NoClassDefFoundError vs ClassNotFoundException

ClassNotFoundException and NoClassDefFoundError occur when a particular class is not found at runtime. However, they occur at different scenarios.

NoClassDefFoundError:

Thrown if the Java Virtual Machine or a ClassLoader instance tries to load in the definition of a class (as part of a normal method call or as part of creating a new instance using the new expression) and no definition of the class could be found.

The searched-for class definition existed when the currently executing class was compiled, but the definition can no longer be found.

ClassNotFoundException:

Thrown when an application tries to load in a class through its string name using:

  • The forName method in class Class.
  • The findSystemClass method in class ClassLoader.
  • The loadClass method in class ClassLoader.

but no definition for the class with the specified name could be found. For example, you may have come across this exception when you try to connect to MySQL or Oracle databases and you have not updated the classpath with required JAR files. Most of the time, this exception occurs when you try to run an application without updating the classpath with required JAR files.

ClassNotFoundException NoClassDefFoundError
It is an exception. It is of type java.lang.Exception. It is an error. It is of type java.lang.Error.
It occurs when an application tries to load a class at run time which is not updated in the classpath. It occurs when java runtime system doesn’t find a class definition, which is present at compile time, but missing at run time.
It is thrown by the application itself. It is thrown by the methods like Class.forName(), loadClass() and findSystemClass(). It is thrown by the Java Runtime System.
It occurs when classpath is not updated with required JAR files. It occurs when required class definition is missing at runtime.

 

 

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