How to execute an Array of CompletableFuture and compose their results

问题

I am investigating Java 8 CompletableFutures and read (and seen) that I should employ thenCompose instead of thenApply.

I have converted my code to use thenCompose but I have a feeling in an incorrect manner.

Here is my controlling code...

final CompletableFuture<List<String>> extractor = get(htmlPageSource);
@SuppressWarnings("unchecked")
final CompletableFuture<List<Documentable>>[] completableFutures =
   new CompletableFuture[ENDPOINT.EXTRACTABLES.size()];
int index = 0;
for( ENDPOINT endpoint : ENDPOINT.EXTRACTABLES ) {
   final CompletableFuture<List<Documentable>> metaData =
      extractor.thenComposeAsync(
         s -> endpoint.contactEndpoit(s), executorService );
   completableFutures[index++] = metaData.exceptionally(x -> failedList(x));
}
CompletableFuture
   .allOf( completableFutures )
   .thenComposeAsync( dummy -> combineDocuments( completableFutures ))
   .thenAccept   ( x -> finish( x ))
   .exceptionally( x -> failed( x ));

private List<Documentable> failedList(final Throwable x) {
   LOGGER.error("failedList", x);
   final List<Documentable> metaData = new ArrayList<>();
   return metaData;
}

private Void failed(final Throwable x) {
   LOGGER.error("failed", x);
   return null;
}

Which I believe is acceptable

However the code that makes me uneasy is this:-

WWW_SITE_ONE("https://example.site.one/") {
   @Override
   public <T extends Documentable> CompletionStage<List<T>> contactEndpoit( final List<String> elements) {
      LOGGER.info("WWW_SITE_ONE " + Thread.currentThread().getName());
      final List<T> SITE_ONEs = new ArrayList<>();
      for (final String element : elements) {
         try {
            final String json = Jsoup.connect(ENDPOINT.WWW_SITE_ONE.getBaseUrl() + element).ignoreContentType(true).ignoreHttpErrors(true).maxBodySize(0).timeout(60000).execute().body();
            if (json.contains("errors")) {
               continue;
            }
            final T SITE_ONE = OBJECT_READER_SITE_ONE.readValue(json);
            SITE_ONEs.add(SITE_ONE);
         }
         catch( final Throwable e ) {
            LOGGER.error("WWW_SITE_ONE failed", e);
            throw new RuntimeException(e);
         }
      }
      return CompletableFuture.supplyAsync(() -> SITE_ONEs);  
   }
},
WWW_SITE_TWO("https://example.site.two/") {
   @Override
   public <T extends Documentable> CompletionStage<List<T>> contactEndpoit(final List<String> elements) {
      LOGGER.info("WWW_SITE_TWO " + Thread.currentThread().getName());      
      final List<T> SITE_TWOs = new ArrayList<>();
      for (final String element : elements) {
         try {
            final String json = Jsoup.connect(ENDPOINT.WWW_SITE_TWO.getBaseUrl() + element).ignoreContentType(true).ignoreHttpErrors(true).maxBodySize(0).timeout(60000).execute().body();
            if (json.equals("Resource not found.")) {
               continue;
            }
            final T SITE_TWO = OBJECT_READER_SITE_TWO.readValue(json);
            SITE_TWOs.add(SITE_TWO);
         }
         catch (final Throwable e) {
            LOGGER.error("WWW_SITE_TWO failed", e);
            throw new RuntimeException(e);
         }
      }
      return CompletableFuture.supplyAsync(() -> SITE_TWOs);  
   }
},
WWW_SITE_THREE("https://example.site.three/") {
   @Override
   public <T extends Documentable> CompletionStage<List<T>> contactEndpoit(final List<String> elements) {
      LOGGER.info("WWW_SITE_THREE " + Thread.currentThread().getName());        
      final List<T> SITE_THREEs = new ArrayList<>();
      for (final String element : elements) {
         try {
            final String SITE_THREEJsonString = Jsoup
               .connect( ENDPOINT.WWW_SITE_THREE.getBaseUrl() + element)
               .ignoreContentType(true)
               .ignoreHttpErrors(true)
               .maxBodySize(0)
               .timeout(60000)
               .execute()
               .body();
            final SITE_THREE SITE_THREE_Json = OBJECT_READER_SITE_THREE.readValue(SITE_THREEJsonString);
            final T SITE_THREE = (T) SITE_THREE_Json;
            if (SITE_THREE_Json.getHitCount() > 0) {
               SITE_THREEs.add(SITE_THREE);
            }
         }
         catch (final Throwable e) {
            LOGGER.error("WWW_SITE_THREE failed", e);
            throw new RuntimeException(e);
         }
      }
      return CompletableFuture.supplyAsync(() -> SITE_THREEs);  
   }
};

Its where I am returning CompletableFuture.supplyAsync(() -> SITE_THREEs);

Is this the correct approach?

Or does this start another asynchronous thread to simply return my List<>?

回答1:

As the name suggests, supplyAsync will perform an asynchronous operation, executing the Supplier’s get() method, hence the body of the lambda expression, in a background thread, regardless of how trivial it is. Since the implementation of supplyAsync has no way to check how trivial the code encapsulated by the Supplier is, it has to work this way.

Instead of CompletableFuture.supplyAsync(() -> SITE_THREEs), you should use CompletableFuture.completedFuture(SITE_THREEs) which returns a future that has already been completed with the result, hence, not requiring additional actions.

If the method only returns completed stages or throws an exception, you may also change it to return the result value instead of a CompletionStage and use thenApply instead of thenCompose, simplifying your code—unless you want to keep the option of introducing asynchronous operations in a future version of that method.

来源：https://stackoverflow.com/questions/45299556/how-to-execute-an-array-of-completablefuture-and-compose-their-results