Programming Languages and Techniques II

16.3 Concurrent Collections and Executors

4 min readLast Updated on August 9, 2024

Concurrent collections and executors are essential tools for managing shared data and task execution in multithreaded programs. They provide thread-safe alternatives to standard collections and offer efficient ways to handle concurrent operations without explicit synchronization.

Executors simplify thread management by decoupling task submission from execution. They offer thread pooling, asynchronous task handling, and result retrieval, making it easier to implement scalable and performant multithreaded applications. These concepts are crucial for effective concurrent programming.

Concurrent Collections

Thread-Safe Map Implementation

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  • ConcurrentHashMap provides thread-safe operations for hash table-based map
  • Allows multiple threads to read and write concurrently without external synchronization
  • Utilizes lock striping technique divides the map into segments, each with its own lock
  • Offers better performance than synchronized HashMap for concurrent access scenarios
  • Supports atomic operations like
    putIfAbsent()
    ,
    replace()
    , and
    remove()
  • Provides methods for aggregate operations such as
    forEach()
    ,
    reduce()
    , and
    search()
  • Maintains weak consistency ensures most recent updates are visible to other threads

Thread-Safe List and Queue Implementations

  • CopyOnWriteArrayList implements a thread-safe variant of ArrayList
  • Creates a new copy of the underlying array for each write operation
  • Ideal for read-heavy scenarios with infrequent modifications
  • Provides snapshot iterators that do not throw ConcurrentModificationException
  • BlockingQueue interface extends Queue with blocking operations for concurrent scenarios
  • Supports operations that wait for the queue to become non-empty when retrieving elements
  • Offers methods like
    put()
    (blocks if queue is full) and
    take()
    (blocks if queue is empty)
  • Implementations include LinkedBlockingQueue, ArrayBlockingQueue, and PriorityBlockingQueue

Concurrent Collection Design Principles

  • Designed to handle concurrent access without explicit synchronization
  • Provide atomic operations to ensure thread safety and data consistency
  • Offer better scalability and performance compared to synchronized collections
  • Support non-blocking algorithms to minimize thread contention
  • Implement fail-fast or fail-safe iterators for concurrent modification detection
  • Provide methods for bulk operations that can be performed atomically
  • Balance between thread safety and performance optimizations

Executors

Executor Service Fundamentals

  • ExecutorService interface extends Executor provides methods for managing thread execution
  • Decouples task submission from task execution mechanics
  • Offers methods like
    submit()
    ,
    invokeAll()
    , and
    invokeAny()
    for task submission
  • Supports both Runnable and Callable tasks for execution
  • Provides shutdown mechanisms (
    shutdown()
    and
    shutdownNow()
    ) for graceful termination
  • Enables asynchronous task execution and result retrieval using Future objects
  • Implements thread pooling to efficiently manage and reuse threads

Thread Pool Implementation and Management

  • ThreadPoolExecutor class provides a flexible and customizable thread pool implementation
  • Allows configuration of core pool size, maximum pool size, and keep-alive time
  • Supports different types of work queues (unbounded, bounded, synchronous) for task storage
  • Offers customizable thread factory for creating worker threads
  • Implements different rejection policies for handling tasks when the pool is saturated
  • Provides hooks for extending behavior (
    beforeExecute()
    ,
    afterExecute()
    ,
    terminated()
    )
  • Enables dynamic adjustment of pool size and other parameters during runtime

Task Submission and Result Handling

  • Future interface represents the result of an asynchronous computation
  • Provides methods to check task completion status (
    isDone()
    ) and cancel tasks (
    cancel()
    )
  • Allows retrieval of task results using
    get()
    method, which blocks until the result is available
  • Supports timeout mechanism for result retrieval to avoid indefinite waiting
  • Callable interface represents a task that returns a result and may throw an exception
  • Enables creation of tasks with return values, unlike Runnable which only performs actions
  • Can be used with ExecutorService's
    submit()
    method to obtain a Future for the task result

Asynchronous Programming

CompletableFuture for Asynchronous Operations

  • CompletableFuture class implements Future and CompletionStage interfaces
  • Provides a rich set of methods for composing, combining, and handling asynchronous operations
  • Supports chaining of asynchronous tasks using
    thenApply()
    ,
    thenAccept()
    , and
    thenRun()
  • Allows combining multiple CompletableFutures using
    thenCombine()
    ,
    allOf()
    , and
    anyOf()
  • Offers exception handling mechanisms with
    exceptionally()
    and
    handle()
    methods
  • Enables asynchronous execution without explicitly creating threads or using ExecutorService
  • Supports both synchronous and asynchronous completion of futures

Fork/Join Framework for Parallel Processing

  • Fork/Join framework designed for recursive divide-and-conquer algorithms
  • Utilizes work-stealing algorithm to efficiently balance tasks across available threads
  • ForkJoinPool class implements ExecutorService optimized for fork/join tasks
  • RecursiveTask and RecursiveAction classes represent tasks that can be split into subtasks
  • Implements work-stealing deques for each worker thread to minimize contention
  • Supports automatic load balancing by allowing idle threads to steal work from busy ones
  • Provides methods like
    fork()
    and
    join()
    for task splitting and result aggregation
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© 2025 Fiveable Inc. All rights reserved.
AP® and SAT® are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.

© 2025 Fiveable Inc. All rights reserved.
AP® and SAT® are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.
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