What is the Difference Between NoClassDefFoundError and ClassNotFoundException in Java

Hi, In this tutorial I will explain the difference between NoClassDefFoundError and ClassNotFoundException in Java.

Errors and exceptions are unavoidable issues that developers encounter when working with Java programming. Two common errors among the many exceptions are NoClassDefFoundError and ClassNotFoundException.

Both errors in Java are connected to classpath and Java ClassLoader issues, but they happen in distinct ways and have different names in the language. I'll illustrate the variations between these two errors.

What is NoClassDefFoundError?

NoClassDefFoundError is an error that occurs when the Java Virtual Machine (JVM) or a ClassLoader instance tries to load a class that is present at compile-time but is absent at runtime.

Example of NoClassDefFoundError:

In this example, I will show you the NoClassDefFoundError error example. In this example, I will generate two class files and I will delete one class file and show you how JVM throws a NoClassDefFoundError.

class Apple   
{   
    void display()   
    {   
        System.out.println( "apple fruit color is red" );   
    }   
}   
class NoClassDefFoundErrorExample
{   
    public static void main(String args[])    
    {   
        Apple obj = new Apple();   
        obj.display();          
    }   
} 

Output:

apple fruit color is red

Explanation:

In the above Java program I created the Apple named parent class and created NoClassDefFoundErrorExample is the main class in the code above and I have created the void type display() method within the Apple class..

I created an Apple class object and invoked its method in the main method.

In the above program, I mentioned I have generated two .class files upon compilation. The first is Apple.class and the second is NoClassDefFoundErrorExample.class. 

The above program I have compiled and executed successfully.

Now,

The Java Runtime System will throw a NoClassDefFoundError similar to this one if we delete the Apple.class file and execute the NoClassDefFoundErrorExample.class file:

See the screenshot below I deleted Apple.class file.

Output:

After I deleted Apple.class file I again ran the program without compiling after that a NoClassDefFoundError occurred.

H:\Hamid>java NoClassDefFoundErrorExample
Exception in thread "main" java.lang.NoClassDefFoundError: Apple
        at NoClassDefFoundErrorExample.main(NoClassDefFoundErrorExample.java:12)
Caused by: java.lang.ClassNotFoundException: Apple
        at java.net.URLClassLoader.findClass(URLClassLoader.java:381)
        at java.lang.ClassLoader.loadClass(ClassLoader.java:424)
        at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:338)
        at java.lang.ClassLoader.loadClass(ClassLoader.java:357)

What is ClassNotFoundException Error?

Java developers are probably familiar with the term ClassNotFoundException. 

The ClassNotFoundException error happens when a class is not found in the classpath when the Java Virtual Machine (JVM) tries to load it at runtime using the Class.forName() or ClassLoader.loadClass() methods. To put it another way, during execution, the JVM is unable to locate the class file.

Example of ClassNotFoundException:

We try to connect to the Oracle database in this Java program using the Java Database Connectivity (JDBC) driver. The driver is loaded using the Class class's forName method. A ClassNotFoundException will be thrown by this program since the driver's JAR file is not present in the classpath.

import java.sql.Connection;
import java.sql.DriverManager;

public class ClassNotFoundExceptionExample 
{
    public static void main(String args[]) 
	{
        try 
		{
            Class.forName("oracle.jdbc.driver.OracleDriver");
            Connection con = DriverManager.getConnection("jdbc:oracle:thin:@localhost:1521:xe","system","tiger");
            System.out.println("database Connect successfully");
        } 
		catch (Exception e) 
		{
            System.out.println("Unable to connect to the database");
            e.printStackTrace();
        }
    }
}

Output:

H:\Hamid>java ClassNotFoundExceptionExample
Unable to connect to the database
java.lang.ClassNotFoundException: oracle.jdbc.driver.OracleDriver
        at java.net.URLClassLoader.findClass(URLClassLoader.java:381)
        at java.lang.ClassLoader.loadClass(ClassLoader.java:424)
        at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:338)
        at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
        at java.lang.Class.forName0(Native Method)
        at java.lang.Class.forName(Class.java:264)
        at ClassNotFoundExceptionExample.main(ClassNotFoundExceptionExample.java:10)

The key differences between NoClassDefFoundError and ClassNotFoundException I outlined in the following table:

Attribute	NoClassDefFoundError	ClassNotFoundException
Timing	Runtime	Runtime
Cause	Missing Class at Runtime	Class Not Found
Methods	Thrown by the JVM	Thrown by Class.forName() or ClassLoader.loadClass()

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