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

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

Type Parameters:

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

All Implemented Interfaces:

ConcurrentInitializer<T>

public abstract class AtomicInitializer<T>
extends Object
implements ConcurrentInitializer<T>

A specialized implementation of the ConcurrentInitializer interface based on an AtomicReference variable.

This class maintains a member field of type AtomicReference. It implements the following algorithm to create and initialize an object in its get() method:

• First it is checked whether the AtomicReference variable contains already a value. If this is the case, the value is directly returned.
• Otherwise the initialize() method is called. This method must be defined in concrete subclasses to actually create the managed object.
• After the object was created by initialize() it is checked whether the AtomicReference variable is still undefined. This has to be done because in the meantime another thread may have initialized the object. If the reference is still empty, the newly created object is stored in it and returned by this method.
• Otherwise the value stored in the AtomicReference is returned.

Because atomic variables are used this class does not need any synchronization. So there is no danger of deadlock, and access to the managed object is efficient. However, if multiple threads access the AtomicInitializer object before it has been initialized almost at the same time, it can happen that initialize() is called multiple times. The algorithm outlined above guarantees that get() always returns the same object though.

Compared with the LazyInitializer class, this class can be more efficient because it does not need synchronization. The drawback is that the initialize() method can be called multiple times which may be problematic if the creation of the managed object is expensive. As a rule of thumb this initializer implementation is preferable if there are not too many threads involved and the probability that multiple threads access an uninitialized object is small. If there is high parallelism, LazyInitializer is more appropriate.

Since:

3.0

Version:

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

Constructor Summary

AtomicInitializer()

Method Summary

T	get() Returns the object managed by this initializer.
protected abstract T	initialize() Creates and initializes the object managed by this AtomicInitializer.

Methods inherited from class java.lang.Object

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

Constructor Detail

AtomicInitializer

public AtomicInitializer()

Method Detail

get

public T get()
      throws ConcurrentException

Returns the object managed by this initializer. The object is created if it is not available yet and stored internally. This method always returns the same object.

Specified by:

get in interface ConcurrentInitializer<T>

Returns:

the object created by this AtomicInitializer

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 AtomicInitializer. This method is called by get() when the managed object is not available yet. An implementation can focus on the creation of the object. No synchronization is needed, as this is already handled by get(). As stated by the class comment, it is possible that this method is called multiple times.

Returns:

the managed data object

Throws:

ConcurrentException - if an error occurs during object creation