Emulating CompletableFuture with Promise

[Ian Preston]

I'm trying to emulate CompletableFuture by backing directly onto Promise, but when compiling it gives this error: 

"No source code is available for type java.util.concurrent.CompletableFuture<T>"

(it finds other JRE classes I've replaced fine)

My source I'm including is below. Am I doing anything illegal?

********************************************

package java.util.concurrent;

import com.google.gwt.user.client.*;

import jsinterop.annotations.*;

import java.util.function.*;

@JsType(isNative=true, name = "Promise", namespace = JsPackage.GLOBAL)

public class CompletableFuture<T> implements CompletionStage<T> {

    @JsMethod(name = "then")

    public native <U> CompletableFuture<U> thenApply(Function<? super T,? extends U> fn);

    @JsMethod(name = "then")

    public native CompletableFuture<Void> thenAccept(Consumer<? super T> action);

    @JsMethod(name = "then")

    public native <U> CompletableFuture<U> thenCompose(Function<? super T, ? extends CompletionStage<U>> fn);

    @JsMethod(name = "reject")

    public native boolean completeExceptionally(Throwable ex);

    @JsMethod(name = "resolve")

    public native boolean complete(T value);

    @JsMethod(name = "resolve")

    public static native <U> CompletableFuture<U> completedFuture(U value);

    @JsOverlay

    public T get() throws InterruptedException, ExecutionException {

        Window.alert("Calling synchronous get() on CompletableFuture is not possible in Javascript!");

        throw new IllegalStateException("Unimplemented!");

    }

}

[Andrei Korzhevskii]

I guess you need to super-source this class so the gwt compiler could use it.

You can take a look at my initial CompletableFuture implementation here:

https://github.com/nordligulv/gwt/tree/completable-future

https://github.com/nordligulv/gwt/commit/ea70c5358bac3c939cb0c1628a58ccc462cd7423

Feel free to use it but note that I haven't tested it properly yet.

[Ian Preston]

Thanks, Andrei. That looks great! Is there a reason you've changed the signatures on some of the CompletableFuture methods?

E.g. in the JRE,

public <U> CompletableFuture<U> thenApply(Function<? super T,? extends U> fn)

has become

public <U> CompletionStage<U> thenApply(Function<? super T, ? extends U> fn)

[Andrei Korzhevskii]

No reason for that. I had to implement the code from scratch and didnt pay attention to the signatures. 

Thanks for the hint!

[Ian Preston]

For another hint, I think you don't want the "new" before "Promise.resolve" on line 88 of impl/JsPromise :-)

What the best way for me to report bugs to you? A PR?

[Andrei Korzhevskii]

Sounds good. Thanks!

[Andrei Korzhevskii]

In this case I wanted 'new Promise', not 'Promise.resolve'. Thanks for spotting that out! I've updated my branch.

On Tuesday, September 6, 2016 at 10:09:28 PM UTC+3, Ian Preston wrote:

[James Horsley]

This is brilliant. I would love to see this in GWT proper. Maybe even a separate library temporarily since it won't make the 2.8.0 release train?

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[Benjamin DeLillo]

I decided to package this up in a standalone library so it can be used until such time that it makes it into a GWT release.

https://github.com/OneGeek/GWT-CompletableFuture

[Peter Donald]

That is pretty awesome. Thanks!

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Cheers,

Peter Donald