Execution - The Java Language Specification - page 328

For each class or interface C , there is a unique initialization lock LC . The mapping from C
to LC is left to the discretion of the Java Virtual Machine implementation. The procedure
for initializing C is then as follows:
1. Synchronize on the initialization lock, LC , for C . This involves waiting until the
current thread can acquire LC .
2. If the Class object for C indicates that initialization is in progress for C by some
other thread, then release LC and block the current thread until informed that the
in-progress initialization has completed, at which time repeat this step.
3. If the Class object for C indicates that initialization is in progress for C by the cur-
rent thread, then this must be a recursive request for initialization. Release LC and
complete normally.
4. If the Class object for C indicates that C has already been initialized, then no further
action is required. Release LC and complete normally.
5. If the Class object for C is in an erroneous state, then initialization is not possible.
Release LC and throw a NoClassDefFoundError .
6. Otherwise, record the fact that initialization of the Class object for C is in progress
by the current thread, and release LC .
Then, initialize the final class variables and fields of interfaces whose values are
compile-time constant expressions (§ 8.3.2.1 , § 9.3.1 , § 13.4.9 , § 15.28 ) .
7. Next, if C is a class rather than an interface, and its superclass SC has not yet been
initialized, then recursively perform this entire procedure for SC . If necessary, veri-
fy and prepare SC first. If the initialization of SC completes abruptly because of a
thrown exception, then acquire LC , label the Class object for C as erroneous, notify
all waiting threads, release LC , and complete abruptly, throwing the same excep-
tion that resulted from initializing SC .
8. Next, determine whether assertions are enabled (§ 14.10 ) for C by querying its de-
fining class loader.
9. Next, execute either the class variable initializers and static initializers of the class,
or the field initializers of the interface, in textual order, as though they were a
single block.
10. If the execution of the initializers completes normally, then acquire LC , label the
Class object for C as fully initialized, notify all waiting threads, release LC , and
complete this procedure normally.
11. Otherwise, the initializers must have completed abruptly by throwing some excep-
tion E . If the class of E is not Error or one of its subclasses, then create a new in-