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).