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Make Room for JavaSpaces, Part III
Coordinate your Jini Apps with JavaSpaces
by Susan Hupfer
First Published in JavaWorld, March 2000

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A Jini Game Service

Consider a distributed multiplayer game, offered as a Jini service, that allows only a set number of players at a time. If the game is already at full capacity, a person wanting to play has to wait until an active player leaves. In its simplest form, given a pool of waiting players, the service admits an arbitrary player to the game; this is the approach I take in the example. But of course, in this scheme some players may wait forever while relative newcomers are allowed to play. If you want to be fair, you could make the waiting players queue up and have your service mediate admission to the game on a first-come, first-served basis. For now let's implement the simpler arbitrary-player approach, using a small amount of JavaSpaces code.

Here is the overview of my game service, depicted in Figure 1: The Jini game service supplies two basic methods, joinGame and leaveGame. When a player wishes to play, he or she looks up a game service provider in the standard Jini way, using a lookup server, and is delivered a proxy object to a game service. (I'm not going to cover the basics of Jini services, lookup, and discovery in this article, so if those concepts and terminology are unfamiliar to you, you may want to make a detour to Resources.) With the proxy in hand, the player calls joinGame to join a specific game. When joinGame succeeds in getting admission to the game (via JavaSpaces, as you'll see shortly), it returns an interface to a remote game object, which can be used to play the game. At this point, the player calls the play method of the game object (the game's only method in my simple example) to begin playing. When the play method returns, the player is finished playing and calls the proxy's leaveGame method. In my example, the player sleeps for a bit and then loops, calling joinGame to continue playing the game.

Figure 1. An overview of the game service example

From the command line, you can start a game service, specifying a name for the game and a maximum number of players. Then you can start as many players for that game as you want: if there are fewer players than the maximum, everyone gets to play at once, but if there are more players than the maximum, your game service will need to coordinate access to the game. You may want to download the full example code (Resources) to play with and examine. Let's take a closer look at how the pieces of the system are implemented and how to use JavaSpaces to coordinate them.

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