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Class loaders
Whether you knew it at the time, if you've run a Java program, you have already been using class loaders. Every type (class or interface) used by a Java program must be loaded into the Java virtual machine (JVM), and the JVM loads types through class loaders.

Java has two kinds of class loaders -- the primordial class loader and class loader objects. The primordial class loader is sometimes called the system class loader or the default class loader. Class loader objects are sometimes called custom class loaders.

The difference between these two kinds of class loaders is important to understand. Whereas the primordial class loader is part of the JVM implementation, class loader objects are part of the running Java application.

For example, when I downloaded the JDK for Windows 95, I got a JVM. This JVM, to the best of my knowledge, is written primarily in C. In my case, the primordial class loader is a part of the C program that defines my JVM.

Armed with this installation of Java on my computer, I can then write and run a Java application that uses class loader objects. In this case, I would define class loaders in Java, which my Java application would instantiate and use.

So the difference is this: The primordial class loader (there is only one of these per JVM implementation) is designed and written by the creators of each JVM. Class loader objects are designed and written by Java programmers. Class loader objects are defined as classes (subclasses of java.lang.ClassLoader) and instantiated into regular Java objects on the heap.

Whenever the JVM loads a class or interface, it will use either the primordial class loader or a class loader object. Because it is part of the JVM implementation, one (and only one) primordial class loader is always available to a running application. Class loader objects, by contrast, behave like objects. At any one time during an application's lifetime, the application may have zero to many class loader objects in existence and in use.