Threads - The Java Programming Language - page 516

method need not be synchronized; conversely the extended class may
enhance the behavior of the method in such a way that interference be-
comes possible and so it must be synchronized.
14.3.2. Static synchronized Methods
Static methods can also be declared synchronized . Associated with every
class is a Class objectsee " The Class Class " on page 399 . A static syn-
chronized method acquires the lock of the Class object for its class.
Two threads cannot execute static synchronized methods of the same
class at the same time, just as two threads cannot execute synchronized
methods on the same object at the same time. If static data is shared
between threads then access to it must be protected by static synchron-
ized methods.
Acquiring the Class object lock in a static synchronized method has no
effect on any objects of that class. You can still invoke synchronized
methods on an object while another thread holds the Class object lock in
a static synchronized method. Only other static synchronized methods
are blocked.
14.3.3. synchronized Statements
The synchronized statement enables you to execute synchronized code
that acquires the lock of any object, not just the current object, or for
durations less than the entire invocation of a method. The synchronized
statement has two parts: an object whose lock is to be acquired and a
statement to execute when the lock is obtained. The general form of the
synchronized statement is
synchronized ( expr ) {
statements
}