Understanding Java Threads, Thread Pools, and Concurrency Mechanisms

Java threads are the smallest units of execution within a process, allowing concurrent tasks to run on multiple CPU cores. The article explains the difference between processes and threads, the various thread states (NEW, RUNNABLE, BLOCKED, WAITING, TIMED_WAITING, TERMINATED), and the distinction between sleep() and wait() (the former does not release locks, the latter does).

Thread creation methods are demonstrated with code examples: extending Thread, implementing Runnable, and using Callable with FutureTask. Sample code snippets are provided for each approach.

The discussion then moves to thread pools, describing the four factory methods ( newCachedThreadPool, newFixedThreadPool, newScheduledThreadPool, newSingleThreadExecutor) and the internal workings of ThreadPoolExecutor. Key parameters such as corePoolSize, maximumPoolSize, keepAliveTime, BlockingQueue, ThreadFactory, and RejectedExecutionHandler are explained, along with the role of the ctl field that encodes run state and worker count.

The article also compares ArrayBlockingQueue and LinkedBlockingQueue, outlines the four rejection policies (AbortPolicy, CallerRunsPolicy, DiscardOldestPolicy, DiscardPolicy), and details the five lifecycle states of a thread pool (RUNNING, SHUTDOWN, STOP, TIDYING, TERMINATED). It further clarifies the differences between shutdown() and shutdownNow().

Finally, inter‑thread communication techniques are covered, including a volatile flag example, and a brief mention of synchronization primitives such as CountDownLatch and CyclicBarrier. The article concludes with references to additional resources on Java locking and best practices for concurrent programming.