Slack SDK for Java

Events API

The Events API is a streamlined, easy way to build apps and bots that respond to activities in Slack. All you need is a Slack app and a secure place for us to send your events.

Slack App Configuration

To enable Events API, visit the Slack App configuration page, choose the app you’re working on, and go to Features > Event Subscriptions on the left pane. There are a few things to do on the page.

  • Turn on Enable Events
  • Set the Request URL to https://{your app's public URL domain}/slack/events (this step is not required for Socket Mode apps)
  • Add subscriptions to bot events
    • Click Subscribe to bot events
    • Click Add Bot User Event button
    • Choose events to subscribe
  • Click the Save Changes button at the bottom for sure

What Your Bolt App Does

All you need to do to handle Events API requests are:

  1. Verify requests from Slack
  2. Parse the request body and check if the type in event is the one you’d like to handle
  3. Whatever you want to do with the event data
  4. Respond to the Slack API server with 200 OK as an acknowledgment

Your app has to respond to the request within 3 seconds by ack() method. Otherwise, the Slack Platform may retry after a while.


NOTE: If you’re a beginner to using Bolt for Slack App development, consult Getting Started with Bolt, first.

Bolt does many of the commonly required tasks for you. The steps you need to handle would be:

  • Specify the Java class corresponding to event.type (and also event.subtype when necessary) to handle
  • Whatever you want to do with the event data
  • Call ack() as an acknowledgment

In event payloads, response_url is not included as it’s not a payload coming from direct user interactions. Also, it’s not possible to post a message using ctx.ack() for the same reason. If an event you receive is a user interaction and you’d like to post a reply to the user at the conversation the event happened, call chat.postMessage method or other similar ones with channel in the event payload.

import com.slack.api.model.event.ReactionAddedEvent;

app.event(ReactionAddedEvent.class, (payload, ctx) -> {
  ReactionAddedEvent event = payload.getEvent();
  if (event.getReaction().equals("white_check_mark")) {
    ChatPostMessageResponse message = ctx.client().chatPostMessage(r -> r
      .text("<@" + event.getUser() + "> Thank you! We greatly appreciate your efforts :two_hearts:"));
    if (!message.isOk()) {
      ctx.logger.error("chat.postMessage failed: {}", message.getError());
  return ctx.ack();

The same code in Kotlin looks as below. (New to Kotlin? Getting Started in Kotlin may be helpful)

app.event( { payload, ctx ->
  val event = payload.event
  if (event.reaction == "white_check_mark") {
    val message = ctx.client().chatPostMessage {
        .text("<@${event.user}> Thank you! We greatly appreciate your efforts :two_hearts:")
    if (!message.isOk) {
      ctx.logger.error("chat.postMessage failed: ${message.error}")

Here is another example. With an app.message listener, you can receive only the events that contains given keyword or regular expressions, and do something with those event data in a fewer lines of code.

import com.slack.api.methods.MethodsClient;
import com.slack.api.methods.response.reactions.ReactionsAddResponse;
import com.slack.api.model.event.MessageEvent;

import java.util.Arrays;
import java.util.regex.Pattern;

String notificationChannelId = "D1234567";

// check if the message contains some monitoring keywords
Pattern sdk = Pattern.compile(".*[(Java SDK)|(Bolt)|(slack\\-java\\-sdk)].*", Pattern.CASE_INSENSITIVE);
app.message(sdk, (payload, ctx) -> {
  MessageEvent event = payload.getEvent();
  String text = event.getText();
  MethodsClient client = ctx.client();

  // Add 👀reacji to the message
  String channelId = event.getChannel();
  String ts = event.getTs();
  ReactionsAddResponse reaction = client.reactionsAdd(r ->"eyes"));
  if (!reaction.isOk()) {
    ctx.logger.error("reactions.add failed: {}", reaction.getError());

    // Send the message to the SDK author
  ChatGetPermalinkResponse permalink = client.chatGetPermalink(r ->;
  if (permalink.isOk()) {
    ChatPostMessageResponse message = client.chatPostMessage(r -> r
      .text("The Java SDK might be mentioned:\n" + permalink.getPermalink())
    if (!message.isOk()) {
      ctx.logger.error("chat.postMessage failed: {}", message.getError());
  } else {
    ctx.logger.error("chat.getPermalink failed: {}", permalink.getError());
  return ctx.ack();

If matching an exact word in a text message works for you, the code looks much simpler as below.

app.message(":wave:", (payload, ctx) -> {
  ctx.say("Hello, <@" + payload.getEvent().getUser() + ">");
  return ctx.ack();

Under the Hood

If you hope to understand what is actually happening with the above code, reading the following (a bit pseudo) code may be helpful.

import java.util.Map;
import com.slack.api.Slack;
import com.slack.api.util.json.GsonFactory;

PseudoHttpResponse handle(PseudoHttpRequest request) {

  // 1. Verify requests from Slack
  // This needs "X-Slack-Signature" header, "X-Slack-Request-Timestamp" header, and raw request body
  if (!PseudoSlackRequestVerifier.isValid(request)) {
    return PseudoHttpResponse.builder().status(401).build();
  // 2. Parse the request body and check if the `type` in `event` is the one you'd like to handle
  // The request body in JSON format
  String payloadString = request.getBodyAsString();
  EventTypeExtractor eventTypeExtractor = new EventsDispatcherImpl();
  String eventType = eventTypeExtractor.extractEventType(payloadString);
  if (eventType != null && eventType.equals("message")) {
    Gson gson = GsonFactory.createSnakeCase();
    MessagePayload payload = gson.fromJson(payloadString, MessagePayload.class);
    // 3. Whatever you want to do with the event data
  } else {
    // other patterns
    return PseudoHttpResponse.builder().status(404).build();
  // 4. Respond to the Slack API server with 200 OK as an acknowledgment
  return PseudoHttpResponse.builder().status(200).build();