Remember the types of intrinsic lock

In Java, an intrinsic lock is implied by each use of the synchronized keyword. In this case, the locking is performed by Java behind the scenes. (This is distinct from the programmer using or defining an explicit lock object themselves.)

Each use of the synchronized keyword is associated with one of the two types of intrinsic lock:

If a method is declared as synchronized, then it will acquire either the instance lock or the static lock when it is invoked, according to whether it is an instance method or a static method.

The two types of lock have similar behaviour, but are completely independent of each other.

Acquiring the instance lock only blocks other threads from invoking a synchronized instance method; it does not block other threads from invoking an un-synchronized method, nor does it block them from invoking a static synchronized method.

Similarly, acquiring the static lock only blocks other threads from invoking a static synchronized method; it does not block other threads from invoking an un-synchronized method, nor does it block them from invoking a synchronized instance method.

Outside of a method header, synchronized(this) acquires the instance lock.

The static lock can be acquired outside of a method header in two ways:

See Also :
Synchronize access to mutable fields