Java.Multithreading. if i use ForkJoin is it guaranteed to solve each task on different core ?

❌ No, not guaranteed, but ✅ it is highly optimized and designed to do so — more than ThreadPoolExecutor.

Let’s break this down 👇

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✅ What Makes ForkJoinPool Special?

• It’s built for parallelism, not just concurrency.
• Uses work-stealing algorithm to balance load across multiple worker threads.
• Defaults to using Runtime.getRuntime().availableProcessors() threads — usually one per core.
• Each thread has its own task queue, and idle threads steal work from busy ones.

💡 So: It’s optimized to maximize core usage on multi-core CPUs.

🔬 Example:

ForkJoinPool pool = new ForkJoinPool(); // uses all available cores

pool.submit(() -> {
    IntStream.range(0, 10).parallel().forEach(i -> {
        System.out.println("Task " + i + " running on " + Thread.currentThread().getName());
        heavyComputation(); // Simulate CPU-bound work
    });
});

This parallel() uses the common ForkJoinPool, and each task may run on a different core.

⚠️ But Still Not Guaranteed

Why?

• OS still decides which thread runs on which core.
• CPU core overcommitment may happen.
• Power management / throttling might reduce active cores.
• If your tasks are too short or I/O-bound, the OS might just keep them on fewer cores.

✅ Why It’s More Likely With ForkJoinPool

Feature	ThreadPoolExecutor	ForkJoinPool
Designed for CPU parallelism	❌ No	✅ Yes
Work-stealing	❌ No	✅ Yes
Dynamic load balancing	❌ No	✅ Yes
Uses available processors	❌ Only if you configure it	✅ Automatically

💡 TL;DR

Question	Answer
Does ForkJoinPool use multiple threads?	✅ Yes
Does it try to use all CPU cores?	✅ Yes
Does it guarantee tasks will run on different cores?	❌ No (OS decides)
Is it more parallel-friendly than ThreadPoolExecutor?	✅ 100%