Argument Parsing

The Command Builder API comes with a powerful argument parser. It converts the string input to java base types (integers, booleans, strings) or game objects (players, worlds, block types, …). The parser supports optional arguments and flags. It also handles TAB completion of arguments.

The parsed arguments are stored in the CommandContext object. If the parser returns a single object, obtain it with CommandContext#getOne(String). Optional and weak arguments may return Optional.empty().

Many of the parsers may return more than one object (e.g. multiple players with a matching username). In that case, you must use the CommandContext#getAll(String) method to get the Collection of possible matches. Otherwise, the context object will throw an exception!

When creating a command, consider whether the argument could return multiple values, for example, whether a player argument could support multiple players when using a selector. If you support multiple values the users need to type only one command and can use an easier command syntax. Example: /tell @a Who took the cookies?

Tip

You can use the GenericArguments#onlyOne(CommandElement) element to restrict the amount of returned values to a single one, so you can safely use args.<T>getOne(String). However the user will still get a message, if they try to select more than one value.

To create a new CommandElement (argument), use the GenericArguments factory class. Many command elements require a short text key, which is displayed in error and help messages.

Apply the CommandElement to the command builder with the CommandSpec.Builder#arguments(CommandElement…) method. It is possible to pass more than one CommandElement to the method, thus chaining multiple arguments (e.g. /msg <player> <msg>). This has the same effect as wrapping the CommandElement objects in a GenericArguments#seq(CommandElement…) element.

Example: Building a Command with Multiple Arguments

import org.spongepowered.api.Sponge;
import org.spongepowered.api.text.Text;
import org.spongepowered.api.entity.living.player.Player;
import org.spongepowered.api.command.CommandException;
import org.spongepowered.api.command.CommandResult;
import org.spongepowered.api.command.CommandSource;
import org.spongepowered.api.command.args.CommandContext;
import org.spongepowered.api.command.args.GenericArguments;
import org.spongepowered.api.command.spec.CommandExecutor;
import org.spongepowered.api.command.spec.CommandSpec;

PluginContainer plugin = ...;

CommandSpec myCommandSpec = CommandSpec.builder()
        .description(Text.of("Send a message to a player"))
        .permission("myplugin.command.message")

        .arguments(
                GenericArguments.onlyOne(GenericArguments.player(Text.of("player"))),
                GenericArguments.remainingJoinedStrings(Text.of("message")))

        .executor((CommandSource src, CommandContext args) -> {

            Player player = args.<Player>getOne("player").get();
            String message = args.<String>getOne("message").get();

            player.sendMessage(Text.of(message));

            return CommandResult.success();
        })
        .build();

Sponge.getCommandManager().register(plugin, myCommandSpec, "message", "msg", "m");

Overview of the GenericArguments command elements

Command Element	Description	Value Type & Amount
none	Expects no arguments. This is the default behavior of a CommandSpec.
Java Base Types
string	Require an argument to be a string.	one String
remainingJoinedStrings	Concatenates all remaining arguments separated by spaces (useful for message commands).	one String
bool	Require an argument to be a boolean.	one Boolean
integer	Require an argument to be an integer.	one Integer
doubleNum	Require an argument to be a double.	one Double
Game Objects
player	Expect an argument to represent an online player. May return multiple players!	multiple Player instances
playerOrSource	Like player, but returns the sender of the command if no matching player was found.	multiple Player instances
userOrSource	Like playerOrSource, but returns a user instead of a player.	multiple User instances
world	Expect an argument to represent a world (also includes unloaded worlds).	multiple WorldProperties
dimension	Expect an argument to represent a dimension (END, NETHER, OVERWORLD).	multiple DimensionType instances
location	Expect an argument to represent a Location.	one Location
vector3d	Expect an argument to represent a Vector3d.	one Vector3d
catalogedElement	Expect an argument that is a member of the specified CatalogType.	multiple matching elements of the specified catalog type
Matchers
choices	Return an argument that allows selecting from a limited set of values.	one specified value
literal	Expect a literal sequence of arguments (e.g. "i", "luv", "u": /cmd i luv u). Throws an error if the arguments do not match.	one specified value
enumValue	Require the argument to be a key under the provided enum.	multiple matching elements of the specified enum
Utilities Can be wrapped around command elements. The value type is inherited from the wrapped element.
seq	Builds a sequence of command elements (e.g. /cmd <arg1> <arg2> <arg3>).	inherited
repeated	Require a given command element to be provided a certain number of times.	multiple inherited
allOf	Require all remaining args to match the provided command element.	multiple inherited
optional	Make the provided command element optional. Throws an error if the argument is of invalid format and there are no more args.	inherited
optionalWeak	Make the provided command element optional. Does not throw an error if the argument is of invalid format and there are no more args.	inherited
firstParsing	Returns a command element that matches the first of the provided elements that parses (useful for command overloading, e.g. /settime <day|night|<number>).	inherited
onlyOne	Restricts the given command element to only insert one value into the context at the provided key.	inherited
flags	Returns a builder for command flags (e.g. /cmd [-a] [-b <value>]). See Command Flags	Short Flag: one Boolean Long Flag: one String Value Flag: inherited
requiringPermission	Requires the command sender to have the specified permission in order to use the given command argument	inherited
requiringPermissionWeak	Requires the command sender to have the specified permission in order to use the given command argument. Does not throw an error if the user does not have the permission.	inherited

Tip

See the Javadocs for GenericArguments for more information.

Warning

Don’t expect that a CommandElements will only ever return a single value, a lot of them support multiple return values; some might even support regular expressions or use a command selector. This is intentional as it makes commands easier to use. Example: /tell @a BanditPlayer has the cookies!. If you want to make sure to only get a single value use GenericArguments#onlyOne(CommandElement).

Custom Command Elements

It is possible to create custom command elements (e.g. a URL parser or a Vector2i element) by extending the abstract CommandElement class.

The CommandElement#parseValue(CommandSource, CommandArgs) method should fetch a raw argument string with CommandArgs#next() and convert it to an object. The method should throw an ArgumentParseException if the parsing fails.

The CommandElement#complete(CommandSource, CommandArgs, CommandContext) method should use CommandArgs#peek() to read the next raw argument. It returns a list of suggestions for TAB completion.

Example: Vector2i command element definition

The parser in this example reads two input arguments and converts them to a vector.

import com.flowpowered.math.vector.Vector2i;
import org.spongepowered.api.command.args.ArgumentParseException;
import org.spongepowered.api.command.args.CommandArgs;
import org.spongepowered.api.text.Text;
import org.spongepowered.api.command.args.CommandElement;

import java.util.Collections;
import java.util.List;

public class Vector2iCommandElement extends CommandElement {

    protected Vector2iCommandElement(Text key) {
        super(key);
    }

    @Override
    protected Object parseValue(CommandSource source, CommandArgs args) throws ArgumentParseException {

        String xInput = args.next();
        int x = parseInt(xInput, args);

        String yInput = args.next();
        int y = parseInt(yInput, args);

        return new Vector2i(x, y);
    }

    private int parseInt(String input, CommandArgs args) throws ArgumentParseException {
        try {
            return Integer.parseInt(input);
        } catch(NumberFormatException e) {
            throw args.createError(Text.of("'" + input + "' is not a valid number!"));
        }
    }

    @Override
    public List<String> complete(CommandSource src, CommandArgs args, CommandContext context) {
        return Collections.emptyList();
    }

    @Override
    public Text getUsage(CommandSource src) {
        return Text.of("<x> <y>");
    }
}

Example: Vector2i command element usage

// /plottp <x> <y>
CommandSpec myCommandSpec = CommandSpec.builder()
        .description(Text.of("Teleport to a plot"))
        .permission("myplugin.command.plot.tp")

        .arguments(new Vector2iCommandElement(Text.of("coordinates")))

        .executor(new MyCommandExecutor())
        .build();

Tip

Look at the source code of the GenericArguments class for more examples.

Using Selectors in Custom Command Elements

Sponge provides support for parsing selectors, meaning that you can make use of them in your custom elements. There are two steps in using selectors, parsing (getting a Selector from the string) and resolving (getting a set of Entity objects selected by the selector).

To parse a selector string, use the Selector#parse(String) method, passing the entire selector, including the @ symbol. This will turn the string into a Selector object that can be queried or resolved. Note that if the string is not a valid selector, an IllegalArgumentException will be thrown.

To resolve this selector, use Selector#resolve(CommandSource). This will return a set of Entity objects selected by the selector.

The following parseValue method from the CommandElement class attempts to parse a selector and return a set of entities based on the location of the CommandSource. If the passed string does not start with @, an exception will be thrown indicating that the passed argument is not a selector.

@Override
protected Object parseValue(CommandSource source, CommandArgs args) throws ArgumentParseException {
    String nextArg = args.next();
    if (nextArg.startsWith("@")) {
        Set<Entity> selectedEntities;
        try {
            selectedEntities = Selector.parse(nextArg).resolve(source);
        } catch (IllegalArgumentException e) {
            throw args.createError(Text.of("Could not parse selector."));
        }

        if (selectedEntities.isEmpty()) {
            throw args.createError(Text.of("No entities selected."));
        }

        return selectedEntities;
    }

    throw args.createError(Text.of("Not a selector."));
}

Tip

Look at the SelectorCommandElement source code for an example of how selector parsing is performed in the standard Sponge CommandElements.