How to make Apache's CloseableHttpAsyncClient explicitly use HTTP/1 or HTTP/2

In this tutorial, we'll demonstrate how to explicitly use HTTP/1 or HTTP/2 in Apache's CloseableHttpAsyncClient, a base implementation of HttpAsyncClient that also implements ModalClosable.

You'll need to have this dependency to your pom.xml:

<dependency>
    <groupId>org.apache.httpcomponents.client5</groupId>
    <artifactId>httpclient5</artifactId>
    <version>5.3.1</version>
</dependency>

Deprecated Way

In earlier versions of httpclient5, the HttpAsyncClientBuilder allowed you to use setVersionPolicy(org.apache.hc.core5.http2.HttpVersionPolicy versionPolicy), where you could pass in HttpVersionPolicy.FORCE_HTTP_1 or HttpVersionPolicy.FORCE_HTTP_2. For example:

final CloseableHttpAsyncClient client = HttpAsyncClients
        .custom()
        .useSystemProperties()
        .setVersionPolicy(HttpVersionPolicy.FORCE_HTTP_1) //Deprecated
        .build();

However, in recent versions of httpclient5, setVersionPolicy is deprecated.

Best Practice Way

Now, their documentation says that we should use TlsConfig and connection manager methods instead.

Here's an example:

TlsConfig tlsHttp1Config = TlsConfig.copy(TlsConfig.DEFAULT).setVersionPolicy(HttpVersionPolicy.FORCE_HTTP_1).build();
//Create a default connection manager
PoolingAsyncClientConnectionManager connectionManager = new PoolingAsyncClientConnectionManager();
connectionManager.setDefaultTlsConfig(tlsHttp1Config);

final CloseableHttpAsyncClient client = HttpAsyncClients
        .custom()
        .useSystemProperties()
        .setConnectionManager(connectionManager)
        .build();

I hope these examples help if you ever need to use an explicit version of HTTP for your AsyncHttpClients.