@Test
void add() {

    ChannelHandler<?, ?> handler = new ChannelHandler<>(TestWebSocketMessage.class, TestWebSocketMessage.class, new TestChannelListener());
    this.handler.add("/ws", handler);
    assertThatThrownBy(() -> this.handler.add("/ws", handler))
            .isInstanceOf(Error.class)
            .hasMessageContaining("found duplicate channel listener");
}