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<div class="subTitle">org.apache.commons.lang3.concurrent</div>
<h2 title="Class BackgroundInitializer" class="title">Class BackgroundInitializer<T></h2>
</div>
<div class="contentContainer">
<ul class="inheritance">
<li><a href="http://docs.oracle.com/javase/1.5.0/docs/api/java/lang/Object.html?is-external=true" title="class or interface in java.lang">java.lang.Object</a></li>
<li>
<ul class="inheritance">
<li>org.apache.commons.lang3.concurrent.BackgroundInitializer<T></li>
</ul>
</li>
</ul>
<div class="description">
<ul class="blockList">
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<dl>
<dt><span class="paramLabel">Type Parameters:</span></dt>
<dd><code>T</code> - the type of the object managed by this initializer class</dd>
</dl>
<dl>
<dt>All Implemented Interfaces:</dt>
<dd><a href="../../../../../org/apache/commons/lang3/concurrent/ConcurrentInitializer.html" title="interface in org.apache.commons.lang3.concurrent">ConcurrentInitializer</a><T></dd>
</dl>
<dl>
<dt>Direct Known Subclasses:</dt>
<dd><a href="../../../../../org/apache/commons/lang3/concurrent/CallableBackgroundInitializer.html" title="class in org.apache.commons.lang3.concurrent">CallableBackgroundInitializer</a>, <a href="../../../../../org/apache/commons/lang3/concurrent/MultiBackgroundInitializer.html" title="class in org.apache.commons.lang3.concurrent">MultiBackgroundInitializer</a></dd>
</dl>
<hr>
<br>
<pre>public abstract class <span class="typeNameLabel">BackgroundInitializer<T></span>
extends <a href="http://docs.oracle.com/javase/1.5.0/docs/api/java/lang/Object.html?is-external=true" title="class or interface in java.lang">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>
A class that allows complex initialization operations in a background task.
</p>
<p>
Applications often have to do some expensive initialization steps when they
are started, e.g. constructing a connection to a database, reading a
configuration file, etc. Doing these things in parallel can enhance
performance as the CPU load can be improved. However, when access to the
resources initialized in a background thread is actually required,
synchronization has to be performed to ensure that their initialization is
complete.
</p>
<p>
This abstract base class provides support for this use case. A concrete
subclass must implement the <a href="../../../../../org/apache/commons/lang3/concurrent/BackgroundInitializer.html#initialize--"><code>initialize()</code></a> method. Here an arbitrary
initialization can be implemented, and a result object can be returned. With
this method in place the basic usage of this class is as follows (where
<code>MyBackgroundInitializer</code> is a concrete subclass):
</p>
<pre>
MyBackgroundInitializer initializer = new MyBackgroundInitializer();
initializer.start();
// Now do some other things. Initialization runs in a parallel thread
...
// Wait for the end of initialization and access the result object
Object result = initializer.get();
</pre>
<p>
After the construction of a <code>BackgroundInitializer</code> object its
<a href="../../../../../org/apache/commons/lang3/concurrent/BackgroundInitializer.html#start--"><code>start()</code></a> method has to be called. This starts the background
processing. The application can now continue to do other things. When it
needs access to the object produced by the <code>BackgroundInitializer</code> it
calls its <a href="../../../../../org/apache/commons/lang3/concurrent/BackgroundInitializer.html#get--"><code>get()</code></a> method. If initialization is already complete,
<a href="../../../../../org/apache/commons/lang3/concurrent/BackgroundInitializer.html#get--"><code>get()</code></a> returns the result object immediately. Otherwise it blocks
until the result object is fully constructed.
</p>
<p>
<code>BackgroundInitializer</code> is a thin wrapper around a <code>Future</code>
object and uses an <code>ExecutorService</code> for running the background
initialization task. It is possible to pass in an <code>ExecutorService</code> at
construction time or set one using <code>setExternalExecutor()</code> before
<code>start()</code> was called. Then this object is used to spawn the background
task. If no <code>ExecutorService</code> has been provided, <code>BackgroundInitializer</code> creates a temporary <code>ExecutorService</code> and
destroys it when initialization is complete.
</p>
<p>
The methods provided by <code>BackgroundInitializer</code> provide for minimal
interaction with the wrapped <code>Future</code> object. It is also possible to
obtain the <code>Future</code> object directly. Then the enhanced functionality
offered by <code>Future</code> can be used, e.g. to check whether the background
operation is complete or to cancel the operation.
</p></div>
<dl>
<dt><span class="simpleTagLabel">Since:</span></dt>
<dd>3.0</dd>
</dl>
</li>
</ul>
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<caption><span>Constructors</span><span class="tabEnd"> </span></caption>
<tr>
<th class="colFirst" scope="col">Modifier</th>
<th class="colLast" scope="col">Constructor and Description</th>
</tr>
<tr class="altColor">
<td class="colFirst"><code>protected </code></td>
<td class="colLast"><code><span class="memberNameLink"><a href="../../../../../org/apache/commons/lang3/concurrent/BackgroundInitializer.html#BackgroundInitializer--">BackgroundInitializer</a></span>()</code>
<div class="block">Creates a new instance of <code>BackgroundInitializer</code>.</div>
</td>
</tr>
<tr class="rowColor">
<td class="colFirst"><code>protected </code></td>
<td class="colLast"><code><span class="memberNameLink"><a href="../../../../../org/apache/commons/lang3/concurrent/BackgroundInitializer.html#BackgroundInitializer-java.util.concurrent.ExecutorService-">BackgroundInitializer</a></span>(<a href="http://docs.oracle.com/javase/1.5.0/docs/api/java/util/concurrent/ExecutorService.html?is-external=true" title="class or interface in java.util.concurrent">ExecutorService</a> exec)</code>
<div class="block">Creates a new instance of <code>BackgroundInitializer</code> and initializes
it with the given <code>ExecutorService</code>.</div>
</td>
</tr>
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<h3>Method Summary</h3>
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<caption><span id="t0" class="activeTableTab"><span>All Methods</span><span class="tabEnd"> </span></span><span id="t2" class="tableTab"><span><a href="javascript:show(2);">Instance Methods</a></span><span class="tabEnd"> </span></span><span id="t3" class="tableTab"><span><a href="javascript:show(4);">Abstract Methods</a></span><span class="tabEnd"> </span></span><span id="t4" class="tableTab"><span><a href="javascript:show(8);">Concrete Methods</a></span><span class="tabEnd"> </span></span></caption>
<tr>
<th class="colFirst" scope="col">Modifier and Type</th>
<th class="colLast" scope="col">Method and Description</th>
</tr>
<tr id="i0" class="altColor">
<td class="colFirst"><code><a href="../../../../../org/apache/commons/lang3/concurrent/BackgroundInitializer.html" title="type parameter in BackgroundInitializer">T</a></code></td>
<td class="colLast"><code><span class="memberNameLink"><a href="../../../../../org/apache/commons/lang3/concurrent/BackgroundInitializer.html#get--">get</a></span>()</code>
<div class="block">Returns the result of the background initialization.</div>
</td>
</tr>
<tr id="i1" class="rowColor">
<td class="colFirst"><code>protected <a href="http://docs.oracle.com/javase/1.5.0/docs/api/java/util/concurrent/ExecutorService.html?is-external=true" title="class or interface in java.util.concurrent">ExecutorService</a></code></td>
<td class="colLast"><code><span class="memberNameLink"><a href="../../../../../org/apache/commons/lang3/concurrent/BackgroundInitializer.html#getActiveExecutor--">getActiveExecutor</a></span>()</code>
<div class="block">Returns the <code>ExecutorService</code> that is actually used for executing
the background task.</div>
</td>
</tr>
<tr id="i2" class="altColor">
<td class="colFirst"><code><a href="http://docs.oracle.com/javase/1.5.0/docs/api/java/util/concurrent/ExecutorService.html?is-external=true" title="class or interface in java.util.concurrent">ExecutorService</a></code></td>
<td class="colLast"><code><span class="memberNameLink"><a href="../../../../../org/apache/commons/lang3/concurrent/BackgroundInitializer.html#getExternalExecutor--">getExternalExecutor</a></span>()</code>
<div class="block">Returns the external <code>ExecutorService</code> to be used by this class.</div>
</td>
</tr>
<tr id="i3" class="rowColor">
<td class="colFirst"><code><a href="http://docs.oracle.com/javase/1.5.0/docs/api/java/util/concurrent/Future.html?is-external=true" title="class or interface in java.util.concurrent">Future</a><<a href="../../../../../org/apache/commons/lang3/concurrent/BackgroundInitializer.html" title="type parameter in BackgroundInitializer">T</a>></code></td>
<td class="colLast"><code><span class="memberNameLink"><a href="../../../../../org/apache/commons/lang3/concurrent/BackgroundInitializer.html#getFuture--">getFuture</a></span>()</code>
<div class="block">Returns the <code>Future</code> object that was created when <a href="../../../../../org/apache/commons/lang3/concurrent/BackgroundInitializer.html#start--"><code>start()</code></a>
was called.</div>
</td>
</tr>
<tr id="i4" class="altColor">
<td class="colFirst"><code>protected int</code></td>
<td class="colLast"><code><span class="memberNameLink"><a href="../../../../../org/apache/commons/lang3/concurrent/BackgroundInitializer.html#getTaskCount--">getTaskCount</a></span>()</code>
<div class="block">Returns the number of background tasks to be created for this
initializer.</div>
</td>
</tr>
<tr id="i5" class="rowColor">
<td class="colFirst"><code>protected abstract <a href="../../../../../org/apache/commons/lang3/concurrent/BackgroundInitializer.html" title="type parameter in BackgroundInitializer">T</a></code></td>
<td class="colLast"><code><span class="memberNameLink"><a href="../../../../../org/apache/commons/lang3/concurrent/BackgroundInitializer.html#initialize--">initialize</a></span>()</code>
<div class="block">Performs the initialization.</div>
</td>
</tr>
<tr id="i6" class="altColor">
<td class="colFirst"><code>boolean</code></td>
<td class="colLast"><code><span class="memberNameLink"><a href="../../../../../org/apache/commons/lang3/concurrent/BackgroundInitializer.html#isStarted--">isStarted</a></span>()</code>
<div class="block">Returns a flag whether this <code>BackgroundInitializer</code> has already
been started.</div>
</td>
</tr>
<tr id="i7" class="rowColor">
<td class="colFirst"><code>void</code></td>
<td class="colLast"><code><span class="memberNameLink"><a href="../../../../../org/apache/commons/lang3/concurrent/BackgroundInitializer.html#setExternalExecutor-java.util.concurrent.ExecutorService-">setExternalExecutor</a></span>(<a href="http://docs.oracle.com/javase/1.5.0/docs/api/java/util/concurrent/ExecutorService.html?is-external=true" title="class or interface in java.util.concurrent">ExecutorService</a> externalExecutor)</code>
<div class="block">Sets an <code>ExecutorService</code> to be used by this class.</div>
</td>
</tr>
<tr id="i8" class="altColor">
<td class="colFirst"><code>boolean</code></td>
<td class="colLast"><code><span class="memberNameLink"><a href="../../../../../org/apache/commons/lang3/concurrent/BackgroundInitializer.html#start--">start</a></span>()</code>
<div class="block">Starts the background initialization.</div>
</td>
</tr>
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<h3>Methods inherited from class java.lang.<a href="http://docs.oracle.com/javase/1.5.0/docs/api/java/lang/Object.html?is-external=true" title="class or interface in java.lang">Object</a></h3>
<code><a href="http://docs.oracle.com/javase/1.5.0/docs/api/java/lang/Object.html?is-external=true#clone--" title="class or interface in java.lang">clone</a>, <a href="http://docs.oracle.com/javase/1.5.0/docs/api/java/lang/Object.html?is-external=true#equals-java.lang.Object-" title="class or interface in java.lang">equals</a>, <a href="http://docs.oracle.com/javase/1.5.0/docs/api/java/lang/Object.html?is-external=true#finalize--" title="class or interface in java.lang">finalize</a>, <a href="http://docs.oracle.com/javase/1.5.0/docs/api/java/lang/Object.html?is-external=true#getClass--" title="class or interface in java.lang">getClass</a>, <a href="http://docs.oracle.com/javase/1.5.0/docs/api/java/lang/Object.html?is-external=true#hashCode--" title="class or interface in java.lang">hashCode</a>, <a href="http://docs.oracle.com/javase/1.5.0/docs/api/java/lang/Object.html?is-external=true#notify--" title="class or interface in java.lang">notify</a>, <a href="http://docs.oracle.com/javase/1.5.0/docs/api/java/lang/Object.html?is-external=true#notifyAll--" title="class or interface in java.lang">notifyAll</a>, <a href="http://docs.oracle.com/javase/1.5.0/docs/api/java/lang/Object.html?is-external=true#toString--" title="class or interface in java.lang">toString</a>, <a href="http://docs.oracle.com/javase/1.5.0/docs/api/java/lang/Object.html?is-external=true#wait--" title="class or interface in java.lang">wait</a>, <a href="http://docs.oracle.com/javase/1.5.0/docs/api/java/lang/Object.html?is-external=true#wait-long-" title="class or interface in java.lang">wait</a>, <a href="http://docs.oracle.com/javase/1.5.0/docs/api/java/lang/Object.html?is-external=true#wait-long-int-" title="class or interface in java.lang">wait</a></code></li>
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<h4>BackgroundInitializer</h4>
<pre>protected BackgroundInitializer()</pre>
<div class="block">Creates a new instance of <code>BackgroundInitializer</code>. No external
<code>ExecutorService</code> is used.</div>
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<h4>BackgroundInitializer</h4>
<pre>protected BackgroundInitializer(<a href="http://docs.oracle.com/javase/1.5.0/docs/api/java/util/concurrent/ExecutorService.html?is-external=true" title="class or interface in java.util.concurrent">ExecutorService</a> exec)</pre>
<div class="block">Creates a new instance of <code>BackgroundInitializer</code> and initializes
it with the given <code>ExecutorService</code>. If the <code>ExecutorService</code>
is not null, the background task for initializing this object will be
scheduled at this service. Otherwise a new temporary <code>ExecutorService</code> is created.</div>
<dl>
<dt><span class="paramLabel">Parameters:</span></dt>
<dd><code>exec</code> - an external <code>ExecutorService</code> to be used for task
execution</dd>
</dl>
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<h4>getExternalExecutor</h4>
<pre>public final <a href="http://docs.oracle.com/javase/1.5.0/docs/api/java/util/concurrent/ExecutorService.html?is-external=true" title="class or interface in java.util.concurrent">ExecutorService</a> getExternalExecutor()</pre>
<div class="block">Returns the external <code>ExecutorService</code> to be used by this class.</div>
<dl>
<dt><span class="returnLabel">Returns:</span></dt>
<dd>the <code>ExecutorService</code></dd>
</dl>
</li>
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<pre>public boolean isStarted()</pre>
<div class="block">Returns a flag whether this <code>BackgroundInitializer</code> has already
been started.</div>
<dl>
<dt><span class="returnLabel">Returns:</span></dt>
<dd>a flag whether the <a href="../../../../../org/apache/commons/lang3/concurrent/BackgroundInitializer.html#start--"><code>start()</code></a> method has already been
called</dd>
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<h4>setExternalExecutor</h4>
<pre>public final void setExternalExecutor(<a href="http://docs.oracle.com/javase/1.5.0/docs/api/java/util/concurrent/ExecutorService.html?is-external=true" title="class or interface in java.util.concurrent">ExecutorService</a> externalExecutor)</pre>
<div class="block">Sets an <code>ExecutorService</code> to be used by this class. The <code>ExecutorService</code> passed to this method is used for executing the
background task. Thus it is possible to re-use an already existing
<code>ExecutorService</code> or to use a specially configured one. If no
<code>ExecutorService</code> is set, this instance creates a temporary one and
destroys it after background initialization is complete. Note that this
method must be called before <a href="../../../../../org/apache/commons/lang3/concurrent/BackgroundInitializer.html#start--"><code>start()</code></a>; otherwise an exception is
thrown.</div>
<dl>
<dt><span class="paramLabel">Parameters:</span></dt>
<dd><code>externalExecutor</code> - the <code>ExecutorService</code> to be used</dd>
<dt><span class="throwsLabel">Throws:</span></dt>
<dd><code><a href="http://docs.oracle.com/javase/1.5.0/docs/api/java/lang/IllegalStateException.html?is-external=true" title="class or interface in java.lang">IllegalStateException</a></code> - if this initializer has already been
started</dd>
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<pre>public boolean start()</pre>
<div class="block">Starts the background initialization. With this method the initializer
becomes active and invokes the <a href="../../../../../org/apache/commons/lang3/concurrent/BackgroundInitializer.html#initialize--"><code>initialize()</code></a> method in a
background task. A <code>BackgroundInitializer</code> can be started exactly
once. The return value of this method determines whether the start was
successful: only the first invocation of this method returns <b>true</b>,
following invocations will return <b>false</b>.</div>
<dl>
<dt><span class="returnLabel">Returns:</span></dt>
<dd>a flag whether the initializer could be started successfully</dd>
</dl>
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<h4>get</h4>
<pre>public <a href="../../../../../org/apache/commons/lang3/concurrent/BackgroundInitializer.html" title="type parameter in BackgroundInitializer">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 result of the background initialization. This method blocks
until initialization is complete. If the background processing caused a
runtime exception, it is directly thrown by this method. Checked
exceptions, including <code>InterruptedException</code> are wrapped in a
<a href="../../../../../org/apache/commons/lang3/concurrent/ConcurrentException.html" title="class in org.apache.commons.lang3.concurrent"><code>ConcurrentException</code></a>. Calling this method before <a href="../../../../../org/apache/commons/lang3/concurrent/BackgroundInitializer.html#start--"><code>start()</code></a>
was called causes an <code>IllegalStateException</code> exception to be
thrown.</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/BackgroundInitializer.html" title="type parameter in BackgroundInitializer">T</a>></code></dd>
<dt><span class="returnLabel">Returns:</span></dt>
<dd>the object produced by this initializer</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 a checked exception occurred during
background processing</dd>
<dd><code><a href="http://docs.oracle.com/javase/1.5.0/docs/api/java/lang/IllegalStateException.html?is-external=true" title="class or interface in java.lang">IllegalStateException</a></code> - if <a href="../../../../../org/apache/commons/lang3/concurrent/BackgroundInitializer.html#start--"><code>start()</code></a> has not been called</dd>
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<h4>getFuture</h4>
<pre>public <a href="http://docs.oracle.com/javase/1.5.0/docs/api/java/util/concurrent/Future.html?is-external=true" title="class or interface in java.util.concurrent">Future</a><<a href="../../../../../org/apache/commons/lang3/concurrent/BackgroundInitializer.html" title="type parameter in BackgroundInitializer">T</a>> getFuture()</pre>
<div class="block">Returns the <code>Future</code> object that was created when <a href="../../../../../org/apache/commons/lang3/concurrent/BackgroundInitializer.html#start--"><code>start()</code></a>
was called. Therefore this method can only be called after <code>start()</code>.</div>
<dl>
<dt><span class="returnLabel">Returns:</span></dt>
<dd>the <code>Future</code> object wrapped by this initializer</dd>
<dt><span class="throwsLabel">Throws:</span></dt>
<dd><code><a href="http://docs.oracle.com/javase/1.5.0/docs/api/java/lang/IllegalStateException.html?is-external=true" title="class or interface in java.lang">IllegalStateException</a></code> - if <a href="../../../../../org/apache/commons/lang3/concurrent/BackgroundInitializer.html#start--"><code>start()</code></a> has not been called</dd>
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<h4>getActiveExecutor</h4>
<pre>protected final <a href="http://docs.oracle.com/javase/1.5.0/docs/api/java/util/concurrent/ExecutorService.html?is-external=true" title="class or interface in java.util.concurrent">ExecutorService</a> getActiveExecutor()</pre>
<div class="block">Returns the <code>ExecutorService</code> that is actually used for executing
the background task. This method can be called after <a href="../../../../../org/apache/commons/lang3/concurrent/BackgroundInitializer.html#start--"><code>start()</code></a>
(before <code>start()</code> it returns <b>null</b>). If an external executor
was set, this is also the active executor. Otherwise this method returns
the temporary executor that was created by this object.</div>
<dl>
<dt><span class="returnLabel">Returns:</span></dt>
<dd>the <code>ExecutorService</code> for executing the background task</dd>
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<h4>getTaskCount</h4>
<pre>protected int getTaskCount()</pre>
<div class="block">Returns the number of background tasks to be created for this
initializer. This information is evaluated when a temporary <code>ExecutorService</code> is created. This base implementation returns 1. Derived
classes that do more complex background processing can override it. This
method is called from a synchronized block by the <a href="../../../../../org/apache/commons/lang3/concurrent/BackgroundInitializer.html#start--"><code>start()</code></a>
method. Therefore overriding methods should be careful with obtaining
other locks and return as fast as possible.</div>
<dl>
<dt><span class="returnLabel">Returns:</span></dt>
<dd>the number of background tasks required by this initializer</dd>
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<h4>initialize</h4>
<pre>protected abstract <a href="../../../../../org/apache/commons/lang3/concurrent/BackgroundInitializer.html" title="type parameter in BackgroundInitializer">T</a> initialize()
throws <a href="http://docs.oracle.com/javase/1.5.0/docs/api/java/lang/Exception.html?is-external=true" title="class or interface in java.lang">Exception</a></pre>
<div class="block">Performs the initialization. This method is called in a background task
when this <code>BackgroundInitializer</code> is started. It must be
implemented by a concrete subclass. An implementation is free to perform
arbitrary initialization. The object returned by this method can be
queried using the <a href="../../../../../org/apache/commons/lang3/concurrent/BackgroundInitializer.html#get--"><code>get()</code></a> method.</div>
<dl>
<dt><span class="returnLabel">Returns:</span></dt>
<dd>a result object</dd>
<dt><span class="throwsLabel">Throws:</span></dt>
<dd><code><a href="http://docs.oracle.com/javase/1.5.0/docs/api/java/lang/Exception.html?is-external=true" title="class or interface in java.lang">Exception</a></code> - if an error occurs</dd>
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