Command line Argument Processing

Syntax#

public static void main(String[] args)

Parameters#

Parameter	Details
args	The command line arguments. Assuming that the `main` method is invoked by the Java launcher, `args` will be non-null, and will have no `null` elements.
## Remarks#
When a regular Java application is launched using the `java` command (or equivalent), a `main` method will be called, passing the arguments from the command line in the `args` array.

Unfortunately, the Java SE class libraries do not provide any direct support for command argument processing. This leaves you two alternatives:

Implement the argument processing by hand in Java.
Make use of a 3rd-party library.

This Topic will attempt to cover some of the more popular 3rd-party libraries. For an extensive list of the alternatives, see this answer to the StackOverflow Question “How to parse command line arguments in Java?”.

Argument processing using GWT ToolBase

If you want to parse more complex command-line arguments, e.g. with optional parameters, than the best is to use google’s GWT approach. All classes are public available at:

https://gwt.googlesource.com/gwt/+/2.8.0-beta1/dev/core/src/com/google/gwt/util/tools/ToolBase.java

An example for handling the command-line myprogram -dir "~/Documents" -port 8888 is:

public class MyProgramHandler extends ToolBase {
   protected File dir;
   protected int port;
   // getters for dir and port
   ...

   public MyProgramHandler() {
       this.registerHandler(new ArgHandlerDir() {
            @Override
            public void setDir(File dir) {
                this.dir = dir;
            }
       });
       this.registerHandler(new ArgHandlerInt() {
            @Override
            public String[] getTagArgs() {
               return new String[]{"port"};
            }
            @Override
            public void setInt(int value) {
               this.port = value;
            }
       });
   }
   public static void main(String[] args) {
      MyProgramHandler myShell = new MyProgramHandler();
      if (myShell.processArgs(args)) {
         // main program operation
         System.out.println(String.format("port: %d; dir: %s",
            myShell.getPort(), myShell.getDir()));
      }
      System.exit(1);
   }
}

ArgHandler also has a method isRequired() which can be overwritten to say that the command-line argument is required (default return is false so that the argument is optional.

Processing arguments by hand

When the command-line syntax for an application is simple, it is reasonable to do the command argument processing entirely in custom code.

In this example, we will present a series of simple case studies. In each case, the code will produce error messages if the arguments are unacceptable, and then call System.exit(1) to tell the shell that the command has failed. (We will assume in each case that the Java code is invoked using a wrapper whose name is “myapp”.)

A command with no arguments

In this case-study, the command requires no arguments. The code illustrates that args.length gives us the number of command line arguments.

public class Main {
    public static void main(String[] args) {
        if (args.length > 0) {
            System.err.println("usage: myapp");
            System.exit(1);
        }
        // Run the application
        System.out.println("It worked");
    }
}

A command with two arguments

In this case-study, the command requires at precisely two arguments.

public class Main {
    public static void main(String[] args) {
        if (args.length != 2) {
            System.err.println("usage: myapp <arg1> <arg2>");
            System.exit(1);
        }
        // Run the application
        System.out.println("It worked: " + args[0] + ", " + args[1]);
    }
}

Note that if we neglected to check args.length, the command would crash if the user ran it with too few command-line arguments.

A command with “flag” options and at least one argument

In this case-study, the command has a couple of (optional) flag options, and requires at least one argument after the options.

package tommy;
public class Main {
    public static void main(String[] args) {
        boolean feelMe = false;
        boolean seeMe = false;
        int index;
        loop: for (index = 0; index < args.length; index++) {
            String opt = args[index];
            switch (opt) {
            case "-c":
               seeMe = true;
               break;
            case "-f":
               feelMe = true;
               break;
            default:
               if (!opts.isEmpty() && opts.charAt(0) == '-') {
                   error("Unknown option: '" + opt + "');
               }
               break loop;
            }
        }
        if (index >= args.length) {
            error("Missing argument(s)");
        }      
        
        // Run the application
        // ...
    }

    private static void error(String message) {
        if (message != null) {
            System.err.println(message);
        }
        System.err.println("usage: myapp [-f] [-c] [ <arg> ...]");
        System.exit(1);
   }
}

As you can see, processing the arguments and options gets rather cumbersome if the command syntax is complicated. It is advisable to use a “command line parsing” library; see the other examples.