[Android]Okhttp心跳策略研究

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這一張很是經典的心跳策略圖示

如今通常的心跳策略，都是從客戶端發送一個ping信號給服務器，告訴服務器是長鏈接存活。 服務器會返回一個pong信號給客戶端，讓其更新心跳線程時間。若是超時沒有接收到信號，那麼客戶端考慮重連機制。

這裏說一下okhttp有提供了WebSocket的封裝，咱們的應用也是使用了WebSocket，那就直接看一下WebSocket對pingpong的封裝

public Builder() {
      ……
      //這裏默認的ping的時間間隔爲0，由於okhttp也能夠有如http短鏈接
      pingInterval = 0;
    }

    Builder(OkHttpClient okHttpClient) {
       ……
      //builder函數提供封裝
      this.pingInterval = okHttpClient.pingInterval;
    }
    //設置間隔
public Builder pingInterval(long interval, TimeUnit unit) {
      pingInterval = checkDuration("interval", interval, unit);
      return this;
    }
複製代碼

在RealWebSocket中啓動循環發送ping信號

public void initReaderAndWriter(String name, Streams streams) throws IOException {
    synchronized (this) {
      this.streams = streams;
      this.writer = new WebSocketWriter(streams.client, streams.sink, random);
      this.executor = new ScheduledThreadPoolExecutor(1, Util.threadFactory(name, false));
      if (pingIntervalMillis != 0) {
        //循環定時任務
        executor.scheduleAtFixedRate(
            new PingRunnable(), pingIntervalMillis, pingIntervalMillis, MILLISECONDS);
      }
      if (!messageAndCloseQueue.isEmpty()) {
        runWriter(); // Send messages that were enqueued before we were connected.
      }
    }

    reader = new WebSocketReader(streams.client, streams.source, this);
  }


private final class PingRunnable implements Runnable {
    PingRunnable() {
    }

    @Override public void run() {
      //寫入ping信號
      writePingFrame();
    }
  }

void writePingFrame() {
    WebSocketWriter writer;
    int failedPing;
    synchronized (this) {
      if (failed) return;
      writer = this.writer;
      //是否等待pong信號
      failedPing = awaitingPong ? sentPingCount : -1;
      //等待ping計數
      sentPingCount++;
      //等待pong
      awaitingPong = true;
    }
    
    //ping失敗，長鏈接失效
    if (failedPing != -1) {
      failWebSocket(new SocketTimeoutException("sent ping but didn't receive pong within "
          + pingIntervalMillis + "ms (after " + (failedPing - 1) + " successful ping/pongs)"),
          null);
      return;
    }

    try {
      //寫入空支付到websocket頭部
      writer.writePing(ByteString.EMPTY);
    } catch (IOException e) {
      failWebSocket(e, null);
    }
  }
複製代碼

在RealWebSocket的call中執行loopReader監聽讀取接收到的信息

/** Receive frames until there are no more. Invoked only by the reader thread. */
  public void loopReader() throws IOException {
    //監聽信息
    while (receivedCloseCode == -1) {
      // This method call results in one or more onRead* methods being called on this thread.
      reader.processNextFrame();
    }
  }

  void processNextFrame() throws IOException {
   //讀取頭部
    readHeader();
    if (isControlFrame) {
      //讀取頭部信息體
      readControlFrame();
    } else {
      readMessageFrame();
    }
  }

複製代碼

讀取到是頂部信息

private void readControlFrame() throws IOException {
    if (frameLength > 0) {
      source.readFully(controlFrameBuffer, frameLength);

      if (!isClient) {
        controlFrameBuffer.readAndWriteUnsafe(maskCursor);
        maskCursor.seek(0);
        toggleMask(maskCursor, maskKey);
        maskCursor.close();
      }
    }

    switch (opcode) {
      //讀取ping信號
      case OPCODE_CONTROL_PING:
        frameCallback.onReadPing(controlFrameBuffer.readByteString());
        break;
      //讀取pong信號
      case OPCODE_CONTROL_PONG:
        frameCallback.onReadPong(controlFrameBuffer.readByteString());
        break;
     //讀取到關閉鏈接信號
      case OPCODE_CONTROL_CLOSE:
        int code = CLOSE_NO_STATUS_CODE;
        String reason = "";
        long bufferSize = controlFrameBuffer.size();
        if (bufferSize == 1) {
          throw new ProtocolException("Malformed close payload length of 1.");
        } else if (bufferSize != 0) {
          code = controlFrameBuffer.readShort();
          reason = controlFrameBuffer.readUtf8();
          String codeExceptionMessage = WebSocketProtocol.closeCodeExceptionMessage(code);
          if (codeExceptionMessage != null) throw new ProtocolException(codeExceptionMessage);
        }
        frameCallback.onReadClose(code, reason);
        closed = true;
        break;
      default:
        throw new ProtocolException("Unknown control opcode: " + toHexString(opcode));
    }
  }
複製代碼

讀取到pong信號，等待pong置爲false

@Override public synchronized void onReadPong(ByteString buffer) {
    // This API doesn't expose pings. receivedPongCount++; awaitingPong = false; } 複製代碼

這就是使用OkHttp的WebSocket keepAlive的流程，而基本的okhttp的socket鏈接也是經過相似發送這種pingpong信號來維持，之須要設置維護的時間。 而後keepAlive的經驗之前的經驗值是59秒，微信的大神的方案是經過記錄socket鏈接和斷開時間，適配出最適當的發送長連接時間，有興趣能夠本身實驗寫一個算法。 微信的智能心跳方案

這邊作IM的應用的，說一下這邊方案，僅供參考。 手機進入後臺後十秒後主動關閉長鏈接，經過推送來維護消息，這裏有個問題就是國內的Umeng推送可達率你們懂的，小米和華爲還好點，若是是其餘山寨機，到8.0後後臺很難保活了。若是是國外FCM送達率是很是高的，若是是最推國外平臺，直接依靠推送也很是可靠。