Thread Communication

We in many topics about how one thread hands over the task to threads in other thread pool or how one thread uses a callback to handover the task to another thread. In reality, no such callbacks really exists. It's all about simply submitting a task to another thread pool.

Callbacks in DeferredResult

In case of Spring DeferredResult, we call the setResult() method to return the response to the client. But this method only submits a task to the servlet thread pool to send the response back to the client.

public void setResult(Object result) {
    this.result = result;                    // store in heap
    this.asyncContext.dispatch();            // this is the key call
}

// roughly what dispatch() does:
containerThreadPool.submit(() -> {
    response.write(this.result);   // write HTTP response
    asyncContext.complete();       // close connection
});

How Completable Futures work?

This applies to completable futures as well. When we chain methods, we only submit tasks to the thread pool at each step.

Completable Futures aren't about returning anything. It's about chaining tasks one after the other as separate threads in the thread pool.