Promises

In Chapter 2, Concurrency on the JVM and the Java Memory Model, we implemented an asynchronous method that used a worker thread and a task queue to receive and execute asynchronous computations. That example should have left you with a basic intuition about how the execute method is implemented in execution contexts. You might be wondering how the Future.apply method can return and complete a Future object. We will study promises in this section to answer this question. Promises are objects that can be assigned a value or an exception only once. This is why promises are sometimes also called single-assignment variables. A promise is represented with the Promise[T] type in Scala. To create a promise instance, we use the Promise.apply method on the Promise companion object:

def apply[T](): Promise[T]

This method returns a new promise instance. Like the Future.apply method, the Promise.apply method returns immediately; it is non-blocking. However, the Promise.apply method does not start...