Java Class Loader

Classloader is a part of the Java Runtime Environment that dynamically loads Java classes into the Java Virtual Machine. It is an object that is responsible for loading classes. The class java.lang.ClassLoader is an abstract class. Given the binary name of a class, a class loader should attempt to locate or generate data that constitutes a definition for the class. A typical strategy is to transform the name into a file name and then read a "class file" of that name from a file system.

The following picture illustrates the hierarchy of class loaders. Root loader is bootstrap class loader which has native implementation and cannot be instantiated by Java code.

The class loaders in Java are organized in a tree. Class loader determines the class looking up in cache, paret and self in order.

However, some classes may not originate from a file; they may originate from other sources, such as the network, or they could be constructed by an application. The method defineClass converts an array of bytes into an instance of class Class. Instances of this newly defined class can be created using Class.newInstance.

The methods and constructors of objects created by a class loader may reference other classes. To determine the class(es) referred to, the Java virtual machine invokes the loadClass method of the class loader that originally created the class.

For example, an application could create a network class loader to download class files from a server. Sample code might look like:

   ClassLoader loader = new NetworkClassLoader(host, port);

   Object main = loader.loadClass("Main", true).newInstance();

          . . .

The network class loader subclass must define the methods findClass and loadClassData to load a class from the network. Once it has downloaded the bytes that make up the class, it should use the method defineClass to create a class instance. A sample implementation is:

public class NetworkClassLoader extends ClassLoader {

    String host;

    int port;

    public Class findClass(String name) {

        byte[] b = loadClassData(name);

        return defineClass(name, b, 0, b.length);

    }

    private byte[] loadClassData(String name) {

        // load the class data from the connection

      byte[] b = null;

      //...

      return b;

    }

}

Binary names

Any class name provided as a String parameter to methods in ClassLoader must be a binary name as defined by the Java Language Specification.

Examples of valid class names include:

   "java.lang.String"

   "javax.swing.JSpinner$DefaultEditor"

   "java.security.KeyStore$Builder$FileBuilder$1"

   "java.net.URLClassLoader$3$1"