WebSphere Application Server V8.5 Administration and Configuration Guide for the Full Profile

790 WebSphere Application Server V8.5 Administration and Configuration Guide for the Full Profile

22.1 JVM class loaders

Class loaders enable the Java virtual machine (JVM) to load classes. Given the name of a

class, the class loader locates the definition of this class. Each Java class must be loaded by

a class loader.

When you start a JVM, it uses the following class loaders:

 The

bootstrap class loader loads only the core Java libraries in the Java_home/jre/lib

directory. This class loader, which is part of the core JVM, is written in native code.

 The

extensions class loader loads the code in the extensions directories

(Java_home/jre/lib/ext or any other directory that is specified by the java.ext.dirs

system property). This class loader is implemented by the

sun.misc.Launcher$ExtClassLoader class.

 The

application class loader loads 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. However, a class loader can delegate requests only to its parent class loader

and never to its child class loaders. A class loader can go up the hierarchy but never down.

The extensions class loader is the parent for the application class loader. The bootstrap class

loader is the parent for the extensions class loader. Figure 22-1 shows the class loaders

hierarchy.

Figure 22-1 Java class loaders hierarchy

Note: Keep in mind that each JVM has its own set of class loaders. In an environment that

is hosting multiple application servers (JVMs), the class loaders for the JVMs are

completely separate even if they are running on the same physical machine.

Bootstrap class loader

Extensions class loader

Application class loader