The following test fails frequently
import static jdk.incubator.http.HttpClient.Version.HTTP_2;

import java.net.URI;
import java.nio.file.Paths;
import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.CompletableFuture;
import java.util.concurrent.atomic.AtomicLong;
import java.util.regex.Matcher;
import java.util.regex.Pattern;

import org.testng.annotations.BeforeClass;
import org.testng.annotations.Test;

import jdk.incubator.http.HttpClient;
import jdk.incubator.http.HttpClient.Builder;
import jdk.incubator.http.HttpClient.Version;
import jdk.incubator.http.HttpRequest;
import jdk.incubator.http.HttpResponse;

/*
 * @test
 * @run testng/othervm TestMultiFiles
 */
public class TestMultiFiles {// -Djdk.internal.httpclient.debug=true
    private static final List<String> datafileList = new ArrayList<>();

    private static final String dataDir = "http://&lt;host&gt;:8080/yc/data/";


    @BeforeClass
    public void getFileList() throws Exception {
        HttpClient client = getClient(HTTP_2);
        HttpResponse<String> resp = client.send(
                HttpRequest.newBuilder(new URI(dataDir)).GET().build(),
                HttpResponse.BodyHandler.asString());

        Matcher m = Pattern.compile("<a href=\"(.+).data\">").matcher(resp.body());
        while (m.find()) {
            datafileList.add(m.group(1) + ".data");
        }
        
        System.out.println(datafileList);
    }

    @Test
    public void test() throws Exception {
        test(HTTP_2);
    }

    private static HttpClient getClient(Version v) {
        return HttpClient.newBuilder().proxy(Builder.NO_PROXY).version(v).build();
    }

    private void test(Version v) throws Exception {
        HttpClient client = getClient(v);

        int taskIndex = 0;
        CompletableFuture[] results = new CompletableFuture[datafileList.size()];
        
        for (String datafileName : datafileList)
            results[taskIndex++] = client
                    .sendAsync(
                            HttpRequest.newBuilder(new URI(dataDir + datafileName)).GET().build(),
                            HttpResponse.BodyHandler.asFile(Paths.get(datafileName)))
                    .thenApply(resp -> {
                        System.out.println(resp.body());
                        return true;
                    });

        CompletableFuture.allOf(results).join();
    }
}

with protocol error, e.g.
java.util.concurrent.CompletionException: java.io.IOException: protocol error: Frame type(83) length(5537666) exceeds MAX_FRAME_SIZE(16384)
at java.base/java.util.concurrent.CompletableFuture.encodeThrowable(CompletableFuture.java:331)
at java.base/java.util.concurrent.CompletableFuture.completeThrowable(CompletableFuture.java:346)
at java.base/java.util.concurrent.CompletableFuture.uniWhenComplete(CompletableFuture.java:870)
at java.base/java.util.concurrent.CompletableFuture$UniWhenComplete.tryFire(CompletableFuture.java:837)
at java.base/java.util.concurrent.CompletableFuture.postComplete(CompletableFuture.java:506)
at java.base/java.util.concurrent.CompletableFuture.completeExceptionally(CompletableFuture.java:2088)
at jdk.incubator.httpclient/jdk.incubator.http.internal.common.MinimalFuture.complete(MinimalFuture.java:171)
at jdk.incubator.httpclient/jdk.incubator.http.internal.common.MinimalFuture.lambda$of$2(MinimalFuture.java:163)
at java.base/java.util.concurrent.CompletableFuture.uniWhenComplete(CompletableFuture.java:859)
at java.base/java.util.concurrent.CompletableFuture$UniWhenComplete.tryFire(CompletableFuture.java:837)
at java.base/java.util.concurrent.CompletableFuture.postComplete(CompletableFuture.java:506)
at java.base/java.util.concurrent.CompletableFuture.completeExceptionally(CompletableFuture.java:2088)
at jdk.incubator.httpclient/jdk.incubator.http.ResponseSubscribers$PathSubscriber.onError(ResponseSubscribers.java:161)
at jdk.incubator.httpclient/jdk.incubator.http.Stream.schedule(Stream.java:194)
at jdk.incubator.httpclient/jdk.incubator.http.internal.common.SequentialScheduler$SynchronizedRestartableTask.run(SequentialScheduler.java:175)
at jdk.incubator.httpclient/jdk.incubator.http.internal.common.SequentialScheduler$CompleteRestartableTask.run(SequentialScheduler.java:147)
at jdk.incubator.httpclient/jdk.incubator.http.internal.common.SequentialScheduler$SchedulableTask.run(SequentialScheduler.java:198)
at jdk.incubator.httpclient/jdk.incubator.http.internal.common.SequentialScheduler.runOrSchedule(SequentialScheduler.java:271)
at jdk.incubator.httpclient/jdk.incubator.http.internal.common.SequentialScheduler.runOrSchedule(SequentialScheduler.java:224)
at jdk.incubator.httpclient/jdk.incubator.http.Stream.cancelImpl(Stream.java:922)
at jdk.incubator.httpclient/jdk.incubator.http.Http2Connection.shutdown(Http2Connection.java:555)
at jdk.incubator.httpclient/jdk.incubator.http.Http2Connection.protocolError(Http2Connection.java:733)
at jdk.incubator.httpclient/jdk.incubator.http.Http2Connection.processFrame(Http2Connection.java:574)
at jdk.incubator.httpclient/jdk.incubator.http.internal.frame.FramesDecoder.decode(FramesDecoder.java:156)
at jdk.incubator.httpclient/jdk.incubator.http.Http2Connection$FramesController.processReceivedData(Http2Connection.java:195)
at jdk.incubator.httpclient/jdk.incubator.http.Http2Connection.asyncReceive(Http2Connection.java:528)
at jdk.incubator.httpclient/jdk.incubator.http.Http2Connection$Http2TubeSubscriber.processQueue(Http2Connection.java:1036)
at jdk.incubator.httpclient/jdk.incubator.http.internal.common.SequentialScheduler$SynchronizedRestartableTask.run(SequentialScheduler.java:175)
at jdk.incubator.httpclient/jdk.incubator.http.internal.common.SequentialScheduler$CompleteRestartableTask.run(SequentialScheduler.java:147)
at jdk.incubator.httpclient/jdk.incubator.http.internal.common.SequentialScheduler$SchedulableTask.run(SequentialScheduler.java:198)
at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1135)
at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:635)
at java.base/java.lang.Thread.run(Thread.java:844)

or EOFException
java.util.concurrent.CompletionException: java.io.EOFException: EOF reached while reading
at java.base/java.util.concurrent.CompletableFuture.encodeThrowable(CompletableFuture.java:331)
at java.base/java.util.concurrent.CompletableFuture.completeThrowable(CompletableFuture.java:346)
at java.base/java.util.concurrent.CompletableFuture.uniWhenComplete(CompletableFuture.java:870)
at java.base/java.util.concurrent.CompletableFuture$UniWhenComplete.tryFire(CompletableFuture.java:837)
at java.base/java.util.concurrent.CompletableFuture.postComplete(CompletableFuture.java:506)
at java.base/java.util.concurrent.CompletableFuture.completeExceptionally(CompletableFuture.java:2088)
at jdk.incubator.httpclient/jdk.incubator.http.internal.common.MinimalFuture.complete(MinimalFuture.java:171)
at jdk.incubator.httpclient/jdk.incubator.http.internal.common.MinimalFuture.lambda$of$2(MinimalFuture.java:163)
at java.base/java.util.concurrent.CompletableFuture.uniWhenComplete(CompletableFuture.java:859)
at java.base/java.util.concurrent.CompletableFuture$UniWhenComplete.tryFire(CompletableFuture.java:837)
at java.base/java.util.concurrent.CompletableFuture.postComplete(CompletableFuture.java:506)
at java.base/java.util.concurrent.CompletableFuture.completeExceptionally(CompletableFuture.java:2088)
at jdk.incubator.httpclient/jdk.incubator.http.ResponseSubscribers$PathSubscriber.onError(ResponseSubscribers.java:161)
at jdk.incubator.httpclient/jdk.incubator.http.Stream.schedule(Stream.java:194)
at jdk.incubator.httpclient/jdk.incubator.http.internal.common.SequentialScheduler$SynchronizedRestartableTask.run(SequentialScheduler.java:175)
at jdk.incubator.httpclient/jdk.incubator.http.internal.common.SequentialScheduler$CompleteRestartableTask.run(SequentialScheduler.java:147)
at jdk.incubator.httpclient/jdk.incubator.http.internal.common.SequentialScheduler$SchedulableTask.run(SequentialScheduler.java:198)
at jdk.incubator.httpclient/jdk.incubator.http.internal.common.SequentialScheduler.runOrSchedule(SequentialScheduler.java:271)
at jdk.incubator.httpclient/jdk.incubator.http.internal.common.SequentialScheduler.runOrSchedule(SequentialScheduler.java:224)
at jdk.incubator.httpclient/jdk.incubator.http.Stream.cancelImpl(Stream.java:922)
at jdk.incubator.httpclient/jdk.incubator.http.Http2Connection.shutdown(Http2Connection.java:555)
at jdk.incubator.httpclient/jdk.incubator.http.Http2Connection$Http2TubeSubscriber.processQueue(Http2Connection.java:1046)
at jdk.incubator.httpclient/jdk.incubator.http.internal.common.SequentialScheduler$SynchronizedRestartableTask.run(SequentialScheduler.java:175)
at jdk.incubator.httpclient/jdk.incubator.http.internal.common.SequentialScheduler$CompleteRestartableTask.run(SequentialScheduler.java:147)
at jdk.incubator.httpclient/jdk.incubator.http.internal.common.SequentialScheduler$SchedulableTask.run(SequentialScheduler.java:198)
at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1135)
at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:635)
at java.base/java.lang.Thread.run(Thread.java:844)
Caused by: java.io.EOFException: EOF reached while reading
at jdk.incubator.httpclient/jdk.incubator.http.Http2Connection$Http2TubeSubscriber.onComplete(Http2Connection.java:1087)
at jdk.incubator.httpclient/jdk.incubator.http.SocketTube$InternalReadPublisher$ReadSubscription.signalCompletion(SocketTube.java:551)
at jdk.incubator.httpclient/jdk.incubator.http.SocketTube$InternalReadPublisher$InternalReadSubscription.read(SocketTube.java:728)
at jdk.incubator.httpclient/jdk.incubator.http.SocketTube$SocketFlowTask.run(SocketTube.java:171)
at jdk.incubator.httpclient/jdk.incubator.http.internal.common.SequentialScheduler$SchedulableTask.run(SequentialScheduler.java:198)
at jdk.incubator.httpclient/jdk.incubator.http.internal.common.SequentialScheduler.runOrSchedule(SequentialScheduler.java:271)
at jdk.incubator.httpclient/jdk.incubator.http.internal.common.SequentialScheduler.runOrSchedule(SequentialScheduler.java:224)
at jdk.incubator.httpclient/jdk.incubator.http.SocketTube$InternalReadPublisher$InternalReadSubscription.signalReadable(SocketTube.java:675)
at jdk.incubator.httpclient/jdk.incubator.http.SocketTube$InternalReadPublisher$ReadEvent.signalEvent(SocketTube.java:829)
at jdk.incubator.httpclient/jdk.incubator.http.SocketTube$SocketFlowEvent.handle(SocketTube.java:243)
at jdk.incubator.httpclient/jdk.incubator.http.HttpClientImpl$SelectorManager.handleEvent(HttpClientImpl.java:769)
at jdk.incubator.httpclient/jdk.incubator.http.HttpClientImpl$SelectorManager.run(HttpClientImpl.java:731)