Java同步块

Question

Is it valid to update a reference in an synchronized block of that reference? eg:

Synchronized(list)
{
  list = new ArrayList();
}

Answer 1

When you write

synchronized (list)

The lock is an object , not the variable/field list .

If you do

synchronized (list) {
    list = new ArrayList<>();
}

you are creating a new object, and reassigning a variable/field - but you have not changed the lock - the lock is still the same object it was before.

Answer 2

Yes, it is completely valid but what would you achieve by doing this? Besides, threads gain lock on object instance not on variable which holds object reference so even if you reassign list with some other instance, current thread has already taken lock on the previous object.

To clarify more, let's say list was assigned value val1 . Then a thread1 takes lock on val1 (which is pointed by variable list ), now other threads can't take lock on val1 (because they are waiting for lock on instance val1 not on reference list ) even though thread1 has re-instantiated list .

Better would be to clear the list inside synchronized block instead of re-initializing it.