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JavaBlackBelt Authors: Sumith Kumar Puri, Duncan Thomas, Maureen O'Gara, Karthik Narayanan, Steve Beaty

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Article

Reflections on Java Command Line Options

There are many different types of command line options that programs need to recognize

Abstract
There are many different types of command line options that programs need to recognize. Many languages (e.g.: bash and perl) has built-in processing of command line options; Java does not. The Java Command Line Options (JCLO) package performs this task for a variety of option styles. It also uses Java's reflection capability to automatically assign values to variables in a specified class.

Introduction
Even in these days of sophisticated graphical user interfaces, many programs have a wide variety of command line options that help specify their behavior. It is also the case that command line only programs continue to enjoy wide use. It is also the case the command line arguments can become quite complicated, e.g.: -Djava.util.logging.config.file=All.finest -1 --list --this=that Some languages have built-in parsers for command line options; perl and bash are two obvious examples of this. Java has no such parser built in. The JCLO package provides the capability to parse several different command line option formats, and uses Java's reflecion mechanizm to both drive the parsing and assign the values provided by command line options to the variable in a specified class.

Brief overview of command line option formats
UNIX command line options started with a simple "dash and letter" format. "ls -l" is a classic example. If the option specified an additional option, the dash and letter were followed by a space and the additional option. "sort -t separator" is and example. UNIX also allowed the use of numbers, "ls -1", as options. As the number of possible option proliferated, and newer "GNU" style was developed. These have a "double dash/long name" style: "sort --version" for example. When additional options are needed for this style, it is typically provided with a "equals value" style: "gcc --std=c89". Finally, Java has its own style of "single dash/name with dots/equals value" style: "-Djava.util.logging.config.file=logging.props".

Specifying non-Java variable name options
JCLO uses Java reflection to extract variable names as the basis for parsing command line options. Not all styles just described are valid Java variable names and therefore JCLO uses several conventions to allow them specified. Java variables must begin with an alphabetic character or an underscore, numbers are not allowed to start a variable name. Variable names cannot contain dashes, as dashes specify subtraction or negation. They cannot contain dots as dots specify class references or decimal literals. Therefore, JCLO uses the convention of prefixing number options with an underscore (_1). For dashes embedded in options (e.g.: "--font-size=10"), JCLO uses two underscores ("font__size") in the variable name. For the embedded dots, JCLO uses a convention of an underscore followed by a dollar sign "_$".

Brief review of Java reflection
Java, as with other object oriented languages, has the ability to query and modify an object's internal information. One can retrieve a Class's constructors, methods, fields, etc. JCLO uses the getDeclaredFields()" method on a class to find the names it will accept and set the value for. One can either have a single class devoted to command line options or specify a prefix for the variables JCLO's will examine.

From class variables to command line options
First let's look at an example to see how JCLO works in practice. We have the obligatory import:

import edu.mscd.cs.jclo.JCLO;

and then we create a class whose class varibles will become the command line options:

class ExampleArgs
{
    private int a;
    private boolean b;
    private float c;
    private String d;
    private String[] additional;
}

A simple main will be used to illustrate JCLO's operation:

public class Example
{
    public static void main (String args[])
    {
        ExampleArgs ea = new ExampleArgs();
        JCLO jclo = new JCLO (ea);
        jclo.parse (args);
	System.out.println ("a = " + ea.a);
	System.out.println ("b = " + ea.b);
	System.out.println ("c = " + ea.c);
	System.out.println ("d = " + ea.d);
	System.out.println ("additional = " + 
	    java.util.Arrays.toString (ea.additional));
    }
}

First, an object that contains the variables whose values will be assigned by the command line options is created. This object is then used to create a JCLO object. The parse method is then called with the command line options to do the actual work Running this simple program with the following command line options:

java -cp .:JCLO-1.3.4.jar Example -a 5 --b=true --c=9.0 -d Example one two three

produces:

a = 5
b = true
c = 9.0
d = Example
additional = [one, two, three]

Here, each of the class varibles where assigned values from the associated command line option. The additional String array is assigned any options beyond the last dash option. JCLO can also use only certain fields of an object by the programmer specifying a string prefix that those fields begin with.

There are times when command line options have aliases; for example a long and short version. JCLO has the ability to deal with these directly. Adding

String aliases[][] ={{"boolean", "b"}};

and modifying

JCLO jclo = new JCLO (ea, aliases);

to the above example allows for --boolean to set the value for b

:

java -cp .:JCLO-1.3.4.jar Example --boolean
a = 0
b = true
c = 0.0
d = null
additional = null

JCLO also has a simple usage() method that returns a String of possible options and the type of values they require.

Conclusion
JCLO allows one to easily parse command line options and set the values inside a class based on those options. It is very flexible in its parsing, allowing intermixed single- and double-dashed options, along with aliases that allow long and short versions of an option. JCLO is available from http://jclo.sourceforge.net/.

More Stories By Steve Beaty

Steve has an extensive background in both the theoretic and pragmatic aspects of computer science. He wrote compilers at Cray Computer, and both managed a large group of developers and was a software test architect at HP. He has a number of active open-source projects and is a professor of computer science at the Metropolitan State College of Denver.