Java ClassNotFoundException - java.lang.ClassNotFoundException

• Java ClassNotFoundException occurs when the application tries to load a class but Classloader is not able to find it in the classpath.
• Common causes of java.lang.ClassNotFoundException are using Class.forName or ClassLoader.loadClass to load a class by passing String name of a class and it’s not found on the classpath.
• ClassNotFoundException is a checked exception, so it has to be catch or thrown to the caller.
• ClassNotFoundException always occurs at runtime because we are indirectly loading the class using Classloader. Java compiler has no way to know if the class will be present in the classpath at runtime or not.
• One of the most common example of ClassNotFoundException is when we try to load JDBC drivers using Class.forName but forget to add it’s jar file in the classpath.

Let’s look at a simple example where we will get ClassNotFoundException.

package com.journaldev.exceptions;

public class DataTest {

  public static void main(String[] args) {
    try {
      Class.forName("com.journaldev.MyInvisibleClass");

      ClassLoader.getSystemClassLoader().loadClass("com.journaldev.MyInvisibleClass");

      ClassLoader.getPlatformClassLoader().loadClass("com.journaldev.MyInvisibleClass");
    } catch (ClassNotFoundException e) {
      e.printStackTrace();
    }
  }

}

Note that com.journaldev.MyInvisibleClass doesn’t exist, so When we execute above program, we get following exception stack trace.

java.lang.ClassNotFoundException: com.journaldev.MyInvisibleClass
	at java.base/jdk.internal.loader.BuiltinClassLoader.loadClass(BuiltinClassLoader.java:582)
	at java.base/jdk.internal.loader.ClassLoaders$AppClassLoader.loadClass(ClassLoaders.java:185)
	at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:496)
	at java.base/java.lang.Class.forName0(Native Method)
	at java.base/java.lang.Class.forName(Class.java:292)
	at com.journaldev.exceptions.DataTest.main(DataTest.java:7)

In above example, all the three statements will throw java.lang.ClassNotFoundException.

It’s very easy to fix ClassNotFoundException because the exception stack trace clearly specifies the class not found. Just check for classpath settings and make sure class it’s present at runtime.

NoClassDefFoundError is a runtime error thrown when a class is not found at runtime. It’s very similar to ClassNotFoundException. Read more at Java NoClassDefFoundError. Reference: API Doc