The Java Classloader is a part of the Java Runtime Environment that dynamically loads Java classes into the Java Virtual Machine.[1] Usually classes are only loaded on demand. The Java run time system does not need to know about files and file systems because of class loaders. Delegation is an important concept to understand when learning about class loaders.
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.[2] 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.
When the JVM is started, three class loaders are used[3][4]:
The bootstrap class loader loads the core Java libraries[5] (<JAVA_HOME>/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>/lib/ext or any other directory specified by the java.ext.dirs system property). It is implemented by the sun.misc.Launcher$ExtClassLoader class.
The system class loader loads code found on java.class.path, which maps to the system CLASSPATH variable. This is implemented by the sun.misc.Launcher$AppClassLoader class.
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By default, all user classes are loaded by the default system class loader, but it is possible to replace it by a user and even to chain class loaders as desired.
This makes it possible (for example):
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.[2][7]
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.[8] Three ways JAR hell can occur are:
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, 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.[9]