Differnce between NoClassDefFoundError and ClassNotFoundException

[Sajee Joseph]

Hello all,

What is the difference between NoClassDefFoundError and ClassNotFoundException. I know one is an Error and other is an exception. Can someone explain in terms of an example?

Regards,
Saj

[Maulin Vasavada]

Hi sajee

ClassNotFound is thrown when a system can't find a class its trying to load. e.g.
Class.forName("someClass"); and if system can't find the class with "someClass" name it will throw this exception.

example,
Create a class D which tries to load class named "someClass" which doesn't exist,

$ cat D.java
public class D {
        public static void main(String[] args) throws Exception {
                Class.forName("someClass");
        }
}

Here is the output when you try to run the class D,

$ java D
Exception in thread "main" java.lang.ClassNotFoundException: someClass
        at java.net.URLClassLoader$1.run(URLClassLoader.java:199)
        at java.security.AccessController.doPrivileged(Native Method)
        at java.net.URLClassLoader.findClass(URLClassLoader.java:187)
        at java.lang.ClassLoader.loadClass(ClassLoader.java:289)
        at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:274)
        at java.lang.ClassLoader.loadClass(ClassLoader.java:235)
        at java.lang.ClassLoader.loadClassInternal(ClassLoader.java:302)
        at java.lang.Class.forName0(Native Method)
        at java.lang.Class.forName(Class.java:141)
        at D.main(D.java:3)

NoClassDefFound is thrown when the system can't find a dependent class which was there when the containing class got compiled. e.g.
Create two classes A.java , B.java like,

$ cat A.java
public class A {
        static B b;
        public static void main(String[] args) {
                b = new B();
        }
}
 
$ cat B.java
public class B {
}
 

Now compile A class like,
$javac A.java

It will get compiled and you will see A.class and B.class created.

You can run class A like,
$java A

This will work fine.

Now rename the B.class file with something else, say C.class.

Now run class A again,
$java A

It will give this error,

$ java A
Exception in thread "main" java.lang.NoClassDefFoundError: B
        at A.main(A.java:4)
 

Hope this helps.

Similar google groups link,
http://groups.google.com/group/comp.lang.java.programmer/browse_thread/thread/c8c15027576869f/dad6446f897f8946?lnk=st&q=classnotfoundexception+noclassdeffounderror&rnum=1#dad6446f897f8946

Regards,
Maulin

[Maulin Vasavada]

The reason one is Exception and the other is Error is,

When a system can't find a class that needs to be loaded then its an exceptional situation which the caller of the program (client) might want to handle in some way ...

BUT when a system can't find a class which was there originally on compilation time (otherwise we would get symbol not resolved error on compile time for class B in our example above), THEN its an Error. Hence it throws NoClassDefFoundError.

Regards
Maulin