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：

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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。