Questions d'entretien sur Java Concurrency (+ réponses)

Cet article fait partie d'une série: • Questions d'entretien sur les collections Java

• Questions d’entretien sur Java Type System

• Questions d'entrevue sur la concurrence Java (+ réponses) (article actuel) • Questions d'entrevue sur la structure des classes Java et l'initialisation

• Questions d'entrevue Java 8 (+ réponses)

• Questions d’entrevue sur la gestion de la mémoire dans Java (+ réponses)

• Questions d'entrevue Java Generics (+ réponses)

• Questions d’entretien sur le contrôle de flux Java (+ réponses)

• Questions d’entretien sur les exceptions Java (+ réponses)

• Questions d'entrevue sur les annotations Java (+ réponses)

• Principales questions d'entrevue Spring Framework

1. Introduction

La concurrence en Java est l'un des sujets les plus complexes et avancés abordés lors des entretiens techniques. Cet article fournit des réponses à certaines des questions d'entrevue sur le sujet que vous pourriez rencontrer.

Q1. Quelle est la différence entre un processus et un thread?

Les processus et les threads sont des unités de concurrence, mais ils ont une différence fondamentale: les processus ne partagent pas une mémoire commune, contrairement aux threads.

Du point de vue du système d'exploitation, un processus est un logiciel indépendant qui s'exécute dans son propre espace de mémoire virtuelle. Tout système d'exploitation multitâche (ce qui signifie presque tous les systèmes d'exploitation modernes) doit séparer les processus en mémoire afin qu'un processus défaillant ne fasse pas glisser tous les autres processus en brouillant la mémoire commune.

Les processus sont donc généralement isolés et coopèrent par le biais d'une communication inter-processus qui est définie par le système d'exploitation comme une sorte d'API intermédiaire.

Au contraire, un thread fait partie d'une application qui partage une mémoire commune avec d'autres threads de la même application. L'utilisation de la mémoire commune permet de réduire les frais généraux, de concevoir les threads pour coopérer et d'échanger des données entre eux beaucoup plus rapidement.

Q2. Comment créer une instance de thread et l'exécuter?

Pour créer une instance d'un thread, vous avez deux options. Commencez par transmettre une instance Runnable à son constructeur et appelez start () . Runnable est une interface fonctionnelle, il peut donc être passé en tant qu'expression lambda:

Thread thread1 = new Thread(() -> System.out.println("Hello World from Runnable!")); thread1.start();

Thread implémente également Runnable , donc une autre façon de démarrer un thread est de créer une sous-classe anonyme, de remplacer sa méthode run () , puis d'appeler start () :

Thread thread2 = new Thread() { @Override public void run() { System.out.println("Hello World from subclass!"); } }; thread2.start();

Q3. Décrivez les différents états d'un thread et quand se produisent les transitions d'état.

L'état d'un Thread peut être vérifié à l'aide de la méthode Thread.getState () . Différents états d'un Thread sont décrits dans l' énumération Thread.State . Elles sont:

  • NOUVEAU - une nouvelleinstance de Thread qui n'a pas encore été démarrée via Thread.start ()
  • RUNNABLE - un thread en cours d'exécution. Il est appelé exécutable car à tout moment, il peut être en cours d'exécution ou attendre le prochain quantum de temps du programmateur de threads. Un NOUVEAU fil entre dans le RUNNABLE état lorsque vous appelez Thread.start () sur elle
  • BLOQUÉ - un thread en cours d'exécution est bloqué s'il doit entrer dans une section synchronisée mais ne peut pas le faire en raison d'un autre thread tenant le moniteur de cette section
  • WAITING - un thread entre dans cet état s'il attend qu'un autre thread effectue une action particulière. Par exemple, un thread entre dans cet état lors de l'appel de laméthode Object.wait () sur un moniteur qu'il contient, ou de laméthode Thread.join () sur un autre thread
  • TIMED_WAITING - identique à ce qui précède, mais un thread entre dans cet état après avoir appelé des versions chronométrées de Thread.sleep () , Object.wait () , Thread.join () et quelques autres méthodes
  • TERMINATED - un thread a terminé l'exécution de saméthode Runnable.run () et s'est terminé

Q4. Quelle est la différence entre les interfaces exécutables et appelables? Comment sont-ils utilisés?

L' interface Runnable a une seule méthode d' exécution . Il représente une unité de calcul qui doit être exécutée dans un thread séparé. L' interface Runnable ne permet pas à cette méthode de renvoyer une valeur ou de lever des exceptions non vérifiées.

L' interface Callable a une méthode d' appel unique et représente une tâche qui a une valeur. C'est pourquoi la méthode d' appel renvoie une valeur. Il peut également lever des exceptions. Callable est généralement utilisé dans les instances ExecutorService pour démarrer une tâche asynchrone, puis appeler l' instance Future renvoyée pour obtenir sa valeur.

Q5. Qu'est-ce qu'un fil de discussion démon, quels sont ses cas d'utilisation? Comment créer un fil de discussion démon?

Un thread démon est un thread qui n'empêche pas JVM de se fermer. Lorsque tous les threads non démon sont terminés, la JVM abandonne simplement tous les threads démons restants. Les threads de démon sont généralement utilisés pour effectuer certaines tâches de support ou de service pour d'autres threads, mais vous devez tenir compte du fait qu'ils peuvent être abandonnés à tout moment.

Pour démarrer un thread en tant que démon, vous devez utiliser la méthode setDaemon () avant d'appeler start () :

Thread daemon = new Thread(() -> System.out.println("Hello from daemon!")); daemon.setDaemon(true); daemon.start();

Curieusement, si vous exécutez ceci dans le cadre de la méthode main () , le message risque de ne pas être imprimé. Cela pourrait arriver si le thread main () se terminait avant que le démon n'arrive au point d'imprimer le message. En général , vous ne devriez pas faire des E / S dans le démon threads, car ils ne seront même pas en mesure d'exécuter leurs finalement blocs et fermer les ressources si elles étaient abandonnées.

Q6. Quel est l'indicateur d'interruption du thread? Comment pouvez-vous le définir et le vérifier? Quel est le lien avec l'exception interrompue?

L'indicateur d'interruption, ou état d'interruption, est un indicateur de thread interne qui est défini lorsque le thread est interrompu. Pour le définir, appelez simplement thread.interrupt () sur l'objet thread .

If a thread is currently inside one of the methods that throw InterruptedException (wait, join, sleep etc.), then this method immediately throws InterruptedException. The thread is free to process this exception according to its own logic.

If a thread is not inside such method and thread.interrupt() is called, nothing special happens. It is thread's responsibility to periodically check the interrupt status using static Thread.interrupted() or instance isInterrupted() method. The difference between these methods is that the static Thread.interrupted() clears the interrupt flag, while isInterrupted() does not.

Q7. What Are Executor and Executorservice? What Are the Differences Between These Interfaces?

Executor and ExecutorService are two related interfaces of java.util.concurrent framework. Executor is a very simple interface with a single execute method accepting Runnable instances for execution. In most cases, this is the interface that your task-executing code should depend on.

ExecutorService extends the Executor interface with multiple methods for handling and checking the lifecycle of a concurrent task execution service (termination of tasks in case of shutdown) and methods for more complex asynchronous task handling including Futures.

For more info on using Executor and ExecutorService, see the article A Guide to Java ExecutorService.

Q8. What Are the Available Implementations of Executorservice in the Standard Library?

The ExecutorService interface has three standard implementations:

  • ThreadPoolExecutor — for executing tasks using a pool of threads. Once a thread is finished executing the task, it goes back into the pool. If all threads in the pool are busy, then the task has to wait for its turn.
  • ScheduledThreadPoolExecutor allows to schedule task execution instead of running it immediately when a thread is available. It can also schedule tasks with fixed rate or fixed delay.
  • ForkJoinPool is a special ExecutorService for dealing with recursive algorithms tasks. If you use a regular ThreadPoolExecutor for a recursive algorithm, you will quickly find all your threads are busy waiting for the lower levels of recursion to finish. The ForkJoinPool implements the so-called work-stealing algorithm that allows it to use available threads more efficiently.

Q9. What Is Java Memory Model (Jmm)? Describe Its Purpose and Basic Ideas.

Java Memory Model is a part of Java language specification described in Chapter 17.4. It specifies how multiple threads access common memory in a concurrent Java application, and how data changes by one thread are made visible to other threads. While being quite short and concise, JMM may be hard to grasp without strong mathematical background.

The need for memory model arises from the fact that the way your Java code is accessing data is not how it actually happens on the lower levels. Memory writes and reads may be reordered or optimized by the Java compiler, JIT compiler, and even CPU, as long as the observable result of these reads and writes is the same.

This can lead to counter-intuitive results when your application is scaled to multiple threads because most of these optimizations take into account a single thread of execution (the cross-thread optimizers are still extremely hard to implement). Another huge problem is that the memory in modern systems is multilayered: multiple cores of a processor may keep some non-flushed data in their caches or read/write buffers, which also affects the state of the memory observed from other cores.

To make things worse, the existence of different memory access architectures would break the Java's promise of “write once, run everywhere”. Happily for the programmers, the JMM specifies some guarantees that you may rely upon when designing multithreaded applications. Sticking to these guarantees helps a programmer to write multithreaded code that is stable and portable between various architectures.

The main notions of JMM are:

  • Actions, these are inter-thread actions that can be executed by one thread and detected by another thread, like reading or writing variables, locking/unlocking monitors and so on
  • Synchronization actions, a certain subset of actions, like reading/writing a volatile variable, or locking/unlocking a monitor
  • Program Order (PO), the observable total order of actions inside a single thread
  • Synchronization Order (SO), the total order between all synchronization actions — it has to be consistent with Program Order, that is, if two synchronization actions come one before another in PO, they occur in the same order in SO
  • synchronizes-with (SW) relation between certain synchronization actions, like unlocking of monitor and locking of the same monitor (in another or the same thread)
  • Happens-before Order — combines PO with SW (this is called transitive closure in set theory) to create a partial ordering of all actions between threads. If one action happens-before another, then the results of the first action are observable by the second action (for instance, write of a variable in one thread and read in another)
  • Happens-before consistency — a set of actions is HB-consistent if every read observes either the last write to that location in the happens-before order, or some other write via data race
  • Execution — a certain set of ordered actions and consistency rules between them

For a given program, we can observe multiple different executions with various outcomes. But if a program is correctly synchronized, then all of its executions appear to be sequentially consistent, meaning you can reason about the multithreaded program as a set of actions occurring in some sequential order. This saves you the trouble of thinking about under-the-hood reorderings, optimizations or data caching.

Q10. What Is a Volatile Field and What Guarantees Does the Jmm Hold for Such Field?

A volatile field has special properties according to the Java Memory Model (see Q9). The reads and writes of a volatile variable are synchronization actions, meaning that they have a total ordering (all threads will observe a consistent order of these actions). A read of a volatile variable is guaranteed to observe the last write to this variable, according to this order.

If you have a field that is accessed from multiple threads, with at least one thread writing to it, then you should consider making it volatile, or else there is a little guarantee to what a certain thread would read from this field.

Another guarantee for volatile is atomicity of writing and reading 64-bit values (long and double). Without a volatile modifier, a read of such field could observe a value partly written by another thread.

Q11. Which of the Following Operations Are Atomic?

  • writing to a non-volatileint;
  • writing to a volatile int;
  • writing to a non-volatile long;
  • writing to a volatile long;
  • incrementing a volatile long?

A write to an int (32-bit) variable is guaranteed to be atomic, whether it is volatile or not. A long (64-bit) variable could be written in two separate steps, for example, on 32-bit architectures, so by default, there is no atomicity guarantee. However, if you specify the volatile modifier, a long variable is guaranteed to be accessed atomically.

The increment operation is usually done in multiple steps (retrieving a value, changing it and writing back), so it is never guaranteed to be atomic, wether the variable is volatile or not. If you need to implement atomic increment of a value, you should use classes AtomicInteger, AtomicLong etc.

Q12. What Special Guarantees Does the Jmm Hold for Final Fields of a Class?

JVM basically guarantees that final fields of a class will be initialized before any thread gets hold of the object. Without this guarantee, a reference to an object may be published, i.e. become visible, to another thread before all the fields of this object are initialized, due to reorderings or other optimizations. This could cause racy access to these fields.

This is why, when creating an immutable object, you should always make all its fields final, even if they are not accessible via getter methods.

Q13. What Is the Meaning of a Synchronized Keyword in the Definition of a Method? of a Static Method? Before a Block?

The synchronized keyword before a block means that any thread entering this block has to acquire the monitor (the object in brackets). If the monitor is already acquired by another thread, the former thread will enter the BLOCKED state and wait until the monitor is released.

synchronized(object) { // ... }

A synchronized instance method has the same semantics, but the instance itself acts as a monitor.

synchronized void instanceMethod() { // ... }

For a static synchronized method, the monitor is the Class object representing the declaring class.

static synchronized void staticMethod() { // ... }

Q14. If Two Threads Call a Synchronized Method on Different Object Instances Simultaneously, Could One of These Threads Block? What If the Method Is Static?

If the method is an instance method, then the instance acts as a monitor for the method. Two threads calling the method on different instances acquire different monitors, so none of them gets blocked.

If the method is static, then the monitor is the Class object. For both threads, the monitor is the same, so one of them will probably block and wait for another to exit the synchronized method.

Q15. What Is the Purpose of the Wait, Notify and Notifyall Methods of the Object Class?

A thread that owns the object's monitor (for instance, a thread that has entered a synchronized section guarded by the object) may call object.wait() to temporarily release the monitor and give other threads a chance to acquire the monitor. This may be done, for instance, to wait for a certain condition.

When another thread that acquired the monitor fulfills the condition, it may call object.notify() or object.notifyAll() and release the monitor. The notify method awakes a single thread in the waiting state, and the notifyAll method awakes all threads that wait for this monitor, and they all compete for re-acquiring the lock.

The following BlockingQueue implementation shows how multiple threads work together via the wait-notify pattern. If we put an element into an empty queue, all threads that were waiting in the take method wake up and try to receive the value. If we put an element into a full queue, the put method waits for the call to the get method. The get method removes an element and notifies the threads waiting in the put method that the queue has an empty place for a new item.

public class BlockingQueue { private List queue = new LinkedList(); private int limit = 10; public synchronized void put(T item) { while (queue.size() == limit) { try { wait(); } catch (InterruptedException e) {} } if (queue.isEmpty()) { notifyAll(); } queue.add(item); } public synchronized T take() throws InterruptedException { while (queue.isEmpty()) { try { wait(); } catch (InterruptedException e) {} } if (queue.size() == limit) { notifyAll(); } return queue.remove(0); } }

Q16. Describe the Conditions of Deadlock, Livelock, and Starvation. Describe the Possible Causes of These Conditions.

Deadlock is a condition within a group of threads that cannot make progress because every thread in the group has to acquire some resource that is already acquired by another thread in the group. The most simple case is when two threads need to lock both of two resources to progress, the first resource is already locked by one thread, and the second by another. These threads will never acquire a lock to both resources and thus will never progress.

Livelock is a case of multiple threads reacting to conditions, or events, generated by themselves. An event occurs in one thread and has to be processed by another thread. During this processing, a new event occurs which has to be processed in the first thread, and so on. Such threads are alive and not blocked, but still, do not make any progress because they overwhelm each other with useless work.

Starvation is a case of a thread unable to acquire resource because other thread (or threads) occupy it for too long or have higher priority. A thread cannot make progress and thus is unable to fulfill useful work.

Q17. Describe the Purpose and Use-Cases of the Fork/Join Framework.

The fork/join framework allows parallelizing recursive algorithms. The main problem with parallelizing recursion using something like ThreadPoolExecutor is that you may quickly run out of threads because each recursive step would require its own thread, while the threads up the stack would be idle and waiting.

The fork/join framework entry point is the ForkJoinPool class which is an implementation of ExecutorService. It implements the work-stealing algorithm, where idle threads try to “steal” work from busy threads. This allows to spread the calculations between different threads and make progress while using fewer threads than it would require with a usual thread pool.

Vous trouverez plus d'informations et des exemples de code pour le framework fork / join dans l'article «Guide to the Fork / Join Framework en Java».

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