Difference between ClassNotFoundException Vs NoClassDefFoundError In Java

ClassNotFoundException is a run time exception which is thrown when an application tries to load a class at run time using Class.forName() or loadClass() or findSystemClass() methods and the class with specified name are not found in the classpath.

For example, you may have come across this exception when you try to connect to MySQL or Oracle databases and you have not updated the classpath with required JAR files. In most of time, this exception occurs when you try to run an application without updating the classpath with required JAR files.

For example, below program will throw ClassNotFoundException if the mentioned class “oracle.jdbc.driver.OracleDriver” is not found in the classpath.

public class MainClass

{

    public static void main(String[] args)

    {

        try

        {

            Class.forName("oracle.jdbc.driver.OracleDriver");

        }

        catch (ClassNotFoundException e)

        {

            e.printStackTrace();

        }

    }

}

If you run the above program without updating the classpath with required JAR files, you will get the exception like below,

NoClassDefFoundError is an error which is thrown when Java Runtime System tries to load the definition of a class and class definition is no longer available. The required class definition was present at compile time but it was missing at run time. 

class A

{

}

public class B

{

    public static void main(String[] args)

    {

        A a = new A();

    }

}

When you compile the above program, two .class files will be generated. One is A.class and another one isB.class. If you remove the A.class file and run the B.class file, Java Runtime System will throw NoClassDefFoundError like below,