Tomcat has a couple of connectors to choose from. I’ll leave aside the APR connector, and focus on the BIO and NIO.
The BIO connector (blocking I/O) is blocking – it uses a thread pool where each thread receives a request, handles it, responds, and is returned to the pool. During blocking operations (e.g. reading from database or calling an external API) the thread is blocked.
The NIO connector (non-blocking I/O) is a bit more complicated. It uses the java NIO library and multiplexes between requests. It has two thread pools – one holds the the poller threads, which handle all incoming requests and push these requests to be handled by worker threads, held in another pool. Both pool sizes are configurable.
When to prefer NIO vs BIO depends on the use case. If you mostly have regular request-response usage, then it doesn’t matter, and even BIO might be a better choice (as seen in my previous benchmarks). If you have long-living connections, then NIO is the better choice, because it can server more concurrent users without the need to dedicate a blocked thread to each. The poller threads handle the sending of data back to the client, while the worker threads handle new requests. In other words, neither poller, nor worker threads are blocked and reserved by a single user.
With the introduction of async processing servlet it became easier to have the latter scenario from the previous paragraph. And maybe that was one of the reasons to switch the default connector from BIO to NIO in Tomcat 8. It’s an important thing to have in mind, especially because they didn’t exactly change the “default value”.
The default value is always “HTTP/1.1″, but in Tomcat 7 that “uses an auto-switching mechanism to select either a blocking Java based connector or an APR/native based connector”, while in Tomcat 8 “uses an auto-switching mechanism to select either a non blocking Java NIO based connector or an APR/native based connector”. And to make things even harder, they introduced a NIO2 connector. And to be honest, I don’t know which one of the two NIO connectors is used by default.
So even if you are experienced with tomcat configuration, have in mind this change of defaults. (And generally I’d recommend reading the documentation for all the properties and play with them on your servers)