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    <p>I found the following, valid reasons to create custom classloaders:</p> <ol> <li><p>You want to load a class from an unconventional source (For example, the bytecode for one class is stored in a database, across the network or <a href="http://www.ietf.org/rfc/rfc1149.txt?number=1149" rel="noreferrer">carried as 0 and 1s by pidgeons</a> - MessengerPidgeonClassLoader). There is some ClassLoader implementations already in the API for such cases, such as <a href="http://java.sun.com/j2se/1.4.2/docs/api/java/net/URLClassLoader.html" rel="noreferrer">URLClassLoader</a>.</p></li> <li><p>You need to define a different hierarchy to load classes. Default implementations of the ClassLoader delegate the search first to the parent, then they try to load the class themselves. Maybe you want a different hierarchy. This is the reason why OSGI and Eclipse have its own ClassLoaders as the Manifest .MF files define all types of weird hierarchy paths (buddy-classloading, for example). All Eclipse classloaders implement the BundleClassLoader interface and have some extra code to find resources within Eclipse Plugins.</p></li> <li><p>You need to do some modification to the bytecode. Maybe the bytecode is encrypted, and you will unencrypt it on the fly (<a href="http://www.javaworld.com/javaworld/javaqa/2003-05/01-qa-0509-jcrypt.html" rel="noreferrer">Not that it helps, really, but has been tried</a>). Maybe you want to "patch" the classes loaded on the fly (A la JDO bytecode enhancement).</p></li> </ol> <p>Using a different classloader than the System Classloader is required if you need to unload classes from memory, or to load classes than could change their definition at runtime. A typical case is an application that generates a class on the fly from an XML file, for example, and then tries to reload this class. Once a class is in the System Classloader, there is no way to unload it and have a new definition.</p>
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    1. USMario Ortegón
    1. USMario Ortegón
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    1. COnice answer but the pidgeons guarantees the plus 1 :)
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    2. COIP Datagrams on Avian Carriers is the best RFC ever :) And guess what, it has been tested in real life too! http://www.notes.co.il/benbasat/5240.asp
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