Class loading -static vs dynamic

Static Class Loading

• 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

String myClassName = "au.com.Jeep" ;

Class vehicleClass = Class.forName(myClassName) ;

myJeep = (Jeep) vehicleClass.newInstance();

myJeep.setFuelCapacity(5);

• 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.