Java Concurrency: Part 1

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Questions and Answers

What distinguishes ExecutorService from the Executor interface in task management?

ExecutorService extends Executor by providing methods to manage the task lifecycle and the executor itself.

Compare the functionality of submit() and execute() methods in ExecutorService.

submit() returns a Future object for tasks that return results, while execute() is void and for tasks without return values.

How does CompletableFuture enhance task execution compared to Future?

CompletableFuture enables functional composition and combining of asynchronous computations, unlike Future's limited methods.

What is the purpose of using ThreadLocal variables in concurrent programming?

ThreadLocal allows threads to have their own independent variables, ensuring thread confinement of data. Signup and view all the answers

Explain how lock striping improves concurrency in data structures.

Lock striping splits a data structure into segments, each protected by a separate lock, reducing lock contention. Signup and view all the answers

Distinguish between the operations of park() and wait() in threading.

park() is a low-level blocking operation used by LockSupport, whereas wait() is utilized within synchronized blocks. Signup and view all the answers

What advantages does ReadWriteLock offer in concurrent read-heavy environments?

ReadWriteLock allows multiple readers or a single writer, thereby improving concurrency for read-heavy workloads. Signup and view all the answers

How does the Exchanger class facilitate interaction between threads?

Exchanger allows two threads to synchronously swap objects at a designated point during execution. Signup and view all the answers

Describe the role of a memory barrier in multi-threading.

A memory barrier ensures that all memory operations are completed in a specific order for correct visibility across threads. Signup and view all the answers

What is the significance of the happens-before relationship with volatile variables?

A write to a volatile variable guarantees visibility of preceding writes to other threads reading that variable. Signup and view all the answers

What are the main advantages of using synchronized blocks over synchronized methods?

Synchronized blocks allow for finer-grained control by locking only specific parts of code or particular objects, which can improve performance and reduce contention. Signup and view all the answers

Define the happens-before relationship and its significance in Java concurrency.

The happens-before relationship ensures that actions performed in one thread are visible to other threads, guaranteeing memory visibility across threads. Signup and view all the answers

What is the key difference between volatile and atomic variables in Java?

Volatile variables provide visibility guarantees but not atomicity for compound actions, while atomic variables ensure both visibility and atomicity through CAS operations. Signup and view all the answers

Describe one feature of ReentrantLock that cannot be achieved with synchronized.

ReentrantLock supports timed locking, allowing threads to attempt to acquire a lock within a specified time frame. Signup and view all the answers

What is the role of CountDownLatch in thread synchronization?

CountDownLatch allows one or more threads to wait until a specified number of operations in other threads are completed, using a countdown mechanism. Signup and view all the answers

How does CyclicBarrier differ from CountDownLatch in terms of usability?

CyclicBarrier can be reused multiple times after threads are released, while CountDownLatch can only be used once before it becomes useless. Signup and view all the answers

What is meant by thread starvation, and why is it a problem in multi-threading environments?

Thread starvation occurs when a thread cannot access shared resources regularly, often due to other threads having higher priority or holding locks for too long. Signup and view all the answers

Explain the structure and operation of the Fork/Join framework.

The Fork/Join framework divides tasks into smaller subtasks that can run in parallel and combines the results after processing. Signup and view all the answers

In what way does the Phaser class enhance synchronization compared to CyclicBarrier?

Phaser allows dynamic registration of parties and can handle multiple phases of execution, offering more flexibility than CyclicBarrier. Signup and view all the answers

Differentiate between Executor and ExecutorService in Java concurrency.

Executor provides a basic interface for running tasks asynchronously, while ExecutorService extends it with methods for task lifecycle management such as shutdown and management of callable futures. Signup and view all the answers

Study Notes

Synchronization in Java Concurrency

Synchronized Methods vs. Blocks: Synchronized methods lock the entire object, while synchronized blocks offer finer-grained control, locking specific portions of code for better performance.

Happens-Before Relationship

Happens-before is a Java concurrency principle ensuring that actions in one thread are visible to others. A happens-before relationship guarantees visibility of effects.

Volatile vs. Atomic Variables

Volatile: Guarantees visibility of changes but lacks atomicity for compound operations.
Atomic: Provides both visibility and atomicity for compound operations using Compare-and-Swap (CAS).

ReentrantLock vs. Synchronized

ReentrantLock: Offers more control (timed blocking, interruptible locks, fairness, non-blocking).
Synchronized: Simpler, but less flexible than ReentrantLock.

CountDownLatch

Allows multiple threads to wait until a set of operations in other threads complete. Starts with a count, decrements until zero, releasing threads.

CyclicBarrier vs. CountDownLatch

CyclicBarrier: Reusable, waits for all threads to reach a barrier point.
CountDownLatch: One-time use, waits until a count reaches zero.

Thread Starvation

Thread starvation occurs when a thread consistently fails to gain access to shared resources, hindering its progress. This often arises from other threads having higher priority or holding locks too long.

Fork/Join Framework

A framework for implementing divide-and-conquer parallel computations by breaking problems into subtasks that can be executed concurrently.

Phaser

A versatile synchronization barrier supporting multiple phases of execution, similar to CyclicBarrier but more adaptable.

Executor vs. ExecutorService

Executor: A basic interface for executing tasks.
ExecutorService: Extends Executor, providing more methods for task and executor lifecycle management.

ExecutorService: submit() vs. execute()

submit(): Returns a Future object; suitable for tasks returning results.
execute(): Void method; used for tasks not returning results.

CompletableFuture vs. Future

CompletableFuture: More flexible. Offers functional composition, chaining and handling asynchronous computations.
Future: Basic capabilities for checking completion and accessing results.

ThreadLocal

Stores values accessible only by the thread that created the ThreadLocal. Useful for thread-specific data.

Lock Stripping

Dividing shared data into segments, each secured by its lock, allowing enhanced concurrency by reducing lock contention.

park() vs. wait()

park(): Used by LockSupport for low-level blocking.
wait(): Must own the monitor of the object for use in synchronized blocks.

ReadWriteLock

Provides separate locks for read and write operations, enabling multiple readers but only one writer at a time. Useful for read-heavy workloads.

Exchanger

Enables two threads to exchange objects at a specific synchronization point. Useful in producer-consumer scenarios.

interrupted() vs. isInterrupted()

interrupted(): Checks and clears the interrupted status of the current thread.
isInterrupted(): Only checks; does not clear the interrupt status.

Memory Barriers/Fences

Ensure that memory operations are completed in the specified order. Crucial for visibility of shared variables.

ForkJoinPool & Work Stealing

Empty worker queues try to steal work from another thread's tasks, reducing the workload on busy threads, improving performance.

Fixed vs. Cached Thread Pools

newFixedThreadPool: Creates a pool with a fixed size of threads, reusing them.
newCachedThreadPool: Creates threads on demand, reusing idle threads as needed. Useful for short-lived operations.

StampedLock

Provides optimistic reading, offering multiple access modes (reading, writing, optimistic reading); improves performance over ReadWriteLock in some cases.

Synchronized vs. Concurrent Collections

Synchronized: Uses a single lock for all operations; simpler but lower throughput in concurrent environments.
Concurrent: Employ finer-grained locking mechanisms (e.g., lock splitting), enabling higher throughput.

BlockingQueue & Producer-Consumer

Implements a producer-consumer pattern by blocking put() operations when full and take() operations when empty. Facilitates asynchronous communication.

@Contended Annotation

Prevents false sharing by adding padding between fields; enhances performance during concurrent access.

Heavyweight vs. Lightweight Synchronization

Heavyweight: OS-level thread scheduling and blocking (e.g., wait/notify).
Lightweight: Uses techniques like atomic operations, spinning (yielding to another thread with short pauses).

Semaphore

Controls access to a shared resource by managing a counter. Useful for regulating resource use.

Happens-Before & Volatile Variables

A write to a volatile variable happens-before every subsequent read of that variable, ensuring all prior changes are visible.

Programmatic vs. Declarative Synchronization

Programmatic: Uses explicit locks and conditional statements for synchronization.
Declarative: Relies on synchronized keywords and methods.

FutureTask

Represents a cancellable asynchronous computation. Combines the capabilities of Future with Runnable to allow the execution and retrieval of results.

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Description

Test your knowledge on key synchronization concepts in Java concurrency, including synchronized methods and blocks, happens-before relationships, volatile vs. atomic variables, and the use of ReentrantLock and CountDownLatch. This quiz covers essential principles to enhance your Java programming skills in a multi-threaded environment.