If Vaadin is failing with “Invalid JSON from server: 1|X”, read on. The problem is that Vaadin expects an UIDL JSON message over the websocket pipe and can’t parse 1|X. Where did 1|X came from?

Atmosphere sends data in the format of length|message, which means that 1|X is an Atmosphere message “X” of length 1. It’s a HeartbeatInterceptor message which should be part of the internal Atmosphere heartbeat mechanism and should be filtered out by Atmosphere and thus ignored by Vaadin.

This kind of message is sent every 60 seconds by default by Atmosphere in all Vaadin apps using Push over WebSockets. You can see this message in your browser’s Network tab: it’s marked with the HTTP Status 101. 101 is http code for upgrade to websocket, that’s OK to see.

The heartbeat character is reconfigurable on server-side via ApplicationConfig.HEARTBEAT_PADDING_CHAR - the javadoc says it’s ' ' (space) but in reality it’s ‘X’ as seen in HeartbeatInterceptor.paddingBytes. You use the org.atmosphere.interceptor.HeartbeatInterceptor.paddingChar configuration option to reconfigure the character: place a breakpoint into HeartbeatInterceptor.configure() to learn the effective value.

On the client side, you can check for the effective value of the padding character as follows: the first message sent over Vaadin Push looks like this: 41|52a917c0-9c19-455a-a0d4-b155f46a3ed3|0|X|. Pay attention to the end of the message: the 0|X| configures the heartbeat padding to the X character. Vaadin then configures Atmosphere to the ‘X’ padding character, causing Atmosphere to automatically ignore 1|X messages. It could be that your server sends a different padding character, e.g. A but then proceeds to send 1|X heartbeat messages, which then get passed to Vaadin, which then fails since Vaadin expects the UIDL JSON document.

Double-wrapped messages

If you’re receiving 3|1|X then that’s wrong - it should be 1|X. Looks like there’s “another” Atmosphere which wraps the message “1|X” (the message ‘X’ of size 1) to “3|1|X” (the message ‘1|X’ of size 3).

TODO how to solve this.

History

The X message used to be hard-coded in Vaadin client; changing it server-side would cause the client to fail. The algorithm was changed since then to detect the padding character from the first message, see this flow commit for more details.

Workarounds

Workaround is to disable Atmosphere heartbeat mechanism, relying on Vaadin’s heartbeat mechanism only. Increase org.atmosphere.interceptor.HeartbeatInterceptor.heartbeatFrequencyInSeconds to some absurdly high value, see ApplicationConfig.HEARTBEAT_INTERVAL_IN_SECONDS in Atmosphere for more details.

Configuring Atmosphere

Create a servlet which extends VaadinSevlet and add init parameters such as org.atmosphere.interceptor.HeartbeatInterceptor.paddingChar to it. See ApplicationConfig for more configuration options. Atmosphere is configured in Vaadin’s PushRequestHandler class - you can place a breakpoint there and check whether your changes took any effect.