问题:

new Timer(...).schedule(task)

Is task guaranteed to be run by a single thread at any given time?

回答1:

From the Javadoc

Corresponding to each Timer object is a single background thread that is used to execute all of the timer's tasks, sequentially. Timer tasks should complete quickly. If a timer task takes excessive time to complete, it "hogs" the timer's task execution thread. This can, in turn, delay the execution of subsequent tasks, which may "bunch up" and execute in rapid succession when (and if) the offending task finally completes.

So, yes, you get a new Thread (separate from the caller's thread). Every task in that timer shares the same thread.

回答2:

There is a single thread per Timer, so the answer to your question is yes

回答3:

Indeed. They all run on a same background thread corresponded to the Timer object in sequence. BUT two different Timer instances will run (I believe) on different threads, so you have to save reference to a timer object to schedule more tasks sequentialy.