Can a Java thread aquire a lock while already holding it?

Question

If a thread holds a lock , what happens when the thread needs to enter another critical section controlled by the same lock?

Answer 1

Intrinsic locks (synchronized) in Java are reentrant, thus the JVM will recognize that the current thread already holds this lock and it will proceed.

There are also explicit locks, that are reentrant.

If a lock is not reentrant, you could for instance not use recursive methods.

Answer 2

nothing: the system is able to determine which thread holds the lock to avoid a thread blocking itself.