Sunday, May 16, 2010

Dynamic Class Loading

Static :

  • Classes are statically loaded with Java's "new" operator.

class MyClass {
public static void main(String args[]) {
Car c = new Car();

  • A NoClassDefFoundException is thrown if a class is referenced with Java's "new" operator (i.e. static loading) but the runtime system cannot find the referenced class.

Dynamic loading:

  • Dynamic loading is a technique for programmatically invoking the functions of a class loader at run time. Let us look at how to load classes dynamically.

Class.forName (String className);                    //static method which returns a Class

The above static method returns the class object associated with the class name. The string className can be supplied dynamically at run time. Unlike the static loading, the dynamic loading will decide whether to load the class Car or the class Jeep at runtime based on a properties file and/or other runtime conditions. Once the class is dynamically loaded the following method returns an instance of the loaded class. It's just like creating a class object with no arguments.

  • class.newInstance (); //A non-static method, which creates an instance of a

//class (i.e. creates an object).
Jeep myJeep = null ;

//myClassName should be read from a .properties file or a Constants class.
// stay away from hard coding values in your program. CO

String myClassName = "" ;
Class vehicleClass = Class.forName(myClassName) ;
myJeep = (Jeep) vehicleClass.newInstance();

  • A ClassNotFoundException is thrown when an application tries to load in a class through its string name using the following methods but no definition for the class with the specified name could be found:

-The forName(..) method in class - Class.
-The findSystemClass(..) method in class - ClassLoader.
- The loadClass(..) method in class - ClassLoader.