public class SlackUtils {
    private static String slackWebhookUrl = "복사한 WebHooks URL";

    public static void sendMessage(SlackMessage message) {
        CloseableHttpClient client = HttpClients.createDefault();
        HttpPost httpPost = new HttpPost(slackWebhookUrl);

        try {
            ObjectMapper objectMapper = new ObjectMapper();
            String json = objectMapper.writeValueAsString(message);

            StringEntity entity = new StringEntity(json);
            httpPost.setEntity(entity);
            httpPost.setHeader("Accept", "application/json");
            httpPost.setHeader("Content-type", "application/json");

            client.execute(httpPost);
            client.close();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

@AllArgsConstructor
@Builder(builderClassName = "Builder")
@Getter
@Setter
public class SlackMessage implements Serializable {
    private String channel;
    private String username;
    private String text;
    private String icon_emoji;
}

@SpringBootApplication
public class DemoApplication {

    public static void main(String[] args) {
        SlackMessage slackMessage = SlackMessage.builder()
                .channel("study") // 보낼 채널 이름
                .username("yb") // username
                .text("yeah..") // 보낼 내용
                .icon_emoji(":ghost:") // 이모티콘 이름
                .build();

        SlackUtils.sendMessage(slackMessage);
    }
}