@Test
    void positivePingIntervalShouldBeGreaterThan1Second() {
        final ServerConfig config1 = newServerWithKeepAlive(15000, 0).config();
        assertThat(config1.idleTimeoutMillis()).isEqualTo(15000);
        assertThat(config1.pingIntervalMillis()).isEqualTo(0);

        assertThatThrownBy(() -> newServerWithKeepAlive(10000, MIN_PING_INTERVAL_MILLIS - 1))
                .isInstanceOf(IllegalArgumentException.class)
                .hasMessageContaining("(expected: >= " + MIN_PING_INTERVAL_MILLIS + " or == 0)");

        final ServerConfig config2 = newServerWithKeepAlive(15000, 15000).config();
        assertThat(config2.idleTimeoutMillis()).isEqualTo(15000);
        assertThat(config2.pingIntervalMillis()).isEqualTo(0);

        final ServerConfig config3 = newServerWithKeepAlive(15000, MIN_PING_INTERVAL_MILLIS).config();
        assertThat(config3.idleTimeoutMillis()).isEqualTo(15000);
        assertThat(config3.pingIntervalMillis()).isEqualTo(MIN_PING_INTERVAL_MILLIS);

        final ServerConfig config4 = newServerWithKeepAlive(20000, 19999).config();
        assertThat(config4.idleTimeoutMillis()).isEqualTo(20000);
        assertThat(config4.pingIntervalMillis()).isEqualTo(19999);
    }
Makes sure that {@link ServerBuilder#decorator(DecoratingHttpServiceFunction)} works at every service and
virtual hosts and {@link VirtualHostBuilder#decorator(DecoratingHttpServiceFunction)} works only at
its own services.
core/src/test/java/com/linecorp/armeria/server/ServerBuilderTest.java