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Java class loader
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<h2 id="user-defined-class-loaders">User-defined class loaders</h2> <p>The Java class loader is written in Java. It is therefore possible to create a custom class loader without understanding the finer details of the Java Virtual Machine. Apart from the Bootstrap class loader, every Java class loader has a parent class loader.<a class="footnote-ref" id="fnref:12" href="#fn:12"><sup>12</sup></a> The parent class loader is defined when a new class loader is instantiated or set to the virtual machine's system default class loader. </p><p>This makes it possible (for example): </p> <ul><li>to load or unload classes at runtime (for example to load libraries dynamically at runtime, even from an <a href="/facts/Hypertext_Transfer_Protocol/TWtvf7JD">HTTP</a> resource). This is an important feature for: <ul><li>implementing scripting languages, such as <a href="/facts/Jython/BPsxEwnT">Jython</a></li> <li>using <a href="/facts/JavaBean/C0HvUnQ5">bean</a> builders</li> <li>allowing user-defined <a href="/facts/Extensibility/AEPWNwxp">extensibility</a></li> <li>allowing multiple <a href="/facts/Namespace/9efXwMtV">namespaces</a> to communicate. This is one of the foundations of <a href="/facts/Common_Object_Request_Broker_Architecture/VUDlTHs5">CORBA</a> / <a href="/facts/Java_remote_method_invocation/psHXkdWr">RMI</a> protocols for example.</li></ul></li> <li>to change the way the <a href="/facts/Java_bytecode/Nh1qKH4y">bytecode</a> is loaded (for example, it is possible to use <a href="/facts/Encryption/16q29Fdv">encrypted</a> Java class bytecode<a class="footnote-ref" id="fnref:13" href="#fn:13"><sup>13</sup></a>).</li> <li>to modify the loaded bytecode (for example, for load-time <a href="/facts/Aspect_weaver/cUkBk1Bc">weaving</a> of aspects when using <a href="/facts/Aspect-oriented_programming/HK7CFwan">aspect-oriented programming</a>).</li></ul> <h2 id="class-loaders-in-jakarta-ee">Class loaders in Jakarta EE</h2> <p><a href="/facts/Jakarta_EE/7AAStd35">Jakarta EE</a> (formerly Java EE and J2EE) application servers typically load classes from a deployed <a href="/facts/WAR_(file_format)/KohLDBAw">WAR</a> or <a href="/facts/EAR_(file_format)/2VRcm8Au">EAR</a> archive by a tree of class loaders, isolating the application from other applications, but sharing classes between deployed modules. So-called "<a href="/facts/Servlet_container/Pv06YKvB">servlet containers</a>" are typically implemented in terms of multiple class loaders.<a class="footnote-ref" id="fnref:14" href="#fn:14"><sup>14</sup></a><a class="footnote-ref" id="fnref:15" href="#fn:15"><sup>15</sup></a> </p> <h2 id="jar-hell"> JAR hell</h2> <p>JAR hell is a term similar to <a href="/facts/DLL_hell/e6UBYdrP">DLL hell</a> used to describe all the various ways in which the classloading process can end up not working.<a class="footnote-ref" id="fnref:16" href="#fn:16"><sup>16</sup></a> Three ways JAR hell can occur are: </p> <ul><li>Accidental presence of two different versions of a library installed on a system. This will not be considered an error by the system. Rather, the system will load classes from one or the other library. Adding the new library to the list of available libraries instead of replacing it may result in the application still behaving as though the old library is in use, which it may well be.</li> <li>Multiple libraries or applications require different versions of library foo. If versions of library foo use the same class names, there is no way to load the versions of library foo with the same class loader.</li> <li>The most complex JAR hell problems arise in circumstances that take advantage of the full complexity of the classloading system. A Java program is not required to use only a single "flat" class loader, but instead may be composed of several (potentially very many) nested, cooperating class loaders. Classes loaded by different class loaders may interact in complex ways not fully comprehended by a developer, leading to errors or bugs that are difficult to analyze, explain, and resolve.<a class="footnote-ref" id="fnref:17" href="#fn:17"><sup>17</sup></a></li></ul> <p>The <a href="/facts/OSGi/qh2yzbsu">OSGi</a> Alliance specified (starting as JSR 8 in 1998) a modularity framework that aims to solve JAR hell for current and future VMs in ME, SE, and EE that is widely adopted. Using metadata in the JAR <a href="/facts/Manifest_file/h8PEdvVx">manifest</a>, JAR files (called bundles) are wired on a per-package basis. Bundles can export packages, import packages and keep packages private, providing the basic constructs of modularity and versioned dependency management. </p><p><a href="/facts/Java_version_history/Br6eHSP8">Java 9</a> introduced the <a href="/facts/Java_Platform_Module_System/LAGvEqkx">Java Platform Module System</a> in 2017. This specifies a distribution format for collections of Java code and associated resources. It also specifies a repository for storing these collections, or <i><a href="/facts/Modular_programming/sNoqKvRD">modules</a></i>, and identifies how they can be discovered, loaded and checked for integrity. It includes features such as namespaces with the aim of fixing some of the shortcomings in the existing <a href="/facts/JAR_(file_format)/LcIBCWXT">JAR</a> format. The Java Platform Module System follows a different philosophy from the OSGi architecture that aims at providing modularity for the Java Runtime Environment in a backwards-compatible way that uses the default mechanism of loading classes that the JRE provides. However, since the Java Platform Module System does not offer the ability for controlled co-existence of libraries with different versions, it does not fully address the JAR hell problem.<a class="footnote-ref" id="fnref:18" href="#fn:18"><sup>18</sup></a> </p> <h2 id="see-also">See also</h2> <ul><li><a href="/facts/Loader_(computing)/eIK4f2xB">Loader (computing)</a></li> <li><a href="/facts/Dynamic_loading/mJ9bcYuv">Dynamic loading</a></li> <li><a href="/facts/DLL_hell/e6UBYdrP">DLL hell</a></li> <li><a href="/facts/OSGi/qh2yzbsu">OSGi</a></li> <li><a href="/facts/Classpath_(Java)/Gmsj515v">Classpath (Java)</a></li> <li><a href="/facts/Java_Platform_Module_System/LAGvEqkx">Java Platform Module System</a></li></ul> <h2 id="footnotes">Footnotes</h2> <h2 id="external-links">External links</h2> <ul><li>Chuck McManis, "<a href="https://www.infoworld.com/article/2077260/learn-java-the-basics-of-java-class-loaders.html">The basics of Java class loaders</a>", 1996</li> <li>Brandon E. Taylor, "<a href="http://www.developer.com/java/other/article.php/2248831">Java Class Loading: The Basics</a> <a href="https://web.archive.org/web/20201109091535/http://www.developer.com/java/other/article.php/2248831">Archived</a> 2020-11-09 at the <a href="/facts/Wayback_Machine/nmQ3a6JC">Wayback Machine</a>", 2003</li> <li>Horstmann, Cay (April 15, 2022). <i>Core Java</i>. Oracle Press Java. <a href="/facts/ISBN_(identifier)/15AdSPa9">ISBN</a> 0-13-787107-4.</li> <li>Jeff Hanson, "<a href="http://www.devx.com/Java/Article/31614">Take Control of Class Loading in Java</a> <a href="https://web.archive.org/web/20201204030013/http://www.devx.com/Java/Article/31614">Archived</a> 2020-12-04 at the <a href="/facts/Wayback_Machine/nmQ3a6JC">Wayback Machine</a>", 2006-06-01</li> <li>Andreas Schaefer, "<a href="https://web.archive.org/web/20180506212821/http://www.onjava.com/pub/a/onjava/2003/11/12/classloader.html">Inside Class Loaders</a>", 2003-11-12</li> <li>Sheng Liang and Gilad Bracha, "<a href="https://citeseer.ist.psu.edu/viewdoc/summary?doi=10.1.1.18.762">Dynamic class loading in the Java virtual machine</a>", In Proceedings of the 13th ACM Conference on Object-Oriented Programming, Systems, Languages, and Applications (OOPSLA'98), ACM SIGPLAN Notices, vol. 33, no. 10, ACM Press, 1998, pp. 36–44 <a href="/facts/Doi_(identifier)/muM9Etpq">doi</a>:<a href="https://doi.org/10.1145%2F286936.286945">10.1145/286936.286945</a></li> <li>Jeremy Whitlock, "<a href="https://web.archive.org/web/20080628071703/http://dev2dev.bea.com/blog/jcscoobyrs/archive/2005/05/realworld_use_f.html">Real-World Use For Custom ClassLoaders</a>", May 2005</li> <li>Christoph G. Jung, "<a href="https://web.archive.org/web/20130201093532/http://www.roseindia.net/javatutorials/hotdeploy.shtml">Classloaders Revisited Hotdeploy</a>", <i>Java Specialist Newsletter</i>, 2001-06-07</li> <li>Don Schwarz, "<a href="https://web.archive.org/web/20180506083919/http://www.onjava.com/pub/a/onjava/2005/04/13/dependencies.html">Managing Component Dependencies Using ClassLoaders</a>", 2005-04-13</li></ul> <h2 id="references">References</h2> <ol> <li id="fn:1"><p>Mcmanis, Chuck (October 1, 1996). "The basics of Java class loaders". JavaWorld. Retrieved 2020-07-13. <a href="https://www.infoworld.com/article/2077260/learn-java-the-basics-of-java-class-loaders.html" target="_blank">https://www.infoworld.com/article/2077260/learn-java-the-basics-of-java-class-loaders.html</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></p></li> <li id="fn:2"><p>Horstmann 2022, §10.1.1 The Class-Loading Process. - Horstmann, Cay (April 15, 2022). Core Java. Oracle Press Java. ISBN 0-13-787107-4. <a href="#fnref:2" class="footnote-back-ref">↩</a></p></li> <li id="fn:3"><p>Horstmann 2022, §8.2.5 Writing Byte Codes to Memory. - Horstmann, Cay (April 15, 2022). Core Java. Oracle Press Java. ISBN 0-13-787107-4. <a href="#fnref:3" class="footnote-back-ref">↩</a></p></li> <li id="fn:4"><p>Christudas, Binildas (January 26, 2005). "Internals of Java Class Loading". onjava.com. Archived from the original on 2018-05-10. <a href="https://web.archive.org/web/20180510223447/http://www.onjava.com/pub/a/onjava/2005/01/26/classloading.html" target="_blank">https://web.archive.org/web/20180510223447/http://www.onjava.com/pub/a/onjava/2005/01/26/classloading.html</a> <a href="#fnref:4" class="footnote-back-ref">↩</a></p></li> <li id="fn:5"><p>"Understanding Extension Class Loading". The Java Tutorials. docs.oracle.com. Retrieved 2020-07-13. <a href="https://docs.oracle.com/javase/tutorial/ext/basics/load.html" target="_blank">https://docs.oracle.com/javase/tutorial/ext/basics/load.html</a> <a href="#fnref:5" class="footnote-back-ref">↩</a></p></li> <li id="fn:6"><p>Sosnoski, Dennis (April 29, 2003). "Classes and class loading". IBM DeveloperWorks. Retrieved 2008-01-26. <a href="http://www.ibm.com/developerworks/java/library/j-dyn0429/" target="_blank">http://www.ibm.com/developerworks/java/library/j-dyn0429/</a> <a href="#fnref:6" class="footnote-back-ref">↩</a></p></li> <li id="fn:7"><p>Horstmann 2022, §10.1.1 The Class-Loading Process. - Horstmann, Cay (April 15, 2022). Core Java. Oracle Press Java. ISBN 0-13-787107-4. <a href="#fnref:7" class="footnote-back-ref">↩</a></p></li> <li id="fn:8"><p>These libraries are stored in Jar files called rt.jar, core.jar, server.jar, etc. <a href="/wiki/JAR_(file_format)" target="_blank">/wiki/JAR_(file_format)</a> <a href="#fnref:8" class="footnote-back-ref">↩</a></p></li> <li id="fn:9"><p>Horstmann 2022, §10.1.1 The Class-Loading Process. - Horstmann, Cay (April 15, 2022). Core Java. Oracle Press Java. ISBN 0-13-787107-4. <a href="#fnref:9" class="footnote-back-ref">↩</a></p></li> <li id="fn:10"><p>Horstmann 2022, §10.1.1 The Class-Loading Process. - Horstmann, Cay (April 15, 2022). Core Java. Oracle Press Java. ISBN 0-13-787107-4. <a href="#fnref:10" class="footnote-back-ref">↩</a></p></li> <li id="fn:11"><p>"Understanding Extension Class Loading". The Java Tutorials. docs.oracle.com. Retrieved 2020-07-13. <a href="https://docs.oracle.com/javase/tutorial/ext/basics/load.html" target="_blank">https://docs.oracle.com/javase/tutorial/ext/basics/load.html</a> <a href="#fnref:11" class="footnote-back-ref">↩</a></p></li> <li id="fn:12"><p>Horstmann 2022, 10.1.2 The Class Loader Hierarchy. - Horstmann, Cay (April 15, 2022). Core Java. Oracle Press Java. ISBN 0-13-787107-4. <a href="#fnref:12" class="footnote-back-ref">↩</a></p></li> <li id="fn:13"><p>Roubtsov, Vladimir (May 9, 2003). "Cracking Java byte-code encryption". JavaWorld. Retrieved 2020-07-13. <a href="https://www.infoworld.com/article/2077342/cracking-java-byte-code-encryption.html" target="_blank">https://www.infoworld.com/article/2077342/cracking-java-byte-code-encryption.html</a> <a href="#fnref:13" class="footnote-back-ref">↩</a></p></li> <li id="fn:14"><p>Christudas, Binildas (January 26, 2005). "Internals of Java Class Loading". onjava.com. Archived from the original on 2018-05-10. <a href="https://web.archive.org/web/20180510223447/http://www.onjava.com/pub/a/onjava/2005/01/26/classloading.html" target="_blank">https://web.archive.org/web/20180510223447/http://www.onjava.com/pub/a/onjava/2005/01/26/classloading.html</a> <a href="#fnref:14" class="footnote-back-ref">↩</a></p></li> <li id="fn:15"><p>deBoer, Tim; Karasiuk, Gary (August 21, 2002). "J2EE Class Loading Demystified". IBM DeveloperWorks. Retrieved 2008-01-26. <a href="https://www.ibm.com/developerworks/websphere/library/techarticles/0112_deboer/deboer.html" target="_blank">https://www.ibm.com/developerworks/websphere/library/techarticles/0112_deboer/deboer.html</a> <a href="#fnref:15" class="footnote-back-ref">↩</a></p></li> <li id="fn:16"><p>"Depot - Apache Incubator". Archived from the original on 2013-06-01. <a href="https://web.archive.org/web/20130601002059/http://incubator.apache.org/depot/version/jar-hell.html" target="_blank">https://web.archive.org/web/20130601002059/http://incubator.apache.org/depot/version/jar-hell.html</a> <a href="#fnref:16" class="footnote-back-ref">↩</a></p></li> <li id="fn:17"><p>"Taxonomy of class loader problems with Jakarta Commons Logging". <a href="http://articles.qos.ch/classloader.html" target="_blank">http://articles.qos.ch/classloader.html</a> <a href="#fnref:17" class="footnote-back-ref">↩</a></p></li> <li id="fn:18"><p>Bartlett, Neil; Hackbarth, Kai (2016-09-22). "Java 9, OSGi and the Future of Modularity (Part 1)". InfoQ. <a href="https://www.infoq.com/articles/java9-osgi-future-modularity/#1" target="_blank">https://www.infoq.com/articles/java9-osgi-future-modularity/#1</a> <a href="#fnref:18" class="footnote-back-ref">↩</a></p></li> </ol>