OSGi Services for Dynamic Applications (IV)

Or: use OSGi to write a dueling card game - part 4
In the previous postings I created the basics of a dueling card game using OSGi services. In this last posting of this series I'm going to add some special spell cards. Finally I'll use the capabilities of OSGi to dynamically add a bundle with new cards to the ongoing games without having to interrupt them.

Adding Spell Cards
Spells make the game really interesting as they can influence the game beyond the standard rules. They can do things like change the power of a monster, inflict damage on a player or another card or generally change the game state. Sometimes spells can behave like monsters or monsters can have additional spell-like behaviour too. My SpellDeck bundle adds a number of them, each of which has special abilities and a special implementation.

In my Activator, I'm registering my spell cards (5 of each):
public class Activator implements BundleActivator {
  public void start(BundleContext ctx) throws Exception {
    for (int i = 0; i < 5; i++) {
      registerCard(ctx, new ThunderBoltCard());
      registerCard(ctx, new ReflectCard());
      registerCard(ctx, new SkipBattlePhaseCard());
      registerCard(ctx, new DoubleAttackCard());
      registerCard(ctx, new MedicinCard());
      registerCard(ctx, new ExplosionCard());
      registerCard(ctx, new LightningBoltCard());
      registerCard(ctx, new DoubleStrengthCard());
      registerCard(ctx, new HalfStrengthCard());
    }
  }

  private void registerCard(BundleContext ctx, Card c) {
    ctx.registerService(Card.class.getName(), c,
        new Hashtable());
  }

Let's zoom in on a few of them: ThunderBoltCard, SkipBattlePhaseCard, ReflectCard and DoubleStrengthCard.

ThunderBoltCard
This spell card can be played during a player's Standby or Recover phase and deals 20 damage directly to the opponent. This functionality is implemented in the execute() of this Command Card.
public class ThunderBoltCard extends AbstractCommandCard implements Card, Command {
  public ThunderBoltCard() {
    super(CardType.SPELL, "Thunderbolt",
        "Thunderbolt deals 20 damage to your opponent.");
  }

  public boolean execute(Game game) {
    Player p = game.toGrave(this); // discard this card
    Player q = game.getOtherPlayer();
    q.setScore(q.getScore() - 20);
    printBoth(game, p + " plays Thunderbolt, " +
        q + " loses 20 life");
    
    return true;
  }
}

SkipBattlePhaseCard
This card can be played by a player who is being attacked during battle. It will end the current Battle phase aborting any pending attacks and move to the Recover phase. Here's the code:
public class SkipBattlePhaseCard extends AbstractCommandCard implements Card, Command {
  protected SkipBattlePhaseCard() {
    super(CardType.SPELL, "Battle Skipper",
      "With battle skipper, your opponent must drop " +
      "the current attack and skip the battle phase.");
  }

  public boolean execute(Game game) {
    Player p = game.getOwner(this);
    if (game.getPlayer() == p) {
      p.getUI().print("Can only play this card during " +
        "your opponent's battle phase when attacked.");
      return false;
    } else {
      game.setAttacking(new ArrayList<Fighter>());
      game.setPhase(Phase.RECOVER);
      game.toGrave(this);
      printBoth(game, p + " plays Battle Skipper, " +
        which causes the current battle phase to end.");
      return true;
    }
  }
}

ReflectCard
This spell can also be played by a player who is being attacked during battle. It will reflect the same amount of defense back to the attacker as was the original attack. Typically this will destroy the attacking monster.
public class ReflectCard extends AbstractCard implements Card, Fighter {
  private int defense;

  public ReflectCard() {
    super(CardType.SPELL, "Reflect",
      "Reflects an attack back to the attacker as " +
      "defense, will normally destroy the attacker.");
  }

  public void damage(Game game, int amount) {}

  public int getAttack() {
    return 0;
  }

  public int getDefense() {
    return defense;
  }

  public boolean responding(Game game) {
    defense = 0;
    // calculate the defense
    for (Fighter f : game.getAttacking()) {
      defense += game.getAttack(f);
    }
    return true;
  }
}

DoubleStrengthCard
This is an attached card, which means that it is attached to another Fighter card on the field. It doubles the attack of the fighter card. The attachment behaviour of this card is implemented in the AbstractAttachedCard base class. This card implements the Modifier interface, which is taken into account when a monster's strength is calculated.
public class DoubleStrengthCard extends AbstractAttachedCard implements Card, Modifier {
  protected DoubleStrengthCard() {
    // This card can be attached to a Monster card on
    // the table of type Fighter
    super("Double Strength",
      Area.TABLE, Fighter.class, CardType.MONSTER,
      "Target creature gets double the attack.");
  }

  public int addToAttack(Game game, Fighter fighter) {
    if (fighter instanceof Card) {
      Player owner = game.getOwner((Card) fighter);
      if (owner != null &&
          owner.getTable().contains(fighter)) {      
        return fighter.getAttack();
      }
    }
    return 0;
  }

  public int addToDefense(Game game, Fighter fighter) {
    return 0;
  }
}

Get the code of the other spells by checking out the SpellDeck Bundle from the usual location: http://coderthoughts.googlecode.com/svn/trunk/duel/.


Adding More Monsters while playing
Last but not least, I'm going to add a whole bunch of extra monsters to my game, and I'm going to do this while a game is being played. In a development scenario this is not really normal, but it will be needed once our application is successful and in production. Let's say our application is used by many users and we don't want to disturb our users by the mere action of adding a few new cards to the game. We just want those cards to appear in any ongoing games and in any new games from now on. This is where OSGi will really help us. It allows us to add and remove bundles without the need to restart the container. If those new bundles contain new Card service my game controller will be notified of them straight away. Similarly if a bundle that contains cards is being removed, we will be told about it too. If our application (bundles) interact with other bundles via OSGi Services, we will be able to patch our entire application without taking it down. To show this functionality in action I'm going to add the MoreMonsters bundle to the OSGi runtime after the game has started, but first of all I need to slightly modify the addCard() method in my Controller class to shuffle newly arrived cards into the decks of any ongoing games.

public void addCard(Card card) {
  synchronized (cards) {
    cards.add(card);


    // add the new card to any game deck in
    // progress in a random place
    for (Game g : games) {
      System.out.println(
        "Shuffling new card into existing game " + card);
      List<card> deck = g.getDeck();
      deck.add(rand.nextInt(deck.size()), card);
    }
  }
}


To get started, first check out the MoreMonsters bundle from the usual SVN location: http://coderthoughts.googlecode.com/svn/trunk/duel. This bundle only contains an Activator in which additional monsters (instances of the MonsterCard) are created and registered as Card services (courtesy to my 9 and 11 year old kids, who created these monsters).

I want the MoreMonsters bundle to be added while the game is in progress, so I need to make sure that it's not automatically started with the other bundles. For this, the launch confiration that I'm using in Eclipse needs to be tweaked, I'm setting the 'Auto-Start' of MoreMonsters to false:

You may also need to select the 'Clear the configuration area before launching' option in the 'Settings' tab of the launch configuration, because Equinox remembers your first settings and sometimes doesn't pick up changes like this without clearning the configuration area.

Hit 'Run' and the game will start as usual, with the original three monsters and our spell cards.

If you go into the Equinox console and type 'ss', you'll see that the MoreMonsters bundle is there, but it's just not started:


osgi> ss

Framework is launched.

id State Bundle
0 ACTIVE org.eclipse.osgi_3.3.1.R33x_v20070828
1 ACTIVE org.eclipse.osgi.services_3.1.200.v20070605
3 RESOLVED MoreMonsters_1.0.0
4 ACTIVE DuelInterfaces_1.0.0
7 ACTIVE javax.servlet_2.4.0.v200706111738
8 ACTIVE org.eclipse.equinox.ds_1.0.0.v20070226
9 ACTIVE SpellDeck_1.0.0
10 ACTIVE Duel_1.0.0
11 ACTIVE SimpleUI_1.0.0
12 ACTIVE MonsterDeck_1.0.0

So let's start the bundle!

osgi> start 3

And we'll see our new cards arriving:


  Shuffling new card into existing game Thundergirl
  Shuffling new card into existing game DB Machine
  Shuffling new card into existing game Devil Spider
  <...and so on...>

Soon you'll see your new cards arriving in your current game:


In this case the MoreMonsters Bundles was automatically installed by Eclipse for me, it was just not started. A more realistic real-world scenario would be where the OSGi container is running outside of Eclipse. In that case I would first have to install the bundle (Equinox has the install command for that) before I can actually start it.

Dynamically removing services
The same mechanism can be used to dynamically remove services from the system. I'll leave the implementation of that to the reader, but I hope you can see how powerful this mechanism can be. If you need to patch the an OSGi Service-based system, just install the patch bundle first, which will registered the patched services, then remove the old bundle, the old services will be unregistered which can make clients to switch to the patched services. Together with OSGi's strong versioning support, this is a great feature for mission critical systems that need to be up and running 24/7 (like game servers ;).

Equinox Specifics
In this posting I did use some Equinox-specific commands to control the OSGi container, other OSGi containers have other commands for achieving the same. All of the code in these postings should be 100% portable across the various OSGi containers.

Download the code
All the code for all these postings is available under the Apache License in the google code Subversion Repository.

OSGi Services for Dynamic Applications (III)

Or: use OSGi to write a dueling card game, part 3

Adding Monster cards to the game and a simple UI

In the previous posting I've created the hardest bit of the game. The actual game controller. In this posting I'm going to create a small bundle that contains a few Monster cards and a simple UI bundle so that we can actually start playing.

Creating the Monster Deck bundle
Every card in my Dueling game is an OSGi Service.
In the Duel Bundle I used OSGi Declarative Services to activate the bundle and consume the services, and you can use DS also to register services, but in this case I'm going to use the plain OSGi API's for this. Registering a service is so simple in OSGi that in this case I'm just going to go API.

My monster cards instances are realized by a class called MonsterCard, which implements the Card, Fighter and Command interfaces. It's nice and simple, also because it extends AbstractCommandCard, which means that Command.execute() for the Standby and Recover phase are implemented as placing the card on the table. Other basic Card requirements such as accessors to the name, ID and description and proper implementations of equals(), hashcode() and toString() are handled by AbstractCard, the superclass of AbstractCommandCard. My MonsterCard class basically adds the implementation of the Fighter interface into the mix. It also specifies when the card is enabled in conditions() - in the Standby phase and the Recover phase, and also in the Battle phase, but only when it's on the table.
public class MonsterCard extends AbstractCommandCard implements Fighter, Card, Command {
  private int attack;
  private int defense;

  public MonsterCard(String name, String desc,
      int att, int def) {
    super(CardType.MONSTER, name, desc);

    attack = att;
    defense = def;
  }
    
  public void damage(Game game, int amount) {
    // nothing to do on damage
    // the battle code will discard when hit
  }

  public int getAttack() {
    return attack;
  }

  public int getDefense() {
    return defense;
  }

  public Condition [] conditions() {
    return new Condition [] {
      new Condition(Phase.STANDBY),
      new Condition(Area.TABLE, Phase.BATTLE),
      new Condition(Phase.RECOVER)
    };
  }

  public boolean responding(Game game) {
    return true; // can respond to any attacker
  }
}

To create any straight monster without any special abilities, I can simply use this class. I'm going to create 3 different monster cards, Super Hero, Floppie and Diamond Buster:
  new MonsterCard("Super Hero",
    "This is the invincible super hero.", 60, 35));
  new MonsterCard("Floppie",
    "Floppie joppie.", 15, 25));
  new MonsterCard("Diamond Buster",
    "Diamond buster is as strong as a rock.", 40, 55));
To turn these POJO's into OSGi services, I need to register them as services with the BundleContext.registerService(), defined as:
  registerService(java.lang.String clazz,
    java.lang.Object service,
    java.util.Dictionary properties)
Simply pass in the name of the class as the first argument, the actual object and optionally a map of properties. The properties can be used when selecting services as a service consumer, but I'm not using them here.

So to get 5 of each card registered when the MonsterDeck bundle is started, a small OSGi Activator class does it all:
package monsterdeck;

import game.model.Card;
import java.util.Hashtable;
import org.osgi.framework.BundleActivator;
import org.osgi.framework.BundleContext;

public class Activator implements BundleActivator {
  public void start(BundleContext ctx) throws Exception {
    for (int i = 0; i < 5; i++) {
      registerCard(ctx, new MonsterCard("Super Hero",
          "This is the invincible super hero.", 60, 35));
      registerCard(ctx, new MonsterCard("Floppie",
          "Floppie joppie.", 15, 25));
      registerCard(ctx, new MonsterCard("Diamond Buster",
          "As strong as a rock.", 40, 55));
    }
  }

  private void registerCard(BundleContext ctx, Card c) {
    ctx.registerService(Card.class.getName(), c,
        new Hashtable());
  }

  public void stop(BundleContext context)
      throws Exception { }
}
The only thing left to do it create the bundle-manifest in the META-INF/MANIFEST.MF file:
  Bundle-Name: MonsterDeck Bundle
  Bundle-SymbolicName: MonsterDeck
  Bundle-Version: 1.0.0
  Bundle-Activator: monsterdeck.Activator
  Import-Package: game.model,
   org.osgi.framework
  Export-Package: monsterdeck
I have to import the org.osgi.framework package since in my Activator I'm using the BundleActivator and BundleContext classes from this package. I'm exporting the monsterdeck package from this bundle so that I can use the MonsterCard class later in another bundle.

Adding a simple UI
I've tried to create the simplest UI possible. Its written using SWT and simply implements UserInterface. I just wanted to keep this part as simple as possible, but (as always) GUI classes tend to become relatively big. My GUI looks like this:

At the top you see the cards your opponent has on the table.
Below that is a status area where messages are displayed.
Then the cards you have on the table are shown and at the bottom are the cards that you have in your hand.
On the right hand side there is a status panel where each player's life points are shown, the current game phase is displayed and buttons are present to draw a card, end the current phase and take the damage of a fight.
Monster cards are shown red and spell cards are green. To select a card, you simply click on it. Generally only cards and actions that make sense in the current context are enabled. Attacking monsters have their names shown in italics and card descriptions can be read by hovering with the mouse over a card.

The UI bundles comes with a little factory that is launched as soon as the bundles is started. Type your name in the UI Factory and it will create a User Interface for you which is then registered as a UserInterface Service with OSGi.

You can get this UI implementation by checking it out the SimpleUI bundle from the usual SVN location: http://coderthoughts.googlecode.com/svn/trunk/duel/.

Running the game
This is really easy if you're using Eclipse. What you should have is 4 OSGi Bundle Projects:

Select the Run -> Open Run Dialog... menu and double-click on the OSGi Framework item in the tree. This will create a new OSGi launch configuration for you. By default it will have all the OSGi bundles in the system selected, which is waaaay too much, so I normally deselect the root 'Target Platform' node and then click 'Add Required Bundles':

Hit 'Run' and you're off. The UI Controller will show up and after creating two UI's the game will start.
The game controller prints the names of the users as soon as they register their UI services and when two users are in, it will shuffle the cards and start the game. My console shows:
  Adding user interface for: Pinky
  Adding user interface for: Brain
  Shuffling deck...

I can Draw a card:

Play a monster (e.g. Super Hero) on the table:

and engage in battle: my opponent attacks me with Diamond Buster, I can choose to defend with Super Hero, but he will die, alternatively I can take the damage myself and lose life points.


Before I forget: closing all the windows will not actually exit the system. The OSGi container will keep running. The command to stop the OSGi container is implementation-specific, if you're using Equinox just type in exit in the console.

With only 3 different cards in the game it is a little bit boring to say the least. In the next posting I'll add some cool spell cards and a bunch more monsters too!

OSGi Services for Dynamic Applications (II)

Or: use OSGi to write a dueling card game, part 2: the Game Controller OSGi Services client
In this posting I'm going to write the Game Controller which is a client to the User Interface OSGi Services and also a client to the Card OSGi services.

The following diagram from the first posting outlines the game architecture:

The yellow triangles represent the OSGi services, the red rectangles are bundles that provide service implementations. The Game Controller is a client to the services and lives in an OSGi bundle too.

I am using OSGi Declarative Services to handle the client side of the OSGi services. I'm doing this because DS is easier than using the OSGi ServiceTracker API, and it's part of the OSGi spec.

I always find the easiest way to develop an OSGi bundle by using the Eclipse tooling that you get with the Eclipse Plugin Development Environment (download from here). With that you simply go: File -> New Project... -> Plug-in project. In the following window you simply select 'OSGi Platform' as the target platform. If you specify 'standard' it means that you won't be using any Equinox specific bits.

On the next page you can specify some details around the plugin ID, name and version. The defaults are generally fine but for this posting you won't need an activator as Declarative Services will take care of the stuff that is typically done in an activator for us, so you can deselect that option. Finally, you don't need to use any of the templates that come with Eclipse for this posting.
Hitting 'Finish' will get you an Eclipse project that is marked as a PDE project (which is an OSGi project) with a META-INF/MANIFEST.MF file which defines the attributes of the OSGi bundle.

My game bundle has two dependencies:

• It depends on the DuelInterfaces bundle from the previous posting. All of its classes are in the game.model package, so I will be importing that package, which gives me the freedom to rename or reversion the DuelInterfaces bundle if needed.
• Since I'm using Declarative Services it depends on a DS implementation. I'm using the one from Equinox / Eclipse, which doesn't come with Eclipse by default, you have to manually download and install it. To get it, go to the Equinox download page, select your Eclipse version (I took 3.3.1.1) and then look for the org.eclipse.equinox.ds_xxx.jar download. I downloaded this one. Then simply drop the downloaded jar file in the plugins directory of your Eclipse installation.
  Because I'm not using any specific classes from the DS implementation, I cannot use the 'Import Package' mechanism, so I've specified this bundle as a direct dependency in 'Required Plugins'.

I've entered the dependencies in my manifest, using the Eclipse manifest editor, which now looks like this.


What I want DS to do for me is tell the Game Controller when someone has registered any new Card Services in the game or when a new User Interface is registered. When a UI is added, the controller will look for another UI that is not playing a game, if there is one it will hook them up and start the game for both players. If there is no free other UI, the newly added UI will be held until another UI presents itself. This is implemented in the Controller class which contains an addUI() method like this:

public synchronized void addUI(UserInterface ui) {
  System.out.println("Adding UI for: " +
      ui.getPlayerName());
  
  if (freeUIs.size() > 0) {
    UserInterface otherUI = freeUIs.remove(0);
    
    Player p1 = new Player(otherUI, 250);
    Player p2 = new Player(ui, 250);

    // Init both UI's with players and static commands
    ui.initializeGame(p2, p1, new DrawCommand(),
        new NextPhaseCommand(), new SelfFighter(p2));
    otherUI.initializeGame(p1, p2, new DrawCommand(),
        new NextPhaseCommand(), new SelfFighter(p1));
    
    // Create a new game with all the cards available
    final Game g =
        new GameImpl(new ArrayList(cards), p1, p2);
    
    new Thread(new Runnable() {
      public void run() {
        g.play();
      }
    }).start(); // play, don't block the OSGi thread
    games.add(g);
  } else {
    freeUIs.add(ui);
  }
}

A similar, but even simpler method is called when new cards are registered, my addCard() looks like this:
public void addCard(Card card) {
  synchronized (cards) {
    cards.add(card);
  }
}

But what do I do to get my methods called when these services get registered. This is where DS comes in. I have added a DS configuration file that instantiates my Controller component and gets DS to call me back when this happens. This pattern is called Inversion of Control (IOC). I have added a DS configuration file in my Duel bundle project in OSGI-INF/controlcomponent.xml. I could really place this file anywhere in the bundle, but putting it in OSGI-INF seems to be the convention. It contains the following XML:
<component name="controller">
  <implementation class="game.Controller"/>

  <reference name="UI"
    interface="game.model.UserInterface"
    bind="addUI"
    unbind="removeUI"
    cardinality="0..n"
    policy="dynamic"/>
  <reference name="CARD"
    interface="game.model.Card"
    bind="addCard"
    unbind="removeCard"
    cardinality="1..n"
    policy="dynamic"/>
</component>

This is what tells DS to call the Controller class when a game.model.UserInterface and a game.model.Card service is registered. It defines a component called 'controller', specifies its class and service dependencies. I've set the policy to 'dynamic' so I get notified every time one of these services gets bound or unbound. Matter of fact, this component configuration file actually causes my game.Controller class to be instantiated in the first place so that's why I don't need an OSGi Activator here. DS does it for me.
There's one last handy little nugget in here. You can see that the cardinality of the service dependencies is specified. For user interfaces it's 0 or more, but for cards it's 1 or more. The latter is handy, because it makes sure that my game Controller doesn't get created before there are a few cards registered. There's no point offering a card game to play without any cards!
So as soon as there's a few cards, my game.Controller class gets created and players can make themselves known by registering a UI.

We will need to tell DS about our OSGI-INF/controlcomponent.xml file. For this I have to reference it in the META-INF/MANIFEST.MF file. Mine looks like this:
  Bundle-Name: Duel Bundle
  Bundle-SymbolicName: Duel
  Bundle-Version: 1.0.0
  Import-Package: game.model
  Require-Bundle: org.eclipse.equinox.ds
  Service-Component: OSGI-INF/controlcomponent.xml

I won't go into too much detail on the game.GameImpl class. This is an implementation of the game.model.Game interface. It's not very complicated but it's just a bit too long for a blog post; so just look at the code :)
You can get the Game Implementation class with the Duel bundle from Subversion here: http://coderthoughts.googlecode.com/svn/trunk/duel/.

We can't start the game just yet, we first need to add a Bundle with a few cards. That's what the next posting is about.

OSGi Services for Dynamic Applications (I)

Or: use OSGi to write a dueling card game

Surprise is one of the interesting things about dueling card games. You might know Magic the Gathering or Yi-Gi-Oh, which is quite popular with kids at the moment, many others exist too. They are interesting because there is a sheer endless number of cards so you never know what card your opponent will play. On the other hand, I never really liked the fact that whoever spends the most money on them always wins. I'd rather prefer a variant with a shared deck so that player's chances are even.

I've wanted to write a little framework for a game like this for a while now and OSGi is a great technology for doing this. Its bundle architecture brings some nice modularization and especially the OSGi Services Framework allows me to plug my services in and out at runtime without having to restart other bundles that make use of my services.
In my case, I will use this to add a new sets of cards to my game without interrupting my ongoing games, the new cards will even appear straight away in any ongoing games. I will do this in one of the next postings.

So the basics of my dueling game are similar to the games above and go like this. Each player starts with 250 points, whoever hits 0 first loses. Each player plays in turn:

1. Draw phase - generally just draw a card
   
2. Standby phase - here you can cast spells and a monster. Some spells are cast on your own area or attached to one of your monsters, other spells can affect the opponent.
   
3. Battle phase - now you can attack with your monsters that are on the table. The opponent can defend with any monsters that are on her side of the table. If the defence of the defender is higher than the attack of the attacker, the defender wins, and vice versa. If the defender does not have any monsters on the table, she has to take the damage, which will cause the attack amount to be deducted from her life points.
   
4. Recover phase - play spells and possibly a monster if you haven't already in this turn.

The very first battle phase is skipped (as it would be unfair to attack an opponent who didn't get to play any cards yet).

Monsters and spells often have special abilities that could be tied to a particular phase. They could change the behaviour of other cards or generally disturb the order of the game.
Another feature cards in these games generally have, is some stunning artwork which, unfortunately, I won't be able to provide.

I have come up with a little architecture that looks like this:

I'm using OSGi-style diagramming here. The yellow triangles are services, the red boxes are bundles, which provide some implementations of the services, the blue box is a component which itself lives inside a bundle.

The Game Controller is a consumer of OSGi services. It consumes any number of Card services, it also consumes 2 UI services, one UI per player.
So every card is a service of its own and every user interface is a separate service instance. As long as we adhere to the Card interface we can add any cards we can think of. Also using the service approach for the UI allows us to develop the UI separately from the game and gives us the possibility to implement any type of GUI, a simple one, a fancy one, an AI player (without a screen) or maybe even a remote UI without changing the actual game.

OSGi doesn't use the static wiring for services that you can see in some other Service Oriented Architectures, in OSGi a service consumer specifies the Java interface that it is interested in, the OSGi runtime will then dynamically bind this to an available service that implements this interface. This is completely dynamic, and services could even come and go during the life time of the system. BTW I'm not going to go too much into the OSGi basics. Neil Bartlett's blog has some excellent OSGi Basics tutorials.

Let's start by diving into the the core model of the game. (BTW I would really have loved to use EMF here again, but I wanted to keep this posting focused so I'm just using the EMF Class diagram editor for the picture, and am in this case handcoding the classes) - click on the image for a clearer picture:

Entities in italics are interfaces, others are real classes. Let's look at the interfaces that define the various types of cards first. I put a few example cards in the middle. The top interface is called Action, everything you can do and every card is an action. Cards generally implement both a subclass of Action as well as the Card interface. However there are certain actions that are not a card, such as the DrawCommand and NextPhaseCommand actions.
Commands are things that can executed, usually in a particular Phase of the game. Example command cards are the ThunderBoltCard spell which directly damages the opponent, or the SkipBattlePhaseCard which ends the current Battle phase.
Fighters are things that can take part in a battle, generally monsters, but also the player himself. Modifiers can change the attack and/or defense of a Fighter.

Then there are the following classes and interfaces:

• Game: controls the game and provides information to the various entities about the game state. Provides accessors to change the game state too.
• Player: data class that holds player information, such as the cards she holds in her hand, table and graveyard. Also holds a reference to the UI service that this player uses and the name of the player.
• UserInterface: this API defines the interaction with the UI. The purpose of the UI is simply to display the game state and get any input decisions (actions) from the player. The UI can also display a line of text to the user.
  

Since my dueling game revolves around these classes, I have put them in a separate OSGi bundle. Other bundles can participate in the game by importing these, without having to depend on any implementation bundles. This OSGi bundle is available in Subversion here. If you're using Eclipse, the easiest way to get it into your Eclipse IDE is with a Subversion client. I always use subclipse, which is extremely easy to install; instructions here.
Once you have subclipse, open the SVN Repository Exploring perspective and add the following SVN repository URL: http://coderthoughts.googlecode.com/svn/trunk/duel/. There's no need to log in, if you expand the root node, you will get the following repositories:

Just check out the DuelInterfaces project and you're done. You will then have the OSGi bundle in your local Eclipse workspace, it will be the basis for the other pieces of the game that I'll be writing next.


For more details on the DuelInterfaces classes, read their Javadoc here.

In the following few postings I will be implementing my dueling game using more OSGi bundles and services. I'll be using a mixture of OSGi Declarative Services (DS) and direct OSGi API calls for this. Besides DS and the plain API approach (which are both part of the OSGi spec) there exist a number of other technologies to work with OSGi services: Spring-OSGi, iPojo and Guice-OSGi are some of them. The nice thing is that it doesn't matter what toolkit you use as the actual service registered with OSGi conforms to the OSGi Service Spec, so you can mix-n-match all these technologies if you like.

Besides the game logic bundle, I will be writing a bundle with monster cards and another with spell cards and I'll also provide a basic UI implementation written using SWT, which looks like this:

The good thing is, it allows me to play the game and I can write a nicer GUI later if I want, without changing the rest of the code. I might at some point even want to do a GUI that runs remotely, which allows me to use my OSGi-based application as a game server that can be used by people playing the game in various locations from a browser or something like that.

As usual I'll be using the Eclipse IDE and Equinox OSGi implementation, but nothing in the code is tied to these so please use your IDE and OSGi implementation of choice.