public CloseableHttpAsyncClient build() {
        AuthenticationStrategy targetAuthStrategyCopy = this.targetAuthStrategy;
        if (targetAuthStrategyCopy == null) {
            targetAuthStrategyCopy = DefaultAuthenticationStrategy.INSTANCE;
        }
        AuthenticationStrategy proxyAuthStrategyCopy = this.proxyAuthStrategy;
        if (proxyAuthStrategyCopy == null) {
            proxyAuthStrategyCopy = DefaultAuthenticationStrategy.INSTANCE;
        }

        String userAgentCopy = this.userAgent;
        if (userAgentCopy == null) {
            if (systemProperties) {
                userAgentCopy = getProperty("http.agent", null);
            }
            if (userAgentCopy == null) {
                userAgentCopy = VersionInfo.getSoftwareInfo("Apache-HttpAsyncClient",
                        "org.apache.hc.client5", getClass());
            }
        }

        final HttpProcessorBuilder b = HttpProcessorBuilder.create();
        if (requestInterceptors != null) {
            for (final RequestInterceptorEntry entry: requestInterceptors) {
                if (entry.position == RequestInterceptorEntry.Position.FIRST) {
                    b.addFirst(entry.interceptor);
                }
            }
        }
        if (responseInterceptors != null) {
            for (final ResponseInterceptorEntry entry: responseInterceptors) {
                if (entry.position == ResponseInterceptorEntry.Position.FIRST) {
                    b.addFirst(entry.interceptor);
                }
            }
        }
        b.addAll(
                new H2RequestTargetHost(),
                new RequestDefaultHeaders(defaultHeaders),
                new RequestUserAgent(userAgentCopy),
                new RequestExpectContinue(),
                new H2RequestContent(),
                new H2RequestConnControl());
        if (!cookieManagementDisabled) {
            b.add(RequestAddCookies.INSTANCE);
        }
        if (!cookieManagementDisabled) {
            b.add(ResponseProcessCookies.INSTANCE);
        }
        if (requestInterceptors != null) {
            for (final RequestInterceptorEntry entry: requestInterceptors) {
                if (entry.position == RequestInterceptorEntry.Position.LAST) {
                    b.addLast(entry.interceptor);
                }
            }
        }
        if (responseInterceptors != null) {
            for (final ResponseInterceptorEntry entry: responseInterceptors) {
                if (entry.position == ResponseInterceptorEntry.Position.LAST) {
                    b.addLast(entry.interceptor);
                }
            }
        }

        final HttpProcessor httpProcessor = b.build();

        final NamedElementChain<AsyncExecChainHandler> execChainDefinition = new NamedElementChain<>();
        execChainDefinition.addLast(
                new H2AsyncMainClientExec(httpProcessor),
                ChainElement.MAIN_TRANSPORT.name());

        execChainDefinition.addFirst(
                new AsyncConnectExec(
                        new DefaultHttpProcessor(new RequestTargetHost(), new RequestUserAgent(userAgentCopy)),
                        proxyAuthStrategyCopy,
                        schemePortResolver != null ? schemePortResolver : DefaultSchemePortResolver.INSTANCE,
                        authCachingDisabled),
                ChainElement.CONNECT.name());

        execChainDefinition.addFirst(
                new AsyncProtocolExec(
                        targetAuthStrategyCopy,
                        proxyAuthStrategyCopy,
                        schemePortResolver != null ? schemePortResolver : DefaultSchemePortResolver.INSTANCE,
                        authCachingDisabled),
                ChainElement.PROTOCOL.name());

        // Add request retry executor, if not disabled
        if (!automaticRetriesDisabled) {
            HttpRequestRetryStrategy retryStrategyCopy = this.retryStrategy;
            if (retryStrategyCopy == null) {
                retryStrategyCopy = DefaultHttpRequestRetryStrategy.INSTANCE;
            }
            execChainDefinition.addFirst(
                    new AsyncHttpRequestRetryExec(retryStrategyCopy),
                    ChainElement.RETRY.name());
        }

        HttpRoutePlanner routePlannerCopy = this.routePlanner;
        if (routePlannerCopy == null) {
            SchemePortResolver schemePortResolverCopy = this.schemePortResolver;
            if (schemePortResolverCopy == null) {
                schemePortResolverCopy = DefaultSchemePortResolver.INSTANCE;
            }
            routePlannerCopy = new DefaultRoutePlanner(schemePortResolverCopy);
        }

        // Add redirect executor, if not disabled
        if (!redirectHandlingDisabled) {
            RedirectStrategy redirectStrategyCopy = this.redirectStrategy;
            if (redirectStrategyCopy == null) {
                redirectStrategyCopy = schemePortResolver != null ? new DefaultRedirectStrategy(schemePortResolver) : DefaultRedirectStrategy.INSTANCE;
            }
            execChainDefinition.addFirst(
                    new AsyncRedirectExec(routePlannerCopy, redirectStrategyCopy),
                    ChainElement.REDIRECT.name());
        }

        final AsyncPushConsumerRegistry pushConsumerRegistry = new AsyncPushConsumerRegistry();
        final IOEventHandlerFactory ioEventHandlerFactory = new H2AsyncClientProtocolStarter(
                HttpProcessorBuilder.create().build(),
                (request, context) -> pushConsumerRegistry.get(request),
                h2Config != null ? h2Config : H2Config.DEFAULT,
                charCodingConfig != null ? charCodingConfig : CharCodingConfig.DEFAULT,
                ioReactorExceptionCallback != null ? ioReactorExceptionCallback : LoggingExceptionCallback.INSTANCE);
        final DefaultConnectingIOReactor ioReactor = new DefaultConnectingIOReactor(
                ioEventHandlerFactory,
                ioReactorConfig != null ? ioReactorConfig : IOReactorConfig.DEFAULT,
                threadFactory != null ? threadFactory : new DefaultThreadFactory("httpclient-dispatch", true),
                ioSessionDecorator != null ? ioSessionDecorator : LoggingIOSessionDecorator.INSTANCE,
                ioReactorExceptionCallback != null ? ioReactorExceptionCallback : LoggingExceptionCallback.INSTANCE,
                ioSessionListener,
                ioSession -> ioSession.enqueue(new ShutdownCommand(CloseMode.GRACEFUL), Command.Priority.IMMEDIATE));

        if (execInterceptors != null) {
            for (final ExecInterceptorEntry entry: execInterceptors) {
                switch (entry.position) {
                    case AFTER:
                        execChainDefinition.addAfter(entry.existing, entry.interceptor, entry.name);
                        break;
                    case BEFORE:
                        execChainDefinition.addBefore(entry.existing, entry.interceptor, entry.name);
                        break;
                    case REPLACE:
                        execChainDefinition.replace(entry.existing, entry.interceptor);
                        break;
                    case FIRST:
                        execChainDefinition.addFirst(entry.interceptor, entry.name);
                        break;
                    case LAST:
                        // Don't add last, after H2AsyncMainClientExec, as that does not delegate to the chain
                        // Instead, add the interceptor just before it, making it effectively the last interceptor
                        execChainDefinition.addBefore(ChainElement.MAIN_TRANSPORT.name(), entry.interceptor, entry.name);
                        break;
                }
            }
        }

        customizeExecChain(execChainDefinition);

        NamedElementChain<AsyncExecChainHandler>.Node current = execChainDefinition.getLast();
        AsyncExecChainElement execChain = null;
        while (current != null) {
            execChain = new AsyncExecChainElement(current.getValue(), execChain);
            current = current.getPrevious();
        }

        Lookup<AuthSchemeFactory> authSchemeRegistryCopy = this.authSchemeRegistry;
        if (authSchemeRegistryCopy == null) {
            authSchemeRegistryCopy = RegistryBuilder.<AuthSchemeFactory>create()
                    .register(StandardAuthScheme.BASIC, BasicSchemeFactory.INSTANCE)
                    .register(StandardAuthScheme.DIGEST, DigestSchemeFactory.INSTANCE)
                    .register(StandardAuthScheme.BEARER, BearerSchemeFactory.INSTANCE)
                    .build();
        }
        Lookup<CookieSpecFactory> cookieSpecRegistryCopy = this.cookieSpecRegistry;
        if (cookieSpecRegistryCopy == null && !cookieManagementDisabled) {
            cookieSpecRegistryCopy = CookieSpecSupport.createDefault();
        }

        CookieStore cookieStoreCopy = this.cookieStore;
        if (cookieStoreCopy == null) {
            cookieStoreCopy = new BasicCookieStore();
        }

        CredentialsProvider credentialsProviderCopy = this.credentialsProvider;
        if (credentialsProviderCopy == null) {
            if (systemProperties) {
                credentialsProviderCopy = new SystemDefaultCredentialsProvider();
            } else {
                credentialsProviderCopy = new BasicCredentialsProvider();
            }
        }

        TlsStrategy tlsStrategyCopy = this.tlsStrategy;
        if (tlsStrategyCopy == null) {
            if (systemProperties) {
                tlsStrategyCopy = DefaultClientTlsStrategy.createSystemDefault();
            } else {
                tlsStrategyCopy = DefaultClientTlsStrategy.createDefault();
            }
        }

        final MultihomeConnectionInitiator connectionInitiator = new MultihomeConnectionInitiator(ioReactor, dnsResolver);
        final InternalH2ConnPool connPool = new InternalH2ConnPool(connectionInitiator, host -> null, tlsStrategyCopy);
        connPool.setConnectionConfigResolver(connectionConfigResolver);

        List<Closeable> closeablesCopy = closeables != null ? new ArrayList<>(closeables) : null;
        if (closeablesCopy == null) {
            closeablesCopy = new ArrayList<>(1);
        }
        if (evictIdleConnections) {
            final IdleConnectionEvictor connectionEvictor = new IdleConnectionEvictor(connPool,
                    maxIdleTime != null ? maxIdleTime : TimeValue.ofSeconds(30L));
            closeablesCopy.add(connectionEvictor::shutdown);
            connectionEvictor.start();
        }
        closeablesCopy.add(connPool);

        return new InternalH2AsyncClient(
                ioReactor,
                execChain,
                pushConsumerRegistry,
                threadFactory != null ? threadFactory : new DefaultThreadFactory("httpclient-main", true),
                connPool,
                routePlannerCopy,
                cookieSpecRegistryCopy,
                authSchemeRegistryCopy,
                cookieStoreCopy,
                credentialsProviderCopy,
                defaultRequestConfig,
                closeablesCopy);
    }
Adds to the list of {@link Closeable} resources to be managed by the client.
<p>
For internal use.
</p>
httpclient5/src/main/java/org/apache/hc/client5/http/impl/async/H2AsyncClientBuilder.java