Code review comment: Why modify this? I think `org.assertj.core.api.Assertions.assertThat` is suggested test API.

Code under review:
void testCreateAndCloseSessions() throws Exception {
        List<SessionHandle> sessionHandles = new ArrayList<>();
        Set<String> sessionHandleIds = new HashSet<>();
        for (int num = 0; num < SESSION_NUMBER; ++num) {
            CompletableFuture<OpenSessionResponseBody> response =
                    sendRequest(openSessionHeaders, emptyParameters, openSessionRequestBody);
            String sessionHandleId = response.get().getSessionHandle();
            assertNotNull(sessionHandleId);
            sessionHandleIds.add(sessionHandleId);
            SessionHandle sessionHandle = new SessionHandle(UUID.fromString(sessionHandleId));
            assertNotNull(
                    SQL_GATEWAY_SERVICE_EXTENSION.getSessionManager().getSession(sessionHandle));
            sessionHandles.add(sessionHandle);
        }
        assertThat(sessionHandleIds).hasSize(SESSION_NUMBER);

        for (int num = 0; num < SESSION_NUMBER; ++num) {
            SessionHandle sessionHandle = sessionHandles.get(num);
            SessionMessageParameters sessionMessageParameters =
                    new SessionMessageParameters(sessionHandle);
            CompletableFuture<CloseSessionResponseBody> response =
                    sendRequest(closeSessionHeaders, sessionMessageParameters, emptyRequestBody);
            String status = response.get().getStatus();
            assertThat(status).isEqualTo(CLOSE_MESSAGE);
            assertThatThrownBy(
                            () ->
                                    SQL_GATEWAY_SERVICE_EXTENSION
                                            .getSessionManager()
                                            .getSession(sessionHandle))
                    .isInstanceOf(SqlGatewayException.class);
            
            CompletableFuture<CloseSessionResponseBody> response2 =
                    sendRequest(closeSessionHeaders, sessionMessageParameters, emptyRequestBody);
            assertThatThrownBy(response2::get).isInstanceOf(ExecutionException.class);
        }
    }

Line commented on: assertNotNull(sessionHandleId);

Revised code:
void testCreateAndCloseSessions() throws Exception {
        List<SessionHandle> sessionHandles = new ArrayList<>();
        Set<String> sessionHandleIds = new HashSet<>();
        for (int num = 0; num < SESSION_NUMBER; ++num) {
            CompletableFuture<OpenSessionResponseBody> response =
                    sendRequest(openSessionHeaders, emptyParameters, openSessionRequestBody);
            String sessionHandleId = response.get().getSessionHandle();
            assertThat(sessionHandleId).isNotNull();
            sessionHandleIds.add(sessionHandleId);
            SessionHandle sessionHandle = new SessionHandle(UUID.fromString(sessionHandleId));
            assertThat(SQL_GATEWAY_SERVICE_EXTENSION.getSessionManager().getSession(sessionHandle))
                    .isNotNull();
            sessionHandles.add(sessionHandle);
        }
        assertThat(sessionHandleIds).hasSize(SESSION_NUMBER);

        for (int num = 0; num < SESSION_NUMBER; ++num) {
            SessionHandle sessionHandle = sessionHandles.get(num);
            SessionMessageParameters sessionMessageParameters =
                    new SessionMessageParameters(sessionHandle);
            CompletableFuture<CloseSessionResponseBody> response =
                    sendRequest(closeSessionHeaders, sessionMessageParameters, emptyRequestBody);
            String status = response.get().getStatus();
            assertThat(status).isEqualTo(CLOSE_MESSAGE);
            assertThatThrownBy(
                            () ->
                                    SQL_GATEWAY_SERVICE_EXTENSION
                                            .getSessionManager()
                                            .getSession(sessionHandle))
                    .isInstanceOf(SqlGatewayException.class);
            
            CompletableFuture<CloseSessionResponseBody> response2 =
                    sendRequest(closeSessionHeaders, sessionMessageParameters, emptyRequestBody);
            assertThatThrownBy(response2::get).isInstanceOf(ExecutionException.class);
        }
    }