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Why "IllegalStateException: the servlet does not support async operations for this request" is thrown ?

This exception happens when CometD is run in Servlet 3 compliant containers (where the Jetty Continuations make use of the asynchronous features of Servlet 3) and web.xml is misconfigured.

The correct web.xml must have the async-supported element set to true, for example:

<web-app xmlns=""



The async-supported element should be specified for the CometD servlet and for all filters that may be executed before the CometD servlet such as the CrossOriginFilter.

Refer to the Servlet 3 specification for further details.