Class LazyInitializer<T>
- java.lang.Object
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- org.apache.commons.lang3.concurrent.LazyInitializer<T>
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- Type Parameters:
T
- the type of the object managed by this initializer class
- All Implemented Interfaces:
ConcurrentInitializer<T>
public abstract class LazyInitializer<T> extends java.lang.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 ofLazyInitializer
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 theComplexObject
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 theget()
method are pretty fast because no synchronization is needed (only an access to a volatile member field).- Since:
- 3.0
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Constructor Summary
Constructors Constructor Description LazyInitializer()
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Method Summary
All Methods Instance Methods Abstract Methods Concrete Methods Modifier and Type Method Description T
get()
Returns the object wrapped by this instance.protected abstract T
initialize()
Creates and initializes the object managed by thisLazyInitializer
.
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Method Detail
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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 interfaceConcurrentInitializer<T>
- Returns:
- the object initialized by this
LazyInitializer
- Throws:
ConcurrentException
- if an error occurred during initialization of the object
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initialize
protected abstract T initialize() throws ConcurrentException
Creates and initializes the object managed by thisLazyInitializer
. This method is called byget()
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 byget()
.- Returns:
- the managed data object
- Throws:
ConcurrentException
- if an error occurs during object creation
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