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<h2 title="Class LazyInitializer" class="title">Class LazyInitializer<T></h2>
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<dt><span class="paramLabel">Type Parameters:</span></dt>
<dd><code>T</code> - the type of the object managed by this initializer class</dd>
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<dt>All Implemented Interfaces:</dt>
<dd><code><a href="../../../../../org/apache/commons/lang3/concurrent/ConcurrentInitializer.html" title="interface in org.apache.commons.lang3.concurrent">ConcurrentInitializer</a><T></code></dd>
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<pre>public abstract class <span class="typeNameLabel">LazyInitializer<T></span>
extends <a href="https://docs.oracle.com/javase/7/docs/api/java/lang/Object.html?is-external=true" title="class or interface in java.lang" class="externalLink">Object</a>
implements <a href="../../../../../org/apache/commons/lang3/concurrent/ConcurrentInitializer.html" title="interface in org.apache.commons.lang3.concurrent">ConcurrentInitializer</a><T></pre>
<div class="block"><p>
This class provides a generic implementation of the lazy initialization
pattern.
</p>
<p>
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.
</p>
<p>
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 <code>initialize()</code> method, which
actually creates the wrapped data object.
</p>
<p>
As an usage example consider that we have a class <code>ComplexObject</code> whose
instantiation is a complex operation. In order to apply lazy initialization
to this class, a subclass of <code>LazyInitializer</code> has to be created:
</p>
<pre>
public class ComplexObjectInitializer extends LazyInitializer<ComplexObject> {
@Override
protected ComplexObject initialize() {
return new ComplexObject();
}
}
</pre>
<p>
Access to the data object is provided through the <code>get()</code> method. So,
code that wants to obtain the <code>ComplexObject</code> instance would simply
look like this:
</p>
<pre>
// Create an instance of the lazy initializer
ComplexObjectInitializer initializer = new ComplexObjectInitializer();
...
// When the object is actually needed:
ComplexObject cobj = initializer.get();
</pre>
<p>
If multiple threads call the <code>get()</code> 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
<code>get()</code> method are pretty fast because no synchronization is needed
(only an access to a <b>volatile</b> member field).
</p></div>
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<dt><span class="simpleTagLabel">Since:</span></dt>
<dd>3.0</dd>
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<th class="colConstructorName" scope="row"><code><span class="memberNameLink"><a href="../../../../../org/apache/commons/lang3/concurrent/LazyInitializer.html#LazyInitializer--">LazyInitializer</a></span>()</code></th>
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<td class="colFirst"><code><a href="../../../../../org/apache/commons/lang3/concurrent/LazyInitializer.html" title="type parameter in LazyInitializer">T</a></code></td>
<th class="colSecond" scope="row"><code><span class="memberNameLink"><a href="../../../../../org/apache/commons/lang3/concurrent/LazyInitializer.html#get--">get</a></span>()</code></th>
<td class="colLast">
<div class="block">Returns the object wrapped by this instance.</div>
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<tr id="i1" class="rowColor">
<td class="colFirst"><code>protected abstract <a href="../../../../../org/apache/commons/lang3/concurrent/LazyInitializer.html" title="type parameter in LazyInitializer">T</a></code></td>
<th class="colSecond" scope="row"><code><span class="memberNameLink"><a href="../../../../../org/apache/commons/lang3/concurrent/LazyInitializer.html#initialize--">initialize</a></span>()</code></th>
<td class="colLast">
<div class="block">Creates and initializes the object managed by this <code>
LazyInitializer</code>.</div>
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<h4>LazyInitializer</h4>
<pre>public LazyInitializer()</pre>
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<h4>get</h4>
<pre>public <a href="../../../../../org/apache/commons/lang3/concurrent/LazyInitializer.html" title="type parameter in LazyInitializer">T</a> get()
throws <a href="../../../../../org/apache/commons/lang3/concurrent/ConcurrentException.html" title="class in org.apache.commons.lang3.concurrent">ConcurrentException</a></pre>
<div class="block">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.</div>
<dl>
<dt><span class="overrideSpecifyLabel">Specified by:</span></dt>
<dd><code><a href="../../../../../org/apache/commons/lang3/concurrent/ConcurrentInitializer.html#get--">get</a></code> in interface <code><a href="../../../../../org/apache/commons/lang3/concurrent/ConcurrentInitializer.html" title="interface in org.apache.commons.lang3.concurrent">ConcurrentInitializer</a><<a href="../../../../../org/apache/commons/lang3/concurrent/LazyInitializer.html" title="type parameter in LazyInitializer">T</a>></code></dd>
<dt><span class="returnLabel">Returns:</span></dt>
<dd>the object initialized by this <code>LazyInitializer</code></dd>
<dt><span class="throwsLabel">Throws:</span></dt>
<dd><code><a href="../../../../../org/apache/commons/lang3/concurrent/ConcurrentException.html" title="class in org.apache.commons.lang3.concurrent">ConcurrentException</a></code> - if an error occurred during initialization of
the object</dd>
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<h4>initialize</h4>
<pre>protected abstract <a href="../../../../../org/apache/commons/lang3/concurrent/LazyInitializer.html" title="type parameter in LazyInitializer">T</a> initialize()
throws <a href="../../../../../org/apache/commons/lang3/concurrent/ConcurrentException.html" title="class in org.apache.commons.lang3.concurrent">ConcurrentException</a></pre>
<div class="block">Creates and initializes the object managed by this <code>
LazyInitializer</code>. This method is called by <a href="../../../../../org/apache/commons/lang3/concurrent/LazyInitializer.html#get--"><code>get()</code></a> 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 <code>get()</code>.</div>
<dl>
<dt><span class="returnLabel">Returns:</span></dt>
<dd>the managed data object</dd>
<dt><span class="throwsLabel">Throws:</span></dt>
<dd><code><a href="../../../../../org/apache/commons/lang3/concurrent/ConcurrentException.html" title="class in org.apache.commons.lang3.concurrent">ConcurrentException</a></code> - if an error occurs during object creation</dd>
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