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The current servlet API (<=2.5) supports only a synchronous call style, so that any waiting that a servlet needs to do must be with blocking. Unfortunately this means that the thread allocated to the request must be held during that wait with all its resources including kernal thread, stack memory and often pooled buffers, character converters, EE authentication context, etc. It is wasteful of system resources to be hold these resources while waiting.
Jetty's solution
Jetty provides support for Asynchronous Servlets by supporting Suspendable Requests, either via the Jetty Continuations or the proposed standard servlet 3.0 API.
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