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#590 explanation for Promise
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---

## Also known as

CompletableFuture

## Intent
A Promise represents a proxy for a value not necessarily known when the promise is created. It
allows you to associate dependent promises to an asynchronous action's eventual success value or
failure reason. Promises are a way to write async code that still appears as though it is executing
in a synchronous way.

A Promise represents a proxy for a value not necessarily known when the promise is created. It allows you to associate
dependent promises to an asynchronous action's eventual success value or failure reason. Promises are a way to write
async code that still appears as though it is executing in a synchronous way.

## Explanation

The Promise object is used for asynchronous computations. A Promise represents an operation that hasn't completed yet,
but is expected in the future.

Promises provide a few advantages over callback objects:
* Functional composition and error handling
* Prevents callback hell and provides callback aggregation

Real world example

> We are developing a software solution that downloads files and calculates the number of lines and character
frequencies in those files. Promise is an ideal solution to make the code concise and easy to understand.

In plain words

> Promise is a placeholder for an asynchronous operation that is ongoing.
Wikipedia says

> In computer science, future, promise, delay, and deferred refer to constructs used for synchronizing program
execution in some concurrent programming languages. They describe an object that acts as a proxy for a result that is
initially unknown, usually because the computation of its value is not yet complete.

**Programmatic Example**

In the example a file is downloaded and its line count is calculated. The calculated line count is then consumed and
printed on console.

Let's first introduce a support class we need for implementation. Here's `PromiseSupport`.

```java
class PromiseSupport<T> implements Future<T> {

private static final Logger LOGGER = LoggerFactory.getLogger(PromiseSupport.class);

private static final int RUNNING = 1;
private static final int FAILED = 2;
private static final int COMPLETED = 3;

private final Object lock;

private volatile int state = RUNNING;
private T value;
private Exception exception;

PromiseSupport() {
this.lock = new Object();
}

void fulfill(T value) {
this.value = value;
this.state = COMPLETED;
synchronized (lock) {
lock.notifyAll();
}
}

void fulfillExceptionally(Exception exception) {
this.exception = exception;
this.state = FAILED;
synchronized (lock) {
lock.notifyAll();
}
}

@Override
public boolean cancel(boolean mayInterruptIfRunning) {
return false;
}

@Override
public boolean isCancelled() {
return false;
}

@Override
public boolean isDone() {
return state > RUNNING;
}

@Override
public T get() throws InterruptedException, ExecutionException {
synchronized (lock) {
while (state == RUNNING) {
lock.wait();
}
}
if (state == COMPLETED) {
return value;
}
throw new ExecutionException(exception);
}

@Override
public T get(long timeout, TimeUnit unit) throws ExecutionException {
synchronized (lock) {
while (state == RUNNING) {
try {
lock.wait(unit.toMillis(timeout));
} catch (InterruptedException e) {
LOGGER.warn("Interrupted!", e);
Thread.currentThread().interrupt();
}
}
}

if (state == COMPLETED) {
return value;
}
throw new ExecutionException(exception);
}
}
```

With `PromiseSupport` in place we can implement the actual `Promise`.

```java
public class Promise<T> extends PromiseSupport<T> {

private Runnable fulfillmentAction;
private Consumer<? super Throwable> exceptionHandler;

public Promise() {
}

@Override
public void fulfill(T value) {
super.fulfill(value);
postFulfillment();
}

@Override
public void fulfillExceptionally(Exception exception) {
super.fulfillExceptionally(exception);
handleException(exception);
postFulfillment();
}

private void handleException(Exception exception) {
if (exceptionHandler == null) {
return;
}
exceptionHandler.accept(exception);
}

private void postFulfillment() {
if (fulfillmentAction == null) {
return;
}
fulfillmentAction.run();
}

public Promise<T> fulfillInAsync(final Callable<T> task, Executor executor) {
executor.execute(() -> {
try {
fulfill(task.call());
} catch (Exception ex) {
fulfillExceptionally(ex);
}
});
return this;
}

public Promise<Void> thenAccept(Consumer<? super T> action) {
var dest = new Promise<Void>();
fulfillmentAction = new ConsumeAction(this, dest, action);
return dest;
}

public Promise<T> onError(Consumer<? super Throwable> exceptionHandler) {
this.exceptionHandler = exceptionHandler;
return this;
}

public <V> Promise<V> thenApply(Function<? super T, V> func) {
Promise<V> dest = new Promise<>();
fulfillmentAction = new TransformAction<V>(this, dest, func);
return dest;
}

private class ConsumeAction implements Runnable {

private final Promise<T> src;
private final Promise<Void> dest;
private final Consumer<? super T> action;

private ConsumeAction(Promise<T> src, Promise<Void> dest, Consumer<? super T> action) {
this.src = src;
this.dest = dest;
this.action = action;
}

@Override
public void run() {
try {
action.accept(src.get());
dest.fulfill(null);
} catch (Throwable throwable) {
dest.fulfillExceptionally((Exception) throwable.getCause());
}
}
}

private class TransformAction<V> implements Runnable {

private final Promise<T> src;
private final Promise<V> dest;
private final Function<? super T, V> func;

private TransformAction(Promise<T> src, Promise<V> dest, Function<? super T, V> func) {
this.src = src;
this.dest = dest;
this.func = func;
}

@Override
public void run() {
try {
dest.fulfill(func.apply(src.get()));
} catch (Throwable throwable) {
dest.fulfillExceptionally((Exception) throwable.getCause());
}
}
}
}
```

Now we can show the full example in action. Here's how to download and count the number of lines in a file using
`Promise`.

```java
countLines().thenAccept(
count -> {
LOGGER.info("Line count is: {}", count);
taskCompleted();
}
);

private Promise<Integer> countLines() {
return download(DEFAULT_URL).thenApply(Utility::countLines);
}

private Promise<String> download(String urlString) {
return new Promise<String>()
.fulfillInAsync(
() -> Utility.downloadFile(urlString), executor)
.onError(
throwable -> {
throwable.printStackTrace();
taskCompleted();
}
);
}
```

## Class diagram

![alt text](./etc/promise.png "Promise")

## Applicability

Promise pattern is applicable in concurrent programming when some work needs to be done asynchronously
and:

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* [Guava ListenableFuture](https://github.com/google/guava/wiki/ListenableFutureExplained)

## Related Patterns
* Async Method Invocation
* Callback

* [Async Method Invocation](https://java-design-patterns.com/patterns/async-method-invocation/)
* [Callback](https://java-design-patterns.com/patterns/callback/)

## Tutorials

* [Guide To CompletableFuture](https://www.baeldung.com/java-completablefuture)

## Credits

* [You are missing the point to Promises](https://gist.github.com/domenic/3889970)
* [Functional style callbacks using CompletableFuture](https://www.infoq.com/articles/Functional-Style-Callbacks-Using-CompletableFuture)
* [Java 8 in Action: Lambdas, Streams, and functional-style programming](https://www.amazon.com/gp/product/1617291994/ref=as_li_qf_asin_il_tl?ie=UTF8&tag=javadesignpat-20&creative=9325&linkCode=as2&creativeASIN=1617291994&linkId=995af46887bb7b65e6c788a23eaf7146)
* [Modern Java in Action: Lambdas, streams, functional and reactive programming](https://www.amazon.com/gp/product/1617293563/ref=as_li_qf_asin_il_tl?ie=UTF8&tag=javadesignpat-20&creative=9325&linkCode=as2&creativeASIN=1617293563&linkId=f70fe0d3e1efaff89554a6479c53759c)

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