java websocket中的ping-pong 機制
參考原始碼：
https://github.com/TooTallNate/Java-WebSocket/blob/master/src/main/java/org/java_websocket/client/WebSocketClient.java
https://github.com/TooTallNate/Java-WebSocket/blob/master/src/main/java/org/java_websocket/AbstractWebSocket.java
https://github.com/TooTallNate/Java-WebSocket/blob/master/src/main/java/org/java_websocket/WebSocket.java
https://github.com/TooTallNate/Java-WebSocket/blob/master/src/main/java/org/java_websocket/WebSocketImpl.java
https://github.com/TooTallNate/Java-WebSocket/blob/master/src/main/java/org/java_websocket/drafts/Draft_6455.java
文章參考：
https://blog.csdn.net/u010770993/article/details/70312279 https://blog.csdn.net/ttdevs/article/details/62887058 機制圖解：注：websocket基於tcp協議，它在第一次連線時發起http請求，之後建立握手
在websocket中設定setConnectionLostTimeout引數，解釋為：Setter for the interval checking for lost connections，意思是間隔檢查連線是否丟失
整體是呼叫順序為：onWebsocketOpen -》 startConnectionLostTimer -》 restartConnectionLostTimer-》 scheduleAtFixedRate -》 executeConnectionLostDetection
關鍵程式碼this.connectionLostTimeout = TimeUnit.SECONDS.toNanos(connectionLostTimeout);
if (this.connectionLostTimeout <= 0) {
log.trace("Connection lost timer stopped");
cancelConnectionLostTimer();
return;
}long minimumPongTime = (long) (System.nanoTime() - ( connectionLostTimeout * 1.5 ));
for( WebSocket conn : connections ) {
executeConnectionLostDetection(conn, minimumPongTime);
}
WebSocketImpl webSocketImpl = (WebSocketImpl) webSocket;
if( webSocketImpl.getLastPong() < minimumPongTime ) {
log.trace("Closing connection due to no pong received: {}", webSocketImpl);
webSocketImpl.closeConnection( CloseFrame.ABNORMAL_CLOSE, "The connection was closed because the other endpoint did not respond with a pong in time. For more information check: https://github.com/TooTallNate/Java-WebSocket/wiki/Lost-connection-detection" );
} else {
if( webSocketImpl.isOpen() ) {
webSocketImpl.sendPing();
} else {
log.trace("Trying to ping a non open connection: {}", webSocketImpl);
}
}
connectionLostTimeout在設定後會轉為納秒時間， minimumPongTime為當前納秒時間減去connectionLostTimeout的1.5倍，當最後一次Pong的時間小於minimumPongTime時產生close，即在間隔時間內未收到Pong響應關閉連線。如果正常則繼續傳送ping，即呼叫sendPing。
在服務端收到ping的時候，立即下發pong，兩者的容忍時間為connectionLostTimeout是1.5倍，即設十秒的話就是容忍十五秒。當網路發生異常時，兩者情況，服務端沒有收到ping亦或者客戶端沒有收到pong，觸發close。