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Spring Reactive. How wait for all monos to finish?

I have the following code where I call external APIs via webclient and return Mono.
I need to execute some logic when I receive data. And after all, requests are processed, execute one logic for all gathered data. I can collect all Monos and put them to flux and then execute some logic at the end. But I have serviceName filed which is accessible only in the loop, so I need to execute logic for mono in loop and here I'm stuck and don't know how to wait for all data to complete and do it in a reactive way.
#Scheduled(fixedDelay = 50000)
public void refreshSwaggerConfigurations() {
log.debug("Starting Service Definition Context refresh");
List<SwaggerServiceData> allServicesApi = new ArrayList<>();
swaggerProperties.getUrls().forEach((serviceName, serviceSwaggerUrl) -> {
log.debug("Attempting service definition refresh for Service : {} ", serviceName);
Mono<SwaggerServiceData> swaggerData = getSwaggerDefinitionForAPI(serviceName,
serviceSwaggerUrl);
swaggerData.subscribe(swaggerServiceData -> {
if (swaggerServiceData != null) {
allServicesApi.add(swaggerServiceData);
String content = getJSON(swaggerServiceData);
definitionContext.addServiceDefinition(serviceName, content);
} else {
log.error("Skipping service id : {} Error : Could not get Swagger definition from API ",
serviceName);
}
});
});
//I need to wait here for all monos to complete and after that proceed for All gathered data...
//Now it's empty And I know why, just don't know how to make it.
Optional<SwaggerServiceData> swaggerAllServicesData = getAllServicesApiSwagger(allServicesApi);
if (swaggerAllServicesData.isPresent()) {
String allApiContent = getJSON(swaggerAllServicesData.get());
definitionContext.addServiceDefinition("All", allApiContent);
}
}
private Mono<SwaggerServiceData> getSwaggerDefinitionForAPI(String serviceName, String url) {
log.debug("Accessing the SwaggerDefinition JSON for Service : {} : URL : {} ", serviceName,
url);
Mono<SwaggerServiceData> swaggerServiceDataMono = webClient.get()
.uri(url)
.exchangeToMono(clientResponse -> clientResponse.bodyToMono(SwaggerServiceData.class));
return swaggerServiceDataMono;
}

I would add a temporary class to group data and serivce name :
record SwaggerService(SwaggerServiceData swaggerServiceData, String serviceName) {
boolean hasData() {
return swaggerServiceData != null;
}
}
And then change your pipeline :
Flux.fromStream(swaggerProperties.getUrls().entrySet().stream())
.flatMap((e) -> {
Mono<SwaggerServiceData> swaggerDefinitionForAPI = getSwaggerDefinitionForAPI(e.getKey(),
e.getValue());
return swaggerDefinitionForAPI.map(swaggerServiceData -> new SwaggerService(swaggerServiceData, e.getKey()));
})
.filter(SwaggerService::hasData)
.map(swaggerService -> {
String content = getJSON(swaggerService.swaggerServiceData());
definitionContext.addServiceDefinition(swaggerService.serviceName(), content);
return swaggerService.swaggerServiceData();
})
// here we will collect all datas and they will be emmited as single Mono with list of SwaggerServiceData
.collectList()
.map(this::getAllServicesApiSwagger)
.filter(Optional::isPresent)
.map(Optional::get)
.subscribe(e -> {
String allApiContent = getJSON(e);
definitionContext.addServiceDefinition("All", allApiContent);
});
This does not deal with logging error when SwaggerServiceData is null but you can further change it if you want. Also I assume that DefinitionContext is thread safe.
Solution with error logging (using flatMap and Mono.empty()) :
Flux.fromStream(swaggerProperties.getUrls().entrySet().stream())
.flatMap((e) -> {
Mono<SwaggerServiceData> swaggerDefinitionForAPI = getSwaggerDefinitionForAPI(e.getKey(),
e.getValue());
return swaggerDefinitionForAPI
.flatMap(swaggerServiceData -> {
if(swaggerServiceData != null) {
return Mono.just(new SwaggerService(swaggerServiceData, e.getKey()));
} else {
log.error("Skipping service id : {} Error : Could not get Swagger definition from API ",
e.getKey());
return Mono.empty();
}
});
})
.map(swaggerService -> {
String content = getJSON(swaggerService.swaggerServiceData());
definitionContext.addServiceDefinition(swaggerService.serviceName(), content);
return swaggerService.swaggerServiceData();
}).collectList()
.map(this::getAllServicesApiSwagger)
.filter(Optional::isPresent)
.map(Optional::get)
.subscribe(e -> {
String allApiContent = getJSON(e);
definitionContext.addServiceDefinition("All", allApiContent);
});
You can also wrap those lambads into some meaningful methods to improve readibility.

How to merge multiple vertx web client responses

I am new to vertx and async programming.
I have 2 verticles communicating via an event bus as follows:
//API Verticle
public class SearchAPIVerticle extends AbstractVerticle {
public static final String GET_USEARCH_DOCS = "get.usearch.docs";
#Autowired
private Integer defaultPort;
private void sendSearchRequest(RoutingContext routingContext) {
final JsonObject requestMessage = routingContext.getBodyAsJson();
final EventBus eventBus = vertx.eventBus();
eventBus.request(GET_USEARCH_DOCS, requestMessage, reply -> {
if (reply.succeeded()) {
Logger.info("Search Result = " + reply.result().body());
routingContext.response()
.putHeader("content-type", "application/json")
.setStatusCode(200)
.end((String) reply.result().body());
} else {
Logger.info("Document Search Request cannot be processed");
routingContext.response()
.setStatusCode(500)
.end();
}
});
}
#Override
public void start() throws Exception {
Logger.info("Starting the Gateway service (Event Sender) verticle");
// Create a Router
Router router = Router.router(vertx);
//Added bodyhandler so we can process json messages via the event bus
router.route().handler(BodyHandler.create());
// Mount the handler for incoming requests
// Find documents
router.post("/api/search/docs/*").handler(this::sendSearchRequest);
// Create an HTTP Server using default options
HttpServer server = vertx.createHttpServer();
// Handle every request using the router
server.requestHandler(router)
//start listening on port 8083
.listen(config().getInteger("http.port", 8083)).onSuccess(msg -> {
Logger.info("*************** Search Gateway Server started on "
+ server.actualPort() + " *************");
});
}
#Override
public void stop(){
//house keeping
}
}
//Below is the target verticle should be making the multiple web client call and merging the responses
.
#Component
public class SolrCloudVerticle extends AbstractVerticle {
public static final String GET_USEARCH_DOCS = "get.usearch.docs";
#Autowired
private SearchRepository searchRepositoryService;
#Override
public void start() throws Exception {
Logger.info("Starting the Solr Cloud Search Service (Event Consumer) verticle");
super.start();
ConfigStoreOptions fileStore = new ConfigStoreOptions().setType("file")
.setConfig(new JsonObject().put("path", "conf/config.json"));
ConfigRetrieverOptions configRetrieverOptions = new ConfigRetrieverOptions()
.addStore(fileStore);
ConfigRetriever configRetriever = ConfigRetriever.create(vertx, configRetrieverOptions);
configRetriever.getConfig(ar -> {
if (ar.succeeded()) {
JsonObject configJson = ar.result();
EventBus eventBus = vertx.eventBus();
eventBus.<JsonObject>consumer(GET_USEARCH_DOCS).handler(getDocumentService(searchRepositoryService, configJson));
Logger.info("Completed search service event processing");
} else {
Logger.error("Failed to retrieve the config");
}
});
}
private Handler<Message<JsonObject>> getDocumentService(SearchRepository searchRepositoryService, JsonObject configJson) {
return requestMessage -> vertx.<String>executeBlocking(future -> {
try {
//I need to incorporate the logic here that adds futures to list and composes the compositefuture
/*
//Below is my logic to populate the future list
WebClient client = WebClient.create(vertx);
List<Future> futureList = new ArrayList<>();
for (Object collection : searchRepositoryService.findAllCollections(configJson).getJsonArray(SOLR_CLOUD_COLLECTION).getList()) {
Future<String> future1 = client.post(8983, "127.0.0.1", "/solr/" + collection + "/query")
.expect(ResponsePredicate.SC_OK)
.sendJsonObject(requestMessage.body())
.map(HttpResponse::bodyAsString).recover(error -> {
System.out.println(error.getMessage());
return Future.succeededFuture();
});
futureList.add(future1);
}
//Below is the CompositeFuture logic, but the logic and construct does not make sense to me. What goes as first and second argument of executeBlocking method
/*CompositeFuture.join(futureList)
.onSuccess(result -> {
result.list().forEach( x -> {
if(x != null){
requestMessage.reply(result.result());
}
}
);
})
.onFailure(error -> {
System.out.println("We should not fail");
})
*/
future.complete("DAO returns a Json String");
} catch (Exception e) {
future.fail(e);
}
}, result -> {
if (result.succeeded()) {
requestMessage.reply(result.result());
} else {
requestMessage.reply(result.cause()
.toString());
}
});
}
}
I was able to use the org.springframework.web.reactive.function.client.WebClient calls to compose my search result from multiple web client calls, as against using Future<io.vertx.ext.web.client.WebClient> with CompositeFuture.
I was trying to avoid mixing Springboot and Vertx, but unfortunately Vertx CompositeFuture did not work here:
//This method supplies the parameter for the future.complete(..) line in getDocumentService(SearchRepository,JsonObject)
private List<JsonObject> findByQueryParamsAndDataSources(SearchRepository searchRepositoryService,
JsonObject configJson,
JsonObject requestMessage)
throws SolrServerException, IOException {
List<JsonObject> searchResultList = new ArrayList<>();
for (Object collection : searchRepositoryService.findAllCollections(configJson).getJsonArray(SOLR_CLOUD_COLLECTION).getList()) {
searchResultList.add(new JsonObject(doSearchPerCollection(collection.toString(), requestMessage.toString())));
}
return aggregateMultiCollectionSearchResults(searchResultList);
}
public String doSearchPerCollection(String collection, String message) {
org.springframework.web.reactive.function.client.WebClient client =
org.springframework.web.reactive.function.client.WebClient.create();
return client.post()
.uri("http://127.0.0.1:8983/solr/" + collection + "/query")
.contentType(MediaType.APPLICATION_JSON)
.accept(MediaType.APPLICATION_JSON)
.body(BodyInserters.fromValue(message.toString()))
.retrieve()
.bodyToMono(String.class)
.block();
}
private List<JsonObject> aggregateMultiCollectionSearchResults(List<JsonObject> searchList){
//TODO: Search result aggregation
return searchList;
}
My use case is the second verticle should make multiple vertx web client calls and should combine the responses.
If an API call falls, I want to log the error and still continue processing and merging responses from other calls.
Please, any help on how my code above could be adaptable to handle the use case?
I am looking at vertx CompositeFuture, but no headway or useful example seen yet!

What you are looking for can done with Future coordination with a little bit of additional handling:
CompositeFuture.join(future1, future2, future3).onComplete(ar -> {
if (ar.succeeded()) {
// All succeeded
} else {
// All completed and at least one failed
}
});
The join composition waits until all futures are completed, either with a success or a failure.
CompositeFuture.join
takes several futures arguments (up to 6) and returns a future that is succeeded when all the futures are succeeded, and failed when all the futures are completed and at least one of them is failed
Using join you will wait for all Futures to complete, the issue is that if one of them fails you will not be able to obtain response from others as CompositeFuture will be failed. To avoid this you should add Future<T> recover(Function<Throwable, Future<T>> mapper) on each of your Futures in which you should log the error and pass an empty response so that the future does not fail.
Here is short example:
Future<String> response1 = client.post(8887, "localhost", "work").expect(ResponsePredicate.SC_OK).send()
.map(HttpResponse::bodyAsString).recover(error -> {
System.out.println(error.getMessage());
return Future.succeededFuture();
});
Future<String> response2 = client.post(8887, "localhost", "error").expect(ResponsePredicate.SC_OK).send()
map(HttpResponse::bodyAsString).recover(error -> {
System.out.println(error.getMessage());
return Future.succeededFuture();
});
CompositeFuture.join(response2, response1)
.onSuccess(result -> {
result.list().forEach(x -> {
if(x != null) {
System.out.println(x);
}
});
})
.onFailure(error -> {
System.out.println("We should not fail");
});
Edit 1:
Limit for CompositeFuture.join(Future...) is 6 Futures, in the case you need more you can use: CompositeFuture.join(Arrays.asList(future1, future2, future3)); where you can pass unlimited number of futures.

Transform Observable onError and emit an item

In my Android App I have a generated (Swagger) ApiClient which gets initialized with a Token to identify the User.
This Token (which comes from the Server) can expire.
I get Observable´s with Data from my WebService via myApiClient.myServiceMethod(params)
When I get 401 from the Server it means my Token is expired and I have to initialize myApiClient again (to get it with an unexpired Token).
How myApiClient is initialized. It is returned by getMyApiClient()
//getOkHttpClientWithToken() first issues another Server call to get an
//unexpired Token and then returns an OkHttpClient with that Token set.
myApiClient = new ApiClient()
.getAdapterBuilder()
.baseUrl(url)
.client(getOkHttpClientWithToken())
.build()
.create(MyApiClient.class);
I get the Observable from myApiClient calls
Observable<Result> getResultObservable(Type param1, Type param2) {
return Observable.just(getMyApiClient())
.flatMap(myApiClient ->
myApiClient.getResult(param1, param2).cache()
);
}
What I tried is
//How to recreate myApiClient and Retry call on new myApiClient when an Error occurs
getResultObservable(param1, param2)
.take(1)
.subscribe(result -> {
doSomethingWithResult();
}
});
Works but is done on every error, not just 401 and may never end
I need something like
getResultObservable(param1, param2)
.take(1)
.subscribe(result -> {
doSomethingWithResult();
}, e -> {
if(e.getMessage.equals("HTTP 401")) {
"Transform this Observable to getResultObservable with new myApiClient and emit Result in onNext"
} else {
"Other error inform User and stop."
}
}
});

You may use Observable#onErrorResumeNext to provide an fallback-observable. You can check the exception for typ and return a fallback-observable, if given condition is met or just wrap the exception with Observable#error.
Please have a look at the test, how #onErrorResumNext is used, to provide an fallback-observable, when some exception happens.
#Test
void nameX() {
Observable<String> stringObservable = get();
Observable<String> fallback$ =
stringObservable
.doOnError(s -> System.out.println("fail -> " + s.getMessage()))
.onErrorResumeNext(
throwable -> {
if (throwable instanceof MyException) {
return fallBack().doOnNext(s -> System.out.println("use fallback value " + s));
} else {
return Observable.error(throwable);
}
});
fallback$
.test()
.assertNotComplete()
.assertValueCount(1)
.assertValueAt(0, s -> "Wurst".equals(s));
}
private Observable<String> get() {
return Observable.error(new MyException("Fail"));
}
private Observable<String> fallBack() {
return Observable.just("Wurst").mergeWith(Observable.never());
}
private static final class MyException extends Exception {
MyException(String message) {
super(message);
}
}

Combining many ReactiveX streams into one result stream

I am trying to understand ReactiveX using RxJava but I can't get the whole Reactive idea. My case is the following:
I have Task class. It has perform() method which is executing an HTTP request and getting a response through executeRequest() method. The request may be executed many times (defined number of repetitions). I want to grab all the results of executeRequest() and combine them into Flowable data stream so I can return this Flowable in perform() method. So in the end I want my method to return all results of the requests that my Task executed.
executeRequest() returns Single because it executes only one request and may provide only one response or not at all (in case of timeout).
In perform() I create Flowable range of numbers for each repetition. Subscribed to this Flowable I execute a request per repetition. I additionally subscribe to each response Single for logging and gathering responses into a collection for later. So now I have a set of Singles, how can I merge them into Flowable to return it in perform()? I tried to mess around with operators like merge() but I don't understand its parameters types.
I've read some guides on the web but they all are very general or don't provide examples according to my case.
public Flowable<HttpClientResponse> perform() {
Long startTime = System.currentTimeMillis();
List<HttpClientResponse> responses = new ArrayList<>();
List<Long> failedRepetitionNumbers = new ArrayList<>();
Flowable.rangeLong(0, repetitions)
.subscribe(repetition -> {
logger.debug("Performing repetition {} of {}", repetition + 1, repetitions);
Long currentTime = System.currentTimeMillis();
if (durationCap == 0 || currentTime - startTime < durationCap) {
Single<HttpClientResponse> response = executeRequest(method, url, headers, body);
response.subscribe(successResult -> {
logger.info("Received response with code {} in the {}. repetition.", successResult
.statusCode(), repetition + 1);
responses.add(successResult);
},
error -> {
logger.error("Failed to receive response from {}.", url);
failedRepetitionNumbers.add(repetition);
});
waitInterval(minInterval, maxInterval);
} else {
logger.info("Reached duration cap of {}ms for task {}.", durationCap, this);
}
});
return Flowable.merge(???);
}
And executeRequest()
private Single<HttpClientResponse> executeRequest(HttpMethod method, String url, LinkedMultiValueMap<String, String>
headers, JsonNode body) {
CompletableFuture<HttpClientResponse> responseFuture = new CompletableFuture<>();
HttpClient client = vertx.createHttpClient();
HttpClientRequest request = client.request(method, url, responseFuture::complete);
headers.forEach(request::putHeader);
request.write(body.toString());
request.setTimeout(timeout);
request.end();
return Single.fromFuture(responseFuture);
}

Instead of subscribing to each observable(each HTTP request) within your perform method, Just keep on chaining the observables like this. Your code can be reduced to something like.
public Flowable<HttpClientResponse> perform() {
// Here return a flowable , which can emit n number of times. (where n = your number of HTTP requests)
return Flowable.rangeLong(0, repetitions) // start a counter
.doOnNext(repetition -> logger.debug("Performing repetition {} of {}", repetition + 1, repetitions)) // print the current count
.flatMap(count -> executeRequest(method, url, headers, body).toFlowable()) // get the executeRequest as Flowable
.timeout(durationCap, TimeUnit.MILLISECONDS); // apply a timeout policy
}
And finally, you can subscribe to the perform at the place where you actually need to execute all this, As shown below
perform()
.subscribeWith(new DisposableSubscriber<HttpClientResponse>() {
#Override
public void onNext(HttpClientResponse httpClientResponse) {
// onNext will be triggered each time, whenever a request has executed and ready with result
// if you had 5 HTTP request, this can trigger 5 times with each "httpClientResponse" (if all calls were success)
}
#Override
public void onError(Throwable t) {
// any error during the execution of these request,
// including a TimeoutException in case timeout happens in between
}
#Override
public void onComplete() {
// will be called finally if no errors happened and onNext delivered all the results
}
});

Retry logic with CompletableFuture

I need to submit a task in an async framework I'm working on, but I need to catch for exceptions, and retry the same task multiple times before "aborting".
The code I'm working with is:
int retries = 0;
public CompletableFuture<Result> executeActionAsync() {
// Execute the action async and get the future
CompletableFuture<Result> f = executeMycustomActionHere();
// If the future completes with exception:
f.exceptionally(ex -> {
retries++; // Increment the retry count
if (retries < MAX_RETRIES)
return executeActionAsync(); // <--- Submit one more time
// Abort with a null value
return null;
});
// Return the future
return f;
}
This currently doesn't compile because the return type of the lambda is wrong: it expects a Result, but the executeActionAsync returns a CompletableFuture<Result>.
How can I implement this fully async retry logic?

Chaining subsequent retries can be straight-forward:
public CompletableFuture<Result> executeActionAsync() {
CompletableFuture<Result> f=executeMycustomActionHere();
for(int i=0; i<MAX_RETRIES; i++) {
f=f.exceptionally(t -> executeMycustomActionHere().join());
}
return f;
}
Read about the drawbacks below
This simply chains as many retries as intended, as these subsequent stages won’t do anything in the non-exceptional case.
One drawback is that if the first attempt fails immediately, so that f is already completed exceptionally when the first exceptionally handler is chained, the action will be invoked by the calling thread, removing the asynchronous nature of the request entirely. And generally, join() may block a thread (the default executor will start a new compensation thread then, but still, it’s discouraged). Unfortunately, there is neither, an exceptionallyAsync or an exceptionallyCompose method.
A solution not invoking join() would be
public CompletableFuture<Result> executeActionAsync() {
CompletableFuture<Result> f=executeMycustomActionHere();
for(int i=0; i<MAX_RETRIES; i++) {
f=f.thenApply(CompletableFuture::completedFuture)
.exceptionally(t -> executeMycustomActionHere())
.thenCompose(Function.identity());
}
return f;
}
demonstrating how involved combining “compose” and an “exceptionally” handler is.
Further, only the last exception will be reported, if all retries failed. A better solution should report the first exception, with subsequent exceptions of the retries added as suppressed exceptions. Such a solution can be build by chaining a recursive call, as hinted by Gili’s answer, however, in order to use this idea for exception handling, we have to use the steps to combine “compose” and “exceptionally” shown above:
public CompletableFuture<Result> executeActionAsync() {
return executeMycustomActionHere()
.thenApply(CompletableFuture::completedFuture)
.exceptionally(t -> retry(t, 0))
.thenCompose(Function.identity());
}
private CompletableFuture<Result> retry(Throwable first, int retry) {
if(retry >= MAX_RETRIES) return CompletableFuture.failedFuture(first);
return executeMycustomActionHere()
.thenApply(CompletableFuture::completedFuture)
.exceptionally(t -> { first.addSuppressed(t); return retry(first, retry+1); })
.thenCompose(Function.identity());
}
CompletableFuture.failedFuture is a Java 9 method, but it would be trivial to add a Java 8 compatible backport to your code if needed:
public static <T> CompletableFuture<T> failedFuture(Throwable t) {
final CompletableFuture<T> cf = new CompletableFuture<>();
cf.completeExceptionally(t);
return cf;
}

Instead of implementing your own retry logic, I recommend using a proven library like failsafe, which has built-in support for futures (and seems more popular than guava-retrying). For your example, it would look something like:
private static RetryPolicy retryPolicy = new RetryPolicy()
.withMaxRetries(MAX_RETRIES);
public CompletableFuture<Result> executeActionAsync() {
return Failsafe.with(retryPolicy)
.with(executor)
.withFallback(null)
.future(this::executeMycustomActionHere);
}
Probably you should avoid .withFallback(null) and just have let the returned future's .get() method throw the resulting exception so the caller of your method can handle it specifically, but that's a design decision you'll have to make.
Other things to think about include whether you should retry immediately or wait some period of time between attempts, any sort of recursive backoff (useful when you're calling a web service that might be down), and whether there are specific exceptions that aren't worth retrying (e.g. if the parameters to the method are invalid).

I think I was successfully. Here's an example class I created and the test code:
RetriableTask.java
public class RetriableTask
{
protected static final int MAX_RETRIES = 10;
protected int retries = 0;
protected int n = 0;
protected CompletableFuture<Integer> future = new CompletableFuture<Integer>();
public RetriableTask(int number) {
n = number;
}
public CompletableFuture<Integer> executeAsync() {
// Create a failure within variable timeout
Duration timeoutInMilliseconds = Duration.ofMillis(1*(int)Math.pow(2, retries));
CompletableFuture<Integer> timeoutFuture = Utils.failAfter(timeoutInMilliseconds);
// Create a dummy future and complete only if (n > 5 && retries > 5) so we can test for both completion and timeouts.
// In real application this should be a real future
final CompletableFuture<Integer> taskFuture = new CompletableFuture<>();
if (n > 5 && retries > 5)
taskFuture.complete(retries * n);
// Attach the failure future to the task future, and perform a check on completion
taskFuture.applyToEither(timeoutFuture, Function.identity())
.whenCompleteAsync((result, exception) -> {
if (exception == null) {
future.complete(result);
} else {
retries++;
if (retries >= MAX_RETRIES) {
future.completeExceptionally(exception);
} else {
executeAsync();
}
}
});
// Return the future
return future;
}
}
Usage
int size = 10;
System.out.println("generating...");
List<RetriableTask> tasks = new ArrayList<>();
for (int i = 0; i < size; i++) {
tasks.add(new RetriableTask(i));
}
System.out.println("issuing...");
List<CompletableFuture<Integer>> futures = new ArrayList<>();
for (int i = 0; i < size; i++) {
futures.add(tasks.get(i).executeAsync());
}
System.out.println("Waiting...");
for (int i = 0; i < size; i++) {
try {
CompletableFuture<Integer> future = futures.get(i);
int result = future.get();
System.out.println(i + " result is " + result);
} catch (Exception ex) {
System.out.println(i + " I got exception!");
}
}
System.out.println("Done waiting...");
Output
generating...
issuing...
Waiting...
0 I got exception!
1 I got exception!
2 I got exception!
3 I got exception!
4 I got exception!
5 I got exception!
6 result is 36
7 result is 42
8 result is 48
9 result is 54
Done waiting...
Main idea and some glue code (failAfter function) come from here.
Any other suggestions or improvement are welcome.

util class:
public class RetryUtil {
public static <R> CompletableFuture<R> retry(Supplier<CompletableFuture<R>> supplier, int maxRetries) {
CompletableFuture<R> f = supplier.get();
for(int i=0; i<maxRetries; i++) {
f=f.thenApply(CompletableFuture::completedFuture)
.exceptionally(t -> {
System.out.println("retry for: "+t.getMessage());
return supplier.get();
})
.thenCompose(Function.identity());
}
return f;
}
}
usage:
public CompletableFuture<String> lucky(){
return CompletableFuture.supplyAsync(()->{
double luckNum = Math.random();
double luckEnough = 0.6;
if(luckNum < luckEnough){
throw new RuntimeException("not luck enough: " + luckNum);
}
return "I'm lucky: "+luckNum;
});
}
#Test
public void testRetry(){
CompletableFuture<String> retry = RetryUtil.retry(this::lucky, 10);
System.out.println("async check");
String join = retry.join();
System.out.println("lucky? "+join);
}
output
async check
retry for: java.lang.RuntimeException: not luck enough: 0.412296354211683
retry for: java.lang.RuntimeException: not luck enough: 0.4099777199676573
lucky? I'm lucky: 0.8059089479049389

I recently solved a similar problem using the guava-retrying library.
Callable<Result> callable = new Callable<Result>() {
public Result call() throws Exception {
return executeMycustomActionHere();
}
};
Retryer<Boolean> retryer = RetryerBuilder.<Result>newBuilder()
.retryIfResult(Predicates.<Result>isNull())
.retryIfExceptionOfType(IOException.class)
.retryIfRuntimeException()
.withStopStrategy(StopStrategies.stopAfterAttempt(MAX_RETRIES))
.build();
CompletableFuture.supplyAsync( () -> {
try {
retryer.call(callable);
} catch (RetryException e) {
e.printStackTrace();
} catch (ExecutionException e) {
e.printStackTrace();
}
});

Here is an approach that will work for any CompletionStage subclass and does not return a dummy CompletableFuture that does nothing more than wait to get updated by other futures.
/**
* Sends a request that may run as many times as necessary.
*
* #param request a supplier initiates an HTTP request
* #param executor the Executor used to run the request
* #return the server response
*/
public CompletionStage<Response> asyncRequest(Supplier<CompletionStage<Response>> request, Executor executor)
{
return retry(request, executor, 0);
}
/**
* Sends a request that may run as many times as necessary.
*
* #param request a supplier initiates an HTTP request
* #param executor the Executor used to run the request
* #param tries the number of times the operation has been retried
* #return the server response
*/
private CompletionStage<Response> retry(Supplier<CompletionStage<Response>> request, Executor executor, int tries)
{
if (tries >= MAX_RETRIES)
throw new CompletionException(new IOException("Request failed after " + MAX_RETRIES + " tries"));
return request.get().thenComposeAsync(response ->
{
if (response.getStatusInfo().getFamily() != Response.Status.Family.SUCCESSFUL)
return retry(request, executor, tries + 1);
return CompletableFuture.completedFuture(response);
}, executor);
}

maybe it's late but hopes someone might find this useful, I recently solved this problem for retrying rest API call on failure. In my case, I have to retry on 500 HTTP status code, below is my rest client code (we are using WSClient from play framework) you can change it to whatever rest-client as per requirement.
int MAX_RETRY = 3;
CompletableFuture<WSResponse> future = new CompletableFuture<>();
private CompletionStage<WSResponse> getWS(Object request,String url, int retry, CompletableFuture future) throws JsonProcessingException {
ws.url(url)
.post(Json.parse(mapper.writeValueAsString(request)))
.whenCompleteAsync((wsResponse, exception) -> {
if(wsResponse.getStatus() == 500 && retry < MAX_RETRY) {
try {
getWS(request, retry+1, future);
} catch (IOException e) {
throw new Exception(e);
}
}else {
future.complete(wsResponse);
}
});
return future;
}
This code will return immediately if the status code is 200 or other than 500 whereas if HTTP status is 500 it will retry 3 times.

Inspired by theazureshadow's answer. His or her answer was great but doesn't work with new version of FailSafe. The below code works with
<dependency>
<groupId>dev.failsafe</groupId>
<artifactId>failsafe</artifactId>
<version>3.3.0</version>
</dependency>
solution:
RetryPolicy<Object> retryPolicy = RetryPolicy.builder()
.withMaxRetries(MAX_RETRY)
.withBackoff(INITIAL_DELAY, MAX_DELAY, ChronoUnit.SECONDS)
.build();
Fallback<Object> fallback = Fallback.of((AuditEvent) null);
public CompletableFuture<Object> executeAsync(Runnable asyncTask) {
return Failsafe.with(fallback)
.compose(retryPolicy)
.with(executorService)
.onFailure(e -> LOG.error(e.getException().getMessage()))
.getAsync(() -> asyncTask());
}

We needed to retry a task based on an error condition.
public static <T> CompletableFuture<T> retryOnCondition(Supplier<CompletableFuture<T>> supplier,
Predicate<Throwable> retryPredicate, int maxAttempts) {
if (maxAttempts <= 0) {
throw new IllegalArgumentException("maxAttempts can't be <= 0");
}
return retryOnCondition(supplier, retryPredicate, null, maxAttempts);
}
private static <T> CompletableFuture<T> retryOnCondition(
Supplier<CompletableFuture<T>> supplier, Predicate<Throwable> retryPredicate,
Throwable lastError, int attemptsLeft) {
if (attemptsLeft == 0) {
return CompletableFuture.failedFuture(lastError);
}
return supplier.get()
.thenApply(CompletableFuture::completedFuture)
.exceptionally(error -> {
boolean doRetry = retryPredicate.test(error);
int attempts = doRetry ? attemptsLeft - 1 : 0;
return retryOnCondition(supplier, retryPredicate, error, attempts);
})
.thenCompose(Function.identity());
}
Usage:
public static void main(String[] args) {
retryOnCondition(() -> myTask(), e -> {
//log exception
return e instanceof MyException;
}, 3).join();
}

I would suggest using resilience4j for this use case. It's very handy!!
Introduction: resilience4j-retry and its Javadoc: Retry
They have method to decorate completionStage directly as below:
default <T> java.util.concurrent.CompletionStage<T> executeCompletionStage​(java.util.concurrent.ScheduledExecutorService scheduler,
java.util.function.Supplier<java.util.concurrent.CompletionStage<T>> supplier)