Working with Executor Framework
1. Creating Thread Pools
Factory Behavior newFixedThreadPool(n)Bounded threads, unbounded queue newCachedThreadPool()0..MAX threads, SynchronousQueue newSingleThreadExecutor()Serial execution newWorkStealingPool()ForkJoinPool newVirtualThreadPerTaskExecutor() Java 21+ One virtual thread per task
2. Using ExecutorService Interface
Method Use submit(Callable)Returns Future execute(Runnable)Fire-and-forget invokeAll(tasks)Wait all, returns futures invokeAny(tasks)First success close()AutoCloseable since Java 19
3. Using Callable and Future
Type Detail Callable<V>Returns V, throws Exception Future<V>.get()Blocks for result get(t, unit)Times out cancel(mayInterrupt)Best-effort isDone, isCancelledStatus check
4. Configuring ThreadPoolExecutor
Param Effect corePoolSize Always-alive threads maximumPoolSize Cap when queue full keepAliveTime Idle non-core thread TTL workQueue Tasks awaiting execution threadFactory Customize thread creation handler RejectedExecutionHandler
5. Using ScheduledExecutorService
Method Behavior schedule(r, delay, unit)One-shot scheduleAtFixedRate(r, init, period, unit)Fixed start times scheduleWithFixedDelay(r, init, delay, unit)Delay between completions
Warning: Uncaught exception in periodic task cancels all future runs — wrap in try/catch.
6. Shutting Down Executors
Method Effect shutdown()Stop accepting; finish running shutdownNow()Interrupt running, return queued awaitTermination(t, unit)Block for completion close()shutdown + await indefinite
7. Handling Rejected Tasks (RejectedExecutionHandler)
Policy Behavior AbortPolicy (default) Throws RejectedExecutionException CallerRunsPolicy Runs in submitter thread (back-pressure) DiscardPolicy Silently drops DiscardOldestPolicy Drops head + retries
8. Using CompletionService
Method Use submit(task)Submit and track take()Block for next completed Future poll(t, unit)Timed
9. Understanding Fork/Join Framework
API Use RecursiveTask<V>Returns result RecursiveActionvoid compute()Override; split + invoke fork() / join()Async + wait commonPool()Shared pool Work-stealing Idle threads steal from busy queues
Example: Sum array
class SumTask extends RecursiveTask < Long > { int [] a; int lo, hi; SumTask ( int [] a , int lo , int hi ) { this .a = a; this .lo = lo; this .hi = hi; } protected Long compute () { if (hi - lo < 1000 ) { long s = 0 ; for ( int i = lo; i < hi; i ++ ) s += a[i]; return s; } int m = (lo + hi) >>> 1 ; SumTask l = new SumTask (a, lo, m); l. fork (); SumTask r = new SumTask (a, m, hi); return r. compute () + l. join (); } }
10. Creating Custom ThreadFactory
Customization Reason Name Diagnostics in thread dumps Daemon JVM shutdown behavior Priority Background vs foreground UncaughtExceptionHandler Logging
11. Using Work-Stealing Pools
Aspect Detail Underlying ForkJoinPool Parallelism Default = available cores Best for Many small CPU-bound tasks Avoid Long blocking I/O (use ManagedBlocker)