Android WebSocket 实践

既生HTTP何生WebSockt

与服务端交互的协议中，HTTP只能被动接收客户端请求，然后响应。而WebSocket在客户端与服务端建立连接以后，服务端可以主动发消息给客户端。这就是WebSocket的特点。

应用

因为服务端可以主动发消息的特性，WebSocket在以下场景使用非常多：
1、推送
2、聊天室
3、开发工具
4…

大部分功能依赖于产品的业务，投入使用可能比较困难，而开发工具是开发人员可以肆意发挥的地方！

ReactNative中WebSocket的使用

ReactNative开发过程中，pc端可以控制android端重新加载、打印堆栈等等。这些的实现都依赖于WebSocket。

com.facebook.react.packagerconnection.ReconnectingWebSocket实现了WebSocket的连接，使用了okhttp。

final public class ReconnectingWebSocket extends WebSocketListener {
      public ReconnectingWebSocket(
      String url,
      MessageCallback messageCallback,
      ConnectionCallback connectionCallback) {
    super();
    mUrl = url;
    mMessageCallback = messageCallback;
    mConnectionCallback = connectionCallback;
    mHandler = new Handler(Looper.getMainLooper());
  }

  public void connect() {
    if (mClosed) {
      throw new IllegalStateException("Can't connect closed client");
    }

    OkHttpClient httpClient = new OkHttpClient.Builder()
      .connectTimeout(10, TimeUnit.SECONDS)
      .writeTimeout(10, TimeUnit.SECONDS)
      .readTimeout(0, TimeUnit.MINUTES) // Disable timeouts for read
      .build();

    Request request = new Request.Builder().url(mUrl).build();
    httpClient.newWebSocket(request, this);
  }
}

JSPackagerClient包装了ReconnectingWebSocket，在onMessage回调处分发服务端下发的命令：

final public class JSPackagerClient implements ReconnectingWebSocket.MessageCallback {
  private ReconnectingWebSocket mWebSocket;
  
  public JSPackagerClient(
      String clientId, PackagerConnectionSettings settings,
      Map<String, RequestHandler> requestHandlers,
      @Nullable ReconnectingWebSocket.ConnectionCallback connectionCallback) {
    super();

    Uri.Builder builder = new Uri.Builder();
    builder.scheme("ws")
      .encodedAuthority(settings.getDebugServerHost())
      .appendPath("message")
      .appendQueryParameter("device", AndroidInfoHelpers.getFriendlyDeviceName())
      .appendQueryParameter("app", settings.getPackageName())
      .appendQueryParameter("clientid", clientId);
    String url = builder.build().toString();

    mWebSocket = new ReconnectingWebSocket(url, this, connectionCallback);
    mRequestHandlers = requestHandlers;
  }
  
  @Override
  public void onMessage(String text) {
    try {
      JSONObject message = new JSONObject(text);

      int version = message.optInt("version");
      String method = message.optString("method");
      Object id = message.opt("id");
      Object params = message.opt("params");

      if (version != PROTOCOL_VERSION) {
        FLog.e(
          TAG,
          "Message with incompatible or missing version of protocol received: " + version);
        return;
      }

      if (method == null) {
        abortOnMessage(id, "No method provided");
        return;
      }

      RequestHandler handler = mRequestHandlers.get(method);
      if (handler == null) {
        abortOnMessage(id, "No request handler for method: " + method);
        return;
      }

      if (id == null) {
        handler.onNotification(params);
      } else {
        handler.onRequest(params, new ResponderImpl(id));
      }
    } catch (Exception e) {
      FLog.e(TAG, "Handling the message failed", e);
    }
  }

}

命令的具体执行在RequestHandler。注入RequestHandler的时机在DevServerHelper#openPackagerConnection

public void openPackagerConnection(
      final String clientId, final PackagerCommandListener commandListener) {
    if (mPackagerClient != null) {
      FLog.w(ReactConstants.TAG, "Packager connection already open, nooping.");
      return;
    }
    new AsyncTask<Void, Void, Void>() {
      @Override
      protected Void doInBackground(Void... backgroundParams) {
        Map<String, RequestHandler> handlers = new HashMap<>();
        handlers.put("reload", new NotificationOnlyHandler() {
          @Override
          public void onNotification(@Nullable Object params) {
            commandListener.onPackagerReloadCommand();
          }
        });
        handlers.put("devMenu", new NotificationOnlyHandler() {
          @Override
          public void onNotification(@Nullable Object params) {
            commandListener.onPackagerDevMenuCommand();
          }
        });
        handlers.put("captureHeap", new RequestOnlyHandler() {
          @Override
          public void onRequest(@Nullable Object params, Responder responder) {
            commandListener.onCaptureHeapCommand(responder);
          }
        });
        handlers.putAll(new FileIoHandler().handlers());

        ConnectionCallback onPackagerConnectedCallback =
          new ConnectionCallback() {
              @Override
              public void onConnected() {
                commandListener.onPackagerConnected();
              }

              @Override
              public void onDisconnected() {
                commandListener.onPackagerDisconnected();
              }
            };

        mPackagerClient = new JSPackagerClient(
            clientId,
            mSettings.getPackagerConnectionSettings(),
            handlers,
            onPackagerConnectedCallback);
        mPackagerClient.init();

        return null;
      }
    }.executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR);
  }

Android客户端实现一个WebSocket Server

大部分使用WebSocket的开发工具，服务端都是pc，Android端都是一个client。经过okhttp的封装，实现client非常简单。

github上找到了两个java websocket server的库，Java-WebSocket和AndroidAsync。尝试写了一下Demo，最终只跑通了AndroidAsync，github：https://github.com/PortgasAce/AndroidWebSocketDemo

Server的主要代码：

AsyncHttpClient.getDefaultInstance().websocket(get, "my-protocol", new WebSocketConnectCallback() {
    @Override
    public void onCompleted(Exception ex, WebSocket webSocket) {
        if (ex != null) {
            ex.printStackTrace();
            return;
        }
        webSocket.send("a string");
        // webSocket.send(new byte[10]);
        webSocket.setStringCallback(new StringCallback() {
            public void onStringAvailable(String s) {
                System.out.println("I got a string: " + s);
            }
        });
        webSocket.setDataCallback(new DataCallback() {
            public void onDataAvailable(DataEmitter emitter, ByteBufferList byteBufferList) {
                System.out.println("I got some bytes!");
                // note that this data has been read
                byteBufferList.recycle();
            }
        });
    }
});

上面是github的readme，注释了一行webSocket.send(new byte[10]);，否则websocket连接会断掉。

参考

知乎WebSocket原理：https://www.zhihu.com/question/20215561

阮一峰WebSocket教程：http://www.ruanyifeng.com/blog/2017/05/websocket.html

Java-WebSocket：https://github.com/TooTallNate/Java-WebSocket

AndroidAsync：https://github.com/koush/AndroidAsync