前面几篇文章用Akka写了HelloWorld和EchoServer，为了更进一步学习Akka，本文将会实现一个非常小的RPG游戏server：MiniRPG。
游戏逻辑
由于是迷你RPG，所以逻辑非常easy。server能够处理四种操作：创建玩家、给玩家加经验、升级、查询玩家信息。以下是Player类的代码（Getters和Setters省略）：
public class Player {
private int id;
private String name;
private int exp;
private int level;
// Getters & Setters ...
public void addExp(int val) {
exp += val;
}
public void levelUp() {
if (exp > 100) {
exp -= 100;
level++;
}
}
}
消息协议
MiniRPG底层使用TCP协议，消息使用JSON格式。完整的消息格式例如以下图所看到的：
前八个字节能够觉得是消息头。当中前四个字节是消息ID。后四个字节是JSON字符串长度。其余字节是消息体，也就是UTF8格式编码的JSON字符串。
消息接口
MiniRPG设计了三个接口来表示游戏消息，这三个接口都是Marker接口。里面未定义不论什么方法。例如以下图所看到的：
MsgRegistry
MiniRPG使用GSON来编码和解码JSON字符串，为了把JSON解析为对应的消息对象。须要一个消息ID和class之间的映射关系。MsgRegistry类便是要建立起这样一个映射关系，以下是它的完整代码：
public class MsgRegistry {
private static final Map<Integer, Class<?>> msgById = new HashMap<>();
private static final Map<Class<?>, Integer> idByMsg = new HashMap<>();
static {
register(1, CreatePlayerRequest.class);
register(2, CreatePlayerResponse.class);
register(3, AddExpRequest.class);
register(4, AddExpResponse.class);
register(5, LevelUpRequest.class);
register(6, LevelUpResponse.class);
register(7, GetPlayerInfoRequest.class);
register(8, GetPlayerInfoResponse.class);
}
private static void register(int msgId, Class<?> msgClass) {
msgById.put(msgId, msgClass);
idByMsg.put(msgClass, msgId);
}
public static Class<?> getMsgClass(int msgId) {
return msgById.get(msgId);
}
public static int getMsgId(Class<?> msgClass) {
return idByMsg.get(msgClass);
}
public static int getMsgId(Object msg) {
return getMsgId(msg.getClass());
}
}
serverActor系统设计
MiniRPGserver的Actor系统例如以下图所看到的：
TcpServer负责监听TCP连接。连接建立之后。交给Codec处理。Codec将收到的字节编码成消息对象。然后交给MsgHandler处理。对于每条请求消息。MsgHandler都会产生一条响应消息。响应消息被Codec编码之后发送到client。
以下具体介绍整个Actor系统是怎样实现的。
TcpServer
TcpServer是一个UntypedActor，实例化TcpServer时，我们把MsgHandler引用传给它：
public class TcpServer extends UntypedActor {
private final ActorRef msgHandler;
public TcpServer(ActorRef msgHandler) {
this.msgHandler = msgHandler;
}
}TcpServer仅仅关心四种消息。以下是onReceive()方法实现：
@Override
public void onReceive(Object msg) throws Exception {
if (msg instanceof Integer) {
final int port = (Integer) msg;
startServer(port);
} else if (msg instanceof Bound) {
getSender().tell(msg, getSelf());
} else if (msg instanceof CommandFailed) {
getContext().stop(getSelf());
} else if (msg instanceof Connected) {
final Connected conn = (Connected) msg;
getSender().tell(conn, getSelf());
registerCodec(getSender());
}
}
Integer消息通知TcpServer绑定到某个port，准备接收client连接。假设收到Bound消息，则port绑定成功。server正常启动。假设是CommandFailed消息。则server启动失败：
private void startServer(int port) {
final InetSocketAddress endpoint = new InetSocketAddress("localhost", port);
final Object bindCmd = TcpMessage.bind(getSelf(), endpoint, 100);
Tcp.get(getContext().system()).getManager()
.tell(bindCmd, getSelf());
}
假设是Connected消息。说明有client连接已经建立，TcpServer创建一个子Actor（也就是Codec）来处理client连接：
private void registerCodec(ActorRef connection) {
final Props codecProps = Props.create(MsgCodec.class, connection, msgHandler);
final ActorRef codec = getContext().actorOf(codecProps);
connection.tell(TcpMessage.register(codec), getSelf());
}
MsgCodec
MsgCodec主要负责消息的编码和解码。为此，MsgCodec内部使用了一个ByteString来缓存接收到的字节：
public class MsgCodec extends UntypedActor {
private static final Gson GSON = new Gson();
private final ActorRef connection;
private final ActorRef msgHandler;
private ByteString buf = ByteString.empty();
public MsgCodec(ActorRef connection, ActorRef msgHandler) {
this.connection = connection;
this.msgHandler = msgHandler;
}
}
假设MsgCodec收到的是Received消息，说明有数据到达。MsgCodec尝试解码出一个消息对象。假设收到的是GameMessage消息。MsgCodec将其编码为byte[]然后发送给client。假设收到的是ConnectionClosed，说明连接已经断开了：
@Override
public void onReceive(Object msg) throws Exception {
if (msg instanceof Received) {
final ByteString data = ((Received) msg).data();
buf = buf.concat(data);
decodeMsg();
} else if (msg instanceof ConnectionClosed) {
getContext().stop(getSelf());
} else if (msg instanceof GameMessage) {
final ByteString data = encodeMsg(msg);
connection.tell(TcpMessage.write(data), getSelf());
}
}每当有数据到达时。decodeMsg()方法都会被调用。decodeMsg()先确定能否够把消息头解码出来，假设不能，就继续等待很多其它的字节到达。假设消息头完整到达，decodeMsg()就能够知道消息体的长度，然后等到消息体完整到达。之后依据消息ID和JSON字符串解码消息对象。然后通知msgHandler：
private void decodeMsg() {
while (buf.length() > 8) {
final ByteIterator it = buf.iterator();
final int msgId = it.getInt(ByteOrder.BIG_ENDIAN);
final int jsonLength = it.getInt(ByteOrder.BIG_ENDIAN);
if (buf.length() >= 8 + jsonLength) {
final Object msg = decodeMsg(msgId, buf.slice(8, 8 + jsonLength));
buf = buf.drop(8 + jsonLength);
msgHandler.tell(msg, getSelf());
}
}
}
private Object decodeMsg(int msgId, ByteString jsonData) {
final Class<?> msgClass = MsgRegistry.getMsgClass(msgId);
final Reader reader = new InputStreamReader(
jsonData.iterator().asInputStream(),
StandardCharsets.UTF_8);
return GSON.fromJson(reader, msgClass);
}
消息的编码就简单多了，代码例如以下所看到的：
private ByteString encodeMsg(Object msg) {
final int msgId = MsgRegistry.getMsgId(msg);
final byte[] jsonBytes = GSON.toJson(msg)
.getBytes(StandardCharsets.UTF_8);
final ByteStringBuilder bsb = new ByteStringBuilder();
bsb.putInt(msgId, ByteOrder.BIG_ENDIAN);
bsb.putInt(jsonBytes.length, ByteOrder.BIG_ENDIAN);
bsb.putBytes(jsonBytes);
return bsb.result();
}
MsgHandler
游戏逻辑由MsgHandler来处理。
由于仅仅是个demo，所以MsgHandler内部使用HashMap来模拟数据库。以下是MsgHandler的完整代码：
public class MsgHandler extends UntypedActor {
private final List<Player> players = new ArrayList<>();
@Override
public void onReceive(Object msg) throws Exception {
if (msg instanceof CreatePlayerRequest) {
int newPlayerId = createPlayer((CreatePlayerRequest) msg);
getSender().tell(new CreatePlayerResponse(newPlayerId), getSelf());
} else if (msg instanceof AddExpRequest) {
int newExp = addExpToPlayer((AddExpRequest) msg);
getSender().tell(new AddExpResponse(newExp), getSelf());
} else if (msg instanceof LevelUpRequest) {
int newLevel = levelUpPlayer((LevelUpRequest) msg);
getSender().tell(new LevelUpResponse(newLevel), getSelf());
} else if (msg instanceof GetPlayerInfoRequest) {
PlayerInfo playerInfo = getPlayerInfo((GetPlayerInfoRequest) msg);
getSender().tell(new GetPlayerInfoResponse(playerInfo), getSelf());
}
}
private int createPlayer(CreatePlayerRequest req) {
int playerId = players.size() + 1;
Player newPlayer = new Player();
newPlayer.setId(playerId);
newPlayer.setLevel(1);
newPlayer.setName(req.getPlayerName());
players.add(newPlayer);
return playerId;
}
private int addExpToPlayer(AddExpRequest req) {
Player player = players.get(req.getPlayerId());
player.addExp(req.getExp());
return player.getExp();
}
private int levelUpPlayer(LevelUpRequest req) {
Player player = players.get(req.getPlayerId());
player.levelUp();
return player.getLevel();
}
private PlayerInfo getPlayerInfo(GetPlayerInfoRequest req) {
Player player = players.get(req.getPlayerId());
return new PlayerInfo(player.getId(), player.getName(),
player.getExp(), player.getLevel());
}
}
ServerApp
ServerApp是MiniRPG游戏server主类，main()方法建立好整个Actor系统，然后通知tcpServer绑定到port12345。让server运转起来：
public class ServerApp {
public static void main(String[] args) {
ActorSystem mySystem = ActorSystem.create("rpgServer");
ActorRef msgHandler = mySystem.actorOf(Props.create(MsgHandler.class));
ActorRef tcpServer = mySystem.actorOf(Props.create(TcpServer.class, msgHandler));
tcpServer.tell(12345, ActorRef.noSender());
}
}
client
为了測试MiniServer。我写了个简单的客户端程序。详细实现就不在这里介绍了。