Hi
see below tutorial it might be useful for u.
A class loader is an object that is responsible for loading classes.
The class ClassLoader is an abstract class. Given the 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.
Every Class object contains a reference to the ClassLoader that
defined it.
Class objects for array classes are not created by class loaders, but
are created automatically as required by the Java runtime. The class
loader for an array class, as returned by Class.getClassLoader() is
the same as the class loader for its element type; if the element type
is a primitive type, then the array class has no class loader.
Applications implement subclasses of ClassLoader in order to extend
the manner in which the Java virtual machine dynamically loads
classes.
Class loaders may typically be used by security managers to indicate
security domains.
The ClassLoader class uses a delegation model to search for classes
and resources. Each instance of ClassLoader has an associated parent
class loader. When requested to find a class or resource, a
ClassLoader instance will delegate the search for the class or
resource to its parent class loader before attempting to find the
class or resource itself. The virtual machine's built-in class loader,
called the "bootstrap class loader", does not itself have a parent but
may serve as the parent of a ClassLoader instance.
Normally, the Java virtual machine loads classes from the local file
system in a platform-dependent manner. For example, on UNIX systems,
the virtual machine loads classes from the directory defined by the
CLASSPATH environment variable.
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:
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
. . .
}
}
