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Command Framework

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Supported Java Versions

Supported Message Frameworks

A generic CDI-based command framework. This library requires Java 8 or newer but is fully Java 9+ compatible and can run as a proper Java module on the module path. Any arbitrary underlying message framework like a Discord library, an IRC library, or a Skype library can be used by providing an according CommandHandler implementation. You are also welcome to contribute such implementations back to the main project for all users benefit.

Table of Contents

Prerequisites

  • Java 8+
  • At least one of the supported message frameworks unless a custom CommandHandler is used; without one there will be no error, but this framework will simply have nothing to do
  • An implementation of CDI that implements CDI 3.0.0 like Weld SE
  • [Optional] ANTLR runtime 4.7.2 if the ParameterParser is used

Supported Message Frameworks

The following message frameworks are currently supported out of the box:

  • Javacord (text and slash commands)
  • JDA (text commands)

If you want to have support for an additional framework, do not hesitate to open a pull request or feature request issue.

Setup

Gradle

repositories { mavenCentral() }
dependencies { implementation 'net.kautler:command-framework:0.5.0' }

Maven

<dependency>
  <groupId>net.kautler</groupId>
  <artifactId>command-framework</artifactId>
  <version>0.5.0</version>
</dependency>

Manually

Download the JAR for the latest release from the Latest Release Page and include it in your project.

Usage

Message Framework

Javacord

Text Commands

For the Javacord support, include Javacord as implementation dependency and create a CDI producer that produces either one DiscordApi, or if you use sharding a Collection<DiscordApi> with all shards where you want commands to be handled. You should also have a disposer method that properly disconnects the produced DiscordApi instances.

Example:

@ApplicationScoped
class JavacordProducer {
    @Inject
    Logger logger;

    @Inject
    @Named
    String discordToken;

    @Produces
    @ApplicationScoped
    DiscordApi produceDiscordApi() {
        return new DiscordApiBuilder()
                .setToken(discordToken)
                .login()
                .exceptionally(ExceptionLogger.get())
                .join();
    }

    private void disposeDiscordApi(@Disposes DiscordApi discordApi) {
        discordApi.disconnect();
    }
}

Tested versions:

  • 3.4.0
Slash Commands

For slash commands you need to do the same as for Text Commands.

Additionally, the commands have to implement SlashCommandJavacord instead of Command. Furthermore, they must have a description defined, and all aliases have to consist of one to three slash separated parts, so either command, command/subcommand, or command/subcommand-group/subcommand.

If your command needs parameters, you overwrite the SlashCommandJavacord#getOptions method, which just returns an empty list in its default implementation. In the implementation of the method you can then use the full API for Javacord slash command options.

When using command context transformers with slash commands, all phases before BEFORE_COMMAND_COMPUTATION are skipped by the command handler already.

Finally, this framework does not register the slash commands with Discord for you automatically, because you might want to register them globally, or per server, or you might want to add further commands not managed by this framework, and so on. Instead, you can register the commands yourself by injecting the provided List<SlashCommandBuilder>. This injected list can directly be used as argument for methods like DiscordApi#bulkOverwriteGlobalApplicationCommands or DiscordApi#bulkOverwriteServerApplicationCommands. The requirements described above regarding description and aliases are only enforced if you inject it somewhere.

A fully self-contained example containing a text, a slash, and a combined command can be found at examples/simplePingBotJavacord.

Example:

@ApplicationScoped
public class SlashCommandRegisterer {
    @Inject
    DiscordApi discordApi;

    @Inject
    List<SlashCommandBuilder> slashCommandBuilders;

    void registerSlashCommands(@Observes @Initialized(ApplicationScoped.class) Object __) {
        discordApi
                .bulkOverwriteGlobalApplicationCommands(slashCommandBuilders)
                .exceptionally(ExceptionLogger.get());
    }
}

Tested versions:

  • 3.4.0

JDA

For the JDA support, include JDA as implementation dependency and create a CDI producer that produces either one JDA, of if you use sharding a Collection<JDA>, a ShardManager or a Collection<ShardManager> with all shards where you want commands to be handled. You should also have a disposer method that properly shuts down the produced JDA and / or ShardManager instances.

Example:

@ApplicationScoped
class JdaProducer {
    @Inject
    Logger logger;

    @Inject
    @Named
    String discordToken;

    @Produces
    @ApplicationScoped
    JDA produceJda() {
        try {
            return JDABuilder
                    .createLight(discordToken)
                    .build()
                    .awaitReady();
        } catch (InterruptedException | LoginException e) {
            logger.error("Exception while logging in to Discord", e);
            return null;
        }
    }

    private void disposeJda(@Disposes JDA jda) {
        jda.shutdown();
    }
}

Tested versions:

  • 4.4.0_352

Creating Commands

Create a CDI bean that implements the Command interface.

Example:

@ApplicationScoped
class PingCommand implements Command<Message> {
    @Override
    public void execute(CommandContext<? extends Message> commandContext) {
        commandContext
                .getMessage()
                .getChannel()
                .sendMessage("pong: " + commandContext.getParameterString().orElse(""))
                .exceptionally(ExceptionLogger.get());
    }
}

With everything else using the default, this is already enough to have a working ping bot. A fully self-contained example can be found at examples/simplePingBotJavacord.

To further customize the behavior of a command, you can either annotate the command class or overwrite the corresponding methods. Annotations are ignored when the corresponding methods are overwritten, but they can still be separately evaluated or used as documentation. For all functionality this framework uses the command method implementations. The annotations are only read in the default implementations of those methods.

Command Aliases

By overwriting the Command#getAliases() method or applying one or multiple @Alias annotations, the aliases to which the command reacts can be configured. If no aliases are configured, the class name, with the Command or Cmd suffix / prefix stripped and the first letter lowercased is used as a default. If at least one alias is configured, only the explicitly configured ones are used.

Javacord slash command specific: When injecting a List<SlashCommandBuilder> anywhere, all aliases of commands implementing SlashCommandJavacord have to follow a pre-defined format that is described at slash commands.

Asynchronous Command Execution

By overwriting the Command#isAsynchronous() method or applying the @Asynchronous annotation, the command handler can be told to execute the command asynchronously.

How exactly this is implemented is up to the command handler that evaluates this command. Usually the command will be executed in some thread pool. But, it would also be valid for a command handler to execute each asynchronous command execution in a new thread, so using this can add significant overhead if overused. If a command is not doing long-running or blocking operations, it may be preferable to not execute the command asynchronously. Although, if long-running or blocking operations are done in the command code directly, it may be preferable to execute the command asynchronously, as (depending on the underlying message framework) message dispatching could be blocked, introducing serious lag to the command execution.

As the command executions are potentially done on different threads, special care must be taken, if the command holds state, to make sure this state is accessed in a thread-safe manner. This can of course also happen without the command being configured asynchronously if the underlying message framework dispatches message events on different threads.

Command Description

By overwriting the Command#getDescription() method or applying the @Description annotation, the description of the command can be configured. This description can be used, for example, in a custom help command.

Javacord slash command specific: When injecting a List<SlashCommandBuilder> anywhere, all commands implementing SlashCommandJavacord have to provide a description.

Command Restrictions

By overwriting the Command#getRestrictionChain() method or applying one or multiple @RestrictedTo annotations, and optionally the @RestrictionPolicy annotation, the restriction rules for a command can be configured. If multiple @RestrictedTo annotations are present and the default implementation of the method is used, a @RestrictionPolicy annotation that defines how the single restrictions are to be combined is mandatory. With this annotation the single restrictions can be combined using all-of, any-of, or none-of logic.

For more complex boolean logic either overwrite the getRestrictionChain method or provide custom CDI beans that implement the Restriction interface and contain the intended logic. For the latter also helpers like ChannelJavacord, RoleJavacord, ServerJavacord, UserJavacord, AllOf, AnyOf, or NoneOf can be used as super classes.

Examples:

@ApplicationScoped
class Vampire extends UserJavacord {
    public Vampire() {
        super(341505207341023233L);
    }
}
@ApplicationScoped
class MyFancyServer extends ServerJavacord {
    // make bean proxyable according to CDI spec
    public MyFancyServer() {
        super(-1);
    }

    @Inject
    MyFancyServer(@Named("myFancyServerId") long myFancyServerId) {
        super(myFancyServerId);
    }
}

Command Usage

By overwriting the Command#getUsage() method or applying the @Usage annotation, the usage of the command can be configured. This usage can be used, for example, in a custom help command.

When using the ParameterParser, the usage string has to follow a pre-defined format that is described at Parsing Parameters.

Parsing Parameters

There are three helpers to split the parameterString that is provided to Command#execute(...) into multiple parameters that can then be handled separately.

Simple Splitting of Parameters

The first is the method Command.getParameters(...) which you give the parameter string and the maximum amount of parameters to split into. The provided string will then be split at any arbitrary amount of consecutive whitespace characters. The last element of the returned array will have all remaining text in the parameter string. If you expect exactly three parameters without whitespaces, you should set the max parameters to four, so you can easily test the length of the returned array to determine if too many parameters were given to the command.

Semantic Parsing and Validation

The second is the ParameterParser that you can get injected into your command. For the ParameterParser to work, the usage of the command has to follow a defined syntax language. This usage syntax is then parsed and the given parameter string analysed according to the defined syntax. If the given parameter string does not adhere to the defined syntax, a ParameterParseException is thrown that can be caught and reported to the user giving wrong arguments. The exception message is suitable to be directly forwarded to users.

The usage string has to follow this pre-defined format:

  • Placeholders for free text without whitespaces (in the value) look like <my placeholder>
  • One placeholder for free text with whitespaces (in the value) is allowed as effectively last parameter and looks like <my placeholder...>
  • Literal parameters look like 'literal'
  • Optional parts are enclosed in square brackets like [<optional placeholder>]
  • Alternatives are enclosed in parentheses and are separated by pipe characters like ('all' | 'some' | 'none')
  • Whitespace characters between the defined tokens are optional and ignored

Examples:

  • @Usage("<coin type> <amount>")
  • @Usage("['all'] ['exact']")
  • @Usage("[<text...>]")
  • @Usage("(<targetLanguage> '|' | <sourceLanguage> <targetLanguage>) <text...>")

The values for these non-typed parameters are always Strings unless multiple parameters with the same name have a value given by the user like with the pattern <foo> <foo>, in which case the value will be a List<String>.

Warning: If you for example have

  • an optional placeholder followed by an optional literal like in [<placeholder>] ['literal'] or
  • alternatively a placeholder or literal like in (<placeholder> | 'literal')

and a user invokes the command with only the parameter literal, it could fit in both parameter slots. You have to decide yourself in which slot it belongs. For cases where the literal parameter can never be meant for the placeholder, you can use Parameters#fixup(...) to correct the parameters instance for the two given parameters.

Semantic Parsing and Validation with Type Conversions

The third is an addendum to the second method described above. The syntax is basically the same. The only difference is, that a colon (':') followed by a parameter type can optionally be added after a parameter name like for example <amount:integer>. Parameters that do not have a type specified, are implicitly of type string. If a colon is needed within the actual parameter name, a type has to be specified explicitly, as invalid parameter types are not allowed and will trigger an error at runtime.

The parameter types can be freely defined by supplying parameter converters to define new types or overwrite built-in ones to have a different behavior.

The built-in types currently available are:

  • for all message frameworks
    • decimal for a floating point number converted to BigDecimal
    • number or integer for a natural number converted to BigInteger
    • string or text for a no-op conversion which can be used if a colon is needed in the parameter name or if simply all parameters should have a type specified for consistency
  • for Javacord
    • user_mention or userMention for a mentioned user converted to User
    • role_mention or roleMention for a mentioned role converted to Role
    • channel_mention or channelMention for a mentioned channel converted to Channel
  • for JDA
    • user_mention or userMention for a mentioned user converted to User
    • role_mention or roleMention for a mentioned role converted to Role
    • channel_mention or channelMention for a mentioned channel converted to TextChannel

To select the typed parameter parser, add the qualifier @ParameterParser.Typed to the injected ParameterParser.

Examples:

@ApplicationScoped
@Usage("<text...>")
class PingCommand implements Command<Message> {
    @Inject
    ParameterParser parameterParser;

    @Override
    public void execute(CommandContext<? extends Message> commandContext) {
        Message incomingMessage = commandContext.getMessage();

        try {
            parameterParser.parse(commandContext);
        } catch (ParameterParseException ppe) {
            incomingMessage.getChannel()
                    .sendMessage(format("%s: %s", incomingMessage.getAuthor().getDisplayName(), ppe.getMessage()))
                    .exceptionally(ExceptionLogger.get());
            return;
        }

        incomingMessage.getChannel()
                .sendMessage("pong: " + commandContext.getParameterString().orElse(""))
                .exceptionally(ExceptionLogger.get());
    }
}
@ApplicationScoped
@Usage("[<user:userMention>] ['exact']")
class DoCommand implements Command<Message> {
    @Inject
    @Typed
    ParameterParser parameterParser;

    @Override
    public void execute(CommandContext<? extends Message> commandContext) {
        Parameters<String> parameters;
        try {
            parameters = parameterParser.parse(commandContext);
        } catch (ParameterParseException ppe) {
            Message incomingMessage = commandContext.getMessage();
            incomingMessage.getChannel()
                    .sendMessage(format("%s: %s", incomingMessage.getAuthor().getDisplayName(), ppe.getMessage()))
                    .exceptionally(ExceptionLogger.get());
            return;
        }
        parameters.fixup("user mention", "exact");
        boolean exact = parameters.containsParameter("exact");
        Optional<String> otherUser = parameters.get("user mention");
        // ...
    }
}

Customizing Parameter Converters

A custom parameter converter can be configured by providing a CDI bean that implements the ParameterConverter interface. The implementation of the convert method calculates the converted parameter value using the string parameter, parameter type, and command context. The class also needs to be annotated with one or multiple ParameterType qualifiers that define the parameter type aliases for which the annotated parameter converter works. Without such qualifier the converter will simply never be used. It is an error to have multiple parameter converters with the same parameter type that can be applied to the same framework message type, and this will produce an error latest when a parameter with that type is being converted. The only exceptions are the built-in parameter types. A user-supplied converter with the same parameter type as a built-in converter will be preferred, but it would still be an error to have multiple such overrides for the same type.

Examples:

@ApplicationScoped
@ParameterType("strings")
class StringsConverter implements ParameterConverter<Object, List<String>> {
    @Override
    public List<String> convert(String parameter, String type, CommandContext<?> commandContext) {
        return asList(parameter.split(","));
    }
}

Customizing the Command Recognition and Resolution Process

The command recognition and resolution process consists of five phases. Actually, these phases can vary, as a command handler or a command context transformer can fast-forward the process to a later phase to skip unnecessary work, or some phase can fail the process with a command not found event.

The five phases that are handled are in order:

  • Initialization
  • Prefix Computation
  • Alias and Parameter String Computation
  • Command Computation
  • Command Execution

For all but the first and last, there is a before and an after sub phase each during which the command context transformer is called.

If at the end of the initialization phase, or any before / after sub phases, the command is set in the context, processing is fast forwarded immediately to the command execution phase and all other inbetween phases and sub phases are skipped.

If at the end of the initialization phase, or any before / after sub phases before the BEFORE_COMMAND_COMPUTATION sub phase, the alias is set in the context, processing is fast forwarded immediately to the before command computation sub phase and all other inbetween phases and sub phases are skipped.

If at the end of the initialization phase, or any before / after sub phases before the BEFORE_ALIAS_AND_PARAMETER_STRING_COMPUTATION sub phase, the prefix is set in the context, processing is fast forwarded immediately to the before alias and parameter string computation sub phase and all other inbetween phases and sub phases are skipped.

Before Prefix Computation Sub Phase

At the start of this sub phase, usually only the message and message content are set.

After Prefix Computation Sub Phase

At the start of this sub phase, usually only the message, message content, and prefix are set.

If at the end of this sub phase no fast forward was done and no prefix is set, a command not found event is being fired and processing stops completely.

Before Alias and Parameter String Computation Sub Phase

At the start of this sub phase, usually only the message, message content, and prefix are set.

If at the end of this sub phase no fast forward was done and no prefix is set, a command not found event is being fired and processing stops completely.

If at the end of this sub phase a prefix is set and it does not match the start of the message content, the message is ignored and processing stops completely. This is the only way to stop processing cleanly without getting a command not found event fired. This can also be achieved by fast forwarding to this phase by setting the prefix in an earlier phase and not doing anything in this phase actually, or not even registering for it.

After Alias and Parameter String Computation Sub Phase

At the start of this phase, usually only the message, message content, prefix, alias, and parameter string are set.

If at the end of this phase no fast forward was done and no alias is set, a command not found event is being fired and processing stops completely.

Before Command Computation Sub Phase

At the start of this phase, usually only the message, message content, prefix, alias, and parameter string are set.

If at the end of this phase no fast forward was done and no alias is set, a command not found event is being fired and processing stops completely.

After Command Computation Sub Phase

At the start of this phase, usually the command context is fully populated.

If at the end of this phase no command is set, a command not found event is being fired and processing stops completely.

Hooking into a Sub Phase

For each of the described sub phases, exactly one context transformer that is compatible with the framework message can be registered. If you need multiple such transformers, then make one distributing transformer that calls the other transformers in the intended order. You can also register one context transformer for multiple phases. The transform method gets the current phase as argument and can then decide what to do based on that phase parameter.

A command context transformer can be registered by providing a CDI bean that implements the CommandContextTransformer interface. In the implementation of the transform method, given the current phase, you can transform the current command context, or even return a completely new one. Additionally the bean has to be annotated with at least one @InPhase qualifier annotation, or it will simply not be used silently. The transformer will be called for each sub phase that is added with that annotation.

There are also helper classes that can be used as super classes for custom command context transformers, for example, a transformer that returns the mention string for the bot as command prefix if Javacord is used as the underlying message framework.

Warning: The command prefix can technically be configured to be empty, but this means that if the alias and parameter string computation phase is executed, every message will be checked against a regular expression and that for every non-matching message a CDI event will be sent. It is better for the performance if a command prefix is set instead of including it in the aliases directly. Due to this potential performance issue, a warning is logged each time a message is handled with an empty command prefix. If you do not care and want the warning to vanish, you have to configure your logging framework to ignore this warning, as it also costs additional performance and might hide other important log messages. ;-)

Example use-cases for command context transformers are:

  • Custom command prefixes depending on some database

  • Dynamic commands stored in some database

  • Fuzzy-searching for mistyped aliases and their automatic correction (this could also be used for just a "did you mean X" response, but for that the command not found events are maybe better suited)

  • Having a command that forwards to one command in one channel but to another command in another channel, like !player that forwards to !mc:player in an MC channel but to !s4:player in an S4 channel

  • Supporting something like !runas @other-user foo bar baz, where the transformer will transform that to alias foo and parameter string bar baz, storing the other-user as additional data in the command context, and then a custom Restriction can check whether the message author has the permissions to use !runas and for example whether the other-user would have permissions for the foo command and only then allow it to proceed

  • forwarding to a !help command if an unknown command was issued

Examples:

@ApplicationScoped
@InPhase(BEFORE_PREFIX_COMPUTATION)
class MyPrefixTransformer implements CommandContextTransformer<Message> {
    @Override
    public <T extends Message> CommandContext<T> transform(CommandContext<T> commandContext, Phase phase) {
        Optional<Server> server = commandContext.getMessage().getServer();
        if (!server.isPresent()) {
            return commandContext.withPrefix("!").build();
        } else if (server.get().getId() == 12345L) {
            return commandContext.withPrefix("bot ").build();
        } else {
            return commandContext.withPrefix(":").build();
        }
    }
}
@ApplicationScoped
@InPhase(BEFORE_PREFIX_COMPUTATION)
class MentionPrefixTransformer extends MentionPrefixTransformerJavacord {
}
@ApplicationScoped
@InPhase(AFTER_ALIAS_AND_PARAMETER_STRING_COMPUTATION)
class MyAliasAndParameterStringTransformer implements CommandContextTransformer<Message> {
    @Override
    public <T extends Message> CommandContext<T> transform(CommandContext<T> commandContext, Phase phase) {
        return (!commandContext.getAlias().isPresent())
               ? commandContext.withAlias("help").build()
               : commandContext;
    }
}

Storing Additional Data in the Command Context

The CommandContext is also a store for arbitrary additional information that you can attach to a command invocation. You can, for example, attach some information during execution of a command context transformer and then later evaluate this information in a custom restriction class or in the implementation of the actual command.

The methods regarding additional data all have a generic type argument, as they all return a value, either the current one or the previous one, depending on the method. This value can be cast to the correct type for you, but be careful to select the proper type. As this is an unsafe operation, the type has to be chosen wisely. If you, for example, select String as the type and then try to get a User object from the returned optional, you will get a ClassCastException at runtime.

The type can be specified explicitly like

commandContext.<User>getAdditionalData("user");

or using implicit type inference like with

Optional<User> user = commandContext.getAdditionalData("user");

CDI Events

Handling Missing Commands

If a message starts with the configured command prefix, but does not map to an available command, the command handlers send an async CDI event that you can observe and handle to react accordingly like sending a message that a command was not found.

Example:

@ApplicationScoped
class EventObserver {
    void commandNotFound(@ObservesAsync CommandNotFoundEventJavacord commandNotFoundEvent) {
        commandNotFoundEvent.getMessage()
                .getChannel()
                .sendMessage(format(
                        "Command %s%s was not found!",
                        commandNotFoundEvent.getPrefix(),
                        commandNotFoundEvent.getUsedAlias()))
                .exceptionally(ExceptionLogger.get());
    }
}

Handling Disallowed Commands

If a command was found but not allowed by some restriction rules, the command handlers send an async CDI event that you can observe and handle to react accordingly like sending a message that a command was not allowed.

Example:

@ApplicationScoped
class EventObserver {
    void commandNotAllowed(@ObservesAsync CommandNotAllowedEventJavacord commandNotAllowedEvent) {
        commandNotAllowedEvent.getMessage()
                .getChannel()
                .sendMessage(format(
                        "Command %s%s was not allowed!",
                        commandNotAllowedEvent.getPrefix(),
                        commandNotAllowedEvent.getUsedAlias()))
                .exceptionally(ExceptionLogger.get());
    }
}

Getting the Library Version Programmatically

You are welcome to mention in some "about" command or similar that you use this library to attract other people to use it too. If you want to also mention which version you are using, you can use the getDisplayVersion method of the Version CDI bean by injecting it into your code.

Supporting other Message Frameworks

If you want to support a message framework that is not natively supported already, you need to provide a CDI bean that extends the CommandHandler class. You are also welcome to contribute back any such implementation to the library for all users benefit. You should read the JavaDoc of the CommandHandler class and have a look at any of the existing implementations to get started with writing your own implementation. Most of the common logic should be done in the CommandHandler class already and just some framework-dependent things like attaching message listeners to the underlying framework need to be done in the subclass.

Version Numbers

Versioning of this library follows the Semantic Versioning specification. But only classes in packages starting with net.kautler.command.api are bound to the backwards compatibility constraints of semantic versioning. All other classes are considered internal and can have breaking changes anytime.

License

Copyright 2019-2022 Björn Kautler

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

    http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.

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A generic CDI-based command framework

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