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The '''Java
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}}</ref> Usually classes are only loaded [[lazy initialization|on demand]]. The virtual machine will only load the class files required for executing the program.{{sfn | Horstmann | 2022 | loc=§10.1.1 The Class-Loading Process}} The Java run time system does not need to know about files and file systems as this is [[Delegation _(object-oriented_programming)|delegated]] to the
A [[Library (computing)|software library]] is a collection of related [[object code]].
In the [[Java (programming language)|Java language]], libraries are typically packaged in [[JAR (file format)|JAR files]]. Libraries can contain objects of different types. The most important type of object contained in a Jar file is a [[Java class]]. A class can be thought of as a named unit of code. The class loader is responsible for locating libraries, reading their contents, and loading the classes contained within the libraries. This loading is typically done "on demand", in that it does not occur until the class is called by the program. A class with a given name can only be loaded once by a given
Each Java class must be loaded by a class loader.{{sfn | Horstmann | 2022 | loc=§8.2.5 Writing Byte Codes to Memory}}<ref name="Christudas">{{cite web
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|title=Classes and class loading
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|accessdate=2008-01-26
}}</ref>{{sfn | Horstmann | 2022 | loc=§10.1.1 The Class-Loading Process}}
# Bootstrap class loader
# Extensions class loader
# System class loader
The bootstrap class loader loads the core Java libraries<ref group=fn>These libraries are stored in [[JAR (file format)|Jar files]] called ''rt.jar'', ''core.jar'', ''server.jar'', etc.</ref> located in the <code><JAVA_HOME>/jre/lib</code> (or <code><JAVA_HOME>/jmods></code> for Java 9 and above) directory. This class loader, which is part of the core JVM, is written in native code. The bootstrap class loader is not associated with any {{java|ClassLoader}} object.{{sfn | Horstmann | 2022 | loc=§10.1.1 The Class-Loading Process}} For instance, {{java|StringBuilder.class.getClassLoader()}} returns {{java|null}}.{{sfn | Horstmann | 2022 | loc=§10.1.1 The Class-Loading Process}}
The extensions class loader loads the code in the extensions directories (<code><JAVA_HOME>/jre/lib/ext</code>,<ref name="TJT:UECL"/> or any other directory specified
by the <code>java.ext.dirs</code> system property)
The system class loader loads code found on <code>java.class.path</code>, which maps to the <code>[[Classpath (Java)|CLASSPATH]]</code> [[environment variable]]
==User-defined class loaders==
The Java class loader is written in Java. It is therefore possible to create
This makes it possible (for example):
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* to modify the loaded bytecode (for example, for load-time [[aspect weaver|weaving]] of aspects when using [[aspect-oriented programming]]).
== Class
[[Jakarta EE]] (formerly Java EE and J2EE) application servers typically load classes from a deployed [[WAR (file format)|WAR]] or [[EAR (file format)|EAR]] archive by a tree of
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|title=J2EE Class Loading Demystified
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== {{anchor|jarhell}} JAR hell ==
JAR hell is a term similar to [[DLL hell]] used to describe all the various ways in which the classloading process can end up not working.<ref>{{Cite web|url=http://incubator.apache.org/depot/version/jar-hell.html|archive-url = https://web.archive.org/web/20130601002059/http://incubator.apache.org/depot/version/jar-hell.html|archive-date = 2013-06-01|title = Depot - Apache Incubator}}</ref> Three ways JAR hell can occur are:
* 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.
* 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
* 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"
The [[OSGi]] 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 [[manifest file|manifest]], 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.
[[Java version history#Java SE 9|Java 9]] introduced the [[Java Platform Module System]] 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 ''[[Modular programming|modules]]'', 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 [[JAR (file format)|JAR]] 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.<ref>{{cite web|first1=Neil|last1=Bartlett|first2=Kai|last2=Hackbarth|title=Java 9, OSGi and the Future of Modularity (Part 1)
|url=https://www.infoq.com/articles/java9-osgi-future-modularity/#1|publisher=InfoQ|date=2016-09-22}}</ref>
==
* [[Loader (computing)]]
* [[Dynamic loading]]
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{{reflist}}
==
* Chuck McManis, "[https://www.infoworld.com/article/2077260/learn-java-the-basics-of-java-class-loaders.html The basics of Java class loaders]", 1996
* Brandon E. Taylor, "[http://www.developer.com/java/other/article.php/2248831 Java Class Loading: The Basics] {{Webarchive|url=https://web.archive.org/web/20201109091535/http://www.developer.com/java/other/article.php/2248831 |date=2020-11-09 }}", 2003
* {{cite book | last=Horstmann | first=Cay | title=Core Java | publisher=Oracle Press Java | date=April 15, 2022 | isbn=0-13-787107-4}}
* Jeff Hanson, "[http://www.devx.com/Java/Article/31614 Take Control of Class Loading in Java] {{Webarchive|url=https://web.archive.org/web/20201204030013/http://www.devx.com/Java/Article/31614 |date=2020-12-04 }}", 2006-06-01
* Andreas Schaefer, "[https://web.archive.org/web/20180506212821/http://www.onjava.com/pub/a/onjava/2003/11/12/classloader.html Inside Class Loaders]", 2003-11-12
* Sheng Liang and Gilad Bracha, "[https://citeseer.ist.psu.edu/viewdoc/summary?doi=10.1.1.18.762 Dynamic class loading in the Java virtual machine]", 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 {{doi|10.1145/286936.286945}}
* Jeremy Whitlock, "[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]", May 2005
*
* Don Schwarz, "[https://web.archive.org/web/20180506083919/http://www.onjava.com/pub/a/onjava/2005/04/13/dependencies.html Managing Component Dependencies Using ClassLoaders]", 2005-04-13
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