One thread per client. Doable?

This will scale well for up to hundreds of connections, not to thousands. One issue is that a Java thread takes quite a bit of stack as well (e.g. 256K), and the OS will have problems scheduling all your threads.

Look at Java NIO or framworks that will help you get started doing complex stuff more easily (e.g. Apache Mina)

It is possible this will scale to thousands of clients. But how many thousands is the next question.

A common alternative is to use Selectors and non-blocking I/O found in the java.nio package.

Eventually you get into the question of whether it's useful to set up your server in a clustered configuration, balancing the load over multiple physical machines.

To have a good perfomance when handling many sockets you usually use a select approach that is how Unix API handles single-threaded multi-socket applications that need many resources.

This can be done through the java.nio package that has a Selector class which basically is able to go through all the opened sockets and notify you when new data is available.

You register all the opened streams inside a single Selector and then you can handle all of them from just one thread.

You can get additional infos with a tutorial here