Core classes of the Args4J command line parser.

What is Args4J?

Args4J is a command line parser. As such, its job is to parse the String-array passed to the main() method and transfer the values to a Java object, which includes type conversion. The entry point into this parsing is the CmdLineParser class, with its parse() method.

Args4J must know the mapping between the flag from the command line and the target for the value. There are many ways to use Args4J:

• place an @Option or @Argument annotation at a setter or on a field
• provide a XML configuration file
• make all fields publicly available

Depending on what you want, you may have perform a configuration step before parsing.

Examples

java Main -text newText

The typical use involves writing a small bean class and providing the annotations.

This feature is available since the first Args4J release:

public class Bean {
   {@literal @}Option(name="-text")
    String text;
}
public class Main {
    public static void main(String[] args) throws CmdLineException {
        Object bean = new Bean();
        CmdLineParser parser = new CmdLineParser(bean);
        parser.parse(args);
    }
}

An easy way to initialize fields without touching the bean source code is to use the FieldParser.

The FieldParser scans all fields of bean class (including inheritance), and makes them publicly available as options with a - prefix in the name.

This feature is available since Args4J release 2.0.16:

public class Bean {
    String text;
}
public class Main {
    public static void main(String[] args) throws CmdLineException {
        Object bean = new Bean();
        CmdLineParser parser = new CmdLineParser(null);
        new FieldParser().parse(parser, bean);
        parser.parse(args);
    }
}

While the FieldParser is easier to use, the XmlParser supports more features.

That said, it supports all features which are available via annotations: usage text, specifying handlers, and more. You have to provide an XML InputSource or a URL to the XML file.

This feature is available since Args4J release 2.0.16:

public class Bean {
    String text;
}
public class Main {
    public static void main(String[] args) throws CmdLineException {
        Object bean = new Bean();
        CmdLineParser parser = new CmdLineParser(null);
        new XmlParser().parse(getClass().getResource(bean.getClass().getName() + ".xml"), parser, bean);
        parser.parse(args);
    }
}
<args>
  <option field="text" name="-text" usage="Output text"/>
</args>