a threadThread is a mechanism for execution a sequence of

instructions (task) Creating a new thread means to ask some work to the OS, so

it is a time consuming operationWhen tasks are short lived, run many of them on the

same threadWhen tasks are computationally intensive, use one

thread per processor Avoid the overhead of context switching among threads

Anyway, all that you need is a thread pool

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EXECUTORS Executors are implementation of thread pools

An homogeneous pool of worker threadsAmortizes thread creation and teardown

Improves responsiveness, due to lack of task execution’s delayThread pools execution using static factory methods

Each method return an executor instance that implements a specific execution policy

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// Create the thread pool with a specified execution policyExecutor executor = Executors.newCachedThreadPool();Runnable hellos = new Runnable() { /* Say hello a lot of times */ };Runnable goodbyes = new Runnable() {/* Say hello a lot of times */ };// Submit task for execution to thread poolexecutors.execute(hellos);executors.execute(goodbyes);


EXECUTORS An Executor executes tasks, choosing the

threads on which to run themYou have not the full control on thread life cycle

Based on the producer / consumer pattern Activities produce tasks, threads consume tasks

Decoupling of task submission from task executionSimplier changing of execution policy

What, where, when, and how of task execution

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Runnable task = new Runnable() { /* Some task */ };Executor executor = // Get an instance to an executorexecutor.execute(task); // A thread is choosen to execute the task


EXECUTORS Execution policies

Dependent on the available computing resources and quality of service requirements In what thread will tasks be executed? In what order should tasks be executed (FIFO, LIFO, priority

order)? How many tasks may execute concurrently? How many tasks may be queued pending execution? If a task has to be rejected because the system is overloaded,

which task should be selected as the victim, and how should the application be notified?

What actions should be taken before or after executing a task?

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EXECUTORS

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Available executors policies

Method DescriptionnewCachedThreadPool New thread are created as needed;

idle threads are kept for 60 secondsnewFixedThreadPool The pool contains a fixed set of

threads; idle threads are kept indefinitely

newSingleThreadExecutor A «pool» with a single thread that executes the submitted tasks sequentially (similar to the Swing event dispatch thread)

newScheduledThreadPool A fixed-thread pool for scheduled execution

newSingleThreadScheduledExecutor A single-thread «pool» for scheduled execution


EXECUTORS

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EXECUTOR SERVICES To execute a Callable, use an instance of the ExecutorService interfacePrevious static factory methods return such instances

1° submit returns a Future containing the task itself Control of execution (call isDone, cancel, isCancelled)

2° submit acts like the first, but return result object 3° submit returns a Future of the result of the Callable task Method invokeAll executes all the input Callables

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Future<?> submit(Runnable task)Future<T> submit(Runnable task, T result)Future<T> submit(Callable<T> task)<T> List<Future<T>> invokeAll(Collection<? Extends Callable<T>> task)

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EXECUTOR SERVICES Executor lifecycle

Derived from the interface of ExecutorService type

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Running

Shutting down

Terminated

Executors are created in running state. In this state an executor accepts new tasksand schedules them for executionThis state is reached after shutdown method was called. No new task are accepted, but previously submitted task are allowed to complete. The shutdownNow method initiates an abrupt shutdown: no queued task not yet begun is started

Once all task are completed, the executor transitions to the terminated state


DEADLOCKS Multiple threads wait forever due to a cyclic

locking dependency If threads are nodes and relations of dependency are

edges, a cyclic graph means to have a deadlock

The JVM cannot detect deadlock, differently from database systems Riccardo Cardin

AR1

BR2

C

R3 Also known as deadly embrace:A needs a resource R hold by B, B a resource hold by C, an so on...


DEADLOCKS Deadlocks rarely manifest themeselves

immediatly (only in production under heavy load)Four conditions have to hold simultaneously (Coffman

conditions) Mutual exclusion: at least one resource must be held in a non-

shareable mode Hold and wait: a process is currently holding at least one

resource and requesting additional resources No preemption: a resource can be released only voluntarily Circular wait: a process must be waiting for a resource which is

being held by another process, which in turn is waiting for the first process to release the resource

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DEADLOCKS

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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 Chap. 6 «Task Execution», Java Concurrency in Practice, Brian

Goetz, 2006, Addison-Wesley Professional Chap. 10 «Avoiding Liveness Hazards», Java Concurrency in

Practice, Brian Goetz, 2006, Addison-Wesley Professional Chap. 10 «Concurrent Programming», Core Java for the Impatient,

Cay Horstmann, 2015, Addison-Wesley Deadlocks https://en.wikipedia.org/wiki/Deadlock

Riccardo Cardin

https://en.wikipedia.org/wiki/Deadlock

https://en.wikipedia.org/wiki/Deadlock

java - concurrent programming - thread's advanced concepts

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