Java - ClassNotFoundException

Imagine you’re watching a movie, and suddenly a character is mentioned who hasn’t appeared yet. You pause and check the cast list, but… they don’t exist! Java feels the same way when it’s asked to find a class that isn’t there—it throws a ClassNotFoundException.

In technical terms, this happens when Java tries to load a class dynamically (meaning at runtime, not during compilation), but it can’t find it in the classpath.

Constructors of ClassNotFoundException

There are two constructors of the ClassNotFoundException class:

1. ClassNotFoundException(): This constructor is used to create a ClassNotFoundException object without any message.
2. ClassNotFoundException(String message): This constructor is used to create a ClassNotFoundException object with a message.

How do you get a ClassNotFoundException?

There are several ways to get a ClassNotFoundException:

• You use Class.forName("SomeClass"), but SomeClass isn’t available.
• You call ClassLoader.loadClass("SomeClass"), and Java can’t locate it.
• A required JAR file is missing from your project.

Example of ClassNotFoundException using Class.forName()

In this example, we are trying to load a class that doesn’t exist, so JVM will throw a ClassNotFoundException.

public class ClassNotFoundExample {
   public static void main(String[] args) {
      Class.forName("com.tutorialspoint.Test);
   }
}

Following is the output of the above code:

ClassNotFoundExample.java:3: error: unreported exception ClassNotFoundException; must be caught or declared to be thrown
      Class.forName("com.tutorialspoint.Test");
                   ^
1 error

As you can see in the output, JVM throws a ClassNotFoundException because we are trying to load a class that doesn’t exist.

Example of ClassNotFoundException using ClassLoader.loadClass()

In this example, we are trying to load a class that doesn’t exist, so JVM will throw a ClassNotFoundException.

public class ClassNotFoundExample {
   public static void main(String[] args) {
      ClassLoader.getSystemClassLoader().loadClass("com.tutorialspoint.Test");
   }
}

Following is the output of the above code:

    ClassNotFoundExample.java:3: error: unreported exception ClassNotFoundException; must be caught or declared to be thrown
    ClassLoader.getSystemClassLoader().loadClass("com.tutorialspoint.Test");
                                                ^
1 error

How to handle ClassNotFoundException

In this example, we are trying to load a class that doesn’t exist, so JVM will throw a ClassNotFoundException. We are handling this exception using a try-catch block.

public class ClassNotFoundExample {
   public static void main(String[] args) {
      try {
         Class.forName("com.tutorialspoint.Test");
      } catch(ClassNotFoundException e) {
         System.out.println("Class not found");
      }
   }
}

Following is the output of the above code:

Class not found

How to avoid ClassNotFoundException?

Here are some ways to avoid ClassNotFoundException:

• Check the classpath to ensure the required class is available.
• Check the spelling of the class name.
• Check the package name of the class.

By following the above ways, you can avoid the ClassNotFoundException in Java.