Java Concurrency Primitives

In one line

Thread, Process, Runnable, Callable, Future — the units and handles of concurrent work.

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Think of it like this

A restaurant kitchen. A Process is a separate kitchen with its own ingredients (isolated); a Thread is a chef working inside the same kitchen, sharing the same fridge and counters (shared memory) as other chefs.

Key ideas

01
Process = separate memory space (isolation, heavier); Thread = lightweight unit inside a process sharing heap.
02
Runnable: void run() — fire and forget; Callable<V>: V call() throws Exception — can return a value.
03
Future<V> is the handle: submit a Callable, later call future.get() (blocks) or future.isDone().
04
Modern Java: CompletableFuture for composition; virtual threads (Project Loom, Java 21+) make threads nearly free.
05
How many threads? CPU-bound ≈ cores; IO-bound >> cores (each thread waits on IO). Spring Boot 3.2+ default uses virtual threads on Tomcat.

Java / Spring map

→
Executors; CompletableFuture.supplyAsync; Thread.startVirtualThread (Java 21+).

Code & diagrams

Primitives.javajava

Thread vs Runnable vs Callable+Future in one small program.

Runnable task = () -> System.out.println("fire & forget: " + Thread.currentThread());

Callable<Integer> compute = () -> {
  Thread.sleep(100);                 // pretend work
  return 6 * 7;
};

ExecutorService pool = Executors.newFixedThreadPool(4);
pool.execute(task);                  // Runnable — no result

Future<Integer> f = pool.submit(compute);   // Callable — result handle
int answer = f.get();                // blocks until done
// Java 21+: 
// try (var p = Executors.newVirtualThreadPerTaskExecutor()) { p.submit(task); }

Explain without notes

When do you need Future/Callable over Runnable? Give a real LLD scenario (async release of spots?).

Practice

In the Elevator problem, run each elevator as a task that pushes status to a shared dashboard.

Trade-offs

↔
Thread-per-task is simple; elastically sized pools + queues are production-grade but tuneable and finicky.

Completion checklist

I can pick Thread / Runnable / Callable / Future for a stated need in one sentence.

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