Difference between ClassNotFoundException vs NoClassDefFoundError in Java

Background

In last post we saw how classloading works in Java -

• How classloader works in Java

In this post we will try to understand the difference between ClassNotFoundException vs NoClassDefFoundError thet generally bugs all Java developers.

If you have not gone through above link I strongly suggest you do it right away. It will give you very good understanding on class loading mechanism that we will be using shortly to understand these two situations.

Difference between ClassNotFoundException vs NoClassDefFoundError in Java

• First point to note is their types. ClassNotFoundException is a checked exception. So you will need to handle it. Either catch it or throw it in method signature. NoClassDefFoundError is an error. Error are generally something you cannot recover from. You can still catch and handle it though.
• Both things are related to class not available during runtime. Difference is the cause of non availability which we will see shortly. 
• ClassNotFoundException is throw by running application where as NoClassDefFoundError is thrown by Java runtime.

ClassNotFoundException : 

We have already seen an example of ClassNotFoundException in last post when we tried to load our custom class with Extension classloader which was parent of Application classloader that actually loaded the class. We said parent classloader does not have visibility of classes loaded by child class loaders. So it threw ClassNotFoundException. Generally ClassNotFoundException is thrown when you try to load a custom class using methods like -

• Class.forName()
• ClassLoader.loadClass()
• ClassLoader.findSystemClass()

and the required class is not found in the classpath. This could be because your classpath is incorrectly configured. Famous example is when you connect to a database using Jave you load the driver class using - Class.forName() [We do this explicit loading pre Java 6. From java 6 this class loading happens automatically. All you have to do is make sure driver jar is present in classpath] -

• Connect to a database in Java
• Understanding JDBC and building database applications with it

So for above usecase if you don have a driver class in your classpath then it will led to  ClassNotFoundException. Also you must have noticed by not you need to explicitly handle this exception since this is checked exception.

To resolve this issue you need to check that the class is available in your classpath.

NoClassDefFoundError:

Now this unlike ClassNotFoundException is an Error which is hard to recover from. This generally happens when class is available at compile time but not available at runtime.

One example can be static method/block of a class throws error due to which class does not load (though it was available at compile time and went through in compilation phase). Now if such a class is reference at runtime then it will throw NoClassDefFoundError.

To resolve this error you need check your classpath. It is possible that in your configuration (say in gradle files) you have added jar is lets say test configuration only and not in runtime or compile time configuration. You also need to lookout for any Initialization errors in the logs.

Related Links

• How classloader works in Java(OSFG) 
• Connect to a database in Java(OSFG)
• Understanding JDBC and building database applications with it(OSFG)