SUMMARY FUTURES CALLABLES CONCURRENT PROGRAMMING THREAD S ADVANCED CONCEPTS

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1 SUMMARY CONCURRENT PROGRAMMING THREAD S ADVANCED CONCEPTS Callable tasks Futures Executors Executor services Deadlocks PROGRAMMAZIONE CONCORRENTE E DISTR. Università degli Studi di Padova Dipartimento di Matematica Corso di Laurea in Informatica, A.A rcardin@math.unipd.it 2 CALLABLES A Callable is a Runnable, that returns a value The Callable type is parametrized on the type of its return value public interface Callable<V> { // The method can throw an exception, // unlike Runnable run method public V call() throws Exception; } 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 A Future represents a computation whose result will be available at some future time Start the computation, give someone the Future object, and forget about it To obtain the result a synchronization is needed public interface Future<V> { V get() throws...; V get(long timeout, TimeUnit unit) throws...; void cancel(boolean mayinterrupt); boolean iscancelled(); boolean isdone(); } The get method blocks until the result is available Or until a timeout has been reached 3 4

2 Using FutureTask is possible to run a Callable, obtaining a Future as result Adapter of the Runnable and Future interfaces Using FutureTask is possible to run a Callable using a Thread Callable<Integer> mycomputation =...; FutureTask<Integer> task = new FutureTask<Integer>(myComputation); Thread t = new Thread(task); // it's a Runnable t.start(); //... Integer result = task.get(); // it's a Future Exception semantics ExecutionException: error during execution CancellationException: task was cancelled 5 6 A Future have some interesting characteristics Immutable Once a future is completed, it will be completed forever Future<Integer> future = /* Initialization */ ; System.out.println(future.get()); // Blocks and print 42 System.out.println(future.get()); // Prints 42 again Lets treat asynchronous programming in a synchronous way Simplify the division of complex task into smaller ones, that can be executed concurrently r producesomething startdoingsomething dosomethingwithresult 7 8

3 Usually it doesn t make sense to have a one toone relationship between a task and a thread Thread 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 operation When tasks are short lived, run many of them on the same thread When 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 Executors are implementation of thread pools An homogeneous pool of worker threads Amortizes thread creation and teardown Improves responsiveness, due to lack of task execution s delay Thread pools execution using static factory methods Each method return an executor instance that implements a specific execution policy // Create the thread pool with a specified execution policy Executor 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 pool executors.execute(hellos); executors.execute(goodbyes); 9 10 An Executor executes tasks, choosing the threads on which to run them You have not the full control on thread life cycle Runnable task = new Runnable() { /* Some task */ }; Executor executor = // Get an instance to an executor executor.execute(task); // A thread is choosen to execute the task Based on the producer / consumer pattern Activities produce tasks, threads consume tasks Decoupling of task submission from task execution Simplier changing of execution policy What, where, when, and how of task execution 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? 11 12

4 Available executors policies Method Description newcachedthreadpool New thread are created as needed; idle threads are kept for 60 seconds newfixedthreadpool 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 EXECUTOR SERVICES EXECUTOR SERVICES To execute a Callable, use an instance of the ExecutorService interface Previous static factory methods return such instances 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) Tipical workflow 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 Executor lifecycle Derived from the interface of ExecutorService type Shutting down Running Terminated Executors are created in running state. In this state an executor accepts new tasks and schedules them for execution This 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 15 16

5 DEADLOCKS 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 R 3 C A R 2 R 1 B Also known as deadly embrace: A needs a resource R hold by B, B a resource hold by C, an so on... The JVM cannot detect deadlock, differently from database systems 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 nonshareable 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 DEADLOCKS EXAMPLES snippets 19 20

6 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 21