/**
     * Disables connection state tracking.
     */
    public final HttpClientBuilder disableConnectionState() {
        connectionStateDisabled = true;
        return this;
    }
    public CloseableHttpClient build() {
        //......
        UserTokenHandler userTokenHandlerCopy = this.userTokenHandler;
        if (userTokenHandlerCopy == null) {
            if (!connectionStateDisabled) {
                userTokenHandlerCopy = DefaultUserTokenHandler.INSTANCE;
            } else {
                userTokenHandlerCopy = NoopUserTokenHandler.INSTANCE;
            }
        }   
        ClientExecChain execChain = createMainExec(
                requestExecCopy,
                connManagerCopy,
                reuseStrategyCopy,
                keepAliveStrategyCopy,
                new ImmutableHttpProcessor(new RequestTargetHost(), new RequestUserAgent(userAgentCopy)),
                targetAuthStrategyCopy,
                proxyAuthStrategyCopy,
                userTokenHandlerCopy);
        //......     
    }
}

Object userToken = context.getUserToken();
            if (userToken == null) {
                userToken = userTokenHandler.getUserToken(context);
                context.setAttribute(HttpClientContext.USER_TOKEN, userToken);
            }
            if (userToken != null) {
                connHolder.setState(userToken);
            }

public interface UserTokenHandler {
    /**
     * The token object returned by this method is expected to uniquely
     * identify the current user if the context is user specific or to be
     * {@code null} if it is not.
     *
     * @param context the execution context
     *
     * @return user token that uniquely identifies the user or
     * {@code null} if the context is not user specific.
     */
    Object getUserToken(HttpContext context);
}

@Contract(threading = ThreadingBehavior.IMMUTABLE)
public class DefaultUserTokenHandler implements UserTokenHandler {
    public static final DefaultUserTokenHandler INSTANCE = new DefaultUserTokenHandler();
    @Override
    public Object getUserToken(final HttpContext context) {
        final HttpClientContext clientContext = HttpClientContext.adapt(context);
        Principal userPrincipal = null;
        final AuthState targetAuthState = clientContext.getTargetAuthState();
        if (targetAuthState != null) {
            userPrincipal = getAuthPrincipal(targetAuthState);
            if (userPrincipal == null) {
                final AuthState proxyAuthState = clientContext.getProxyAuthState();
                userPrincipal = getAuthPrincipal(proxyAuthState);
            }
        }
        if (userPrincipal == null) {
            final HttpConnection conn = clientContext.getConnection();
            if (conn.isOpen() && conn instanceof ManagedHttpClientConnection) {
                final SSLSession sslsession = ((ManagedHttpClientConnection) conn).getSSLSession();
                if (sslsession != null) {
                    userPrincipal = sslsession.getLocalPrincipal();
                }
            }
        }
        return userPrincipal;
    }
    private static Principal getAuthPrincipal(final AuthState authState) {
        final AuthScheme scheme = authState.getAuthScheme();
        if (scheme != null && scheme.isComplete() && scheme.isConnectionBased()) {
            final Credentials creds = authState.getCredentials();
            if (creds != null) {
                return creds.getUserPrincipal();
            }
        }
        return null;
    }
}

@Contract(threading = ThreadingBehavior.IMMUTABLE)
public class NoopUserTokenHandler implements UserTokenHandler {
    public static final NoopUserTokenHandler INSTANCE = new NoopUserTokenHandler();
    @Override
    public Object getUserToken(final HttpContext context) {
        return null;
    }
}