问题:

After a thread started, if the reference of the thread is set to null, what will happen to the thread? Does it stop? Is it eligible for GC?

Like this:

t.start();
t = null;

回答1:

Live thread will continue running even its reference is set to null.
Just like any other object, when there are no references to it, it is eligible to GC. The tricky point is that a running thread has a reference in a ThreadGroup even if your program does not retain one, thus a running thread is never GCed.

回答2:

what will happen to the thread?

Nothing.

Does it stop?

No.

Is it eligible for GC?

No.

回答3:

No, setting the reference to null will not effect the thread, other than it has one less reference pointing at it. And yes, any object with active references pointing at it will not be garbage collected. If you want the thread to be eligible for GC (and more importantly stop doing stuff) then interrupt what it is doing:

someThread.interrupt();

from the Oracle docs:

An interrupt is an indication to a thread that it should stop what it is doing and do something else. It's up to the programmer to decide exactly how a thread responds to an interrupt, but it is very common for the thread to terminate