Little Tutorials

The creation of Java console applications was severely crippled up to the current version of Java (6) and things are now better but not perfect.

The example presented in this tutorial implements a shell type application using various features of the Console and Scanner APIs. The best approach to learn how to work with these new features is to dig right in. Read and execute the following example and everything will become clear.

package com.littletutorials.console;

import java.io.*;
import java.util.*;

public class Shell
{
    private static final String NO_CONSOLE = "Error: Console unavailable";
    private static final String GREETINGS = "Welcome to the System. Please login.%n";
    private static final String DENIED_ATTEMPT = "Wrong user name or password [%1$d]%n";
    private static final String ACCESS_DENIED = "Access denied%n";
    private static final String ACCESS_GRANTED = "Access granted%n";
    private static final String UNKNOWN_COMMAND = "Unknown command [%1$s]%n";
    private static final String COMMAND_ERROR = "Command error [%1$s]: [%2$s]%n";

    private static final String TIME_FORMAT = "%1$tH:%1$tM:%1$tS";
    private static final String PROMPT = TIME_FORMAT + " $ ";
    private static final String USER_PROMPT = TIME_FORMAT + " User: ";
    private static final String PASS_PROMPT = TIME_FORMAT + " Password [%2$s]: ";

    private static final String USER = "john";
    private static final String PASS = "secret";

    public static void main(String[] args)
    {
        Console console = System.console();
        if (console != null)
        {
            if (login(console))
            {
                execCommandLoop(console);
            }
        }
        else
        {
            throw new RuntimeException(NO_CONSOLE);
        }
    }

    private static boolean login(Console console)
    {
        console.printf(GREETINGS);

        boolean accessGranted = false;
        int attempts = 0;
        while (!accessGranted && attempts < 3)
        {
            String name = console.readLine(USER_PROMPT, new Date());
            char[] passdata = console.readPassword(PASS_PROMPT, new Date(), name);
            if (USER.equals(name) && PASS.equals(new String(passdata)))
            {
                attempts = 0;
                accessGranted = true;
                break;
            }

            console.printf(DENIED_ATTEMPT, ++attempts);
        }

        if (! accessGranted)
        {
            console.printf(ACCESS_DENIED);
            return false;
        }

        console.printf(ACCESS_GRANTED);
        return true;
    }

    private static void execCommandLoop(final Console console)
    {
        while (true)
        {
            String commandLine = console.readLine(PROMPT, new Date());
            Scanner scanner = new Scanner(commandLine);

            if (scanner.hasNext())
            {
                final String commandName = scanner.next().toUpperCase();

                try
                {
                    final Command cmd = Enum.valueOf(Command.class, commandName);
                    String param = scanner.hasNext() ? scanner.next() : null;
                    cmd.exec(console, new String[]{param}, new Command.Listener()
                    {
                        @Override
                        public void exception(Exception e)
                        {
                            console.printf(COMMAND_ERROR, cmd, e.getMessage());
                        }
                    });
                }
                catch (IllegalArgumentException e)
                {
                    console.printf(UNKNOWN_COMMAND, commandName);
                }
            }

            scanner.close();
        }
    }
}

The class defined above needs the definition of the com.littletutorials.console.Command enumeration where the shell commands are defined:

package com.littletutorials.console;

import java.io.Console;

public enum Command
{
    BYE(new Action()
    {
        @Override
        public void exec(Console c, String[] params)
        {
            c.printf("Bye%n");
            System.exit(0);
        }
    }),
    DETAILS(new Action()
    {
        @Override
        public void exec(Console c, String[] params) throws Exception
        {
            int detailsLevel = 1;
            try
            {
                detailsLevel = Integer.parseInt(params[0]);
            }
            catch (NumberFormatException e)
            {
                // ignore
            }

            for (int i = 1; i <= detailsLevel; i++)
            {
                c.printf("Detail number %1$X%n", i);
            }
        }
    });

    private interface Action
    {
        public void exec(Console c, String[] params) throws Exception;
    }

    public interface Listener
    {
        public void exception(Exception e);
    }

    private Action action;

    private Command(Action a)
    {
        this.action = a;
    }

    public void exec(final Console c, final String[] params, final Listener l)
    {
        try
        {
            action.exec(c, params);
        }
        catch (Exception e)
        {
            l.exception(e);
        }
    }
}

The application has to be executed from a console/command prompt with a command like this:

java -cp . com.littletutorials.console.Shell

There is at most one Console instance associated with a instance of the JVM. If we get a console instance from the System.console() call then a session of our shell could look like this:

Welcome to the System. Please login.
01:13:02 User: john
01:13:08 Password [john]: 
Access granted
01:13:12 $ details 3
Detail number 1
Detail number 2
Detail number 3
01:13:26 $ bye
Bye

These APIs have more functionality than used in this example. For example the Scanner can use regular expressions to locate tokens in the input stream. Reasonably rich console applications can now be implemented in Java and this covers a small but annoying gap in functionality.

Filed under: Java API by Daniel Pietraru