The Java Classloader is a part of the Java Runtime Environment that dynamically loads Java classes into the Java Virtual Machine. Usually classes are only loaded on demand. The Java run time system does not need to know about files and file systems because of classloaders. Delegation is an important concept to understand when learning about classloaders.
A software library is a collection of related object code. In the Java language, libraries are typically packaged in 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 actually used by the program. A class with a given name can only be loaded once by a given classloader.
Each Java class must be loaded by a class loader. Furthermore, Java programs may make use of external libraries (that is, libraries written and provided by someone other than the author of the program) or they may be composed, at least in part, of a number of libraries.
- Bootstrap class loader
- Extensions class loader
- System class loader
The bootstrap class loader loads the core Java libraries located in the
<JAVA_HOME>/jre/lib directory. This class loader, which is part of the core JVM, is written in native code.
The extensions class loader loads the code in the extensions directories (
<JAVA_HOME>/jre/lib/ext, or any other directory specified by the
java.ext.dirs system property). It is implemented by the
The system class loader loads code found on
java.class.path, which maps to the system
CLASSPATH variable. This is implemented by the
User-defined class loaders 
The Java class loader is written in Java. It is therefore possible to create your own class loader without understanding the finer details of the Java Virtual Machine. Every Java class loader has a parent class loader, defined when a new class loader is instantiated or set to the virtual machine's system default class loader.
This makes it possible (for example):
- to load or unload classes at runtime (for example to load libraries dynamically at runtime, even from a HTTP resource). This is an important feature for:
- to change the way the bytecode is loaded (for example, it is possible to use encrypted Java class bytecode).
- to modify the loaded bytecode (for example, for load-time weaving of aspects when using aspect-oriented programming).
Class Loaders in JEE 
Java Platform, Enterprise Edition (JEE) application servers typically load classes from a deployed WAR or EAR archive by a tree of classloaders, isolating the application from other applications, but sharing classes between deployed modules. So-called "servlet containers" are typically implemented in terms of multiple classloaders.
JAR hell 
- A developer or deployer of a Java application has accidentally made two different versions of a library available to the 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 see the application still behaving as though the old library is in use, which it may well be.
- Two libraries (or a library and the application) require different versions of the same third library. If both versions of the third library use the same class names, there is no way to load both versions of the third library with the same classloader.
- 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" classloader, but instead may be composed of several (potentially very many) nested, cooperating classloaders. Classes loaded by different classloaders may interact in complex ways not fully comprehended by a developer, leading to inexplicable errors or bugs.
The OSGi Alliance specified (starting as JSR 8 in 1998) a modularity framework that solved JAR hell for current and future VMs in ME, SE, and EE that is widely adopted. Using metadata in the JAR 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.
To remedy the JAR hell problems a Java Community Process — JSR 277 was initiated in 2005. The resolution — Java Module System — intended to introduce a new distribution format, a modules versioning scheme, and a common modules repository (similar in purpose to Microsoft .NET's Global Assembly Cache). In December 2008 Sun announced that JSR 277 was put on hold.
See also 
- Loader (computing)
- Dynamic loading
- DLL hell
- Apache Maven, automated software build tool with dependency management
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- These libraries are stored in Jar files called rt.jar, core.jar, server.jar, etc.
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- Dr. Christoph G. Jung, "Classloaders Revisited Hotdeploy", Java Specialist Newsletter, 2001-06-07
- Don Schwarz, "Managing Component Dependencies Using ClassLoaders", 2005-04-13