An example of Akka HTTP configuration which makes it accept self signed certificate.

AkkaHttpClientAcceptSelfSignedCertificate.java

import akka.actor.ActorSystem;
import akka.http.javadsl.*;
import akka.http.javadsl.model.HttpRequest;
import com.typesafe.sslconfig.akka.AkkaSSLConfig;

import javax.net.ssl.SSLContext;
import javax.net.ssl.TrustManager;
import javax.net.ssl.X509TrustManager;
import java.security.KeyManagementException;
import java.security.NoSuchAlgorithmException;
import java.util.Optional;

public class Main {
    public static void main(String[] args) {
        /*
            Example of Akka Http client accepting self signed certificate.
            Tested with:
            com.typesafe.akka:akka-stream_2.12:2.5.17
            com.typesafe.akka:akka-actor_2.12:2.5.17
            com.typesafe.akka:akka-http_2.12:10.1.5
         */
        ActorSystem actorSystem = ActorSystem.create("client");

        Http.get(actorSystem)
            .singleRequest(
                HttpRequest.GET("https://localhost:443/some_endpoint"),
                unsafeHttpsContext(actorSystem))
            .handle((response, error) -> {
                if (error != null) {
                    System.out.println(error);
                } else {
                    System.out.println(response);
                }
                return "";
            })
            .toCompletableFuture()
            .join();
    }

    private static HttpsConnectionContext unsafeHttpsContext(ActorSystem actorSystem) {
        final AkkaSSLConfig badSslConfig = AkkaSSLConfig
            .get(actorSystem)
            .convertSettings(s -> s
                .withLoose(s
                    .loose()
                    //.withAcceptAnyCertificate(true) // https://github.com/lightbend/ssl-config/issues/69
                    .withDisableHostnameVerification(true)));
        SSLContext sslContext = null;
        try {
            sslContext = SSLContext.getInstance("TLS");
            sslContext.init(
                null,
                new TrustManager[]{new UnsafeTrustManager()},
                null);
        } catch (KeyManagementException | NoSuchAlgorithmException e) {
            e.printStackTrace();
        }

        return ConnectionContext.https(
            sslContext,
            Optional.of(badSslConfig),
            Optional.empty(),
            Optional.empty(),
            Optional.empty(),
            Optional.empty());
    }

    private static final class UnsafeTrustManager implements X509TrustManager {
        @Override
        public void checkClientTrusted(java.security.cert.X509Certificate[] x509Certificates, String s) throws java.security.cert.CertificateException {

        }

        @Override
        public void checkServerTrusted(java.security.cert.X509Certificate[] x509Certificates, String s) throws java.security.cert.CertificateException {

        }

        @Override
        public java.security.cert.X509Certificate[] getAcceptedIssuers() {
            return new java.security.cert.X509Certificate[0];
        }
    }
}