How to execute CompletableFuture array and compose their results
I am learning Java 8 CompletableFutures
and am reading (and seeing) what thenApply
to use instead thenCompose
.
I used my code to use it thenCompose
, but I have a misconception.
Here is my control 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;
}
I find it acceptable
However the code that worries me 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'm return
ingCompletableFuture.supplyAsync(() -> SITE_THREEs);
Is this the correct approach?
Or will this start another asynchronous thread to just return my list <>?
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As the name suggests, supplyAsync
will perform an asynchronous operation by executing method Supplier
s get()
, hence the body of the lambda expression, on a background thread, no matter how trivial it is. Since the implementation supplyAsync
has no way of checking how trivial the code enclosed in is Supplier
, it should work that way.
Instead, CompletableFuture.supplyAsync(() -> SITE_THREEs)
you should use CompletableFuture.completedFuture(SITE_THREEs)
one that returns a future that has already been completed with a result, hence requiring no additional action.
If a method only returns completed steps or throws an exception, you can also modify it to return a result value instead CompletionStage
and use it thenApply
instead thenCompose
, simplifying your code if you don't want to retain the ability to inject asynchronous operations in a future version of that method.
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