In Java, both ClassNotFoundException and NoClassDefFoundError occur when a particular class is not found at run time. But, they occur at different scenarios. ClassNotFoundException is an exception which occurs when you try to load a class at run time using Class.forName() or loadClass() methods and mentioned classes are not found in the classpath. On the other hand, NoClassDefFoundError is an error which occurs when a particular class is present at compile time but it was missing at run time. In this tutorial, we will see the differences between ClassNotFoundException Vs NoClassDefFoundError in java and when they occur.

ClassNotFoundException 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,

java.lang.ClassNotFoundException: oracle.jdbc.driver.OracleDriver at java.net.URLClassLoader.findClass(Unknown Source) at java.lang.ClassLoader.loadClass(Unknown Source) at sun.misc.Launcher$AppClassLoader.loadClass(Unknown Source) at java.lang.ClassLoader.loadClass(Unknown Source) at java.lang.Class.forName0(Native Method) at java.lang.Class.forName(Unknown Source) at pack1.MainClass.main(MainClass.java:17)

NoClassDefFoundError In Java :

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. For example, compile the below program.

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 is B.class. If you remove the A.class file and run the B.class file, Java Runtime System will throw NoClassDefFoundError like below,

Exception in thread "main" java.lang.NoClassDefFoundError: A at MainClass.main(MainClass.java:10) Caused by: java.lang.ClassNotFoundException: A at java.net.URLClassLoader.findClass(URLClassLoader.java:381) at java.lang.ClassLoader.loadClass(ClassLoader.java:424) at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:331) at java.lang.ClassLoader.loadClass(ClassLoader.java:357)

Below is the quick recap of above findings.

Difference Between ClassNotFoundException Vs NoClassDefFoundError In Java :