GWT, Java 8 and Future

Collect GWT

• So, take the gwt source from here and gwt-tools from here .
• After cloning, you need to set the environment variable GWT_TOOLS=path/to/gwt-tools.
• Next, go to the GWT source directory and run ant-build.

Rule RestyGWT

 void makeServerRequest(MethodCallback<Void> callback); 

 Future<Void> makeServerRequest(); 

 public interface Future<T> { public void onComplete(Consumer<Try<T>> consumer); public void handle(Consumer<Throwable> errorHandler, Consumer<T> successHandler); public void forEach(Consumer<T> consumer); public <R> Future<R> map(Function<T, R> function); public <R> Future<R> flatMap(Function<T, Future<R>> function); public T get(); } 

 public class FutureImpl<T> implements Future<T> { private List<Consumer<Try<T>>> completeFunctions = new ArrayList<>(); private Option<Try<T>> result = Option.getEmpty(); public FutureImpl() { } @Override public void onComplete(Consumer<Try<T>> consumer) { result.forEach(consumer::accept); completeFunctions.add(consumer); } @Override public void handle(Consumer<Throwable> errorHandler, Consumer<T> successHandler) { onComplete((result) -> { if (result.isSuccess()) successHandler.accept(result.get()); else errorHandler.accept(result.getCause()); }); } public void completeWithResult(Try<T> result) { this.result = Option.create(result); for (Consumer<Try<T>> completeFunction : completeFunctions) { completeFunction.accept(result); } } public void completeWithSuccess(T result) { completeWithResult(new Success<T>(result)); } public void completeWithFailure(Throwable ex) { completeWithResult(new Failure<T>(ex)); } @Override public void forEach(Consumer<T> consumer) { onComplete((result) -> { if (result.isSuccess()) { consumer.accept(result.get()); } }); } @Override public <R> Future<R> map(Function<T, R> function) { FutureImpl<R> future = new FutureImpl<R>(); onComplete((result) -> { if (result.isSuccess()) { future.completeWithSuccess(function.apply(result.get())); } }); return future; } @Override public <R> FutureImpl<R> flatMap(Function<T, Future<R>> function) { FutureImpl<R> mapped = new FutureImpl<R>(); onComplete((result) -> { if (result.isSuccess()) { Future<R> f = function.apply(result.get()); f.onComplete(mapped::completeWithResult); } }); return mapped; } @Override public T get() { return result.get().get(); } } 

Using

 @Path("../service") @Consumes(MediaType.APPLICATION_JSON) public interface CallbackCountryService extends RestService { @GET @Path("/countires/") public void getCountries(MethodCallback<List<Country>> callback); @GET @Path("/regions/{countryId}/") public void getRegions(@PathParam("countryId") Integer countryId, MethodCallback<List<Region>> callback); } 

1. The simplest example. We want to take a list of countries and display it in our View:
   
   Without Future:
   
   
   
   

    countryService.getCountries(new MethodCallback<List<Country>>() { @Override public void onFailure(Method method, Throwable exception) { } @Override public void onSuccess(Method method, List<Country> response) { view.displayCountries(response); } }); 

   
   
   
   
   With Future:
   
   
   
   

    countryService.getCountries().forEach(view::displayCountries); 

   
   
   The forEach method is a kind of onSuccess callback. That is, if successful, the displayCountries method on the View will be called.
   
   
   
   
2. The example is more complicated. Suppose we need to handle an exception and display it.
   
   Without Future:
   
   
   
   

    countryService.getCountries(new MethodCallback<List<Country>>() { @Override public void onFailure(Method method, Throwable exception) { view.displayError(exception.getMessage()); } @Override public void onSuccess(Method method, List<Country> response) { view.displayCountries(response); } }); 

   
   
   
   
   With Future:
   
   
   
   

    countryService.getCountries().handle(t -> view.displayError(t.getMessage()), view::displayCountries); 

   
   
   We pass two functions to the Future.handle method. One is responsible for handling the error, the second is responsible for handling successful execution with the result.
   
   
   
   
3. We need to take the first country from the list and display a list of regions for it:
   
   Without Future:
   
   
   
   

    countryService.getCountries(new MethodCallback<List<Country>>() { @Override public void onFailure(Method method, Throwable exception) { view.displayError(exception.getMessage()); } @Override public void onSuccess(Method method, List<Country> response) { countryService.getRegions(response.get(0).getId(), new MethodCallback<List<Region>>() { @Override public void onFailure(Method method, Throwable exception) { view.displayError(exception.getMessage()); } @Override public void onSuccess(Method method, List<Region> response) { view.displayRegions(response); } }); } }); 

   
   
   Using Future:
   
   
   
   

    countryService.getCountries() .map(countries -> countries.get(0).getId()) .flatMap(countryService::getRegions) .handle(err -> view.displayError(err.getMessage()), view::displayRegions); 

   
   
   First we convert Future<List> Future, countryId . Future . , , .





.

Future . .



:



Future<List> Future, countryId . Future . , , .





.

Future . .



:





    Future<List<Future<List<Region>>>> regionFutures = countryService.getCountries() .map( countries -> countries.map(country -> countryService.getRegions(country.getId())) ); 

   
   
   Here we get a list of Future of all regions.
   
   
   
   In the next transformation you need to bring

    List<Future>  Future<List>.    Future  ,   Future    . 
       
   
         
         
         
       
   
      
       
   
         
         
         
       
   
      Future<Future<List<List<Region>>>> regions = regionFutures.map(FutureUtils::toFutureOfList); 

   
   
   
   
   And finally, we give

    Future<Future>  Future,         : 
       
   
         
         
         
       
   
      
       
   
         
         
         
       
   
      FutureUtils .flatten(regions) .map(ListUtils::flatten) .handle(err -> view.displayError(err.getMessage()), view::displayRegions()); 

   
   
   
   
   The disadvantage of the last example is that it is quite difficult to understand it to a person who did not participate in the writing of this code. However, the decision was quite compact and has the right to exist.
   
   
   
   
   
   
   
   PS For those who want to try Java 8 in GWT, a demo project has been prepared with examples from the article. The project is mavenesirovan, you can run through mvn jetty: run-exploded.
   
   
   
   It should be understood that it is better not to use all the libraries provided in real projects. Future support in RestyGWT is quite raw, not yet tested, and so far it does not know how to work, for example, with JSONP requests. Support for default interfaces and lambda works fairly confidently, although compilation does not always work when using lambdas in static methods.