It’s common to start a thread within a ctor, as seen in many articles and projects.
Danger – the half-cooked object under-construction should not be exposed and accessible in the child thread. Often in practice it’s not disastrous but accessing a half-cooked object is a code smell.
I’d say it’s fine if the code is simple and you know for sure the thread.start() is at end of ctor and the child thread doesn’t do anything fancy. Make sure other developers don’t mess things up.
http://docs.oracle.com/javase/tutorial/essential/concurrency/syncmeth.html explains that constructor is single-threaded by default and is designed to run in a single-threaded “context”. Don’t break that assumption.