Java Classloaders

The Java platform ships with a number of typical classloaders, which are used to do different jobs during the startup and normal operation of the platform:

Primordial (or bootstrap) classloader—This is instantiated very early in the process of starting up the VM, and is usually implemented as native code. It’s often best to think of it as being a part of the VM itself. It’s typically used to get the basic system JARs—basically rt.jar—loaded and it does no verification.


Extension classloader—This is used to load installation-wide standard extensions. This often includes security extensions.

Application (or system) classloader—This is the most widely used classloader. It’s the one that will load the application classes and do the majority of the work in in most SE environments.


Custom classloader—In more complex environments, such as EE or the more sophisticated SE frameworks, there will often be a number of additional (a.k.a. custom) classloaders. Some teams even write classloaders that are specific to their individual applications. In addition to their core role, classloaders are also often used to load resources (files that aren’t classes, such as images or config files) from JAR files or other locations on the classpath.

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