The Java Class Loader is a part of the Java Runtime Environment that dynamically loadsJava 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 called 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 or they may be composed, at least in part, of a number of libraries. When the JVM is started, three class loaders are used:
Bootstrap class loader
Extensions class loader
System class loader
The bootstrap class loader loads the core Java libraries located in the /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. It is implemented by the sun.misc.Launcher$ExtClassLoader class. The system class loader loads code found on java.class.path, which maps to the CLASSPATH environment variable. This is implemented by the sun.misc.Launcher$AppClassLoader class.
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 :
to load or unload classes at runtime. This is an important feature for:
* implementing scripting languages, such as Jython
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
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. 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 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 nested, cooperating classloaders. Classes loaded by different classloaders may interact in complex ways not fully comprehended by a developer, leading to errors or bugs that are difficult to analyze, explain, and resolve.
The OSGi Alliance specified 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, JAR files 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 Platform Module System — intended to introduce a new distribution format, a modules versioning scheme, and a common modules repository. In December 2008, Sun announced that JSR 277 was put on hold. The Java Module System was later rebooted as "project Jigsaw" which was included in Java 9.
