Clad User Manual - Concepts

Internal

• clad User Manual

Command

A command accepts command options and arguments.

Default Command

An application may not have a default command. In this case, the ApplicationRuntime implementation of public String getDefaultCommand() must return null.

Command Arguments

The command arguments are a list of space separated free-format strings, that are not options, and that have a special significance for the command.

The command is given a chance to recognize arguments that make sense to it, and "consume" them. The command implementation has a chance to do that when implementing the configure(...) method of the Command interface, or when overriding the corresponding method of the CommandBase abstract class.

The CommandBase configure(...) implementation parses the declared options for the command, so the command only accepts options, it is sufficient to use the default implementation. However, for a more nuanced handling of the command line argument, use:

@Override
public void configure(int from, List<String> commandLineArguments) throws Exception {

    super.configure(from, commandLineArguments);

    //
    // perform additional handling here and remove the arguments that make sense to us
    // from the given list
    //
}

Options

The options (global or specific to a certain command) use the GNU command line option syntax:

 -o <value> | --option=<value>

Global Options

The global options apply to the application runtime, and they are not specific to a certain command.

Universal Global Options

All clad-based application automatically support the following global options. There is no need to declared them among the "required" or "optional" global options for a specific application.

-v|--verbose

-v or --verbose turn on DEBUG on the underlying CONSOLE appender, so the application will display internal execution information at stdout. In order to be displayed in this mode, the internal execution information must be logged by the application with SLFJ4 debug() method. For more details on how to program debug logging, see How to Implement a Command Line Application - Debug Logging.

-d|--debug

-d or --debug, in addition to logging debug information at the CONSOLE the way -v|--verbose options do, start the underlying JVM in debug mode, so a debugger can be attached to it.

Logging

The clad framework comes pre-bundled with SLF4J API support, so the command line applications build with clad can use SLF4J for logging. However, clad does not favor any binding, so the application should provide its own SLF4J binding library and the backend logging framework. For details on how do do that, see How to Implement a Command Line Application - SLF4J Integration.

Debug Logging

Use SLF4J API debug() method:

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
...
private static final Logger log = LoggerFactory.getLogger(MyClass.class);
...
log.debug("...");

These log messages will be automatically sent to stdout when the -v|--verbose option is used.

Console Output

Use ApplicationRuntime's info(...), warn(...) and error(...) API calls.

An alternative mechanism to signal critical user errors is to throw UserErrorExceptions.

Error Handling

If the underlying command fails because of what the user did, or by invalid input data entered by the user, and the behavior can be corrected by modifying the user input, the conventional approach should be to throw an UserErrorException with a human-readable message. The default implementation will detect this situation, display the error message embedded within the exception following the error display rules, and terminate the JVM by executing System.exit() with a non-zero exit code.

Configuration File

Each command line option has a configuration file correspondent. Command line value takes precedence over the configuration file value.