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<h1>Pluggable Typehandler Benefits</h1>
<p>As the name suggests Pluggable <span class="MCPopup"><a href="javascript:void(0);" class="MCPopupSpot" onclick="FMCPopup( event, this ); return false;" MadCap:src="../typehandlers.htm">Typehandler<img style="border: none;margin-left: 5px;" src="../../../SkinSupport/ExpandingClosed.gif" MadCap:altsrc="../../../SkinSupport/ExpandingOpen.gif" class="MCExpandingIcon" onload="if ( typeof( FMCPreloadImage ) == 'function' ) { FMCPreloadImage( '../../../SkinSupport/ExpandingOpen.gif' ); }" /></a></span> allows anybody to
write custom typehandlers, and thus control the way the class objects are
stored to the database and retrieved in a query. Why would you do this? There
can be various reasons:</p>
<ul class="unIndentedList">
<li value="1">You know a more performant way to convert
objects to byte array or to compare them.</li>
<li value="2">You need to store only part of the object's
information and want to skip unneeded fields to keep the database smaller. You
can also do the same using <a href="../../../tuning/selective_persistence.htm">Transient</a>marker, but this is only possible for classes with available code. Using custom
typehandler you can configure partial storage for any third-party class.</li>
<li value="3">You need to keep information that will allow you
to restore fields that cannot be stored as is, for example: references to
environmental variables (like time zone), proxy objects or variables of
temporary state (like current memory usage). Previously, this job was done by
<a href="../translators.htm">Translators</a>, but certainly custom Typehandler gives you more control
and unifies the approach.</li>
<li value="4">You need to perform a complex refactoring
on-the-fly (use typehandler versioning)</li>
<li value="5">You want to cipher each object before putting it
into the database</li>
</ul>
<p>Other not so common and more
difficult in realization behaviours that can be realized with the new
Typehandler:</p>
<ul class="unIndentedList">
<li value="1">Customary indexes</li>
<li value="2">Versioning of typehandlers (can be used for
refactoring and db4o version upgrades)</li>
</ul>
<p>Of course, writing typehandlers
is not totally simple, but once you understand how to do that - you will also
gain a much deeper understanding of db4o itself. You can start with a <a href="custom_typehandler_example.htm">simple
example</a> provided in this documentation and continue by looking into existing
db4o typehandler implementations: StringHandler, VariableLengthTypeHandler,
IndexableTypeHandler etc.</p>
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