org.apache.commons.lang3.concurrent
Class LazyInitializer<T>

java.lang.Object
  org.apache.commons.lang3.concurrent.LazyInitializer<T>

Type Parameters:

T - the type of the object managed by this initializer class

All Implemented Interfaces:

ConcurrentInitializer<T>

public 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).

Since:

3.0

Version:

$Id: LazyInitializer.java 1088899 2011-04-05 05:31:27Z bayard $

Constructor Summary

LazyInitializer()

Method Summary

T	get() Returns the object wrapped by this instance.
protected abstract T	initialize() Creates and initializes the object managed by this LazyInitializer.

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Detail

LazyInitializer

public LazyInitializer()

Method Detail

get

public T get()
      throws ConcurrentException

Returns the object wrapped by this instance. On first access the object is created. After that it is cached and can be accessed pretty fast.

Specified by:

get in interface ConcurrentInitializer<T>

Returns:

the object initialized by this LazyInitializer

Throws:

ConcurrentException - if an error occurred during initialization of the object

initialize

protected abstract T initialize()
                         throws ConcurrentException

Creates and initializes the object managed by this 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().

Returns:

the managed data object

Throws:

ConcurrentException - if an error occurs during object creation