classLoader

A ClassLoader is a fundamental component of the Java Runtime Environment (JRE) responsible for dynamically loading Java classes into the Java Virtual Machine (JVM). When a Java program needs to use a class, the JVM delegates the task of finding and loading that class's bytecode to a ClassLoader. This process is crucial for the Java platform's ability to be platform-independent and for its dynamic loading capabilities.

Java employs a hierarchical ClassLoader architecture. Typically, there are three main types of ClassLoaders: the Bootstrap ClassLoader, the Extension ClassLoader, and the System ClassLoader (also known as the Application ClassLoader). The Bootstrap ClassLoader, written in native code, is the parent of all other ClassLoaders and is responsible for loading core Java API classes. The Extension ClassLoader loads classes from directories specified by the Java extension properties. The System ClassLoader loads classes from the application's classpath.

When a ClassLoader is asked to load a class, it follows a delegation model. It first asks its parent ClassLoader to load the class. If the parent can load the class, it does so. Only if the parent cannot find and load the class does the current ClassLoader attempt to load it itself. This delegation ensures that classes are loaded consistently and prevents duplicate loading of the same class.

ClassLoaders are also essential for features like hot-swapping code, custom security policies, and the operation of application servers, where different applications may need to load their own versions of libraries without conflicts. The ClassLoader mechanism provides a powerful and flexible way to manage and load code in the Java ecosystem.