Thursday, April 12, 2007

What is a class loader and what is its responsibilities?

The Class loader is a subsystem of a JVM which is responsible,predominantly for loading classes and interfaces in the system.Apart from this,a class loader is responsible for the following activities:

-Verification of imported types(classes and interfaces)

-Allocating memory for class variables and initializing them to default values.Static fields for a class are created and these are set to standard default values but they are not explicitly initialized.The method tables are constructed for the class.

-Resolving symbolic references from type to direct references The class loaders can be of two types: a bootstrap or primordial class loader and user defined class loaderEach JVM has a bootstrap class loader which loads trusted classes , including classes from Java API.JVM specs do not tell how to locate these classes and is left to implementation designers.

A Java application with user defined class loader objects can customize class loading.These load untrustworthy classes and not an intrinsic part of JVM.They are written in Java,converted to class files and loaded into the JVM and installed like any other objects.

If you want to read this in more details then read Chapter 8,"The Linking Model" of Inside Java 2 Virtual Machine by Bill Venners.

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