any real benefit of using servlet 3.1 async io?

Question

I am wondering that if servlet containers like Tomcat, Jetty etc already use nio to read and write data back, is there really a need of using setWritelistner and setReadListner on servlet input and output streams? Is there any additional performance gain?

Answer 1

The benefit is not directly¹ about "performance gain". The purpose of those methods is to avoid a request thread (in async mode) from blocking when it reads input (POST) data or writes the document.

There is an example in the Java EE7 tutorial: "17.13.1 Reading a Large HTTP POST Request Using Non-Blocking I/O" (link updated).

This is orthogonal to Tomcat's use of nio under the covers.

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^{1 - There is an indirect performance benefit. When it is likely for threads to block on network I/O, an alternative strategy for increasing throughput is to increase the number of worker threads. But that increases the memory footprint (among other things) resulting in more "overheads".}