Asynchronous suggestions

In addition to normal suggestions, safely-typed suggestions and suggestions with tooltips, the CommandAPI can support computing the list of suggestions to send to a player asynchronously. This allows you to perform suggestions independent of the main server thread, which is useful for slow operations, such as:

  • Retrieving suggestions from a remote database
  • Retrieving information from the web
  • Retrieving suggestions from a file

asynchronous suggestions

The asynchronous methods for suggestions are similar in format to normal suggestions, except for two things: the name of the method ends with Async, and the return type for the inner function is a CompletableFuture:

ArgumentSuggestions stringsAsync(Function<SuggestionInfo, CompletableFuture<String[]>> suggestions);
ArgumentSuggestions stringsWithTooltipsAsync(Function<SuggestionInfo, CompletableFuture<IStringTooltip[]>> suggestions);
SafeSuggestions<T> suggestAsync(Function<SuggestionInfo, CompletableFuture<T[]>> suggestions);
SafeSuggestions<T> tooltipsAsync(Function<SuggestionInfo, CompletableFuture<Tooltip<T>[]>> suggestions);

Creating completable futures

The easiest way to create a CompleteableFuture for asynchronous suggestions is to use Java's CompletableFuture.supplyAsync() method. If you have a simple array of string suggestions (String[]), these can be turned into a completable future (CompletableFuture<String[]>) using this method:

new String[] { "dirt", "grass", "cobblestone", };


CompletableFuture.supplyAsync(() -> {
    return new String[] { "dirt", "grass", "cobblestone", };

Developer's Note:

As with all asynchronous operations running in a Minecraft server, you should not try to access the Bukkit API within an asynchronous block. If you want to run code which accesses Bukkit's API while inside an asynchronous block, you can schedule a synchronous task using the Bukkit scheduler, for example:

CompletableFuture.supplyAsync(() -> {

   Bukkit.getScheduler().scheduleAsyncDelayedTask(plugin, () -> 
       // Your code here

   return new String[] { "dirt", "grass", "cobblestone", };

Asynchronous suggestions examples

Reading keys from a config file

Say you wanted to write a command to modify your plugin's config file. Since the config file is an external file, you ideally want to access the file in a separate thread to the main server thread to retain performance for players on the server. We have the following command syntax:

/setconfig <key> <value>

We make use of the ArgumentSuggestions.stringsAsync method to provide asynchronous suggestions. In our completable future implementation, we access the keys from the plugin configuration.

new CommandAPICommand("setconfig")
    .withArguments(new StringArgument("key").replaceSuggestions(ArgumentSuggestions.stringsAsync(info -> {
        return CompletableFuture.supplyAsync(() -> {
            return plugin.getConfig().getKeys(false).toArray(new String[0]);
    .withArguments(new TextArgument("value"))
    .executes((sender, args) -> {
        String key = (String) args.get(0);
        String value = (String) args.get(1);
        plugin.getConfig().set(key, value);
    .withArguments(StringArgument("key").replaceSuggestions(ArgumentSuggestions.stringsAsync { _ ->
        CompletableFuture.supplyAsync { plugin.config.getKeys(false).toTypedArray() }
    } ))
    .executes(CommandExecutor { _, args ->
        val key = args[0] as String
        val value = args[1] as String
        plugin.config.set(key, value)