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