Akka边学边写（4）-- MiniRPG

前面几篇文章用Akka写了HelloWorld和EchoServer，为了更进一步学习Akka，本文将会实现一个很小的RPG游戏服务器：MiniRPG。

游戏逻辑

因为是迷你RPG，所以逻辑很简单。服务器可以处理四种操作：创建玩家、给玩家加经验、升级、查询玩家信息。下面是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。完整的消息格式如下图所示：

前八个字节可以认为是消息头，其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，里面没有定义任何方法，如下图所示：

MsgRegistry

MiniRPG使用GSON来编码和解码JSON字符串，为了把JSON解析为相应的消息对象，需要一个消息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，下面是它的完整代码：

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());
    }
    
}

服务器Actor系统设计

MiniRPG服务器的Actor系统如下图所示：

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编码之后发送到客户端。下面详细介绍整个Actor系统是如何实现的。

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只关心四种消息，下面是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绑定到某个端口，准备接收客户端连接。如果收到Bound消息，则端口绑定成功，服务器正常启动。如果是CommandFailed消息，则服务器启动失败：

    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());
    }

如果是Connected消息，说明有客户端连接已经建立，TcpServer创建一个子Actor（也就是Codec）来处理客户端连接：

    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;
    }

}

如果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。如果收到的是 GameMessage消息，MsgCodec将其编码为byte[]然后发送给客户端。如果收到的是 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()先确定是否可以把 消息头解码出来，如果不能，就继续等待更多的字节到达。如果消息头完整到达，decodeMsg()就可以知道 消息体的长度，然后等到消息体完整到达。之后根据消息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，代码如下所示：

    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。因为只是个demo，所以MsgHandler内部使用HashMap来模拟数据库。下面是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游戏服务器主类，main()方法建立好整个Actor系统，然后通知tcpServer绑定到端口12345，让服务器运转起来：

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());
    }
    
}

客户端

为了测试MiniServer，我写了个简单的客户端程序，具体实现就不在这里介绍了。