Java Reference
In-Depth Information
Listing 16.5. Eager Initialization.
Using eager initialization, shown in
Listing 16.5
,
eliminates the synchronization cost incurred
on each call to
getInstance
in
SafeLazyInitialization
. This technique can be
combined with the JVM's lazy class loading to create a lazy initialization technique that does
not require synchronization on the common code path. The
lazy initialization holder class
idiom [EJ Item 48] in
Listing 16.6
uses a class whose only purpose is to initialize the
Re-
source
. The JVM defers initializing the
ResourceHolder
class until it is actually used
[JLS 12.4.1], and because the
Resource
is initialized with a static initializer, no addition-
al synchronization is needed. The first call to
getResource
by any thread causes
Re-
sourceHolder
to be loaded and initialized, at which time the initialization of the
Re-
source
happens through the static initializer.
Listing 16.6. Lazy Initialization Holder Class Idiom.
16.2.4. Double-checked Locking
No book on concurrency would be complete without a discussion of the infamous double-
checked locking (DCL) antipattern, shown in
Listing 16.7
.
In very early JVMs, synchron-
ization, even uncontended synchronization, had a significant performance cost. As a result,
