Java class loaders

Class loaders are hierarchical. Classes are introduced into the JVM as they are referenced by name in a class that is 

already running  in the JVM. So, how is the very first class loaded? The very first class is especially loaded with the help of static main( ) method declared in your class. All the subsequently loaded classes are loaded by the classes, which are already loaded and running. A class loader creates a namespace. All JVMs include at least one class loader that is embedded within the JVM called the primordial (or bootstrap) class loader.

Now let’s look at non-primordial class loaders. The JVM has hooks in it to allow user defined class loaders to be used in place of primordial class loader. Let us look at the class loaders created by the JVM.

Bootstrap (primordial)     :

Loads JDK internal classes,

java.* packages. (as defined in the sun.boot.class.pathsystem property, typically loads rt.jar and i18n.jar)

Extensions :

Loads jar files from JDK extensions directory (as defined in the java.ext.dirs system

property – usually lib/ext directory of the JRE)

System :

 Loads classes from system classpath (as defined by the java.class.path property,which
is set by the CLASSPATH environment variable or -classpath or -cp command line options)
Class loaders are hierarchical and use a delegation model when loading a class. Class loaders request their parent to load the class first before attempting to load it themselves. When a class loader loads a class, the child class loaders in the hierarchy will never reload the class again. Hence uniqueness is maintained. Classes loaded