The Java® Virtual Machine Specification, Java SE 7 Edition, Third Edition

3.14. Synchronization

Synchronization in the Java Virtual Machine is implemented by monitor entry and exit, either explicitly (by use of the monitorenter and monitorexit instructions) or implicitly (by the method invocation and return instructions).

For code written in the Java programming language, perhaps the most common form of synchronization is the synchronized method. A synchronized method is not normally implemented using monitorenter and monitorexit. Rather, it is simply distinguished in the run-time constant pool by the ACC_SYNCHRONIZED flag, which is checked by the method invocation instructions (§2.11.10).

The monitorenter and monitorexit instructions enable the compilation of synchronized statements. For example:

void onlyMe(Foo f) ...