Differences Between ClassNotFoundException and NoClassDefFoundError in Java

In Java, both ClassNotFoundException and NoClassDefFoundError are issues that occur when the JVM or ClassLoader is not able to find the appropriate class at the time of loading (run-time). The ClassNotFoundException is a checked exception, and NoClassDefFoundError is an Error that comes under unchecked.

There are different types of ClassLoaders, each responsible for loading classes from different sources such as directories, JAR files, or network locations. If a required class is missing due to an incorrect classpath or a missing JAR file, the ClassLoader might fail to load it. This situation leads to one of these two issues.

The ClassNotFoundException in Java

The ClassNotFoundException is thrown when we try to load a class explicitly at runtime using Reflection, and that particular class is not found or missing in the classpath. There is nothing to check at compile-time since it is loading the class at run-time.

Example

Following is an example of ClassNotFoundException:

public class ClassNotFoundExceptionTest {
   public static void main(String[] args) {
      try {
         Class.forName("Test");
      } catch (ClassNotFoundException cnfe) {
         System.err.println("You are trying to search for a class is not existing. "+cnfe);
      }
   }
}

Output of the above Java program is as follows:

You are trying to search for a class is not existing. java.lang.ClassNotFoundException: Test

The NoClassDefFoundError

The NoClassDefFoundError occurs when a class has been compiled with a specific class from the classpath, but the same class is missing during run-time. The most basic reason to get this Error is when a class used by another class has been deleted or is no longer accessible during execution.

Example

The following Java program shows the NoClassDefFoundError:

class Test1 {
   public void show() {
      System.out.println("show() method called");
   }
}
public class Test2 {
   public static void main(String[] args) {
      Test1 t = new Test1();
      t.show();
   }
}

When we compile both the classes, it will generate two class files, Test1.class and Test2.class. While running the Test2 class, just remove the Test1.class file, then we will get NoClassDefFoundError as shown below:

Exception in thread "main" java.lang.NoClassDefFoundError: Test1
at Test2.main(Test2.java:9)

ClassNotFoundException VS NoClassDefFoundError

Now, let's discuss the difference between ClassNotFoundException and NoClassDefFoundError in Java: