When does servlet release its thread

Question: I assume that the servlet's thread is not released until the entire response is sent to the client (say a web browser). Is this a correct assumption?

Ans: No it is wrong. Servlet container will just write the content to the socket and return. It is not guaranteed that return from write() method will ensure that the response has reached the client.

Question: Is the thread released once the response is delivered to Nginx, or is it held until the response is sent to its final client (say a browser)?

Ans: When Nginx is behind , then the client for Servlet container is Nginx. It is not aware of actual remote client. So, the thread will be released once the response is written to Nginx.

The server container not being able to send a response to the client will trigger an exception that will be handled by the container. You can enclose the writing to the outputstream or writer by a try catch finally (with close()) but you don't need to, the container will manage, including the return of the thread to the pool.RegardsS

A servlet does not see the network. According to the specifications It is handled 2 objects: a Request and a Response to be filled in (in the case of HTTP, this means a HTTPRequest and a HTTPResponse). It shall process the request data within the request object, and write to the buffer in the response object. Once that content is commited by the servlet, the container may do some postprocessing (using filters) and will transmit it back to the client.

The servlet thread returns naturally to the pool once the call to the request handling method finishes (that may happen after the payload is sent back to the client, if the method has to do further work.

Note that because the servlet doesn't see the network and is only concerned about a single request, the state of the http connection (keep-alive or close) is independent of the servlet lifetime; several servlets may handle the different requests pipelined in a single connection. See this question for a related issue.