Multithreading Models

// One-to-One: each Thread = one OS kernel thread
// Default stack size ~512KB–1MB per thread
ExecutorService pool = Executors.newFixedThreadPool(
    Runtime.getRuntime().availableProcessors() // match CPU cores for CPU-bound
);

List<Future<Integer>> results = new ArrayList<>();
for (int i = 0; i < 100; i++) {
    int task = i;
    results.add(pool.submit(() -> {
        // Each task runs on a kernel thread from the pool
        return task * task;
    }));
}
for (Future<Integer> f : results) System.out.println(f.get());
pool.shutdown();

// Check active thread count
System.out.println("Active threads: " + Thread.activeCount());

// Many-to-Many: millions of virtual threads on N carrier threads
// N ≈ Runtime.getRuntime().availableProcessors() by default

// I/O-bound workload — perfect for virtual threads
try (var executor = Executors.newVirtualThreadPerTaskExecutor()) {
    // Simulate 10,000 concurrent HTTP calls (each blocks on network I/O)
    var futures = IntStream.range(0, 10_000)
        .mapToObj(i -> executor.submit(() -> {
            // simulatedHttpCall() blocks — virtual thread parks, carrier is freed
            Thread.sleep(50);  // simulates network latency
            return "response-" + i;
        }))
        .toList();

    long done = futures.stream().filter(f -> {
        try { f.get(); return true; } catch (Exception e) { return false; }
    }).count();
    System.out.println("Completed: " + done);
}