Call Slack APIs with no deps in Java 11+

App.java

package com.github.seratch;

import java.net.URI;
import java.net.URLEncoder;
import java.net.http.HttpClient;
import java.net.http.HttpRequest;
import java.net.http.HttpResponse;
import java.nio.charset.StandardCharsets;
import java.util.Map;

public class App {

    public static void main(String[] args) throws Exception {

        String token = System.getenv("SLACK_BOT_TOKEN");

        HttpClient httpClient = HttpClient.newHttpClient();

        Map<String, ?> params = Map.of(
                "token", token,
                "channel", "#random", // using channel id is better, though
                "text", "java.net.http.HttpClient works!"
        );
        HttpRequest request = HttpRequest.newBuilder()
                .uri(URI.create("https://slack.com/api/chat.postMessage"))
                .POST(HttpRequest.BodyPublishers.ofString(toRequestBody(params)))
                .header("Content-Type", "application/x-www-form-urlencoded")
                .build();

        HttpResponse<String> response = httpClient.send(request, HttpResponse.BodyHandlers.ofString());

        System.out.println("status: " + response.statusCode() + ", body: " + response.body());
    }

    public static String toRequestBody(Map<String, ?> params) {
        StringBuilder sb = new StringBuilder();
        for (String key : params.keySet()) {
            if (sb.length() > 0) {
                sb.append("&");
            }
            sb.append(URLEncoder.encode(key, StandardCharsets.UTF_8));
            sb.append("=");

            Object value = params.get(key);
            if (value != null) {
                sb.append(URLEncoder.encode(String.valueOf(value), StandardCharsets.UTF_8));
            }
        }
        return sb.toString();
    }

}