T - the type of the object managed by this initializer classpublic abstract class LazyInitializer<T> extends Object implements ConcurrentInitializer<T>
This class provides a generic implementation of the lazy initialization pattern.
Sometimes an application has to deal with an object only under certain circumstances, e.g. when the user selects a specific menu item or if a special event is received. If the creation of the object is costly or the consumption of memory or other system resources is significant, it may make sense to defer the creation of this object until it is really needed. This is a use case for the lazy initialization pattern.
This abstract base class provides an implementation of the double-check idiom
for an instance field as discussed in Joshua Bloch's "Effective Java", 2nd
edition, item 71. The class already implements all necessary synchronization.
A concrete subclass has to implement the initialize() method, which
actually creates the wrapped data object.
As an usage example consider that we have a class ComplexObject whose
instantiation is a complex operation. In order to apply lazy initialization
to this class, a subclass of LazyInitializer has to be created:
public class ComplexObjectInitializer extends LazyInitializer<ComplexObject> {
@Override
protected ComplexObject initialize() {
return new ComplexObject();
}
}
Access to the data object is provided through the get() method. So,
code that wants to obtain the ComplexObject instance would simply
look like this:
// Create an instance of the lazy initializer ComplexObjectInitializer initializer = new ComplexObjectInitializer(); ... // When the object is actually needed: ComplexObject cobj = initializer.get();
If multiple threads call the get() method when the object has not yet
been created, they are blocked until initialization completes. The algorithm
guarantees that only a single instance of the wrapped object class is
created, which is passed to all callers. Once initialized, calls to the
get() method are pretty fast because no synchronization is needed
(only an access to a volatile member field).
| Constructor and Description |
|---|
LazyInitializer() |
| Modifier and Type | Method and Description |
|---|---|
T |
get()
Returns the object wrapped by this instance.
|
protected abstract T |
initialize()
Creates and initializes the object managed by this
LazyInitializer. |
public LazyInitializer()
public T get() throws ConcurrentException
get in interface ConcurrentInitializer<T>LazyInitializerConcurrentException - if an error occurred during initialization of
the objectprotected abstract T initialize() throws ConcurrentException
LazyInitializer. This method is called by get() when the object
is accessed for the first time. An implementation can focus on the
creation of the object. No synchronization is needed, as this is already
handled by get().ConcurrentException - if an error occurs during object creationCopyright © 2001–2016 The Apache Software Foundation. All rights reserved.