Java Class Loader

When you start a JVM, you use three class loaders: the bootstrap class loader, the extensions class loader, and the application class loader.

1. The bootstrap class loader is responsible for loading only the core Java libraries, that is vm.jar, core.jar, and so on, in the <JAVA_HOME>/jre/lib directory. This class loader, which is part of the core JVM, is written in native code.


2. The extensions class loader is responsible for loading the code in the extensions directories (<JAVA_HOME>/jre/lib/ext or any other directory specified by the java.ext.dirs system property). This class loader is implemented by the sun.misc.Launcher$ExtClassLoader class.


3.  The application class loader is responsible for loading the code that is found on java.class.path, which ultimately maps to the system CLASSPATH variable. This class loader is implemented by the sun.misc.Launcher$AppClassLoader class.

The parent-delegation model is a key concept to understand when dealing with class loaders. It states that a class loader delegates class loading to its parent before trying to load the class itself. The parent class loader can be either another custom class loader or the bootstrap class loader. But what is very important is that a class loader can only delegate requests to its parent class loader, never to its child class loaders