Slack SDK for Java

Slash Commands

Slash Commands allow users to invoke your app from the message composer box.

Responding to slash command invocations is a common use case. Your app has to respond to the request within 3 seconds by ack() method. Otherwise, the user will see the timeout error on Slack.

Slack App Configuration

To enable slash commands, visit the Slack App configuration page, choose the app you’re working on, go to Features > Slash Commands on the left pane. There are a few things to there on the page.

  • Click Create New Command button
  • Input the command information on the dialog:
    • Command: /hello
    • Request URL: https://{your domain here}/slack/events - if you use ngrok for development, the URL would be https://{random} (this step is not required for Socket Mode apps)
    • Short Description: whatever you like
  • Click Save Button

What Your Bolt App Does

All your app needs to do to handle slash command requests are:

  1. Verify requests from Slack
  2. Parse the request body and check if the command is the one you’d like to handle
  3. Build a reply message or do whatever you want to do
  4. Respond to the Slack API server with 200 OK as an acknowledgment

If the response body is empty, the response will be recognized as just an acknowledgment. No message will be posted to the channel.


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 command to handle (by either of the exact name or regular expression)
  • Build a reply message or do whatever you want to do
  • Call ack() as an acknowledgment

Slash command request payloads have response_url, so that your app can reply to the action (even asynchronously after the acknowledgment). If you post a message using response_url, call ctx.ack() without arguments and use ctx.respond() to post a message.

Here is a tiny example demonstrating how to handle slash command requests in a Bolt app.

app.command("/echo", (req, ctx) -> {
  String commandArgText = req.getPayload().getText();
  String channelId = req.getPayload().getChannelId();
  String channelName = req.getPayload().getChannelName();
  String text = "You said " + commandArgText + " at <#" + channelId + "|" + channelName + ">";
  return ctx.ack(text); // respond with 200 OK

Here is the example to use response_url for posting a message. It’s also fine to asynchronously run ctx.respond after the acknowledgment.

app.command("/echo", (req, ctx) -> {
  String text = buildMessage(req);
  ctx.respond(text); // perform an HTTP request
  return ctx.ack(); // respond with 200 OK

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

app.command("/echo") { req, ctx ->
  val commandArgText = req.payload.text
  val channelId = req.payload.channelId
  val channelName = req.payload.channelName
  val text = "You said ${commandArgText} at <#${channelId}|${channelName}>"

app.command("/echo") { req, ctx ->
  val text = buildMessage(req)
  ctx.respond(text) // send via response_url

To learn how to build Block Kit messages with this SDK, consult Composing Messages.

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.app_backend.slash_commands.*;
import com.slack.api.app_backend.slash_commands.payload.SlashCommandPayload;

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 `command` is the one you'd like to handle
  SlashCommandPayloadParser parser = new SlashCommandPayloadParser();

  // The request body looks like this:
  //   token=gIkuvaNzQIHg97ATvDxqgjtO&team_id=T0001&team_domain=example
  //   &enterprise_id=E0001&enterprise_name=Globular%20Construct%20Inc
  //   &channel_id=C2147483705&channel_name=test
  //   &user_id=U2147483697&user_name=Steve
  //   &command=weather&text=94070&response_url=
  //   &trigger_id=123.123.123
  String requestBody = request.getBodyAsString();

  SlashCommandPayload payload = parser.parse(requestBody);

  if (payload.getCommand().equals("/echo")) {
    // 3. Build a reply message or do whatever you want to do
    String commandArgText = payload.getText();
    String channelId = payload.getChannelId();
    String channelName = payload.getChannelName();
    String text = "You said " + commandArgText + " at <#" + channelId + "|" + channelName + ">";

    // 4. Respond to the Slack API server with 200 OK as an acknowledgment
    return PseudoHttpResponse.builder()
      .body(PseudoJsonOps.serialize(Map.of("text", text))) // send a reply in the response
  } else {
    return PseudoHttpResponse.builder().status(404).build();

Also, Bolt’s ctx.respond method internally does messaging via response_url like this.

import com.slack.api.app_backend.slash_commands.response.SlashCommandResponse;
import com.slack.api.webhook.WebhookResponse;

// Reply to the request using response_url
Slack slack = Slack.getInstance();
SlashCommandResponseSender responder = new SlashCommandResponseSender(slack);
SlashCommandResponse reply = SlashCommandResponse.builder().text(text).build();
WebhookResponse result = responder.send(payload.getResponseUrl(), reply);