What is the difference between Tomcat’s BIO Connector and NIO Connector?

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NIO and Comet are completely unrelated: you can mix-and-match them.

Using the NIO (or APR for that matter) connector allows you to handle more requests with fewer threads due to the threading model. See http://tomcat.apache.org/tomcat-7.0-doc/config/http.html#Connector_Comparison for a comparison between the Connectors.

Comet (and Websocket) have a completely different dispatch model which requires a different application architecture and achieves higher throughput in a different way.

The scenario you pose in your question is the typical blocking one-thread-per-request model. In step 4, the Java BIO connector (which is the default up through Tomcat 7) will continue to wait for additional requests on the existing connector — for keepalive HTTP requests. If the client does not set Connection:close on the previous request and does not close the connection, the thread will hang until the keepalive timeout is reached. If you use the NIO connector, the thread will go back into the thread pool immediately after the response is sent and you won’t “waste” a thread on keepalive requests that may never arrive.

Comet/Websocket works entirely differently by delivering a message to a specially-written servlet (and optional filters) and the threads are only used when there are messages to send or data to be written.

UPDATE 2016-08-19

Tomcat 8.5 and 9.0 have completely dropped the BIO connector. This is because many of the new APIs and technologies (e.g. Websocket) require non-blocking semantics, and building a non-blocking service on top of a blocking API is very very difficult. The code required to get the job done was making the rest of the Tomcat code very ugly, etc. and so the decision was made to drop the BIO connector completely. So for Tomcat 8.5 and onward, only NIO, NIO2, and the APR-based connectors are available.

Note that, also with Tomcat 8.5 and 9.0, support for Comet has been dropped. Uses of Comet should all be replaced with Websocket which is a more standard protocol.

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