# Executors & Futures — Advanced Java

Source: https://www.geekswithgeeks.com/en/advanced-java/adv-executors

> Decouple task submission from thread management with ExecutorService, run Callables, collect results from Future, and always shut the pool down.

## Submit work to a pool

An `ExecutorService` decouples task submission from thread management. `submit(Callable<T>)` returns a `Future<T>`; `future.get()` blocks for the result. Always `shutdown()` the pool.

```java
try (var pool = Executors.newFixedThreadPool(4)) {
    List<Future<Integer>> fs = pool.invokeAll(tasks);
    for (Future<Integer> f : fs) total += f.get();
}   // close() shuts down and waits (Java 19+)
```

## Pick the right pool

`newFixedThreadPool(n)` for bounded CPU-bound work, `newCachedThreadPool()` for many short-lived bursty tasks, and Java 21's `Executors.newVirtualThreadPerTaskExecutor()` for thousands of blocking IO tasks — one virtual thread per task, no pool sizing needed.

```java
try (var pool = Executors.newVirtualThreadPerTaskExecutor()) {
    urls.forEach(url -> pool.submit(() -> fetch(url)));
}
```