Using Program Parameters in Java
by Keld H. Hansen
Introduction
Every programmer will encounter a situation where a choice must
be made between various alternative solutions. Should tabular
data be sorted in one way or another? What exactly is the
graphical layout of your HTML pages? How often should you poll a
queue? Most often you'll know the path to take based on the
requirements specifications, a design manual or some other kind
of documentation, but now and then you'll have to choose without
help. Instead of just selecting one solution and hoping the
customer will like it when he or she starts using your code, you
could make it a "program parameter" or
"option".
This means that the actual choice is
given to your program in some way at startup time, for example
from a configuration file. Programs like command line utilities,
where there's no GUI and no database, simply take their data as
"program parameters".
In this article we'll look into the various ways you may enter
parameters to a Java program:
- Command line parameters
- Property files
- A combination of 1 and 2
- System properties
- XML files
- Databases
Finally I'll show how you can hide the implementation--at least
to some degree.
Command line parameters
Let's start with the most basic and simplest way to enter
parameters to a Java program: the command line. We all should know
that a Java main program takes a String array as its parameter,
and it contains the program arguments given to the Java VM. I
deliberately used the word "arguments", since the
"parameters" given on the command line are meant for
the VM. Entering "java -?" gives the syntax for the
Java command:
C:\>java -?
Usage: java [-options] class [args...
(to execute a class)
or java [-options] -jar jarfile [
(to execute a jar file)
. . .
If you have a fixed list of parameters for your program then you
simply add them after the class name, but if you'd like to work
with optional parameters, you need some way to identify the
different parameters.
It's common practice to give the parameters descriptive names,
and use a hyphen in front of the name of the parameter. The
parameter value(s) then follow after the name. This syntax is,
for example, used in the Java command shown above.
It might be fun to write a command line parser in Java once and
maybe twice, but the third time it gets rather boring. The
Jakarta Commons project has a command line parser called CLI (Command
Line Interface), which is simple and efficient to
use. There are 3 steps involved when using CLI: define,
parse, interrogate
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