Promise.java

package com.firefly.utils.concurrent;

import java.util.Objects;
import java.util.concurrent.CompletableFuture;

/**
 * <p>A callback abstraction that handles completed/failed events of asynchronous operations.</p>
 *
 * @param <C> the type of the context object
 */
public interface Promise<C> {
    /**
     * <p>Callback invoked when the operation completes.</p>
     *
     * @param result the context
     * @see #failed(Throwable)
     */
    default void succeeded(C result) {
    }

    /**
     * <p>Callback invoked when the operation fails.</p>
     *
     * @param x the reason for the operation failure
     */
    default void failed(Throwable x) {
    }

    /**
     * <p>Empty implementation of {@link Promise}.</p>
     *
     * @param <U> the type of the result
     */
    class Adapter<U> implements Promise<U> {
        @Override
        public void failed(Throwable x) {

        }
    }

    /**
     * <p>Creates a promise from the given incomplete CompletableFuture.</p>
     * <p>When the promise completes, either succeeding or failing, the
     * CompletableFuture is also completed, respectively via
     * {@link CompletableFuture#complete(Object)} or
     * {@link CompletableFuture#completeExceptionally(Throwable)}.</p>
     *
     * @param completable the CompletableFuture to convert into a promise
     * @param <T>         the type of the result
     * @return a promise that when completed, completes the given CompletableFuture
     */
    static <T> Promise<T> from(CompletableFuture<? super T> completable) {
        if (completable instanceof Promise)
            return (Promise<T>) completable;

        return new Promise<T>() {
            @Override
            public void succeeded(T result) {
                completable.complete(result);
            }

            @Override
            public void failed(Throwable x) {
                completable.completeExceptionally(x);
            }
        };
    }

    /**
     * <p>A CompletableFuture that is also a Promise.</p>
     *
     * @param <S> the type of the result
     */
    class Completable<S> extends CompletableFuture<S> implements Promise<S> {
        @Override
        public void succeeded(S result) {
            complete(result);
        }

        @Override
        public void failed(Throwable x) {
            completeExceptionally(x);
        }
    }

    class Wrapper<W> implements Promise<W> {
        private final Promise<W> promise;

        public Wrapper(Promise<W> promise) {
            this.promise = Objects.requireNonNull(promise);
        }

        @Override
        public void succeeded(W result) {
            promise.succeeded(result);
        }

        @Override
        public void failed(Throwable x) {
            promise.failed(x);
        }

        public Promise<W> getPromise() {
            return promise;
        }

        public Promise<W> unwrap() {
            Promise<W> result = promise;
            while (true) {
                if (result instanceof Wrapper)
                    result = ((Wrapper<W>) result).unwrap();
                else
                    break;
            }
            return result;
        }
    }
}