Systematic Overview of Java Concurrency and the J.U.C Framework

Knowledge Systematic Overview

A global view of Java concurrency knowledge, including theory, thread basics, and key keywords.

Concurrency Basics

Why multithreading was introduced and what problems it solves.

Definition of thread‑unsafe, visibility, atomicity, ordering.

How Java solves concurrency with volatile , final , synchronized , JMM and the eight Happens‑Before relations.

Thread safety is not binary; various implementation ideas and the difference between concurrency and parallelism.

Key Keywords

synchronized : object lock vs class lock, re‑entrancy, visibility guarantee, drawbacks and comparison with Lock .

volatile : purpose, lack of atomicity, visibility via memory barriers, ordering, and typical use cases.

final : compile‑time constants, immutability, inheritance limits, field reordering rules, and practical restrictions.

J.U.C Framework

Lock Framework and Tools

Core interfaces and abstract classes such as Lock , Condition , ReadWriteLock , AbstractQueuedSynchronizer , and its long/int variants are described, explaining how they provide exclusive and shared acquisition modes.

LockSupport

Provides low‑level park/unpark primitives that replace Object.wait/notify in advanced synchronizers.

ReentrantLock

Details of fair vs. non‑fair lock implementations, re‑entrancy, and comparison with synchronized .

ReentrantReadWriteLock

Read/write state encoding, thread‑local counters, lock upgrade/downgrade behavior.

Atomic Classes

Explanation of CAS, Unsafe , and the 13 atomic classes (basic types, arrays, reference types, and field updaters), including AtomicStampedReference and AtomicMarkableReference for ABA problems.

Concurrent Collections

ConcurrentHashMap : segment lock (JDK 7) vs. CAS + tree bins (JDK 8).

CopyOnWriteArrayList/Set : copy‑on‑write semantics, weakly consistent iterator.

ConcurrentLinkedQueue/Deque : lock‑free linked‑node structure, HOPS strategy.

BlockingQueue family : ArrayBlockingQueue, LinkedBlockingQueue, DelayQueue, SynchronousQueue, etc., with their insertion methods and typical use cases.

Executors (Thread Pools)

Hierarchy of Executor , ExecutorService , ScheduledExecutorService , and concrete implementations such as ThreadPoolExecutor , ScheduledThreadPoolExecutor , and the Fork/Join framework ( ForkJoinPool , RecursiveTask , RecursiveAction ).

Utility Classes

CountDownLatch , CyclicBarrier , Semaphore , Phaser , Exchanger : synchronization primitives with their core methods and typical scenarios.

ThreadLocal : thread‑local storage, memory‑leak risk, and applications.

The article concludes with interview‑style questions covering each component of the J.U.C framework.