How Tomcat 8 Handles websocket upgrade request

Question

I want to understand handling of websocket upgrade request in tomcat ? Is it done via filters or servlets ? How the ws upgrade request is handled by TOmcat-8.

Thanks,

Answer 1

WebSockets are a completely different protocol. They are not handled by filters or servlets but instead by EndPoints. To create them you use to extend the endpoint class but, like everything else in Java EE, you can now use annotations. For more info check out the latest doc Java API for WebSockets

To create them on the client side the request starts in HTTP, perhaps in JavaScript, but on Tomcat the endpoint is listening, not your normal servlets.

Answer 2

Tomcat uses a filter to look for the HTTP upgrade request that starts a WebSocket connection and passes it the the appropriate handler if it sees one.

The Filter is configured using a ServletContainerInitializer. The location of the Filter will depend on what other Filters are configured by the web application and where the web application places them in the FilterChain. It is possible, for example, that a web application would wish to place a security filter before the Web Socket filter and Tomcat allows this.