Question

Say the current thread holds a lock, then the same thread calls

synchronize(lock)

again, will deadlock happen?

I vaguely remember that the lock is re-entrant for the same thread, what does that mean?

Answer 1

From documentation:

Thread cannot acquire a lock owned by another thread. But a
thread can acquire a lock that it already owns. Allowing a thread to
acquire the same lock more than once enables reentrant
synchronization. This describes a situation where synchronized code,
directly or indirectly, invokes a method that also contains
synchronized code, and both sets of code use the same lock.